Pumps

Types of pumps used in the plant

Centrifugal Pumps

Centrifugal pumps are used in industry for applications where large volumes of fluids must be moved. Centrifugal pumps have been widely accepted in the petroleum industry because of their versatility, simple construction, and low initial cost. Operating costs are lower for centrifugal pumps than other types because of minimal maintenance and ease of repair.

Unlike positive displacement pumps, centrifugal pumps will not continue to produce a head when operating against a closed discharge. Centrifugal pumps perform best when pumping low-viscosity fluids. Centrifugal pumps can be classified in several ways. They can be divided according to:• The kind of impeller they contain.• The number of stages they have.• Their axis of rotation.• The method used to drive them.• Their configuration or appearance.

Because centrifugal pumps are available in a great Varity of styles and have many different uses, they do not always resemble each other. They all have the same operating characteristics.

Centrifugal Pumps Components

The major parts of a centrifugal pump are the casing, impeller, shaft, coupling, bearings, and seals or packing (Figure 1.15 & 1.16)

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Figure 1.15 Schematic Drawing of Centrifugal Pump

Figure 1.16 Typical Centrifugal Pump

A. The casing is the largest and most visible part of the pump. It

can be constructed of cast iron, steel, bronze, or other special

material depending on type of service. The primary purpose of the

casing is to house and protect internal pump parts.

B. An impeller is the part of a centrifugal pump which imparts energy to the fluid being pumped. It is firmly attached to the shaft and rotates at the same speed as the shaft. Most impellers are made of cast iron, but stainless steel, plastic, or other materials can be used for corrosive fluids.

C. The coupling which connects the driver to the pump transmits power from the driver shaft to the pump shaft. Couplings must be able to withstand sudden changes in pump load or stoppage of the driver. They must be flexible enough to handle misalignment between the shafts as well as changes in the speed of the driver.

D. The shaft is connected to a driver (e.g., electric motor, engine, or steam turbine) and turns the impeller. The shaft is usually made of steel and rotates at the speed of the driver.

E. Bearings support the shaft and reduce the friction as the shaft rotates in the casing. They also control the forward and backward movement (thrust) of the shaft, and control the side to side (radial) movement of the shaft so that rotating parts will not rub against the pump casing. Bearings may be contained in the pump casing on small process pumps or in special housings on larger pumps.

F. Seals or packing are used to prevent or reduce fluid leakage around the shaft. Most centrifugal pumps in the petroleum industry use mechanical seals. A mechanical seal has a stationary ring secured in a seal gland which is bolted to the casing and a rotating ring attached to the shaft. Packing is composed of a series of pliable rings tightly pressed around the shaft in the stuffing box.

POSITIVE DISPLACEMENT PUMPS

IntroductionPositive displacement pumps are used in the petroleum industry for low volume applications or where high pressure is required to move fluids

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Reciprocating pumps have a piston, plunger or diaphragm moving back and forth (reciprocating) in a cylinder

Operating Principle

Positive displacement pumps work according to an old law of nature; that is, no two things can occupy the same space at the same time. An example: Fill a bucket to the top with water as shown in Figure 3.28 and then carefully drop a rock into the bucket. Some of the water is forced out of the bucket because the rock and water cannot occupy the same space at the same time. The rock displaces a volume of water equal to the volume of the rock.Fluid is drawn into the cylinder, then displacement by a piston, plunger, etc, and the fluid is forced out the cylinder. Fluid pressure is increased in the pump by the piston, plunger, etc. pressing against the fluid in the cylinder.

Because of all the moving parts in positive displacement pumps, lubrication is very important.

In addition to lubrication, positive displacement pumps need to be cooled. Pumps may be either air-cooled or water-cooled. Air-cooling relies on ambient air to cool the pump while water-cooling requires the circulation of water or another coolant through the pump casing. In larger pumps or in pumps moving high

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Figure 1.29 Positive Displacement Pump

temperature fluids, water jackets are commonly used. Water jackets are channels located in the casing around the hot sections in the pump, such as bearings and packing locations.

If the discharge line becomes blocked, the piston, plunger, etc. cannot displace the fluid in the cylinder and pressure will build up. Excessive pressure can cause the pump driver to stall or the pump casing and/or discharge piping to rupture. For safety, positive displacement pumps should always have a safety relief valve and bypass line located a short distance beyond the discharge port.A reciprocating pump works with a back-and-forth, straight-line motion. Piston: Fluid-end pistons convert mechanical energy into fluid movement. In the steam end of the direct-acting pump, the piston converts the steam into mechanical energy.Piston ring:A piston ring acts as a seal between a piston and the cylinder in which it is operating. The ring moves with the piston.Plunger: Some pumps, like the one shown In Figure 3.31, use a plunger rather than a piston in the fluid end. The plunger slides back and forth in a stationary packing rather than carrying its own seal.Cylinder: The cylinder is a tubular chamber that contains the piston or plunger.Cylinder head: The cylinder head is a cap that seals the cylinder to allow pressure build-up.Stuffing box: A stuffing box is filled with packing to prevent fluid leakage from the cylinder. It surrounds the plunger, piston rod, and connecting rod.Valves: A slide valve controls the flow of steam into the steam end of the direct-acting pump. On the fluid end of the pump, suction and discharge valves control the flow of fluid into and out of the fluid cylinder. The valve on the steam end is mechanically actuated. The valves on the fluid end are material actuated.Crankshaft: In the power pump; the crankshaft transmits motion from the prime mover to the driving components in the power end of the pump.Crosshead:

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The crosshead of the power pump converts the rotary motion of the crankshaft connecting rod into reciprocating motion.

Reciprocating Pump Types

A. Piston Pumps

One of the most familiar reciprocating pumps is the piston pump (Figure 1.37). The main components of this pump are the cylinder, piston, piston rings, suction valve, discharge valve, packing, and pump casing. The piston moves back and forth in the cylinder. Each complete movement of this piston along the cylinder length is called a stroke. Movement of the piston toward the driving section of the pump is called the backstroke (Figure 1.38). A forward stroke is movement of the piston away from the driving section of the pump. A full stroke is the movement of the piston from one end of the cylinder to the other end and back to its original position

Gear Pump

There are many types of positive displacement rotary pumps, and they are normally grouped into three basic categories that include gear pumps, screw pumps, and moving vane pumps.

There are several variations of gear pumps. The simple gear pump shown in Figure 14 consists of two spur gears meshing together and revolving in opposite directions within a casing. Only a few thousandths of an inch clearance exists between the case and the gear faces and teeth extremities. Any liquid that fills the space bounded by two successive gear teeth and the case must follow along with the teeth as they revolve. When the gear teeth mesh with the teeth of the other gear, the space between the teeth is reduced, and the entrapped liquid is forced out the pump discharge pipe. As the gears revolve and the teeth disengage, the space again opens on the suction side of the pump, trapping new quantities of liquid and carrying it around the pump case to the discharge. As liquid is carried away from the suction side, a lower pressure is created, which draws liquid in through the suction line

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Gear pump

SCREW PUMPS.

Several different types of screw pumps exist. The differences between the various types are the number of intermeshing screws and the pitch of the screws. Figure 5 shows a double-screw, low-pitch pump; and figure 6 shows a triple-screw, high-pitch pump. Screw pumps are used aboard ship to pump fuel and lube oil and to supply pressure to the hydraulic system. In the double-screw pump, one rotor is driven by the drive shaft and the other by a set of timing gears. In the triple-screw pump, a central rotor meshes with two idler rotors.In the screw pump, liquid is trapped and forced through the pump by the action of rotating screws. As the rotor turns, the liquid flows in between the threads at the outer end of each pair of screws. The threads carry the liquid along within the housing to the center of the pump where it is discharged.

Most screw pumps are now equipped with mechanical seals. If the mechanical seal fails, the stuffing box has the capability of accepting two rings of conventional packing for emergency use.

Figure 4-Double-screw, low-pitch pump.

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Figure 5 Triple-screw, high-pitch pump

Selection criteria of plant pumps

Parameter Centrifugal Pumps

Reciprocating Pumps

Rotary Pumps

Optimum Flow and

Capacity

Medium/High Low Capacity Low/Medium Capacity

Pressure Application

Low/Medium Pressure

High Pressure Low/Medium

Low Flow Rate Capability

No Yes Yes

Requires Relief Valve

No Yes Yes

Smooth or Pulsating Flow

Smooth Pulsating Smooth

Variable or Variable Constant Constant

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Constant FlowSelf-priming No Yes Yes Performance with viscosity

Not suitable for high viscosity

Suitable for high viscosity

Optimum with high viscosity

The functions of the pumps in the plant

Should be prepared by Mopco

Start up and shut down procedures

The operator can prolong the life of his centrifugal pumps and lower the maintenance frequency and cost by using sound orderly procedures for starting and stopping centrifugal pumps.

A. Starting up Procedure1. The operator should check where applicable:a. Cooling water supply to pedestal is commissioned.b. Pump jacket cooling water supply is commissioned.c. Motor cooling is commissioned.d. Gland flush is commissioned.e. Seal quench is commissioned.f. Lubrication- pump and motor bearings – have the right type of lubrication and that are lube oil levels.g. Check that the electric supply is available i.e., the switch gear is rest and okay.

Note: it depends upon production practice, but in most cases, the spare pump in any service should be left with the suction valve open and the pump under suction pressure. His should not stop the operator carrying out all the routine checks before starring the pump. There is no excuse not to check oil levels or cooling water, etc. before starting a pump.3. With the discharge valve cracked open, start the pump – if electric, by pressing the button.4. Watch discharge pressure gauge reading come up to normal. If the pump motor has an amp meter check the load. If the load remains higher than normal or if there is no discharge pressure,

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check for the reason and shut down if this condition remains. Shut down the pump and inform your supervisor immediately.

5. If the pressure is normal and the motor load is normal, open the discharge valve slowly.6. REMEMBER TO CHECK PUMP REGULARLY for bearing lubrication and temperature and any unusual noises.

B. Stopping Procedure1. Stop the pump.2. Close the discharge valve.3. Depending for what reason the pump was shut down, carry out any special instructions that might be given.4.If the pump is to be worked on, make sure that it is isolated correctly and it is safe so that no injury can happen to the person working on it.

Normal operation of the pumps

Measurement systems used to monitor the pump performance

You can monitor:

• The temperature at the pump suction and discharge flanges. • The pressure at the pump suction and discharge flanges. • A proximity gage can record the distance between the open

impeller and the pump volute. • Shaft deflection can be measured by proximity gages around

the volute. • Product flow can be measured by a variety of instruments

without penetrating the piping. • Strain gages could tell you if the rotating shaft has locked up

when the pump is stopped. • Vibration can be measured at several locations on the volute. • Noise is easily detected and a valuable source of emerging

problems. • The amount of amperage being drawn by the motor

combined with pump flow and capacity can be an excellent indication of pump performance.

THE STUFFING BOX AND SEAL AREA

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You can monitor:

• Stuffing box temperature. • Stuffing box pressure. • Liquid leakage out of the stuffing box, or air leakage in. • Stuffing box jacket inlet and outlet flow • Stuffing box jacket inlet and outlet temperature. • Seal gland flush pressure, flow and temperature. • The temperature, pressure and flow of the fluid between dual

seals. • Convection tank temperature, pressure and level. • Quench temperate and flow. • Vibration.

THE BEARING CASE

You can monitor:

• Oil temperature to let you know if the oil is about to form varnish or coke.

• Oil level. • Case pressure. • Shaft movement or thrust • The amount of water present in the oil. • Shaft speed. • Vibration • Cooling coil inlet and outlet temperature, pressure and flow.

Capacity control of the pumps in the plant

1- Centrifugal pumps

•1 DISCHARGE THROTTLING

•2 Since the pump exists to serve the requirements of the process, and one of the primary purposes of instrumentation is to adapt the equipment to the process, let us consider the pump from the point of view of the process. It can be viewed as a constant pressure device with an internal restriction.

•3 It is the restriction that gives it the "curve". It seems natural to put a valve on the discharge to further restrict the pump. This

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has the effect of rotating the curve of the pump/valve system clockwise around Ppm, as can be seen in Figure 1-3.

•4 At this point I must warn the reader that we are about to encounter a paradigm shift.

•5 The combination of pump and valve will be presented as a "black box" with a single characteristic curve which I shall term the "modified" pump curve.

•6 The more traditional way of looking at the situation is from the point of view of the pump.

•7 It sees the process system curve as having rotated counter clockwise around Plm. Figure 1-3 shows that the flow, Q1, is the same for both cases. The difference between the two pressures is the Delta P across the valve. Since the purpose of the pump is to serve the process requirements, and the purpose of the valve is to adapt the pump to the process, it makes sense to consider the valve to be part of the pump system and to use the modified pump curve rather than the modified system curve in our discussion. In any case it can be seen that a discharge valve can be used to achieve any

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operating point on the system curve so long as the point is below the pump curve.

•8 SUCTION THROTTLING

•9 The second possibility for control using valves is to place the valve in the pump suction line. This would have an identical effect on the characteristic curve, but the method has a fatal flaw – cavitation. Cavitation is a phenomenon that occurs when the pressure of a liquid is reduced below its vapour pressure and brought back up above the vapour pressure again. Bubbles of vapour form in the liquid and then collapse upon arriving at the higher pressure region. The collapse occurs at sonic speed ejecting minute jets of extremely high velocity liquid. Wherever these jets impinge on a solid surface extreme erosion occurs. Over time even the hardest materials will be destroyed. Therefore it is of utmost importance that this pressure reduction never occurs. It is prevented by having sufficient pressure available at the pump suction so that the pressure drops that occur as the liquid is drawn into the eye of the impeller are at all times above the vapour pressure of the liquid at its current temperature.

•10 An explanation of the term Net Positive Suction Head (NPSH) is in order. This is the pressure of the liquid at the pump suction in terms of feet or meters of liquid head above the vapour pressure of the liquid. The actual NPSH under operating conditions is called NPSHA and the minimum required by the pump to prevent cavitation is called NPSHR. Clearly NPSHA must be greater than NPSHR to avoid cavitation. It is safe to leave a margin of about one meter.

•11 These peculiar definitions are very reasonable in terms of the pumps actual characteristic but they cause some problems to the controls engineer. It means that the gauge pressure equivalent of a given NPSHA is proportional to the density of the liquid and is also affected by its temperature. The vapour pressure can rise dramatically as the temperature rises. This means that the NPSHA can fall without a noticeable change in pressure.

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•12 Anything that would reduce the net positive pressure at the pump inlet below the NPSHR must be absolutely avoided. Thus suction throttling is never used to control pump flow.

•13 RECYCLE CONTROL

•14 The third remaining possibility for pump control with valves is to bleed some of the discharge flow back to the pump suction or to some other point on the supply side. Once again we can view the result as a modified system curve or as a modified pump characteristic. Figure 1-4 shows both. Each curve is a rotation of the original: The modified system curve as a clockwise rotation around Plm. Note the little "tail" at the left of the modified system curve. This represents the flow through the recycle valve before the discharge check valve opens to the process. The modified pump curve has a counter clockwise rotation around the hypothetical intersection of the pump curve with the flow axis.

•15 This family of curves shows several problems with

recycle control. Firstly, the pump is not rated to discharge

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more than the flow rate at the end of the curve. It is possible, of course, to run the pump with a wide open discharge, minimum D P, but it is unhealthy for this particular pump to run at such a high rate. Excessive flow may cause cavitation damage. (Excess flow cavitation is not caused by NPSH problems but by high velocity within the internal passages of the pump.) This restriction means that the minimum discharge pressure may not be lower than the one corresponding to the maximum flow. In other words, the modified pump curve cannot reach all points on the system curve.

•16 Secondly, although many pumps are capable of operating near zero discharge pressure, the very flat pressure vs. flow curve for much of the lower range for most pumps means a change of flow has very little effect on the discharge pressure.

•17 Thus it would take a very large amount of flow to produce a small drop in pressure. In control terms this means that control would be very 'sloppy'. Discharge throttling on the other hand, allows the pump to develop the head that 'suits' it. The unwanted pressure is dropped across the valve. (Note that the curve for this particular pump rises rather steeply. It will be more easily controlled than most.)

•18 Thirdly, this method is often inefficient. Figure 1-5 shows a system curve, a pump characteristic, a discharge modified characteristic, and a recycle modified characteristic. Above these is a pump power requirement curve. In the case of discharge control, the pump is adapted to the process by dropping its discharge pressure. If one follows the flow line vertically to the actual pump curve and then beyond to the power requirement curve one arrives at its power requirement. In the case of recycle control, the pump is adapted by reducing the discharge flow. Following the pressure line to the right to the actual pump curve and then upwards to the power requirement curve one arrives at the power requirement for recycle control. Note that the power requirement curve tends to slope upward as flow increases. Therefore recycle control consumes more pump horsepower than discharge throttling when both achieve the same

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operating point. This is not always so. If the power requirement curve were flat, there would be no difference. Notice on the curve that there is a slight drop in horsepower near the right hand end. If circumstances were such that the operating point corresponded to a downward sloping power curve, recycle control would be more efficient. This is rare.

•19 SPEED CONTROL

•20 There is, of course, one other means of adapting a pump to the changing demands of the process: Speed control. The virtue of this method is that it reduces the energy input to the system instead of dumping the excess. Figure 1-6 shows a system curve superimposed on a family of curves for a variable speed pump. The curves reach all parts of the system curve below the full speed curve. Therefore this is an effective means of control. Note, however, that these curves have one feature in common with recycle control: At the far left end of the system curve the pump curve and the system curve are almost parallel. (The particular pump chosen for this example has a rather steeply rising curve near shutoff. Most are considerably flatter.) In mathematical terms this means that the intersection is poorly defined. In practical terms this means that it is difficult to maintain a precise operating point and that control is 'loose' at high turndown.

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•21 In practice, variable speed drives for centrifugal pumps are still relatively uncommon. For small pumps the power savings are not significant and for large pumps the associated electronics become very expensive. Also, they do not have the high reliability of valves. Variable speed steam turbine drives are quite common in the larger horsepower ranges. Electric variable speed drives are used in certain specialized applications such as pumps that are embedded inside a high pressure vessel. In such cases there are no alternatives.

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Low flow damage problem for centrifugal pumps

Centrifugal pumps have a minimum operating flow rate, below which the pump should not be run for long periods without sacrificing reliability. Naturally it is best, from the standpoint of long-term reliability and operating efficiency, to operate pumps close to their best efficiency point (BEP), but there may be periods where reduced flow demand or system changes cause the pump to run at reduced flow rates. Minimum flow is usually expressed as a percentage of the flow at the BEP of the pump, for a given impeller diameter. Extended operation below recommended minimum flow can lead to excessive vibration, impeller damage and premature bearing and seal failures. With most sealless (magnetic drive and canned motor) pumps, the allowable run time below the minimum flow rate may be only a matter of minutes before significant damage to the pump can occur.

The recommended minimum flow rate varies considerably from one size and type of pump to another, ranging from 10 to 60 percent of the BEP flow. The major characteristics of the pump that influence the determination of minimum flow include the energy level (horsepower per stage) of the pump, the specific hydraulic design of the impeller inlet, the mechanical design of the pump shaft and bearing system and the cost and criticality of the pump. For sealless pumps, the amount of heat generated in the canned motor or across the magnets also is a consideration, as is the specific heat of the pumped fluid. There is no accepted industry standard for minimum flow that applies to all pump types, and even different manufacturers of the same pump type may have a range of acceptable minimum flows for a given application. That being said, however, the pump manufacturer is still the best place to start for the recommended minimum flow for a particular pump installation. Some manufacturers show this information on the pump performance curve.

What to do, if anything, to protect the pump from the consequences of low-flow damage is an economic decision made by the user. This analysis considers the cost of the pump, minimum flow protection system and downtime/lost production, in addition to energy and maintenance costs. Other potential factors may include health, safety and environmental risks.

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A significant majority (estimated at over 80 percent) of centrifugal pumps have no minimum flow protection whatsoever. The vast majority of the pumps installed annually are fairly low horsepower pumps used for transfer or cooling, which are not normally expected to operate over a wide range of flow. These pumps are unlikely to operate below the minimum flow point, except for the inadvertent closure of the main discharge valve or other inadvertent blockage of the system. Furthermore, many of these relatively low-cost pumps are not deemed worth the capital expenditure for minimum flow protection. This is especially true for non-critical pumps used in residential, commercial and light duty industrial services.

For the 20 percent or so of pump applications that do require minimum flow protection, there are number of choices that the user or system designer has. The determination of which choice to use considers accuracy, reliability, cost, and criticality and is very specific to the application. One other factor in the selection process is whether the pump needs to be protected for minimum flow in a modulating fashion (i.e., keeping the pump operating but with a certain amount of flow bypassed), or whether it’s sufficient to simply alarm or trip off the pump in the event that the flow rate drops below the recommended minimum flow. Finally, additional protection obtained from the same instrument should be considered (e.g., a power monitor can protect against both high and low flow damage to a pump, while a relief valve will only protect against low flow.) For the 20 percent or so of pump applications that do require minimum flow protection, here are 10 different methods that may be considered for protection of the pump. All of these are used in the industry, and some systems use a combination of these methods for protection against low-flow excursions.

1. Continuous Bypass

This may be the lowest capital cost method of protecting a pump, whereby a bypass line with an orifice allows a fixed amount of flow to be pumped continuously back to the suction

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source. This always ensures that the pump delivers its recommended minimum flow, even if the main line is shut off completely. The most significant negative aspect of this system is that the pump must be oversized in the first place to allow for the continuously bypassed flow. Secondly, and sometimes more importantly, is the fact that energy is wasted due to the extra horsepower required to accommodate the bypassed flow. There may also be a potential for product damage when being forced through an orifice. Still, this alternative is chosen by many industrial users for pumps in the range of 50 horsepower and below. The operating cost of a continuous bypass low-flow protection system can be significant compared to other methods. For example, a 50-hp ANSI process pump costs $3,320 more per year to operate with a continuous bypass, compared with a bypass system that only opens when the flow drops below the specified minimum.

2. Multi-Component Control Valve System

This type of system relies on a continuous flow measurement in the system. When the flow drops below the recommended minimum flow, a signal is sent to a valve in the bypass line that either opens it completely or modulates the valve so that it gradually opens. This valve may be a solenoid type if it is strictly on/off, which is generally the least costly method, or may be a pneumatically actuated control valve. This method of bypass eliminates the energy waste of continuous bypass, but relies on considerably more complexity than a continuous bypass system. The system includes multiple components, each of which could fail. It requires a power supply and, if pneumatically actuated, an air supply. Maintenance costs are typically higher than other alternatives. As such, it is one of the more costly methods of minimum flow protection. However, it is deemed by many users to be the best approach, especially if the system already includes a reliable method of flow measurement.

3. Variable Frequency Drive

Variable frequency drives (VFDs) change the frequency of the electric motor on the pump to slow the pump down when the demand for lower flow is called for by the process. For most

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systems, this keeps the pump operating near its BEP at all times, and prevents the moving of the pump to a lower percentage of BEP that causes the damage to pumps. VFDs are being used more and more in process applications and have eliminated the need for other minimum flow protection when they are being used. (Note that with canned motor and magnetic drive pumps, there will still be a minimum flow required to carry away the heat caused by the motor or magnetic flux, and to lubricate the bearings.) VFDs are relatively expensive, although the cost has reduced dramatically in recent years with rapid improvements in technology. Other benefits of VFDs include lighter loading of pump seals and bearings, and the ability to “soft start” equipment at slower speeds, reducing the strain and high current caused by on-line starts.

Valve4. Automatic Recirculation

This type of valve combines the features of a check valve and a bypass valve, and has a number of advantages compared to other approaches. Compared to the multi-component flow control valve system, it has fewer components, requires lower installation and operating costs, has less environmental effect (no dynamic seals) and does not require air or electricity. Compared to systems that just shut down the pump, it keeps the pump and the system operating (does not shut down the process). Disadvantages include its relatively high cost, and the fact that these valves are not normally available in alloys higher than stainless steel, thus eliminating many chemical services. Also, ARC valves are generally unsuitable for fluids containing solids.

5. Relief Valve

This simply relies on a pressure relief valve in the pump discharge piping being set to relieve back to suction when the pressure put out by the pump reaches a certain setpoint pressure. The characteristic performance curve of all centrifugal pumps is such that as the pump delivers a lower capacity (flow), the pressure (head) that the pump produces gets higher. Some pumps exhibit a steeper capacity versus

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pressure curve than others. A pressure relief valve is particularly appropriate for pumps with fairly steep capacity versus pressure curves. Thus, this is the normally chosen minimum flow protection system for regenerative turbine style pumps. It is also the accepted method for protection of many fire pump systems. (Note: With liquid returning to the suction, the internal temperature of the pump may rise, so this may limit the use of the relief valve for an extended period of time. Also, relief valves are subject to maintenance and testing/calibration regimens.)

6. Pressure Sensor

This device relies on the fact that as the flow decreases with a centrifugal pump, the amount of pressure produced by the pump increases. This high-pressure signal is then used to either open a bypass valve at a high-pressure (low-flow) indication, or to simply trip the pump. For pumps with relatively steep head versus capacity curves, this method can be economical and reliable. For pumps with flatter head capacity curves in the low-flow range, it is considered to be less reliable than other approaches.

7. Amp Meter

The amp draw of the electric motor varies across the range of flow produced by a pump. For many pumps, the amp draw of the pump is lower at lower flow rates, and increases with increasing flow. Thus, it is possible with many pump types to monitor amp draw, and to alarm or trip the pump when the amps drop below a certain setpoint level. While this is a relatively inexpensive way to protect the pump against low flow damage, it has some potential drawbacks. It may be subject to unacceptable inaccuracy due to current fluctuations in the system and the fact that the amp draw curve can be fairly flat at lower flow rates. In general, the lower the nominal speed of the driver, the less practical amp monitoring becomes, due to the flatter curve and resulting smaller amp range. The device may also need to be disabled during start-up of the pump due to high current draw. On the plus side, an amp meter can also be used to protect a pump from damage due to excess flow.

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8. Power Monitor

Power monitors measure motor horsepower. Since most pump curves have a horsepower curve that rises with increasing flow, it is possible to set the motor to shut off if the power drops below a minimum setpoint, so this is a reliable protection against low flow problems. Power monitors are typically more reliable than amp meters, since they are not subject to fluctuating results with variations in line current. For pumps with relatively flat head capacity curves, where pressure measurements aren rsquot reliable, the power monitor may be the best choice for low-flow protection. Power monitors can be programmed to protect against excessive flow (high power), as well as minimum flow (low power). They can also be programmed to ignore momentary power spikes where an amp meter might trip the motor. They are adjustable to allow altering setpoints should the process requirements change. They aren’t appropriate for many mixed flow pumps, which may have a nearly flat horsepower curve as a function of pump flow. If the power monitor measures motor input power rather than motor output power, it may not be as accurate, since the efficiencies of small motors at low power can be quite low.

9. Vibration Sensor

Some pump systems have vibration monitors to alarm or trip the pump if the pump begins to vibrate excessively. One of the things that occur at lower flow rates is that the pump may indeed vibrate significantly higher than normal. (Note that high vibration levels may also be an indication of other problems with the pump, such as misalignment, imbalance of the impeller or cavitation.) This device, while relatively expensive, is part of the low-flow protection system on many critical process pumps. If vibration is associated with pump wear or other factors, such as bearing degradation, it is also possible to project the time of failure and plan preventive maintenance.

10. Temperature Sensor

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At very low flow rates, the temperature of the pumped liquid increases because of the recirculation of the liquid within the pump that goes on at lower flow rates. Thus, if the pump discharge is shut off by a closed main valve, the temperature of the liquid inside the pump will begin to rise. One method of protecting the pump against this occurrence is by monitoring the temperature in the pump casing (or containment shell in the case of a magnetic drive pump), and tripping the pump off when the temperature rises above a certain setpoint value. This may be relatively inexpensive but not necessarily too reliable, because by the time it shuts the pump off, damage may have already occurred in the pump.

Pumps operation problems

Operation practices that cause frequent seal and bearing maintenance

Seals and bearings account for over eighty five percent (85%) of premature centrifugal pump failure. In the following paragraphs we will be looking at only those operation practices that can, and will cause premature seal and bearing failure. Design and maintenance practices will be discussed in other papers in this series.

When pumps were supplied with jam packing, the soft packing stabilized the shaft to prevent too much deflection. In an effort to save flushing water and to conserve power, many of these same pumps have since been converted to a mechanical seal and the radial stabilization the packing provided has been lost.

The bad operating practices include:

Running the pump dry will cause over-heating and excessive vibration problems that will shorten seal life. Here are some of the common reasons why a pump is run dry:

• Failing to vent the pump prior to start-up. • Running the tank dry at the end of the operation cycle. • Emptying the tank for steaming or introduction of the next

product. • Running on the steam that is being used to flush the tank.

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• Starting the standby pump without venting it. Venting a hazardous product can cause a lot of problems with the liquid disposal. Many operators have stopped venting for that reason.

• Tank vents sometimes freeze during cold weather. This will cause a vacuum in the suction tank, and in some cases could collapse the tank.

• Sump fluids are often dirty, corrosive or both. The control rods for the float switch will often "gum up" or corrode and give a false reading to the operator. He may think that there is an adequate level, when in fact, the tank is empty.

Dead heading the pump can cause severe shaft deflection as the pump moves off of its best efficiency point (B.E.P.). This translates to excessive heat that will affect both the seal and the bearings as well as causing the seal faces to open, and the possibility of the impeller contacting the volute when the shaft deflects.

• Starting the centrifugal pump with a shut discharge valve is standard practice with many operation departments. The concern is to save power without realizing the damage that is being done to the mechanical seal, impeller, wear rings and bearings.

• Some pumps are equipped with a recirculation valve that must be opened to lessen the problem, but many times the valve is not opened, or the bypass line is clogged or not of the correct diameter to prevent the excessive head. Another point to remember is that if the bypass line is discharged to the suction side of the pump the increased temperature can cause cavitation.

• After a system has been blocked out the pump is started with one or more valves not opened.

• Discharge valves are shut before the pump has been stopped.

Operating off of the best efficiency point (B.E.P.). Changing the flow rate of the liquid causes shaft deflection that can fail the mechanical seal and over-load the bearings.

• Starting the pump with the discharge valve closed to save power.

• The level in the suction tank is changing. Remember that the pump pumps the difference between the discharge and

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suction heads. If the suction head varies, the pump moves to a different point on its curve.

• Any upset in the system such as closing, throttling or opening a valve will cause the pump to move to a new point on the curve as the tank fills.

• Pumping to the bottom of a tank will cause the pump to move to a different point on the curve as the tank fills. Some systems were designed for a low capacity positive displacement pump and have since been converted to a centrifugal design because of a need for higher capacity. Centrifugal pumps must discharge to the top of the tank to prevent this problem.

• If the discharge piping is restricted because of product build up on the inside walls, the pump will run throttled. This is one of the reasons that it is important to take periodic flow and amperage readings.

• Increasing the flow will often cause cavitation problems.

Seal environmental controls are necessary to insure long mechanical seal life. It is important that operations understand their function and need because many times we find the controls installed, but not functioning.

• Cooling-heating jackets should show a differential temperature between the inlet and outlet lines. If the jacket clogs up, this differential will be lost and seal failure will shortly follow.

• Barrier fluid is circulated between two mechanical seals. There may or may not be a differential temperature depending upon the flow rate. If a convection tank is installed, there should be a temperature differential between the inlet and outlet lines. The line coming out of the top of the seal to the side of the tank should be warmer than the line from the bottom of the tank to the bottom of the seals, otherwise the system is running backwards and may fail completely. The level in the tank is also critical. It should be above the tank inlet line or no convection will occur. Some convection tanks are pressurized with a gas of some type. Many original equipment (O.E.M.) seal designs will fail if this differential pressure is lost.

• Some seal glands (A.P.I. type) are equipped with a quench connection that looks like the seal is leaking water or steam. If there is too much steam pressure on this quench

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connection, the excessive leakage will get into the bearings causing premature failure. The steam is often used to keep the product warm to prevent it from solidifying, crystallizing, getting too viscous, building a film on the faces etc. Operating people frequently shut off the quench to stop the condensate from leaking.

• Flushing fluids are used for a variety of purposes, but most of the time they are used to get rid of unwanted solids. The flush can be closely controlled with a flow meter or throttling valve. The amount of flush is determined by the seal design. As an example, those designs that have springs in the product require more flush.

• It is important to check that the stuffing box has been vented in vertical pumps. The vent should be coming out of the seal gland and not the stuffing box lantern ring connection.

There are some additional things that all operators should know to insure longer rotating equipment life. As an example :

• Mechanical seals have an 85% or more failure rate that is normally correctable. This is causing unnecessary down time and excessive operating expense. Seals should run until the sacrificial carbon face is worn away, but in more that 85% of the cases the seal fails before this happens.

• There are five different causes of cavitation. • You should know where the best efficiency point (B.E.P.) is

on a particular pump, and how far it is safe to operate off the B.E.P. with a mechanical seal installed.

• You should be aware that washing down the pump area with a water hose will cause premature bearing failure when the water penetrates the bearing case.

• Learn about the affect of shaft L3/D4 on pump operation. • Know how the pumped product affects the life of the

mechanical seal and why environmental controls are necessary.

• If you are not using cartridge seals, adjusting the open impeller for efficiency will shorten the seal life. In most cases the seal will open as the impeller is being adjusted to the volute. Durco pumps are the best example of the exception to this rule. The popular Durco pumps adjust to the back plate causing a compression of the seal faces that can create mechanical seal "over heating" problems.

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• Cycling pumps for test will often cause a mechanical seal failure unless an environmental control has been installed to prevent the failure.

• Mechanical seals should be positioned after the impeller has been adjusted for thermal growth. This is important on any pump that is operated above 200°F (100°C) or you will experience premature seal failure.

• Some elastomers will be affected by steaming the system. A great deal of caution must be exercised if a flushing fluid such as caustic is going to be circulated through the lines or used to clean a tank. Both the elastomer and some seal faces (reaction bonded silicone carbide is a good example) can be damaged. If the elastomer is attacked, the failure usually occurs within one week of the cleaning procedure.

• The stuffing box must be vented on all vertical centrifugal pumps or otherwise air will be trapped at the seal faces that can cause premature failure of many seal designs.

• Most original equipment seal designs cause shaft damage (fretting) necessitating the use of shaft sleeves that weaken the shaft and restrict pump operation to a narrow range at the B.E.P..

Here are a few common misconceptions that cause friction between maintenance and operation departments

• Shutting the pump discharge valve suddenly, will blow the seal open.

• All ceramics cold shock. • High head, low capacity consumes a lot of power. • The pump must come into the shop to change a mechanical

seal. • If you use two hard faces or dual mechanical seals in slurry

applications, you will not need flushing water with its corresponding product dilution.

• If you use metal bellows seals for hot oil applications, you will not need the stuffing box cooling jacket operating.

• It is O.K. to use an oversized impeller because throttling back will save power.

A few more thoughts on the subject

• Operators should receive proper schooling on the trouble shooting and maintenance of pumps. In the military and

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many modern plants, the operator and the maintenance mechanic are often the same person. If the operator knows how the pump works he will have no trouble figuring out the solution to his problem. Too often he is told to keep the flow gage at a certain point, or between two values without understanding what is actually happening with the equipment. If the operator recognizes cavitation he can tell the maintenance department and help them with their trouble shooting.

• As you wander around the plant look out for painters that paint the springs of outside and double mechanical seals. There is a trend to putting two seals in a pump for environmental reasons and the painting of springs is becoming a common problem.

• If someone is adjusting the impeller make sure he is resetting the seal spring tension at the same time.

• If the pump is getting hot or making excessive noises, report it immediately. After the failure, it does no good to tell maintenance that it was making noise for two weeks.

• If you are the floor operator it is common knowledge that taking temperature and pressure readings is very boring, especially on those gages that are located in hot or awkward locations. Avoid the temptation to "radio" these readings. From hot to failure is a very short trip.

• Maintenance's favorite expression is "there is never time to do it right, but there is always time to fix it." Try to keep this in mind when the pressure is on to get the equipment running again.

• Do not let cleaning people direct their "wash down" hoses directly at the pump. Water entering the bearings through the lip or grease seals is a major cause of premature bearing failure. Most water wash downs are used to dilute and wash away seal leakage. Stop the leak and you have eliminated the reason for the hose.

• A great many motor and electrical problems are caused by these same wash down hoses.

• Cooling a bearing outside diameter will cause it to shrink and the bearing will get hotter as the radial load increases. Keep the water hose and all other forms of cooling off of the bearing casing.

Two Basic Requirements for Trouble-Free Operation of Centrifugal Pumps

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Centrifugal pumps are the ultimate in simplicity. In general there are two basic requirements that have to be met at all the times for a trouble free operation and longer service life of centrifugal pumps.

The first requirement is that no cavitation of the pump occurs throughout the broad operating range and the second requirement is that a certain minimum continuous flow is always maintained during operation.

A clear understanding of the concept of cavitation, its symptoms, its causes, and its consequences is very much essential in effective analyses and troubleshooting of the cavitation problem.

Just like there are many forms of cavitation, each demanding a unique solution, there are a number of unfavorable conditions which may occur separately or simultaneously when the pump is operated at reduced flows. Some include:

Cases of heavy leakages from the casing, seal, and stuffing box

Deflection and shearing of shafts Seizure of pump internals Close tolerances erosion Separation cavitation Product quality degradation Excessive hydraulic thrust Premature bearing failures

Each condition may dictate a different minimum flow low requirement. The final decision on recommended minimum flow is taken after careful “techno-economical” analysis by both the pump user and the manufacturer.

The consequences of prolonged conditions of cavitation and low flow operation can be disastrous for both the pump and the process. Such failures in hydrocarbon services have often caused damaging fires resulting in loss of machine, production, and worst of all, human life.

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Thus, such situations must be avoided at all cost whether involving modifications in the pump and its piping or altering the operating conditions. Proper selection and sizing of pump and its associated piping can not only eliminate the chances of cavitation and low flow operation but also significantly decrease their harmful effects.

Understanding Cavitation

In the above, two basic requirements for trouble free operation and longer service life of centrifugal pumps are mentioned in brief.

1. PREVENT CAVITATION

Cavitation of the pump should not occur throughout its operating capacity range.

2. MINIMIZE LOW FLOW OPERATION

Continuous operation of centrifugal pumps at low flows i.e. reduced capacities, leads to a number of unfavorable conditions. These include reduced motor efficiency, excessive radial thrusts, excessive temperature rise in the pumping fluid, internal re-circulation, etc. A certain minimum continuous flow (MCF) should be maintained during the pump operation.

Operating a pump under the condition of cavitation for even a short period of time and have damaging consequences for both the equipment and the process. Operating a pump at low flow conditions for an extended duration may also have damaging consequences for the equipment.

The condition of cavitation is essentially an indication of an abnormality in the pump suction system, whereas the condition of low flow indicates an abnormality in the entire pumping system or process. The two conditions are also interlinked such that a low flow situation can also induce cavitation.

The concept of cavitation is explored in detail under following topics:

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1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps.2. Mechanism of cavitation.3. General symptoms of cavitation and its effects on pump performance and pump parts.

Concept of Cavitation

Cavitation is a common occurrence but is the least understood of all pumping problems. Cavitation means different things to different people. Some say when a pump makes a rattling or knocking sound along with vibrations, it is cavitating. Some call it slippage as the pump discharge pressure slips and flow becomes erratic. When cavitating, the pump not only fails to serve its basic purpose of pumping the liquid but also may experience internal damage, leakage from the seal and casing, bearing failure, etc.

In summary, cavitation is an abnormal condition that can result in loss of production, equipment damage and worst of all, personnel injury.

The plant engineer’s job is to quickly detect the signs of cavitation, correctly identify the type and cause of the cavitation and eliminate it. A good understanding of the concept is the key to troubleshooting any cavitation related pumping problem.

The concept of cavitation is explored under the following topics:

1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps.2. Mechanism of cavitation.3. General symptoms of cavitation and its effects on pump performance and pump parts.

1. Meaning of the term ‘cavitation’ in the context of centrifugal pumps

The term ‘cavitation’ comes from the Latin word cavus, which means a hollow space or a cavity. Webster’s Dictionary defines the word ‘cavitation’ as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.

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In any discussion on centrifugal pumps various terms like vapor pockets, gas pockets, holes, bubbles, etc. are used in place of the term cavities. These are one and the same thing and need not be confused. The term bubble shall be used hereafter in the discussion.

In the context of centrifugal pumps, the term cavitation implies a dynamic process of formation of bubbles inside the liquid, their growth and subsequent collapse as the liquid flows through the pump.

Generally, the bubbles that form inside the liquid are of two types: Vapor bubbles orGas bubbles.

1. Vapor bubbles are formed due to the vaporisation of a process liquid that is being pumped. The cavitation condition induced by formation and collapse of vapor bubbles is commonly referred to as Vaporous Cavitation.

2. Gas bubbles are formed due to the presence of dissolved gases in the liquid that is being pumped (generally air but may be any gas in the system). The cavitation condition induced by the formation and collapse of gas bubbles is commonly referred to as Gaseous Cavitation.

Both types of bubbles are formed at a point inside the pump where the local static pressure is less than the vapor pressure of the liquid (vaporous cavitation) or saturation pressure of the gas (gaseous cavitation).

Vaporous cavitation is the most common form of cavitation found in process plants.

Generally it occurs due to insufficiency of the available NPSH or internal recirculation phenomenon. It generally manifests itself in the form of reduced pump performance, excessive noise and vibrations and wear of pump parts. The extent of the cavitation damage can range from a relatively minor amount of pitting after years of service to catastrophic failure in a relatively short period of time.

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Gaseous cavitation occurs when any gas (most commonly air) enters a centrifugal pump along with liquid. A centrifugal pump can handle air in the range of ½ % by volume. If the amount of air is increased to 6%, the pump starts cavitating. The cavitation condition is also referred to as Air binding. It seldom causes damage to the impeller or casing. The main effect of gaseous cavitation is loss of capacity.

The different types of cavitation, their specific symptoms and specific corrective actions shall be explored in the next part of the article. However, in order to clearly identify the type of cavitation, let us first understand the mechanism of cavitation, i.e. how cavitation occurs.

Unless otherwise specified, the term cavitation shall refer to vaporous cavitation.

2. Mechanism of Cavitation

The phenomenon of cavitation is a stepwise process as shown in Figure 11.

Step One, Formation of bubbles inside the liquid being pumped.

The bubbles form inside the liquid when it vaporises i.e. phase change from liquid to vapor.But how does vaporization of the liquid occur during a pumping operation?

Vaporization of any liquid inside a closed container can occur if either pressure on the liquid surface decreases such that it becomes equal to or less than the liquid vapor pressure at the operating temperature, or the temperature of the liquid rises, raising the vapor pressure such that it becomes equal to or greater than the operating pressure at the liquid surface. For example, if water at room temperature (about 77 o F) is kept in a closed container and the system pressure is reduced to its vapor pressure (about 0.52 psia), the water quickly changes to a vapor. Also, if the operating pressure is to remain constant at about 0.52 psia and the temperature is allowed to rise above 77 o F, then the water quickly changes to a vapor.

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Just like in a closed container, vaporization of the liquid can occur in centrifugal pumps when the local static pressure reduces below that of the vapor pressure of the liquid at the pumping temperature.

NOTE: The vaporisation accomplished by addition of heat or the reduction of static pressure without dynamic action of the liquid is excluded from the definition of cavitation. For the purposes of this article, only pressure variations that cause cavitation shall be explored.

Temperature changes must be considered only when dealing with systems that introduce or remove heat from the fluid being pumped.

To understand vaporization, two important points to remember are:1. We consider only the static pressure and not the total pressure when determining if the system pressure is less than or greater than the liquid vapor pressure. The total pressure is the sum of the static pressure and dynamic pressure (due to velocity)2. The terms pressure and head have different meanings and they should not be confused. As a convention in this article, the term “pressure” shall be used to understand the concept of cavitation whereas the term “head” shall be used in equations.

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Figure 11: Phenomenon of Cavitation

Thus, the key concept is - vapor bubbles form due to vaporization of the liquid being pumped when the local static pressure at any point inside the pump becomes equal to or less than the vapor pressure of the liquid at the pumping temperature.

How does pressure reduction occur in a pump system?

The reduction in local static pressure at any point inside the pump can occur under two conditions:

1. The actual pressure drop in the external suction system is greater than that considered during design. As a result, the pressure available at pump suction is not sufficiently high enough to overcome the design pressure drop inside the pump.

2. The actual pressure drop inside the pump is greater than that considered during thepump design.

Step Two, Growth of bubbles

Unless there is no change in the operating conditions, new bubbles continue to form and old bubbles grow in size. The bubbles then get carried in the liquid as it flows from the impeller eye to the impeller exit tip along the vane trailing edge. Due to impeller rotating action, the bubbles attain very high velocity and eventually reach the regions of high pressure within the impeller where they start collapsing. The life cycle of a bubble has been estimated to be in the order of 0.003 seconds.

Step Three, Collapse of bubbles

As the vapor bubbles move along the impeller vanes, the pressure localized hammering effect can pit the pump impeller. The pitting effect is illustrated schematically in Figure 12.

After the bubble collapses, a shock wave emanates outward from the point of collapse.

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In nutshell, the mechanism of cavitation is all about formation, growth and collapse of bubbles inside the liquid being pumped. But how can the knowledge of mechanism ofcavitation can really help in troubleshooting a cavitation problem. The concept of mechanism can help in identifying the type of bubbles and the cause of their formation and collapse. The troubleshooting method shall be explored in detail in the next part of the article.

Next let us explore the general symptoms of cavitation and its affects on pump performance.

3- General Symptoms of Cavitation and its Affects Pump Performance and Pump Parts

Perceptible indications of the cavitation during pump operation are more or less loud noises, vibrations and an unsteadily working pump. Fluctuations in flow and discharge pressure take place with a sudden and drastic reduction in head rise and pump capacity.

Figure 12: Collapse of a Vapor Bubble

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Depending upon the size and quantum of the bubbles formed and the severity of their collapse, the pump faces problems ranging from a partial loss in capacity and head to total failure in pumping along with irreparable damages to the internal parts. It requires a lot of experience and thorough investigation of effects of cavitation on pump parts to clearly identify the type and root causes of cavitation.A detailed description of the general symptoms is given as under.

Reduction in capacity of the pump:

The formation of bubbles causes a volume increase decreasing the space available for the liquid and thus diminish pumping capacity. For example, when water changes state from liquid to gas its volume increases by approximately 1,700 times. If the bubbles get big enough at the eye of the impeller, the pump “chokes” i.e. loses all suction resulting in a total reduction in flow. The unequal and uneven formation and collapse of bubbles causes fluctuations in the flow and the pumping of liquid occurs in spurts. This symptom is common to all types of cavitations.

Decrease in the head developed:

Bubbles unlike liquid are compressible. The head developed diminishes drastically because energy has to be expended to increase the velocity of the liquid used to fill up the cavities, as the bubbles collapse. As mentioned earlier, The Hydraulic Standards Institute defines cavitation as condition of 3 % drop in head developed across the pump. Like reduction in capacity, this symptom is also common to all types of cavitations.

Thus, the hydraulic effect of a cavitating pump is that the pump performance drops off of its expected performance curve, referred to as break away, producing a lower than expected head and flow. The Figure 14 depicts the typical performance curves. The solid line curves represent a condition of adequate NPSHa whereas the dotted lines depict the condition of inadequate NPSHa i.e. the condition of cavitation.

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Figure 13: Pump Performance Curves

Abnormal sound and vibrations:

It is movement of bubbles with very high velocities from low-pressure area to a high-pressure area and subsequent collapse that creates shockwaves producing abnormal sounds and vibrations. It has been estimated that during collapse of bubbles the pressures of the order of 10 atm develops.

The sound of cavitation can be described as similar to small hard particles or gravel rapidly striking or bouncing off the interior parts of a pump or valve. Various terms like rattling, knocking, crackling are used to describe the abnormal sounds. The sound of pumps operating while cavitating can range from a low-pitched steady knocking sound (like on a door) to a high-pitched and random crackling (similar to a metallic impact).

People can easily mistake cavitation for a bad bearing in a pump motor. To distinguish between the noise due to a bad bearing or

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cavitation, operate the pump with no flow. The disappearance of noise will be an indication of cavitation.

Similarly, vibration is due to the uneven loading of the impeller as the mixture of vapor and liquid passes through it, and to the local shock wave that occurs as each bubble collapses. Very few vibration reference manuals agree on the primary vibration characteristic associated with pump cavitation. Formation and collapsing of bubbles will alternate periodically with the frequency resulting out of the product of speed and number of blades. Some suggest that the vibrations associated with cavitation produce a broadband peak at high frequencies above 2,000 Hertz. Some suggest that cavitation follows the vane pass frequency (number of vanes times the running speed frequency) and yet another indicate that it affects peak vibration amplitude at one times running speed. All of these indications are correct in that pump cavitation can produce various vibration frequencies depending on the cavitation type, pump design, installation and use.

The excessive vibration caused by cavitation often subsequently causes a failure of the pump’s seal and/or bearings. This is the most likely failure mode of a cavitating pump.

Damage to pump parts:

Cavitation erosion or pitting

During cavitation, the collapse of the bubbles occurs at sonic speed ejecting destructive micro jets of extremely high velocity (up to 1000 m/s) liquid strong enough to cause extreme erosion of the pump parts, particularly impellers. The bubble is trying to collapse from all sides, but if the bubble is lying against a piece of metal such as the impeller or volute it cannot collapse from that side. So the fluid comes in from the opposite side at this high velocity and bangs against the metal creating the impression that the metal was hit with a "ball pin hammer". The resulting long-term material damage begins to become visible by so called

Pits (see Figure 14), which are plastic deformations of very small dimensions (order of magnitude of micrometers). The damage caused due to action of bubble collapse is commonly referred as Cavitation erosion or pitting. The Figure 14 depicts the cavitation pitting effect on impeller and diffuser surface.

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Cavitation erosion from bubble collapse occurs primarily by fatigue fracture due to repeated bubble implosions on the cavitating surface, if the implosions have sufficient impact force. The erosion or pitting effect is quite similar to sand blasting. High head pumps are more likely to suffer from cavitation erosion, making cavitation a “high-energy” pump phenomenon.

The most sensitive areas where cavitation erosion has been observed are the low-pressure sides of the impeller vanes near the inlet edge. The cavitation erosion damages at the impeller are more or less spread out. The pitting has also been observed on impeller vanes, diffuser vanes, and impeller tips etc. In some instances, cavitation has been severe enough to wear holes in the impeller and damage the vanes to such a degree that the impeller becomes completely ineffective. A damaged impeller is shown in Figure 15.

Figure 14: Photographic Evidence of Cavitation

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The damaged impeller shows that the shock waves occurred near the outside edge of the impeller, where damage is evident. This part of the impeller is where the pressure builds to its highest point. This pressure implodes the gas bubbles, changing the water’s state from gas into liquid. When cavitation is less severe, the damage can occur further down towards the eye of the impeller. A careful investigation and diagnosis of point of the impeller erosion on impeller, volute, diffuser etc. can help predict the type and cause of cavitation.

The extent of cavitation erosion or pitting depends on a number of factors like presence of foreign materials in the liquid, liquid temperature, age of equipment and velocity of the collapsing bubble.

Troubleshooting of the pumps

The following is a guide for troubleshooting centrifugal pump and pump systems.

Failure to Deliver Liquid A. Pump not primed.B. Insufficient speed.C. Discharge head too high.D. Suction lift too high (over 15 feet) insufficient NPSH, check with vacuum gauge.E. Impeller passages partially clogged (plugged).F. Wrong direction of rotation.G. Air leaks or pockets in suction line.

Cavitations

Cavitations are caused by a lowering of liquid pressure at the impeller eye-giving rise to vapor formation. This is followed by the sudden collapse of the vapour bubbles as pressure increases causing damage to pump by pitting and erosion.

Bearing Housing Lubrication

The correct level of oil in the bearing housing is kept by the lubricating oil level bottle provided that the bottle contains an oil level.

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Pump Routine Checks

For centrifugal pumps check the following:• Discharge pressure.• Suction pressure.• Pressure differential at suction strainer.• Bearing temperature.• Noise (cavitation)• Mechanical seal leakage.• Cooling medium temperature.• Lube oil system (P,T and level)• Power consumption in Amps. (means pump loading)

Insufficient Pressure1. Sped too low.2. Air or gases in liquid.3. Mechanical defects.

• Wearing rings worn.• Impeller damaged.• Internal leakage due to defective gasket.

4. Wrong direction of rotation.

Insufficient Capacity1. Air leaks in suction or stuffing boxes.2. Speed too low.3. Total head higher than that for which pump is rated.4. Suction lift too high (over 15 feet) or insufficient NPSH, check

with vacuum gauge.5. Impeller passages or piping partially clogged (plugged).6. Insufficient suction head for hot liquid.7. Mechanical defects.

• Wearing rings worn.• Impeller damaged.• Internal leakage due t defective gasket.

8. Wrong direction of rotation.Pump Vibrates

1. Misalignment.2. Foundation not sufficiently rigid (grounding broken).3. Impeller partially clogged, causing imbalance.4. Mechanical defects.

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Noisy Pump Operation

1. Hydraulic noise:• Cavitation.• Insufficient NPSH.• Air in liquid.

2. Mechanical defects:• Shaft bent.• Bearing warn.• Rotating parts binding.

Reducing Capacity

Centrifugal pumps should not be operated at a greatly reduced capacity or with closed discharge valve, because the energy required driving the pump is converted into heat and the temperature of the liquid may reach the boiling point. If this occurs, the rotating parts are exposed to vapour with no lubrication and they will be damaged.

Troubleshooting the positive displacement rotary pump

No liquid discharge

• The pump is not primed. Prime it from the outlet side by keeping the outlet air vent open until liquid comes out the vent.

• The rotating unit is turning in the wrong direction. • Valves are closed or there is an obstruction in the inlet or

outlet line. Check that the flange gaskets have their center cut out.

• The end of the inlet pipe is not submerged. You can either increase the length of the inlet pipe into the liquid level or raise the level in the tank.

• The foot valve is stuck. • A strainer or filter is clogged. • The net inlet pressure is too low. • A bypass valve is open.

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• There is an air leak some where in the inlet line. Air can come in through gaskets or valves above the fluid line.

• The stuffing box is under negative pressure. Packing is allowing air to get into the system. You should convert the packing to a mechanical seal

• The pump is worn. The critical clearances have increased. • Something is broken. Check the shaft, coupling, internal

parts, etc. • There is no power to the pump.

The pump is putting out a low capacity

• The pump's internal clearances have increased. It is time to change some parts.

• The net inlet pressure is too low; the pump is cavitating. • A strainer or filter is partially clogged. • The speed is too low. Check the voltage. • The tank vent is partially frozen shut. • A bypass line is partially open. • A relief valve is stuck partially open. • The inlet piping is damaged. Something ran over it • A corrosion resistant liner has collapsed in the inlet piping. • Air is leaking through the packing. You should go to a

mechanical seal.

The pump looses its prime after it has been running for a while

• The liquid supply is exhausted. Check the tank level; sometimes the float is stuck, giving an incorrect level reading.

• The liquid velocity has increased dramatically. • The liquid is vaporizing at the pump inlet. • A bypass line is heating the incoming fluid. • An air leak has developed in the suction piping.

The pump is using too much power

• The speed is too high. • The liquid viscosity is higher than expected. • The discharge pressure is higher than calculated • The packing has been over tightened. You should convert to

a mechanical seal.

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• A rotating element is binding. Misalignment could be the problem or something is stuck in a close clearance and binding the rotating element.

Excessive noise and vibration

• Relief valve chatter. • Foundation or anchor bolts have come loose. • The pump and driver are misaligned. • The piping is not supported properly. • The liquid viscosity is too high. The pump is starving. Check

the temperature of the incoming liquid. Check to see if the supply tank heater has failed.

Excessive noise or a loss of capacity is frequently caused by cavitation. Here is how the NPSH required was determined initially:

With the pump initially operating with a 0 psig. inlet pressure and constant differential pressure, temperature, speed and viscosity; a valve in the inlet line is gradually closed until cavitation noise is clearly audible, there is a sudden drop off in capacity or there is a 5% overall reduction in output flow. Cavitation occurs with:

• A loss of suction pressure. • An increase in fluid velocity. • An increase in inlet temperature.

Here are some common causes of cavitation problems:

• A foot valve or any valve in the suction piping is sticking. • Something is occasionally plugging up the suction piping. If

the pump suction is coming from a river, pond or the ocean, grass is a strong possibility.

• A loose rag is another common cause. • A collapsed pipe liner. • A filter or strainer is gradually clogging up. • The tank vent partially freezes in cold weather. • The sun is heating the suction piping, raising the product

temperature close to its vapor point. • The level in the open suction tank decreases causing vortex

problems that allow air into the pump suction. • Several pumps in the same sump are running, decreasing

the level too much.

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• The suction tank float is stuck. It will sometimes show a higher level than you really have.

• A discharge recirculation line, piped to the pump suction, opens and heats the incoming liquid.

• Sometimes the suction lift is too high. The increase in pipe friction will reduce the suction head.

• The vapor pressure of the product is very close to atmospheric pressure. The pump cavitates every time it rains because of a drop in atmospheric pressure.

• The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much.

• The process fluid specific gravity is changing. This can happen with a change in product operating temperature or if a cleaner or solvent is being flushed through the lines.

• The source tank is changing from a positive pressure to a vacuum due to the process.

• A packed valve in the suction piping is at a negative pressure and air is leaking in through the packing.

• The tank is being pumped dry. • The inlet piping has been moved or altered in some way.

Has a foot valve, strainer, elbow, or some other type of hardware been installed in the suction piping?

• Has a layer of hard water calcium or some other type of solid formed on the inside of the suction piping reducing its inside diameter over some period of time?

You are experiencing rapid pump wear.

• There are abrasives in the liquid you are pumping causing erosion problems. You may have to go to a larger pump running at a slower speed.

• There is some corrosion in one or more of the pump elements.

• There is a lack of lubrication. • You have a severe pipe strain problem. It could have been

caused by thermal growth of the hardware. • Too much misalignment. • The pump is running dry. • When all else fails the best way to reduce NPSH required is

to select a larger pump and run it at a slower speed.

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