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This document is the course text. You may review this material atyour leisure either before or after you purchase the course. Topurchase this course, click on the course overview page:
http://www.pdhengineer.com/pages/M-1008.htm
or type the link into your browser. Next, click on the Take Quiz button
at the bottom of the course overview page. If you already have anaccount, log in to purchase the course. If you do not have aPDHengineer.com account, click theNew User Sign Up link to createyour account.
After logging in and purchasing the course, you can take the onlinequiz immediately or you can wait until another day if you have not yetreviewed the course text. When you complete the online quiz, yourscore will automatically be calculated. If you receive a passing score,you may instantly download your certificate of completion. If you donot pass on your first try, you can retake the quiz as many times asneeded by simply logging into your PDHengineer.com account andclicking on the link Courses Purchased But Not Completed .
If you have any questions, please call us toll-free at 877 500-7145.
I once presented this course to a group of mechanical engineers, and called it"What You Always Needed to Know About NPSH, But Were Afraid to Ask". Theyall knew that NPSH was important in designing a pumping system, but many of them who had been out of school for a few years would have had difficulty incalculating the NPSH available of a piping system without breaking out a textbook.
NPSH is the link that connects the piping system designer to the pump specifier.It is not uncommon to hear the complaint "This pump is cavitating. Call themanufacturer!" More likely than not, the fault is with the piping system design,
which is causing the pump to cavitate.
A common source of difficulty (and errors) is dealing with all the various units of measure. It seems that everything in the formula for calculating NPSH iscommonly expressed in different units.
There may be a few surprises, such as the fact the NPSH applies to positivedisplacement pumps as well as centrifugal pumps, and how suction specificspeed can be used as a rough indicator for selecting a pump type.
Net Positive Suction Head (NPSH)
When specifying pump ratings and when diagnosing pump operating problems,few items are more discussed and less understood than net positive suctionhead (NPSH). Symptoms that may be attributed to inadequate NPSH include:
• Sudden drop in pump discharge pressure under certain operatingconditions
• Excessive noise and/or vibration• Poor pump efficiency, especially at low loads• Excessive pump wear, such as pitting of impellers
Before getting into a discussion of NPSH, some basic definitions of terms anddesignation of symbols used throughout this discussion will be laid out. For thesake of uniformity, the definitions and symbols will be taken from the HydraulicInstitute Standards, the most widely used compilation of pump design, operating,and testing standards in the U.S.
Head is a measure of pressure, expressed in feet. The relation betweenpressure expressed in pounds per square inch (psi), and feet of head is: h = psix 144/w, where w = the specific weight in pounds per cubic foot of liquid being
pumped, under pumping conditions. All pressure readings must be converted tofeet of the liquid being pumped. The datum for gauge readings shall be taken atthe centerline of the pump for horizontal pumps and double suction verticalpumps, or at the entrance eye of the first stage impeller for single suction verticalpumps.
Static Head, or Gravity Head (Hz)
The vertical distance between the surface of the pumped liquid and the pumpdatum. Where the liquid surface is below the pump datum, hz is negative.
Velocity Head (Hv)
Velocity head shall be figured from the average velocity (V) obtained by dividingthe flow in cubic feet per second (cfs) by the area of the pipe cross section insquare feet at the point of the gauge connection: hv = V2/2g, where g = 32.17ft/sec/sec at sea level, and V = the velocity in the pipe in ft/sec (fps).
Flooded Suction
The liquid must flow from an atmospheric vented source to the pump without theaverage pressure at the pump datum falling below atmospheric pressure with the
pump running at specified capacity.
Total Suction Head, or Lift (Hs or HL)
Positive suction head exists when the total suction head at the pump datum isabove atmospheric pressure. It is the reading of a gauge at the pump suctionconverted to feet of liquid and referred to datum, plus the velocity head at thepoint of attachment. When the total suction head at the pump datum is belowatmospheric pressure, (negative suction head) it is called suction lift, and is thereading of a gauge at the pump datum minus the velocity head.
Total Discharge Head (Hd)
The reading of a pressure gauge at the pump discharge converted to feet of liquid and referred to datum plus the velocity head at the point of attachment.
Bowl Assembly Head (Hb)
The difference between the total discharge head measured in the column pipe of
a vertical turbine pump, connected to the top of the bowl assembly, and the totalsuction head.
Total Dynamic Head (TDH)
Total dynamic head is the measure of the work increase per pound of liquidimparted to the liquid by the pump. It is the difference between the totaldischarge head and the total suction head. Where suction lift exists, totaldynamic head is the sum of the total discharge head and the suction lift (which,remember, is expressed as a positive number). TDH can only be determined bythe user and specified to the pump supplier along with the rated capacity.
Net Positive Suction Head (NPSH)
Net positive suction head is the total suction head in feet of liquid absolutedetermined at the suction nozzle and referred to datum, less the vapor pressure
of the liquid in feet absolute. In simple terms, NPSH is the pressure at the pumpsuction port measured in feet of liquid absolute, less the vapor pressure. It is theanalysis of energy conditions on the suction side of a pump to determine if theliquid will vaporize at the lowest pressure within the pump. The Hydraulic Instituteuses the term Net Inlet Pressure (NIP) instead of NPSH in connection withrotary pumps. NIP is just NPSH expressed in psia instead of feet. It ismentioned here just to make the student aware of the term.
Vapor Pressure (Hvp)
Vapor pressure is one of the physical properties of a liquid. By definition, it is the
pressure exerted by the vapor of a contained liquid. It varies with temperature.Vapor pressure of a liquid is of little concern in many pumping applications, suchas pumping water or # 2 fuel oil at moderate temperatures from an opencontainer. In such cases, vapor pressure is well below atmospheric pressure,and can be ignored and left out of the calculation for NPSH. It is usuallyexpressed in mm of mercury or psia.
However, confusion over the term “vapor pressure” can lead to errors whencalculating NPSH in cases where the vapor pressure of a liquid is significant.This is true for hot (saturated) water, volatile liquids such as gasoline and other petroleum products, ammonia, and LP gas, which have relatively high vapor pressures, and must be stored in pressurized containers. For such liquids, thevapor pressure may be expressed in psig, and should not be ignored whencalculating NPSH. For example, when calculating the NPSH available from adeaerating feedwater heater pressurized to 5 psig (~20 psia), where the water isnear the saturation temperature for that pressure, it nearly cancels out thepressure within the deaerator. Therefore, the NPSH available at the feedwater pump is due primarily to the gravity head alone, or the elevation of the deaerator above the feedwater pump suction nozzle minus frictional losses in the suction
NPSH must be indicated as NPSH available (NPSHa) or NPSH required(NPSHr) in order to be meaningful. NPSHa is a function of everything in thesystem on the suction side of the pump up to the suction nozzle of the pump.This includes the pressure on the surface of the liquid in the supply tank, thedifference between the liquid level and the centerline of the pump suction nozzle,the line losses, velocity head, and vapor pressure. NPSHr is based oneverything from the pump suction nozzle to the point in the pump where thepressure starts to increase. This includes the entrance losses and the frictionlosses or pressure drops getting into the pumping elements.
Since NPSHa is the absolute pressure available less the vapor pressure of the
liquid, the NPSHa should always be greater than the NPSHr. If this were not thecase, the pressure at some point in the pump suction area will be less than thevapor pressure of the liquid, and cavitation will occur. Cavitation is the formationof pockets of vapor, or bubbles, at a point inside the pump where the liquidpressure drops below its vapor pressure. These vapor bubbles are carried alongto the higher pressure area of the pump, where they collapse. It is the violentcollapse of the bubbles that cause the damaging effects of cavitation; noise,erosion, and short service life. Cavitation also reduces capacity and efficiency,as well as causes pulsations in the discharge pressure.
There is a widely used formula for calculating NPSHa, which should be reviewed
and thoroughly understood. NPSHa is a function of the suction piping system,the operating conditions, and the liquid being pumped. For a system in thedesign stage, or for one in use, NPSHa can be calculated from the formula:
NPSHa = Ha + Hz – Hf + Hv – Hvp
An example of how this formula is applied is given below:
Ha = (14.7 psia) x (2.31 ft H2O per psi) / 0.88 s.g. of fuel oil = 38.8 ft of fuel oil
(To convert pressure in psia to ft of liquid, multiply by 2.31 and divide by specificgravity of the liquid).
Hz = -10 ft of fuel oil (worst case)
Hf = 2.9 ft of fuel oil (from piping pressure loss calculation)
If the liquid being pumped were gasoline instead of fuel oil, the NPSHa would bequite different. The specific gravity would change from 0.88 to 0.71, and thevapor pressure to 8.5 psia.
Hvp = 8.5 x 2.31 / .71 =27.6 ft of gasoline.
NPSHa = 7.3 ft of gasoline. (From a practical standpoint, the normal lift shouldnot exceed 6 ft when pumping gasoline).
Notice the big difference the vapor pressure has made in the same systempumping gasoline instead of fuel oil.
For an installed pump with a suction gauge, it is possible to determine theNPSHa of an operating system without making all of these calculations.
Introduce a new term, hi , and define it as the absolute pressure of the liquid atthe pump suction nozzle in feet of liquid. It then can be substituted for the firstthree terms of the NPSHa formula: Hi = Ha ± Hz – Hf .
Restating the formula with this substitution: NPSHa = Hi – Hvp. The value of Hi can be determined by converting the suction gauge reading to ft of liquidabsolute. When making the conversion, use the local absolute barometricpressure, the specific gravity of the liquid, and the appropriate conversion factors.
Units
Perhaps the most frequent mistake made in calculating NPSHa is failure to keepthe units consistent. Atmospheric pressure is often given in inches of mercuryabsolute or psia, vapor pressure in mm of mercury, elevation in feet, and line lossin psi. Keep in mind that psig will vary with elevation of the pumping site above(or below) sea level, and that atmospheric pressure at any given site can vary ± 1in Hg from an average value. The standard value of 14.7 psia and 29.9 in Hg isa good reference.
Entrained Gases
The factors connected with NPSH determinations are not always precise. While
the vapor pressure of a pure liquid is constant and predictable, it is possible tohave entrained or dissolved air or gas in the liquid with the result that the fluidacts as though it had a higher vapor pressure than the figures for pure liquidwould indicate. The higher apparent vapor pressure would decrease the NPSHa.In this case, as the pressure in the suction system is reduced, entrained air or gas in the liquid would tend to expand or to be released. As this occurs, acentrifugal pump would tend to cavitate and a positive displacement pump wouldexperience a loss of volumetric efficiency. This situation could happen when the
liquid is agitated while being transported or unloaded or where the liquid is beingcontinuously recirculated. A similar situation can be encountered when handlingvolatile petroleum products, since the liquid is probably made up of manydifferent fractions, each with its own vapor pressure. For such a liquid a slightvacuum might vaporize the lighter fractions.
If the suction side of a “lift” system is so designed that a positive displacementpump develops a vacuum that results in an absolute pressure less than the vapor pressure of the liquid being handled, the liquid will vaporize. This formation of vapor on the suction side of the pump can cause vapor lock. Collapse of thevapor bubbles will cause cavitation and loss of pump performance plus excessivewear. NPSHa should always exceed NPSHr by a comfortable margin.
NPSHr
NPSHr is another way of indicating the pressure loss within the pump itself. It is a
function of the pump design. In a centrifugal pump, the velocity increases and thepressure decreases as the liquid passes from the pump suction to the eye of theimpeller. There are also pressure losses due to shock and turbulence as theliquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSHr isthe positive head in feet absolute required at the pump suction to overcomethese pressure drops in the pump and maintain the liquid above its vapor pressure.
The dynamics within a positive displacement pump are different, but theconsequences are the same. The pump internally raises the liquid velocity as it
decreases the liquid pressure. By their nature, positive displacement pumps aremore tolerant of low NPSHa, and in fact may still operate as the liquid vaporizeswithin the pump, albeit with reduced capacity and efficiency.
The pump manufacturer will provide NPSHr curves along with the other operatingcharacteristics. For a given pump, NPSHr will increase as pump speedincreases, and also increase with the viscosity and the temperature of thepumped liquid. NPSHr can be determined on the pump test stand by methodsdetailed in The Hydraulic Institute Standards. In general, an NPSH test consistsof throttling the inlet flow to the pump over the operating range of capacities. Asharp drop in the discharge pressure indicates the onset of cavitation, and isplotted as a point on the NPSHr curve. If a pump specification calls for performance testing of a pump, it should include an NPSH test.
Dealing With an NPSH Problem
Pump Speed
If a change to the suction piping is not practical, it may be practical to reduce the
NSPHr of the pump. For a given pump, reducing the speed (which will lower thecapacity as well) will reduce the NPSHr. It will also increase the NPSHa becausethe pipeline friction loss hf will also be reduced. If it is not practical to reduce thespeed, a larger pump running at a slower speed will be able to deliver the samecapacity with a lower NPSHr.
Tank Level
If the source of the liquid is a tank with a variable level, and if the pumpingproblem occurs when the tank level is unusually low, the problem may be causedby swirling or vortexing at the suction pipe. The Hydraulic Institute Standardssuggests methods to reduce this tendency. Of course, a solution may be tosimply limiting the maximum draw down level, at the cost of reducing the tank’suseable capacity.
Entrained Air
As previously mentioned, entrained air in the liquid causes a reduction in NPSHa,and produces pump surging and loss of capacity. Measures to reduce theentrained air, such as reducing agitation during transport, or allowing longer settling time in the tank, may help.
High Temperatures
High temperature of the pumped liquid increases the vapor pressure, and sodecreases NPSHa. A method sometimes used to reduce the tendency of aboiler feedwater pump to “flash” during rapid load changes is to inject cooling
water (condensate) into the suction piping when flashing is likely to occur.
Specific Speed
As previously mentioned, the net positive suction head required (NPSHr) is afunction of the pump design. For centrifugal pumps, it is the design of theimpeller that is of interest. The Hydraulic Institute recognized that a relationshipexists between the specific speed and suction conditions that could affect thetendency for the pump to cavitate.
Specific speed is a dimensionless quantity that correlates pump capacity, head,
and rotational speed at optimum efficiency. This classifies pump impellers withrespect to their geometrical similarities. The definition of specific speed is “therpm at which an impeller would run if reduced in size to deliver one gpm againsta total head of one foot”. It is difficult to visualize just what this means, much lessits significance, so don’t worry about the definition. Specific speed is usuallyexpressed as Ns = ( N√ Q) / H3/4, where N is the rotational speed in rpm, Q is theflow in gpm at optimum efficiency (take ½ of the gpm for double suction pumps),and H is the total head in feet per stage. Specific speed is indicative of the
shape and characteristics of the impeller, as illustrated below:
Centrifugal pumps are traditionally classified by three types:
1. radial flow
2. mixed flow3. axial flow.
However, there is a continuous change from the radial flow impeller, which develops pressure principally by centrifugal force, to the axial flow, which develops most of itshead by the propelling action of the vanes. Each impeller has a specific speed range to
which it is best adapted. Impellers for high heads usually have a low specific speed,
while impellers for low heads have a high specific speed.
Suction limitations of different pumps bear a relation to the specific speed. The
Hydraulic Institute publishes charts giving recommended specific speed limits for various
conditions. Exceeding these limits would increase the potential for serious cavitation problems.
Suction Specific Speed (Ss)
Suction specific speed has come into use as an indication of the suction characteristics of centrifugal pumps. Ss = (N√Q) /(NPSHr)
3/4. Normally, the highest value of Ss is at or
near the capacity corresponding to the best efficiency point (BEP). In comparisons of
different pumps, or different impellers for a given pump, Ss is an indication of the relativesize of the eye of the impeller. An impeller with a higher Ss would require lower NPSHr,
and would typically have a larger eye than an impeller with a lower Ss.
By plotting Ss vs. Ns from test and operating data, it has been found that for pumps of a
“normal” design, values of Ss vary within the range of 6,000 to 12,000. Impellers with Ss
above this range have a relatively large impeller eye. They have a rather narrow stableoperating range, which is at or near the BEP. When outside of this range, impellers with
an oversize eye can experience recirculation at the eye and tend to surge and vibrate. Onthe other hand, impellers that have an Ss below this range would have an impeller eye
comparatively small, and could be subject to choking at certain operating ranges.
By substituting NPSHa for NPSHr in the equation defining Ss, the quantity Suction
Specific Speed Available (SA) is derived. SA = (N√Q) / (NPSHa)3/4
. NPSHa is the net positive suction head available in feet. The suction specific speed available (SA), must
equal or exceed the suction specific speed required (Ss). Again, through tests and
operational data, an SA of 8,500 has been found to be a practical value for SA, and has
been chosen by the Hydraulic Institute to be a valuable criterion to determine themaximum recommended rotational speed of a given pump. Substitute 8,500 for SA in
the above equation and solve for N. This produces: N = 8,500 (NPSHa)3/4
/ √ Q.
N is therefore the recommended maximum operating speed for a centrifugal pump
operating at or near the BEP of Q gpm.
Example Assuming a suction specific speed available of 8,500, what is the recommended rpm limit
for a single suction centrifugal pump with a capacity of 3,000 gpm and an NPSHa of 30
ft.?
N = SA (NPSHa)3/4 / √ Q
N = 8,500 (30)3/4 / 3,0001/2
N = (8,500 x 12.82) /54.77
N = 1,990
The recommended maximum operating rpm is 1,990. If motor driven, the next lower
synchronous speed is 1,800 rpm.
Is this rule firm? No, of course not. But it is an indication that if you were planning to
run this pump with a 3,600 rpm motor, you should seriously consider reconfiguring thesystem for a higher NPSHa or look at a specially designed pump.
Conclusion
Working with NPSH involves a number of factors and variables, ofteninterrelated, which must be given consideration. Pump manufacturers tend to be
somewhat conservative when making recommendations for a system or a pumpon an application which may involve NPSH problems. Conservatism is, of course, a two-edged sword. What may be "safe" for the supplier is expensive for the buyer. So the engineer must be familiar with and be able to explain thechoices and justify the decisions.
As you observed if you worked through some of the calculations, you appreciatethe difficulty in keeping the units consistent within the problem.
