Unit NineUnit Nine

Flow Control ValvesFlow Control Valves

Flow control, simply put, is the continuous regulation Flow control, simply put, is the continuous regulation of the movement of fluid to achieve optimum of the movement of fluid to achieve optimum machine performance. Thus, flow control is machine performance. Thus, flow control is important because flow controls machine speed.important because flow controls machine speed.

Needle Valve SymbolNeedle Valve Symbol

Flow Control in GeneralFlow Control in General

It would be prudent at this point to mention the fact that It would be prudent at this point to mention the fact that when discussing flow control two different meanings when discussing flow control two different meanings exist. First, one has the “topic” of flow control involving exist. First, one has the “topic” of flow control involving the various devices that perform this task. Second, there the various devices that perform this task. Second, there is the industry tendency to refer to a particular device as is the industry tendency to refer to a particular device as a “flow control.” The latter is a nickname for a control a “flow control.” The latter is a nickname for a control valve that has, as a integral part of its design, a needle valve that has, as a integral part of its design, a needle valve and check valve together in the same body or as a valve and check valve together in the same body or as a unit. unit.

Control of FlowControl of Flow

You should identify the above circuit as “hydraulic.” The primary You should identify the above circuit as “hydraulic.” The primary influence for flow is the pump. With the relief valve set at 500psi influence for flow is the pump. With the relief valve set at 500psi and no resisting load on the actuator, the rod extends at full speed. and no resisting load on the actuator, the rod extends at full speed. Often in industrial fluid power systems we do not allow actuators to Often in industrial fluid power systems we do not allow actuators to extend at full speed because of the risk of damage to the machine, extend at full speed because of the risk of damage to the machine, work piece, or operator. work piece, or operator.

Control of FlowControl of Flow

With a needle valve placed in series between the directional control With a needle valve placed in series between the directional control valve and the actuator, fluid is resisted causing some of the pump valve and the actuator, fluid is resisted causing some of the pump flow to go over the relief valve. The diversion of flow slows down the flow to go over the relief valve. The diversion of flow slows down the actuator. Remember that fluids take the path of least resistance.actuator. Remember that fluids take the path of least resistance.

OrificeOrifice

An orifice is no more than a tiny opening, placed in the path of An orifice is no more than a tiny opening, placed in the path of fluid to restrict it. It could be a precision machined piece or simply fluid to restrict it. It could be a precision machined piece or simply a pipe plug with a hole in it. Whatever it may be it is not a pipe plug with a hole in it. Whatever it may be it is not adjustable. The size of the opening, the pressure on the fluid in adjustable. The size of the opening, the pressure on the fluid in front of the opening, and temperature, if its oil, all affect the flow front of the opening, and temperature, if its oil, all affect the flow across the orifice.across the orifice.

Variable OrificeVariable Orifice

The above term simply means that you can adjust the flow control The above term simply means that you can adjust the flow control device. Technically, any of the devices above could be used as a flow device. Technically, any of the devices above could be used as a flow control but only one design is desirable because of its low restriction control but only one design is desirable because of its low restriction and that would be the needle valve. Examine the needle valve and that would be the needle valve. Examine the needle valve symbol back in the frame that showed the circuit. The addition of symbol back in the frame that showed the circuit. The addition of the arrow determines it to be adjustable.the arrow determines it to be adjustable.

Orifice in a CircuitOrifice in a Circuit

In the circuit above, the orifice allows only 2gpm to pass In the circuit above, the orifice allows only 2gpm to pass at 500psi. This in turn forces the other 3gpm to go back at 500psi. This in turn forces the other 3gpm to go back to tank. Since actuator speed is controlled by flow, the to tank. Since actuator speed is controlled by flow, the speed of the actuator is limited to the supplied 2gpm speed of the actuator is limited to the supplied 2gpm instead of the full 5gpm. instead of the full 5gpm. (See next slide for explanation.) (See next slide for explanation.)

Calculating Cylinder SpeedCalculating Cylinder Speed

In the previous example, the formula used to calculate In the previous example, the formula used to calculate the speed of the cylinder was the speed of the cylinder was Rod Speed(feet per minute) = Rod Speed(feet per minute) =

GPM x 19.25/ Piston Area. GPM x 19.25/ Piston Area. If you do the math, it will NOT If you do the math, it will NOT come out correctly as there is a mistake in the text. come out correctly as there is a mistake in the text. Your answer should have been 12.8fps. Try again but Your answer should have been 12.8fps. Try again but this time use the following formula:this time use the following formula:

Rod SpeedRod Speed(in/min)(in/min) = GPM x 231 = GPM x 231

Piston AreaPiston Area(in squared inches)(in squared inches)

This time your answer should be 154 inches per This time your answer should be 154 inches per minute. Divide by 12 to get feet per minute. Sorry but minute. Divide by 12 to get feet per minute. Sorry but text books are like people, none are perfect.text books are like people, none are perfect.

Orifice Size Increased or DecreasedOrifice Size Increased or Decreased

The rod speed formula illustrates mathematically the relationship The rod speed formula illustrates mathematically the relationship between flow and actuator speed. By adjusting the size of the between flow and actuator speed. By adjusting the size of the orifice speed can be altered. Increasing orifice size allows more orifice speed can be altered. Increasing orifice size allows more flow which increases speed. Decreasing orifice size decreases flow flow which increases speed. Decreasing orifice size decreases flow which in turn decreases speed.which in turn decreases speed.

Pressure Differential affects FlowPressure Differential affects Flow

If pressure on the fluid were to increase directly in front of the If pressure on the fluid were to increase directly in front of the orifice, fluid flow across the orifice would increase and so would orifice, fluid flow across the orifice would increase and so would actuator speed. In fact, when ever fluid is moving across a actuator speed. In fact, when ever fluid is moving across a restriction of any type, there will be a difference of pressure.restriction of any type, there will be a difference of pressure.

Pressure DifferentialPressure Differential

Pressure differential is the difference in pressure between Pressure differential is the difference in pressure between any two points in a fluid power system. It is sometimes any two points in a fluid power system. It is sometimes referred to as “Delta P.”referred to as “Delta P.”

Examples from Everyday LifeExamples from Everyday Life

The illustrations compare two fluids. The toothpaste The illustrations compare two fluids. The toothpaste represents a liquid and the air mattress a gas. In both cases represents a liquid and the air mattress a gas. In both cases the fluid is aided in its movement when a force is applied the fluid is aided in its movement when a force is applied increasing the pressure within.increasing the pressure within.

Needle Valves “Cause” Pressure DifferentialNeedle Valves “Cause” Pressure Differential

In the text it refers to needle valves as being “affected” by pressure In the text it refers to needle valves as being “affected” by pressure differential when in fact it is the needle valve that causes the change differential when in fact it is the needle valve that causes the change due to its resistance. The precise nature of adjustment allows us to due to its resistance. The precise nature of adjustment allows us to determine what that change will be under certain circumstances. As determine what that change will be under certain circumstances. As for return oil, what ever does not go out into the system will have to for return oil, what ever does not go out into the system will have to go back to tank.go back to tank.

Relief Valve Setting affects Flow across a Needle Relief Valve Setting affects Flow across a Needle ValveValve

This illustration demonstrates that if you increase pressure in This illustration demonstrates that if you increase pressure in front of an orifice(needle valve), the flow through the orifice will front of an orifice(needle valve), the flow through the orifice will increase. Recall that one of the three things that affects flow increase. Recall that one of the three things that affects flow across any restriction is the pressure applied to it.across any restriction is the pressure applied to it.

Affect of Work Load on Pressure.Affect of Work Load on Pressure.

Since the load is resistance it should follow that any increase in load Since the load is resistance it should follow that any increase in load will likewise have an affect on pressure. Simply put, an increase in will likewise have an affect on pressure. Simply put, an increase in load results in a proportional increase in pressure. Pressure load results in a proportional increase in pressure. Pressure generated is always the amount required to overcome the load, generated is always the amount required to overcome the load, nothing more. In addition, if the load increases to a point above nothing more. In addition, if the load increases to a point above relief valve setting, the actuator will stall out.relief valve setting, the actuator will stall out.

Pressure Compensated Flow Control ValvePressure Compensated Flow Control Valve

To help neutralize the effects of upstream and downstream pressure To help neutralize the effects of upstream and downstream pressure changes, a valve capable of changing its flow is used. Note the changes, a valve capable of changing its flow is used. Note the addition of a straight arrow pointing up shown in the schematic addition of a straight arrow pointing up shown in the schematic symbol as indicated by the red arrow.symbol as indicated by the red arrow.

Restrictor type P.C. Flow ControlRestrictor type P.C. Flow Control

The restrictor type P.C. Flow Control is the most type used in The restrictor type P.C. Flow Control is the most type used in industry. It has the addition of a spool valve which is spring industry. It has the addition of a spool valve which is spring offset to the full flow position. As pressure builds from offset to the full flow position. As pressure builds from resistance the spool moves against the spring and cuts down resistance the spool moves against the spring and cuts down flow based on the pressure differential across its ports. The flow based on the pressure differential across its ports. The adjustment is the same as a non-compensated flow control.adjustment is the same as a non-compensated flow control.

P.C. Flow Control in a CircuitP.C. Flow Control in a Circuit

The purpose of the P.C. flow control here is to keep the cylinder The purpose of the P.C. flow control here is to keep the cylinder moving at a constant rate of speed even if the load changes. Certain moving at a constant rate of speed even if the load changes. Certain operations can not tolerate uncontrolled movement. It should be operations can not tolerate uncontrolled movement. It should be noted that P.C. flow controls are used only in hydraulics.noted that P.C. flow controls are used only in hydraulics.

Increase of Work Load or Relief Increase of Work Load or Relief Valve PressureValve Pressure

The compensator spring has a value of 100psi in order to develop The compensator spring has a value of 100psi in order to develop flow. This should be considered if the load increases because you flow. This should be considered if the load increases because you must have at least a 100psi differential between the relief valve must have at least a 100psi differential between the relief valve setting and the work load in order to develop enough flow to move setting and the work load in order to develop enough flow to move the load.the load.

Temperature Affects FlowTemperature Affects Flow

As a liquid takes on heat it gets thinner. The result of thinned As a liquid takes on heat it gets thinner. The result of thinned oil is lessened viscosity and this results in increased flow. As you oil is lessened viscosity and this results in increased flow. As you know, increased flow means increased actuator speed. So when know, increased flow means increased actuator speed. So when machines warm up, their operating characteristics will change.machines warm up, their operating characteristics will change.

Temperature CompensationTemperature Compensation

The idea behind the metal rod is that metal expands when heated. The idea behind the metal rod is that metal expands when heated. When the rod expands it extends directly in the path of fluid flow, When the rod expands it extends directly in the path of fluid flow, reducing in direct proportion to temperature.reducing in direct proportion to temperature.

Sharp Edge OrificeSharp Edge Orifice

The operating principles of this device are not clearly The operating principles of this device are not clearly understood but its effect is very precise control of flow. This understood but its effect is very precise control of flow. This design is very common although tricky to manufacture.design is very common although tricky to manufacture.

A temperature-pressure compensated flow control A temperature-pressure compensated flow control should be used wherever there exists extreme should be used wherever there exists extreme temperature differentials between start up and operating temperature differentials between start up and operating temperatures.temperatures.

Temperature-Pressure Compensated Temperature-Pressure Compensated Flow Control ValveFlow Control Valve

Meter-In Flow ControlMeter-In Flow Control

In the circuit above, a positive load resting on the cylinder rod In the circuit above, a positive load resting on the cylinder rod makes meter-in an acceptable choice for flow control because makes meter-in an acceptable choice for flow control because nothing can “pull” on the rod. Another example is the old style in-nothing can “pull” on the rod. Another example is the old style in-ground car lifts that were used in garages years ago. The weight of ground car lifts that were used in garages years ago. The weight of the vehicle was the positive load and gravity ensured that there the vehicle was the positive load and gravity ensured that there would be no “runaway.”would be no “runaway.”

Meter-Out Flow ControlMeter-Out Flow Control

Where a load is pulling on a cylinder rod some means is needed to Where a load is pulling on a cylinder rod some means is needed to prevent the piston from out running the oil supply as in the case of a prevent the piston from out running the oil supply as in the case of a runaway load. The technique is simply to use the incompressible runaway load. The technique is simply to use the incompressible nature of oil to hold back the rod at the rate of flow the needle valve nature of oil to hold back the rod at the rate of flow the needle valve is set for. This type of metering is also common in pneumatic is set for. This type of metering is also common in pneumatic systems. systems.

Bleed-Off(by-pass) Flow ControlBleed-Off(by-pass) Flow Control

A bleed-off circuit simply works by allowing a portion of the pump’s A bleed-off circuit simply works by allowing a portion of the pump’s flow to go back to tank before it gets to the actuator. The general flow to go back to tank before it gets to the actuator. The general idea is that “leaking” oil back to the tank is better than “forcing” it idea is that “leaking” oil back to the tank is better than “forcing” it over the relief valve and generating unnecessary heat. The bleed-off over the relief valve and generating unnecessary heat. The bleed-off circuit is only good on positive loads or one where precise speed circuit is only good on positive loads or one where precise speed control is not very important.control is not very important.

Reverse FlowReverse Flow

As stated in an earlier unit, the primary use of a check valve is by-As stated in an earlier unit, the primary use of a check valve is by-pass. In the above circuit, the check valve allows the oil to go pass. In the above circuit, the check valve allows the oil to go around the flow control so that it only meters in one direction. around the flow control so that it only meters in one direction. Check valves are also used on pressure controls.Check valves are also used on pressure controls.

Pneumatic Control of FlowPneumatic Control of Flow

Although the pressure regulator is classified as a pressure control Although the pressure regulator is classified as a pressure control valve, it also affects flow by throttling air movement. As pressure valve, it also affects flow by throttling air movement. As pressure builds at the outlet, the regulator begins to shut off until its setting builds at the outlet, the regulator begins to shut off until its setting has been reached where it is closed. Between full open and full close has been reached where it is closed. Between full open and full close the regulator limits air movement based on its opening.the regulator limits air movement based on its opening.