pressure drop vs flow relationship - philadelphia.edu.jo · pressure drop vs flow relationship 2-22...

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Exercise 2-3

Pressure Drop vs Flow Relationship

EXERCISE OBJECTIVE

C To introduce flowmeters, needle valves, check valves and flow control valves;C To show the relationship between pressure drop and generated flow using a test

circuit in order to see the effect of a load on the measured flow.

DISCUSSION

Flowmeters

The flow rate in a fluid power circuit describes the volume of fluid passing throughthe circuit during a given period of time. The flow rate is measured in liters perminute, R/min (Standard Cubic Feet per Minute, SCFM, or Standard Cubic Feet perHour, SCFH).

The device used to measure the flow rate is called a flowmeter. The Flowmetersupplied with your trainer is a variable-area flowmeter. In this type of flowmeter, theflow of air is used to float a lightweight ball or bullet-shaped element through atapered tube. As the flow rate of air increases, a larger orifice inside the flowmeteris needed to allow the air to escape. The taper of the tube requires a nonlinear scale,which means that the scale calibrations are not evenly spaced. The Flowmetersupplied with your trainer is shown in Figure 2-9.

Figure 2-9. Typical Flowmeter and Symbol.

The variable-area flowmeter must be mounted vertically to allow gravity to act on theball or bullet. It is calibrated to measure air at atmospheric pressure. This means that


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the flowmeter should be placed in a circuit to measure flow as the air is returning toatmospheric pressure.

Needle Valves

The needle valve is a component that allows control of the rate of flow in a system.It consists of a valve body containing two ports, an adjustable needle, a seat and astem seal. Its operation is shown in Figure 2-10.

Figure 2-10. Typical Needle Valve and Symbol.

When the needle is held against the seat by screwing the threaded stem, fluid (air)cannot flow through the valve. When the needle is lifted off the seat slightly, anorifice is created, permitting a small amount of fluid to flow past the needle. As theneedle is lifted farther off the seat, the size of the orifice is increased, allowing morefluid to flow past the needle. If the orifice is not large enough to permit full systemflow to pass through the valve, the orifice acts as a restriction, and creates pressureupstream from the valve. The difference between the pressures upstream anddownstream from the needle valve is called the pressure drop, or pressuredifferential, across the valve.

The flow through the valve will increase if either the size of the orifice is increasedor, if the size of the orifice remains constant but the pressure drop is increased byraising the upstream pressure. Needle valves control the flow in both directions.

Check Valves

Check valves allow fluid to flow through the component in one direction only. Asimple check valve as shown in Figure 2-11, consists of a valve body containing twoports, a sealing element such as a ball or poppet and a light spring.


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Figure 2-11. Typical Check Valve and Symbol.

When fluid enters the inlet port of a check valve, the sealing element is lifted off thevalve seat, and fluid flows through the valve. When the direction of flow is reversed,the sealing element is pushed against the valve seat and fluid cannot flow throughthe valve.

A pressure of 20 to 35 kPa (or 3 to 5 psi) is generally required to compress the lightspring in a check valve and unseat the sealing element. The actual inlet pressureneeded to open the valve is sometimes referred to as cracking pressure.

Flow Control Valve

The Flow Control Valve supplied with your trainer consists of a needle valve and acheck valve connected in parallel and integrated in one package. The majordifference between a simple needle valve and a flow control valve involves thedirection of metered flow. A needle valve placed into a pressure line meters flow inboth directions. A flow control valve meters flow in only one direction. The checkvalve permits flow to bypass the needle valve in one direction. A typical Flow ControlValve and its corresponding graphic symbol are shown in Figure 2-12.


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Figure 2-12. Typical Flow Control Valve and Symbol.

REFERENCE MATERIAL

For additional information, refer to the chapters entitled Control of Pneumatic Energyand Flow Control Valves, Silencers, Quick Exhausts in the Parker-Hannifin manualIndustrial Pneumatic Technology.

Procedure summary

In the first part of the exercise, you will set up a circuit in order to observe the effectof a check valve.

In the second part of the exercise, you will vary the flow rate through a flow controlvalve by varying the size of the orifice in the needle valve and by varying thepressure drop across the valve.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain thelist of equipment required to perform this exercise.

PROCEDURE

G 1. Verify the status of the trainer according to the procedure given inExercise 1-2.

G 2. Connect the circuit shown in Figure 2-13.


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Note: The muffler is located at the outlet port of the main shutoffvalve of the Conditioning Unit.

Figure 2-13. Schematic Diagram of a Circuit using a Flow Control Valve.

G 3. Close the Flow Control Valve by turning the control knob fully clockwise.

G 4. Open the main shutoff valve and the branch shutoff valve at the manifold.Set the pressure regulator at 200 kPa (or 30 psi) on the regulated PressureGauge.

G 5. Record the reading indicated by the Flowmeter.

G 6. Open the Flow Control Valve by turning the control knob fullycounterclockwise.


G 8. Close the Flow Control Valve by turning the control knob fully clockwise, andclose the main shutoff valve without modifying the setting of the pressureregulator.

G 9. Reverse the tubes at the Flow Control Valve ports to reverse the air flowthrough the valve.

G 10. Open the main shutoff valve.


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G 12. Since the Flow Control Valve is closed, explain why the Flowmeter indicatesa flow through the valve.

G 13. Open the Flow Control Valve by turning the control knob fullycounterclockwise.


G 15. Compare the flow rates measured in steps 11 and 14. Explain why the flowrate is a little larger when the Flow Control Valve is fully open.

G 16. Close the shutoff valves and turn the regulator adjusting knob completelycounterclockwise.

G 17. From the results obtained, what can you conclude about the metering of aFlow Control Valve?

G 18. Modify your circuit as shown in Figure 2-14.


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Figure 2-14. Schematic Diagram of a Circuit using a Flow Control Valve.

G 19. Open the shutoff valves.

G 20. Close the Flow Control Valve by turning the control knob fully clockwise. Setthe pressure regulator at 400 kPa (or 60 psi) on the regulated PressureGauge.

G 21. Gradually open the Flow Control Valve by turning the control knobcounterclockwise as indicated in Table 2-3. Refer to the mark on the controlknob to help you set the correct position. For each setting, ensure that theregulated pressure is set at 400 kPa (or 60 psi). Record the flow rates in theappropriate cells in Table 2-3.

PRESSURE REGULATOR FLOW CONTROL VALVE FLOW RATE

400 kPa (or 60 psi) 3 turns



400 kPa (or 60 psi) fully open

Table 2-3. Flow Rate vs Size of the Orifice.

G 22. What relation is there between the opening of the Flow Control Valve andthe flow rate?

G 23. Without modifying the setting of the Flow Control Valve, set the pressureregulator to obtain a reading of 50 R/min (or 2 SCFM) on the Flowmeter.Record the value of P1 and P2 in Table 2-4.


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FLOW RATE P1 P2 ∆P

50 R/min(or 2 SCFM)

75 R/min(or 3 SCFM)

100 R/min(or 4 SCFM)

Table 2-4. Flow Rate vs Pressure Drop.

G 24. Set the pressure regulator to obtain the remaining flow rate values indicatedin Table 2-4. For each setting, record the value of P1 and P2.

G 25. Close all the shutoff valves.

G 26. Calculate the pressure drop (∆P) for each flow rate and record the result inTable 2-4.

G 27. What relation is there between the pressure drop and the flow rate?

G 28. On the Conditioning Unit, turn the regulator adjusting knob completelycounterclockwise. You should read 0 kPa (or 0 psi) on the regulatedPressure Gauge.

G 29. Disconnect and store all tubing and components.

CONCLUSION

In the first part of the exercise, you saw the effect of a check valve. You saw that itallows fluid to flow in one direction only.

You have seen that a check valve connected in parallel with a needle valve permitsflow to bypass the needle valve in one direction.

In the second part of the exercise, you have observed that the flow through the FlowControl Valve varies with the size of the orifice or, if the size remains constant, withthe pressure drop across the valve.


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REVIEW QUESTIONS

1. A needle valve is used to

a. adjust the flow rate in a circuit.b. adjust pressure downstream from the valve.c. control the direction of fluid flow.d. align the needle in an industrial pneumatic sewing operation.

2. Which components are combined within a pneumatic flow control valve?

a. A relief valve and a needle valve.b. A needle valve and a check valve.c. A check valve and a regulator.d. A regulator and a needle valve.

3. A needle valve consists of a valve body containing

a. two ports, a spring and a spool.b. two ports, a needle and seat, and a threaded needle stem.c. three ports, a spring and a ball.d. one port, a needle and seat, and a spring.

4. The flow through a control valve will increase if either the size of the orifice isincreased or

a. if the size of the orifice remains constant but the pressure drop is increasedby raising the upstream pressure.

b. if the size of the orifice remains constant but the pressure drop is decreasedby lowering the upstream pressure.

c. if the size of the orifice remains constant but the pressure drop is increasedby lowering the upstream pressure.

d. if the size of the orifice remains constant but the pressure drop is decreasedby raising the upstream pressure.

5. The major difference between a simple needle valve and a flow control valveinvolves

a. the size of the valve.b. the construction method.c. the number of ports.d. the direction of metered flow.

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Exercise 2-1

Pressure vs Force Relationship

EXERCISE OBJECTIVE

C To introduce the relationship between pressure and force;C To verify the formula F = P x A using a cylinder and a load device;C To measure the force delivered by a cylinder;C To observe that the force exerted on a given surface is directly proportional to the

pressure applied on this surface.

DISCUSSION

Pneumatics technology involves the use of a gas to transmit power. The gas mostoften used in pneumatics is ordinary air. Air is a highly compressible fluid. Thismeans that the molecules in a body of air can be pushed closer together in aconfined space to make the air occupy a smaller volume. This property of air isillustrated in Figure 2-1.

Figure 2-1. Compressibility of Air.

Pascal’s Law

Pascal’s Law states that pressure applied on a confined fluid is transmittedundiminished in all directions, and acts with equal force on equal areas, and at rightangles to them.


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The cylinder in Figure 2-2 is completely filled with fluid. When force is applied to thetop of the piston, pressure is created within the fluid and transmitted equally in alldirections.

Figure 2-2. Force Applied to a Confined Fluid.

Pascal's law deals with the relationship between pressure, force and area. In fluidpower technology, pressure is a measurement of force per unit of surface area. Inmathematical formulas, the word per may be replaced by a division sign, so that:

where P is the pressure in kilopascals (or pound force per square inch),F is the force in newtons (or pound force),A is the area in square centimeters (or square inches).

Gases have weight. As an example, the mass of air molecules located inside a1 m2 (or 1 in2) column of air exerts force on the air below as the mass is pulled downby gravity. At sea level, the pressure of this column of air is about 101.3 kPa (or14.7 psi) and it is called atmospheric pressure.

Fluid pressure gauges commonly measure pressure above atmospheric pressureon kPa (or psi) gauge scales, which read a value of zero at sea level. In pneumatics,the term compressed air refers to air that is compressed beyond atmosphericpressure.


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Pneumatic Pressure Versus Cylinder Force

The relationships between force, pressure and area, and Pascal's Law, allowcalculation of the force generated by the piston shown in Figure 2-3.

Figure 2-3. Cap End of the Cylinder.

Compressed air is confined within the cap end of the cylinder. As a result, pressuredevelops in the cap end of the cylinder. This pressure is exerted evenly over theentire surface of the cap end of the cylinder. It acts on the piston, resulting in amechanical force to push the piston.

To calculate the force generated by the piston during its extension, we can rewritethe formula P = F / A as F = P × A. Therefore, the generated force is equal to thepressure in the cap end of the cylinder times the piston area being acted upon. Thisarea is called full area, or “face” area (AF).

where AF is the full area,RCYLINDER is the radius of the cylinder,DCYLINDER is the diameter of the cylinder.

In Figure 2-4, compressed air is confined in the rod end of the cylinder. This time,however, the generated force is lower because the piston area available for thepressure to act on is reduced by the area the cylinder rod covers on the piston. Thisarea is called annular area (AA). Therefore, the system must generate more pressureto pull than to push the load.

where AA is the annular area,AF is the full area,

RROD is the radius of the rod,DROD is the diameter of the rod.


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Figure 2-4. Rod End of the Cylinder.

REFERENCE MATERIAL

For additional information on the relationship between force and pressure, refer tothe chapter entitled Force Transmission Through a Fluid in the Parker-Hannifinmanual Industrial Pneumatic Technology.

Procedure summary

In the first part of the exercise, you will verify the formula F = P × A by measuring thecompression force of a cylinder using the load device. In the second part, you willobserve the relationship between pressures of both sections of a cylinder.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart, in Appendix A of this manual, to obtain thelist of equipment required to perform this exercise.

PROCEDURE

Conversion of Pressure to Force

G 1. Use the cylinder diameter, DCYLINDER: 2.7 cm (or 1.06 in) to calculate the fullarea AF:

Full area

G 2. Use your calculated full area AF and the formula F = P × A to calculate theforce of the cylinder for the pressure levels indicated in Table 2-1. Recordyour results in the appropriate cells in Table 2-1.


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Note: A pressure of 1 Pa represents a force of 1 N applied on asurface area of 1 m2 (or 1 psi = 1 lbf / 1 in2).

APPLIED PRESSUREON FULL PISTON AREA

CALCULATEDCYLINDER FORCE

MEASUREDCYLINDER FORCE

600 kPa (or 90 psi)

400 kPa (or 60 psi)

200 kPa (or 30 psi)

Table 2-1. Cylinder Force versus Pressure.

G 3. Verify the status of the trainer according to the procedure given inExercise 1-2.

G 4. As Figure 2-5 shows, screw the load device to the cylinder until the loadpiston inside the spring load begins to push on the spring. Do not use a toolto screw the load device.

Figure 2-5. Load Device Assembly.

G 5. Clip the N/lbf-graduated ruler to the spring load device, and align the "0"mark with the colored line on the load piston. To reverse measurementunits, install the ruler on the opposite side of the spring load device.


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Note: Ensure that your N/lbf-graduated ruler corresponds to thespring characteristics of your loading device. To do so, verify ifthere is an encircled letter at the left of the unit symbol N on theruler. If so, it should be the same as the one engraved on one ofthe Loading Device extremities.

G 6. Connect the circuit shown in Figure 2-6.

G 7. Open the main shutoff valve and the branch shutoff valve at the manifold.

G 8. Set the pressure regulator at 600 kPa (or 90 psi) on the regulated PressureGauge. This corresponds to the pressure applied on the full piston area.

Figure 2-6. Schematic Diagram of the Circuit Used to Measure the Cylinder Output Force.

G 9. On the spring load device, observe that the applied pressure caused thecylinder to compress the spring. Record the force value indicated by theload device in the appropriate cell in Table 2-1.

G 10. Close the main shutoff valve to exhaust compressed air, then set thepressure at 400 kPa (or 60 psi) and record the force value in the appropriatecell in Table 2-1. Repeat for a pressure setting of 200 kPa (or 30 psi).

G 11. Are the calculated and measured values of force in Table 2-1 approximatelyequal, showing that the force corresponds to the applied pressure multipliedby the area?

G Yes G No

Note: The theoretical examples presented in this manual assumeperfect systems. Unfortunately, pneumatic systems alwaysexperience a slight amount of leakage through fittings and seals.A difference of 15% between calculated and measured values isacceptable.


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G 12. Turn the regulator adjusting knob completely counterclockwise, close theshutoff valves and remove the load device.

G 13. Use the rod diameter, DROD: 0.8 cm (or 0.31 in) and the full area AFcalculated in step 1, to calculate the annular area AA of the piston.

Annular area

G 14. Calculate the area ratio.

G 15. Ensure that the piston rod is fully retracted then connect a Pressure Gaugeat the rod end of the cylinder. Screw a tip (bullet) to the rod.

Note: For accurate results, the air volume in the tube connectingthe Pressure Gauge must be kept small with respect to the airvolume in the circuit. It is therefore important to use tubes thatare as short as possible.

G 16. Open the main shutoff valve and set the pressure regulator at 350 kPa (or50 psi) on the regulated Pressure Gauge. This corresponds to the pressureapplied on the full piston area.

G 17. To eliminate the friction caused by the extension of the rod, pull and pushslightly the tip of the rod then release it. Record in the appropriate cell inTable 2-2, the pressure in the rod end of the cylinder.

APPLIED PRESSURE (PF)ON FULL PISTON AREA

MEASURED PRESSURE (PA) IN THE ROD END

OF CYLINDER

PF /PA

350 kPa (or 50 psi)

Table 2-2 Pressure Measurement Results.

G 18. Use the values indicated in Table 2-2 to calculate the ratio PF /PA and recordthe results in the appropriate cell.


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G 19. Compare the pressure ratio PF / PA with the area ratio AA /AF calculated instep 14. Is the pressure ratio approximately equal to the reciprocal of thearea ratio?

G Yes G No

G 20. Do your results confirm the relationship F = PA × AA = PF × AF?

G Yes G No

G 21. Use your values of AF and AA to calculate the pressure in the rod end of thecylinder when the pressure applied on full piston area is 700 kPa (or100 psi).

G 22. Use your values of AF, AA, PF and PA to calculate the force in the cap endand in the rod end of the cylinder.

Force in the cap end = Force in the rod end =

G 23. Are the results approximately equal? Explain.

G 24. On the Conditioning Unit, close the shutoff valves and turn the regulatoradjusting knob completely counterclockwise. You should read 0 kPa (or0 psi) on the regulated Pressure Gauge.

G 25. Disconnect and store all tubing and components.

CONCLUSION

In this exercise, you learned how to measure force using a load device. You saw thatthe force exerted on a given surface is directly proportional to the pressure appliedon this surface.

Since the relationship between force and pressure is linear, predicting the forceexerted by the cylinder at any pressure setting is possible.


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REVIEW QUESTIONS

1. According to the relationship between force, pressure and area, the force maybe calculated using the formula

a. F = P / A.b. F = P × A.c. F = P2 × A.d. F = A / P.

2. Fluid pressure gauges commonly measure pressure

a. above atmospheric pressure.b. below atmospheric pressure.c. at atmospheric pressure.d. below absolute pressure.

3. Pressure is a measurement of

a. weight.b. force per unit of surface area.c. flow per unit of time.d. force per unit of time.

4. The pressure at sea level is called

a. sea level pressure.b. absolute pressure.c. atmospheric pressure.d. reference pressure.

5. The annular area of a cylinder corresponds to

a. the face area plus the rod area.b. the rod area minus the face area.c. the face area minus the rod area.d. the rod area plus the face area.

pressure drop vs flow relationship - philadelphia.edu.jo · pressure drop vs flow relationship 2-22...

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