Selecting the Proper Diaphragm Pressure Control Valve
by KNF Flodos R&D Center, Switzerland
FDV Valves in PVDF materials
1. How It Works
The possible uses of the pressure control valve are widely varied and well established. It can be used in two possible ways depending on the application:
Pressure Control Valve
Maintain constant back pressure for exact flow rates under free flow conditions, with positive pressure on the inlet side or with varying back pressure, or with the operation under vacuum etc.
Bypass Valve
Serves as a safety device for protection of the pump, motor, pipework, vessels and other accessories. Installed as a bypass valve, it prevents excessive pressure build up in the system caused by dirt, misuse or other problems.
The pressure control valve FDV30/1.30 are able to handle air, gas and liquids.
The pressure control valve can used with KNF products as well as other pump systems. The FDV30/1.30 series are recommended for use with the following pumps.
Metering pumps - all FM Products
Transfer pumps NF10, NF30, NF1.30
Vacuum pumps. Up to a max flowrate of: CALL KNF
Note! Pressure control valves are not absolutely tight shut-off valves. They should always be installed on the pressure side of the pump.
2. Construction
The FDV pressure control valve is based on the principle of the diaphragm valve. The essential components are the lower housing, the upper housing, the spindle and the diaphragm.
The required pressure is achieved by adjusting the tension in the spring. The spring tension exerts pressure on the diaphragm with is then transferred to the fluid system. By turning the spindle clockwise the opening pressure increases at a given flow rate and by turning it counter-clockwise the opening pressure decreases. A locking nut prevents adjustment from the set position.
In the normal position the diaphragm rests on both of the ports and the system is then closed. When the pressure produced by the pump exceeds the pre-set opening pressure the diaphragm is pushed open and the medium can flow. The pressure control valve is now in the working mode and is opened.
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Index 10 Lower housing 20 Upper housing 30 Protective surface of diaphragm 40 Lip diaphragm 50 Flexi-washer 60 Support plate 70 Washer 80 Spring 90 Washer 100 Shaft spindle 110 Locking nut 120 Screw |
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The parts in contact with the media are the diaphragm and the lower housing. They can be produced in a variety of materials which can be selected according to the liquid or gas to be transferred.
The following material combinations are available:
Base Model Code
Head components
Material (liquid contacting parts)
FDV30KP, FDV1.30KP
lower hosing PP
diaphragm EPDM
FDV30KV, FDV1.30KV
lower housing PP
diaphragm Viton
FDV30KT, FDV1.30KT
lower housing PP
diaphragm Viton/FFKM
FDV30TV, FDV1.30TV
lower housing PVDF
diaphragm Viton
FDV30TT, FDV1.30TT
lower housing PVDF
diaphragm Viton/FFKM
The upper housing which is not in contact with the media is produced in Ryton for all types.
3. Technical Data
The correct control valve is selected according to the following criteria:
- Pressure
- Flow rate
- Aggressive nature of the media
- Size
Note that the pre-set opening pressure should not exceed the maximum pressure of the pump. Other important factors include connecting sizes, overall geometry, temperature etc. The following table assists in making the correct selection.
Following base models are available:
| Parameter | FDV30 Z | FDV1.30 Z | |
|---|---|---|---|
| Adj. pressure range | bar g | 0.2 0 2.5 | 2.0 - 6.5 |
| Standard pressure (factory set) | bar g | 0.5 | 3.0 |
| Max. flow with liquids | ml/min | 600 | 600 |
| Max. flow with air/gases | l/min | ||
| Max. environment temp | °C | 80 | 80 |
| Max. media Temp | °C | 80 | 80 |
| Threads for hose connector | in. | G 1/8" | G 1/8" |
| Weight (depends on model) | grams | 50 - 60 | 50 - 60 |
The above flow rates should not be exceeded. If required the factory pre-set opening pressure can be adjusted to other values. The adjusted opening pressure will be noted on the identification plate.
Dimension Drawing
4. Options
The pressure control valve is also available with other options:
- turning knob instead of locking nut
- other materials / combination
- with hose connectors
5. Applications
Pressure control valves are used in diverse operations and can therefore undertake several functions:
- As pressure control valve for precise dosing in systems with fluctuating pressure, for operating in a vacuum and for operating with back pressure on the suction side.
- As bypass valve to prevent the build up of excessive pressure on the operating side of the system, for protection of the pump, pipework, vessels, glassware, etc.
- Anti-injection function to avoid unintended injection of liquid when metering into pipework at high flow rates.
Examples of applications
1. Pressure control valve
The precision of diaphragm pumps can be influenced by other factors such as system pressures. The following illustrations demonstrates the use of the control valve in achieving precise metering.
a. Operation in a system with fluctuating back pressure
Varying back pressure can significantly influence the performance and thus the precision of the flow rate. The use of the pressure control valve promotes more stable system pressure. The pressure variation is reduced and so precise dosing is guaranteed.
1a. Flow diagram
b. Operation with vacuum on the outlet of the pump
When it is required to meter into a vacuum on the outlet side of the pump, use of the pressure control valve restricts the free flow of the media through the pump. Without a pressure control valve, the vacuum on the outlet side of the pump would cancel the effect of the pump and the media would flow unaided through the pump. This applies whether the pump is being used or not.
1b. Flow diagram
c. Operation with positive pressure on the suction side of the pump
If positive pressure exists on the suction side of the pump, accurate dosing is not possible. Even when the pump is not being used, it is still possible for the pressure head to force the media through the pump. The pressure control valve will withstand the higher pressure therefore guaranteeing optimum performance.
1c. Flow diagram
2. Bypass function
The bypass valve serves to restrict the build up of excessive pressure on the pump outlet side of the system. Therefore the pump, pipework and motor are adequately protected against malfunction or failures as a result of high pressure build up. In the case of excessive back pressure on the outlet side of the pump, the bypass valve opens and the media is recirculated. The media recirculates and the pump is protected until the restriction is removed.
A bypass valve should also be used if the pump has to operate against a closed system on the pressure side.
Pressurized measuring systems, pipework, receivers etc. can also be protection against excessive pressure build up by using the bypass valve. If excessive pressure build up occurs, the media will flow back through the valve and into a storage vessel.
2. Flow diagram
3. Anti-injection function
When pumping into a tube that contains a continuously high speed of flow of media, “venturi action” can occur. This means that when the pump is stationary, suction produced by the fast flow of liquid in the tube overcomes the resistance of the valves and pulls the dosing medium through the pump. This venturi action has a negative affect on the dosing accuracy.
With the built in diaphragm pressure control valve, the venturi action can be stopped. Furthermore the pressure control valve function assists the dosing accuracy even though the pipework pressure may vary.
3. Flow diagram
4. Other functions
Further uses are:
Improved sealing against back flow, for example in analyzers
Over-pressure valves in liquid, air/gas systems
Charging pipework with constant pressure without the flow being drawn off through the bypass valve
Questions? Contact KNF application engineering at 609-890-8600.
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