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Before deciding on the appropriate diaphragm pressure control valve for your application, it is important to first understand some of the basic uses of these devices. Gaining a better understanding of how a pressure control valve works, and determining how it can best suit your specific application, will simplify the selection process.

FDV Valves in PVDF materials

What Does a Pressure Control Valve Do?

When used in tandem with a pump, a pressure control valve provides more accurate control over the pressure supplied by the pump. Generally, pressure control valves can be used in the following 3 ways, depending on the application:

1) Pressure Control

A pressure control valve can be used to maintain constant pressure, after the pump, for exact flow rates under a variety of conditions. For instance: flow that is unrestricted, flow with variable or fixed pressure on the inlet side, or flow with varying pressure or even vacuum on the downstream side.

2) Bypass Valve

A pressure control valve can serve as a safety device for protection of a system's pump, motor, tubing and connections, 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.

3) Anti-injection Function

A pressure control valve can prevent the unwanted injection of a liquid into a pipe which already contains liquid flowing at a high velocity (Venturi effect).

Please note the following specifications regarding usage of KNF pressure control valves:

  • The FDV pressure control valves are able to handle air, gas and liquids.
  • KNF pressure control valves can be used with KNF products as well as other pump systems.
  • KNF pressure control valves are recommended for use with the following pumps:
       - Metering pumps: all FEM and FMM Products
       - Transfer pumps: all liquid pumps up to 12L/min flow rate and 6.5 bar g (94 psig)
       - Vacuum pumps: all gas pumps up to 300L/min flow rate and 6.5 bar g (94 psig)
  • Pressure control valves are not absolutely tight shut-off valves. They should always be installed on the pressure side of the pump.

Construction and Operation

Next, it is important to have a basic understanding of how pressure control valves are constructed. FDV pressure control valves are based on the principle of a diaphragm valve. The essential components are the lower housing, the upper housing, the spindle and the diaphragm (see Figure 1).

Required pressure is achieved by adjusting the tension in the spring. The spring tension exerts pressure on the diaphragm which is then transferred to the fluid system. By turning the spindle clockwise the valve opening pressure increases at a given flow rate and by turning it counter-clockwise the opening pressure decreases. A locking nut prevents accidental 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.

Figure 1: Pressure Control Valve Construction

Figure 1 Index:

10 Lower housing
20 Upper housing
30 Diaphragm surface              
40
Lip diaphragm
50 Flexi-washer
60 Support plate

70 Washer
80 Spring
90 Washer
100 Shaft spindle
110 Locking nut
120 Screw


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 in contact w/ liquid

FDV 30KP, FDV 1.30KP, FDV 300 KP, FDV 1.300KP

lower housing PP

diaphragm EPDM

FDV 30KV, FDV 1.30KV, FDV 300KV, FDV 1.300KV

lower housing PP

diaphragm Viton®

FDV 30KT, FDV 1.30KT, FDV 300KT, FDV 1.300KT

lower housing PP

diaphragm Viton®/FFKM

FDV 30TV, FDV 1.30TV, FDV 300TV, FDV 1.300TV

lower housing PVDF

diaphragm Viton®

FDV 30TT, FDV 1.30TT, FDV 300TT, FDV 1.300TT

lower housing PVDF

diaphragm Viton®, FFKM


The upper housing which is not in contact with the media is produced in Ryton® for all types.

Options

Other available options for pressure control valves include:

  • Turning knob instead of locking nut
  • Other materials
  • Tubing connectors
  • Custom factory set pressure opening valves

 

Applications

This is a very important consideration as one application is rarely the same as the next. Moreover, the application can completely change the technical specifications required from the pressure control valve, which can perform several functions.

Examples of Applications:

1. Pressure Control

The precision of diaphragm pumps can be influenced by other factors such as system pressures. The illustrations below demonstrate the use of the control valve in achieving precise metering.

a) Operation in a System with Fluctuating Back Pressure

Varying back pressure (i.e. pumping into a piping system) 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, leading to more precise dosing.



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 prohibits the media from flowing freely through the pump. This applies whether the pump is on or off.

 

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 off, 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.


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.


3. Anti-injection function

When pumping into a tube that contains a continuously high speed 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.

4. Other functions

Additional uses include:

  • 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.

Technical Data

Once we understand how pressure control valves can be used, depending on the application, and how they are constructed, then we use some technical data to make the proper selection. The correct control valve is selected according to the following criteria:

  1. Pressure
  2. Flow rate
  3. Aggressive nature of the media
  4. 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.

The following base models are available:

Parameter

Dimensions

FDV30 Z

FDV1.30 Z

FDV300 Z

FDV1.300 Z

Adj. pressure range

bar g

0.2 - 2.5

2.0 - 6.5

0.8 - 2.5

2.0 - 6.5

Standard pressure (factory set)

bar g

0.5

3.0

1.5

3.0

Max. flow with liquids

L/min

0.600

0.600

12.0

12.0

Max. flow with air/gases

L/min

150

150

300

300

Max. environment temp.

°C

80

80

80

80

Max. media temp.

°C

80

80

80

80

Threads for tubing connector

in.

G 1/8"

G 1/8"

G 3/8"

G 3/8"

Weight (depends on model)

grams

50 - 60

50 - 60

50 - 70

50 - 70


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.

 

In Summary...

  1. Remember the basics about how pressure control valves work, both as a pressure control valve, or as a bypass valve.
  2. Consider how control valves are constructed, including additional options which may be available such as: locking nuts, turning knobs, alternative materials, and tubing options.
  3. Pay close attention to the specific application in which the pressure control valve will be used, there are many examples of applications- one size does not fit all.
  4. Determine the technical data. The control valve should be selected based on the criteria of: pressure, flow rate, media type, and size.

Click here to view all KNF pressure control valve models

Contact a KNF Application Engineer to discuss your application or specific needs.