How to Minimize Offset on Clean Air and Gas Applications

First and foremost, select the correct regulator style/type for your particular application!

• Point of Use Applications usually have stable flow rates. Sparging (Co² sparge flow rates can vary, but usually not too widely. Co² is used to control PH during batch life), blanketing, purging, motive force (using gas to move liquids) and filter drying are typically stable flows. Because of that, and because drainability isn’t an issue, these applications are usually suitable for less expensive small diaphragm, inline style valves.
• Low Pressure Point of Use Applications with set points below 5 psi are a special case. A large diaphragm inline style valve should be your first choice on these applications because low pressure set points require a larger diaphragm for lower offset and more precise control.
• Primary Pressure Reduction Upstream of Multiple Points of Use require higher Cv’s. Higher Cv’s mean larger body valves with proportionally larger diaphragms. You can still use a lower cost inline style valve for these applications but know that if less offset is required because of flow variability (multiple points of use downstream may mean that not all Points of Use will be operating at the same time), it may be desirable to use a regulator with a very large diaphragm size.

Second, for all applications: select the correct spring range during model specification:

Select a spring range so that the set point is in the upper 50% of that spring range. The more compressed the spring is at your set point pressure, the less offset is exhibited.

Another way to mitigate the offset effect of spring force is to eliminate, or minimize, the use of a spring. Use an Air Augmented or Air Loaded regulator. These PRV’s use air pressure delivered by a self-relieving air pressure regulator or an I/P to pressurize the top of the diaphragm to affect and maintain outlet setpoint. Their use will greatly reduce or eliminate offset completely because the compressible spring is taken out of the control equation and replaced with stable air pressure. Note that the installed cost of air loaded PRV’s (even with an I/P and the associated 24 Vdc input wiring and pneumatic tubing) are usually less costly than a pressure control valve on sizes up to 1-1/2″ (DN40).

How to minimize offset during project design and specification on clean steam and liquid pressure applications

1. Select the correct regulator style/type for your particular application!

• Clean Steam PRV applications have highly variable flow rates whether they are installed on SIP pressure reduction at points of use or, on a clean steam header upstream of multiple SIP points of use.
• Select a drainable PRV design that offers the widest diaphragm that you can find to minimize offset
• Look at air-augmented or air-loaded PRVs for these applications if any amount of offset is a concern. These PRV’s use air pressure delivered by a self-relieving air pressure regulator or an I/P to pressurize the top of the diaphragm to affect and maintain outlet setpoint. Their use will greatly reduce or eliminate offset completely because the compressible spring is taken out of the control equation and replaced with stable air pressure. Note that the installed cost of air-loaded PRV’s (even with an I/P and the associated 24 Vdc input wiring and pneumatic tubing) are usually less costly than a pressure control valve on sizes up to 1-1/2″ (DN40)
• WFI and USP PW Point of Use PRV applications are usually stable flows. WFI is used up and downstream to fill media vessels, bioreactors/fermenters and formulation vessels. It is also delivered to all points of the biopharma and parenteral pharma production train as a final rinse after CIP. Note that USP Purified Water is also distributed to the entire production train for the first rinse in sanitization routines. Filing flows for WFI are usually stable and the purpose of the PRV is to reduce pressure prior to filling. PRV’s are sometimes used for sanitization rinse flows to control the pressure for high flow/velocity rinse purposes. For both of these applications, use a drainable PRV with a wide diaphragm.
• Media and buffer dilution applications can require a valve that will operate at stable and at variable flow rates. These applications also require complete drainability and will see regular sanitization routines. For these installations, choose a wide diaphragm, drainable valve. When sizing these valves, you should also provide rinse and SIP flow and pressure information to the manufacturer so that multi case sizing can be performed to determine a single Cv that satisfies each condition.Since these valves are often installed on compact self-contained skids with PLC driven automated routines, an air loaded valve with an I/P can be used to change set point so as to force the valve fully open for sanitization.

2. For all applications: select the correct spring range:

• Select as spring range for the regulator you are specifying so that the set point is in the upper 50% of that spring range. The more compressed the spring is at your set point pressure, the less offset exhibited.
• To remove the offset effect of spring force, eliminate the spring! Use an air-augmented or air-loaded regulator.