Pressure Reducing Valves Part Five: Minimizing Offset on Clean Steam and Liquid ApplicationsSeptember 16, 2016 by admin
Factors that a user can control during design and specification to minimize offset
In our last blog post, we discussed the factors that a user can control during design and specification to lower offset on Clean Air and Gas Applications.
In this post, we will focus on factors that can be controlled during project design and specification on Clean Steam and Liquid pressure control applications.
How to minimize offset during project design and valve specification on Clean Steam and Liquid 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 Point 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. See Figure 1 below.
– Look at air-augmented or air loaded PRV’s (Figure 2) 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/fermentors 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. Filling 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. See figure 1 below.
– 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, chose a wide diaphragm, drainable valve like the valve shown in figure 1 below. 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 (figure 2a) 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 a spring range for the regulator you are specifying so that the set point is in the upper 50% of that spring range. As discussed in part two of this series, 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 (figure 2a, 2b) as discussed above.
In our next blog post, we will take a step back to the basics and discuss how PRVs work.
Karl Lutkewitte, Steriflow Valve Sales and Product Manager