Pressure Reducing Valves Part Four: Minimizing Offset on Clean Air and Gas ApplicationsSeptember 2, 2016 by admin
In our last blog, we discussed the factors that a user can control during commissioning to lower offset. Examples were provided that described how setting a regulator at a specific flow rate for particular applications can affect the amount of offset error.
In the next two blog posts, we will focus on factors that can be controlled during project design and specification to reduce offset in various applications.
Part One: How to minimize offset during project design and valve specification on Clean Air and Gas Applications
1. Select the correct regulator style/type for your particular application!
– Clean Air and Gas PRVs
– Point of Use Applications usually have stable flow rates. Sparging*, 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 like the one featured in Figure 1 below.
– 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 press set points require a larger diaphragm for lower offset and more precise control. See Figure 2 below.
– 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 (Figure 3), 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, as shown in Figure 4 below
*CO2 sparge flow rates can vary, but usually not too widely. CO2 is used to control PH during batch life.
2. 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. As discussed in part two of this series, the more compressed the spring is at your set point pressure, the less offset 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). See Figure 5 below.
Karl Lutkewitte, Steriflow Valve Product and Sales Manager