Can You Size up Your Needs for Pressure Regulators?
by Katie Dennis, on Tue, Aug 14, 2012 @ 08:08 AM
Here’s a quick look at how to figure out the right regulator for the job
First comes the easy part: You know that you need a pressure regulator. Then you go to the Swagelok catalogue and suddenly you realize how many variations there can be. Where do you start? Fortunately, you can count on Edmonton Valve & Fitting to help you navigate the choices. But you can also handle a lot of the basic decisions on your own.
There’s size, and then there’s size
When we talk about “sizing” a pressure regulator, we don’t just mean the size of the end connections. We mean determining how much flow can pass through the regulator, and what pressures it is capable of regulating. Regulators should be sized based on the largest possible flow and the smallest change in pressure. If your requirements approach the limit of a regulator, we recommend using the next largest regulator available.
Start with the most critical part of the job: Are you trying to regulate a gas or a liquid? Gases are compressible and liquids are not. That will make a big difference in selecting the right regulator.
Next, what do you want the regulator to do, reduce pressure or control backpressure?
Then you’ll need to list a few ranges: the maximum and minimum inlet and outlet pressure ranges, the operating temperature range, and the flow requirements.
Liquid versus gas
Because liquids won’t compress, their flow rate depends only on the pressure drop. Look at the pressure going into the regulator and the pressure of the liquid at the outlet port. As long as the difference between P1 and P2 is the same, the flow is the same, no matter if the system pressure is high or low.
Gases are another, more complex story, because the density of gas changes with pressure. For these applications, your Edmonton Valve & Fitting representative will need to calculate the size of two orifices in the regulator.
First there’s the size of the seat orifice. That’s a restriction inside the regulator body: the flow area between a valve stem and the seat with the valve fully opened. When any fluid goes through a restriction and into a lower-pressure environment, the flow must increase as it goes through the restriction. At the same time, pressure decreases. If it decreases too much, we end up with a situation called “choked flow” or “critical flow,” where the fluid can’t flow through at any higher velocity.
If the pressure drop is big enough, seat flow depends on the inlet pressure and not on the outlet pressure. If the pressure drop is smaller, the flow through the seat is restricted by the high outlet pressure.
The outlet orifice is the area of the internal passage in the regulator outlet connection. We want controlled outlet pressure with as little droop as possible. To do that, we have to keep the velocity of the fluid on the outlet side at an acceptable level. For gases, the rule of thumb is 25 to 30 metres per second.
If you can tell your Edmonton Valve & Fitting representative the type of gas you are sending through the regulator, the flow, the inlet pressure and the outlet pressure, he or she can calculate the required orifice areas.
You might wonder why the type of gas matters. Most formulas for regulators are based on the assumption that air is flowing through. Some gases are denser than air, others are less dense. That can change the results of the formulas. Your Edmonton Valve & Fitting representative has a list of the corrections to make for a wide variety of gases.
Having some project-specific information will help your Edmonton Valve & Fitting representative find the right regulator for the job. This can include such things as the temperature range of the fluid going through the regulator, and whether stainless steel or special alloys are best for the particular application. But if you have the sizing information in hand when you start, you’ll find the selection process much easier.