Two Ways to Find the Right Size Pressure Regulator for the Job

A few simple questions is usually all you need to do, but we also have formulas

	Download the Swagelok web-catalogue, How to Size a Pressure Regulator PDF here.

We've blogged before about the importance of selecting a regulator that's the correct size for the job. By size, we mean how much pressure and flow it is designed to handle, not the size of the end connections.

Some customers have the math skills and the desire to go through a set of precise calculations leading to the right answer. Other customers want something simpler. We can accommodate both approaches.

Five questions

For those who want to avoid most of the complexities, we typically ask some combination of five basic questions:

1. What type of regulator are you going to be using? Usually it will be a pressure-reducing regulator or a back-pressure regulator.

2. What is the inlet and outlet pressure ranges? Generally speaking, you'll want a regulator designed to operate across a range beyond the pressures you are using. "For instance, if the desired outlet pressure is 500 psi, you wouldn't want the control range operating 0 – 500 psi" says Andrew Worthington of our Sales & Service team. "You would want it rated at twice that. The middle is where you get the most accurate reading."

In regards to inlet pressure, regulators whose inlet pressure rating closely match actual system pressure show less droop and a broader ideal operating range than those whose inlet pressure rating is much higher than the actual system pressure.

3. Will you be running a gas or a liquid through it? Gasses can be compressed, but liquids can't. That makes liquids a little easier to deal with.

4. What is the operating temperature?

5. What is the flow requirement?

Then we typically refer to the flow curves for the various regulators. A flow curve is a graph that shows the range of pressures that a regulator will maintain, given certain flow rates in a system. Typically it's best for a regulator to operate in the middle of its range, where the flow curve is relatively flat. When a regulator is forced to work at either extreme of the flow curve, you risk getting a choked flow, or having the regulator operate wide open and unable to control pressure at all.

Figuring the details

But for customers who like to work the formulas, we have plenty to offer.

For liquids, we start with the fact that volume flow (Qv) through a pipe cross-section (A) in a unit of time is always constant. So, Qv = A x V (velocity). This implies that velocity must increase as the cross-section becomes smaller to maintain the same volume flow. The conclusion is that the flow rate depends only on the pressure drop. As long as the difference between P1 and P2 is the same, the flow is the same whether the system pressure is high or low.

Then we have a rule of thumb that says velocities should not exceed certain limits for different kinds of pumps. Another rule of thumb says not to exceed a velocity of 4.5m/sec for pressure above 7 bar.

For gasses, the formulas get more complex. Since density of gas changes with pressure, it's important to calculate the size of two orifices in the regulator, the seat orifice, and the outlet orifice.

That’s a lot of information to take in, and there's even more to consider, enough to fill a couple of pages. But don't worry; your Edmonton Valve & Fitting representative’s already know that information. So bring us the basics, and we'll be glad to do the math for you, or even with you.

To download the complete Swagelok web-catalogue on How to Size a Pressure Regulator, click here.