Edmonton Valve & Fitting Blog

Weekly posts for northern and central Alberta engineers, plant operators, and buyers.

Don't Let Pressure Regulators Throw You a Curve

by Taryn Hardes, on Tue, Aug 14, 2012 @ 14:08 PM

Learn a little about flow curves, and you'll get the best performance

When we talk about matching a pressure regulator to the job, one of the primary tools is the flow curve. If you learn a bit about flow curves and how they work, you'll feel a lot more confident when you talk to your Swagelok representative about your needs.

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How to read a flow curve

It starts with a simple graph. The vertical axis shows the amount of pressure on the downstream side of the regulator. The horizontal axis shows the flow rate. When you plot the pressure that the regulator can maintain at various flow rates, you end up with the flow curve. You'll get a different curve for each set pressure.

In an ideal world, you'd want the curve to be a flat line: No change in pressure no matter what the flow rate. In the real world, that's not possible. Each regulator has its limits.  Remember, it's a pressure regulator, not a flow regulator. Flow is controlled by a valve or flow meter. When the flow rate gets high enough, the curve on the right side of the graph will start to slope downward. That's called "droop." If you operate a regulator in that zone, you'll get a relatively large pressure drop downstream. Go far enough to the right and you end up with "choked flow." That means the regulator is wide open and is no longer controlling pressure.

At the far left of the graph, you'll see a similar change as the line comes down sharply and flattens out. That's called "seat-load drop" or "lockup," another area where the regulator has a hard time maintaining pressure.

What you want, then, is a regulator with a relatively flat flow curve in the range of pressures and flows where you intend to operate it. Each Swagelok pressure regulator has its own flow curve. Look at the conditions under which you plan to operate the regulator, and mark them right on the flow curve graph. If your numbers are far to the right or left where the line is no longer close to flat, look for a regulator model that is better suited for your specs.

Here's one more term to learn: Hysteresis. Basically, reading a flow curve left to right shows what happens as flow increases. When flow decreases, you would read the curve from right to left. But the curve doesn't follow exactly the same line when flow increases as it does when flow decreases. The difference between the two is called hysteresis. It's a good practice to use pressure gauges in the system to help fine-tune regulator settings.

Flatten the curve

As with many pieces of machinery, there are also a few things you can do to get better performance out of a pressure regulator. That is, you can flatten out the flow curve a bit.

One way is to use an upstream pilot regulator for a dome-loaded regulator. This controls pressure in the main regulator’s dome chamber, keeping it constant over a wider system flow range. An even better way is to use the pilot regulator plus external feedback from tubing run to the dome-loaded regulator. For the flattest flow curve of all, add feedback tubing to the pilot regulator too. Your Swagelok representative can give you more details on all three methods.

There are a few other considerations when reading flow curves. For instance, make sure all the measurement units agree. Pressure readings are usually in psig or bar. Liquid flow is typically expressed as gallons per minute or litres per minute, while gas flow is expressed as cubic feet per minute or standard litres per minute.

If you feel ready for a more advanced, technical explanation, get a copy of the new Regulator Reference Guide, which covers all basic aspects of regulators, including sizing, types, troubleshooting, and more. Download the Regulator Reference Guide »


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