Managing Droop

Last week we talked about regulator droop and the reasons behind it, so let’s look at how we manage droop when selecting a regulator.

When you get to the point of selecting a regulator for a specific application, you will know what the flowing media is and the desired flow rate, and what the inlet and outlet pressure will be.  We can use this knowledge to manage droop by getting a fairly accurate estimate what the flow will be at a given offset expressed as a percentage.

Let’s take a common water service example to illustrate this. We will assume we have city water coming in at 80 psi, and we want to reduce the pressure to a more manageable 35 psi for a parts washer. About 12 gallons per minute will be required to properly operate the washer. A quick run through a valve sizing program will tell you a Cv of 1.8 is required. We will use ¾” piping and select a valve with 4.4 Cv.

Referring to our Cv vs Offset Data, we find our ¾ Size in the first column, then we move right to our 4.4 Cv in the next column,  then right again to our set point of 35 psi. Now simply follow the 35 psi  line to the right until you see a number greater than your requirement of 1.8 Cv. We come to 1.90 Cv under the heading of Cv @ 20% offset. This is how much offset (or droop) you can expect to see from this application. You then check the data for your parts washer and find that it can tolerate pressures that are plus or minus 25% of the recommended 35 psi.  You have your regulator!