the devils in the detail

AllAboutAir Thomas W. Kreher | Applied Pneumatic Controls, Inc. | www.applied-pneumatic.com The Devil’s in the Details path, we will not get more flow. If you have ever felt that your Cv calculations let you down and couldn’t recover by over-sizing the valve, now you know that it was the devil in the One of the great things about using Coefficient of velocity (Cv) formula to size and or select the Pneumatic Valves for a system is that Cv’s can be added to get the total coefficient for an entire branch. Anyone who has tried to add several components and the accumulated potential pressure drop may have concluded that there had to be a better way. The formula for multiple Cv’s is 1/Cv12 + 1/Cv22 + 1/Cv32 = 1/Cvtotal2. Just for fun, let the Cv’s be 1 + 2 + 3. Then 1/12 + ‘1/22 + 1/32 or 1/1 + 1/4 + 1/9 = 1.361 which is 1/Cvtotal2. Then 1/[1.3612] = .857 Total Cv. This seems counter intuitive. Shouldn’t we have a Cv greater than that? Let’s try bigger numbers. Let the Cv’s be 1, 4, and 8. Then 1/1 + 1/16 + 1/64 = 1.078 which is 1/Cvtotal2. Then 1/[1.0782] = .963 Total Cv. The point illustrated here is that the smallest Cv or passage in the flow path limits the total flow. In both cases above, we had additional Cv’s much greater with the smallest being 1. We were unable to meet or exceed the smaller Cv. Obviously then, if we calculated a requirement for a Cv of 1.5 to power the actuator at the velocity required, all fittings, plumbing and components in the flow path must meet or exceed the Cv of 1.5. Just for discussion, let’s say that we specified a valve with a Cv of 1.7 but the actuators did not move fast enough due to a restrictive fitting with that provided an approximate Cv equivalent of .9 that was overlooked. Suppose we take the most obvious corrective action and change the valve to one with a Cv of 5. Guess what? While the fitting with an equivalent flow to a valve with a Cv of .9 remains in the flow 1. Multiply the Cv of the valve you have chosen by .034. 2. Take the square root of that number and label it “Inside Diameter Minimum.” 3. Make double dawg sure that the inside diameter of the pipe, tubing, fittings, etc. meet or exceeds this diameter. details. This may be a great time to caution about flow control devices, both those used at the cylinder and those that are used in the exhaust port of the Valve. More often than not, the flow control valve will allow less flow than the valve itself, even in the “free flow” direction. I prefer speed control at the exhaust port of the valve (except poppet valves). If you require better flow control than the speed control mufflers provide, use a quality needle valve with a muffler in it. If the actuators in a system do not operate fast enough, my first reaction is to open the exhaust speed control to the maximum. If that does not give enough speed, take the speed control device out of the exhaust port. This often shows that the supply is adequate but the speed control was restrictive. If the flow or speed control devices are located at the cylinder or inline, this is hard to do. Now I know what you are thinking: “The valve catalog specified the Cv of the valve we selected. How in thunder are we supposed to know the Cv equivalent of the fittings, pipe, hose and tubing and flow controls?” Don’t argue with me now until you try this, my friends. These are pearls I cast before you. The diameter of an orifice equivalent to a Cv of 1 is .1833 or the diameter squared is .034 for a Cv of 1. For illustration: We have selected a valve with a Cv of 1.3 because it was the first valve slightly larger than the Cv number that we calculated. Then Cv x .034 is equivalent to 1.3 x .034 = .044. The square root of .044 = .210. Do not use any item with an inside diameter less than .210” ID Min. in the flow path of this application. “I intended to use 1/4” Nylon tube with .170 inside diameter. You can’t tell me it makes that much difference.” Don’t be stuck on silly. Go to 3/8” nylon tubing with .275 inside diameter and live happily ever after. If you use the tubing with an ID of .170 (.1702 = .029, .029/.034 = .85 Cv) your system will not exceed a Cv value of .85. Your carefully selected valve with a Cv of 1.33 is limited to a Cv of .85 by the 1/4” tubing. For flow control valves, shuttle valves, quick dump, check valves, etc., do not assume that they meet or exceed the flow or Cv required to match the valves you are using. Drop a dime and call the distributor or manufacturer or surf the web. It is high time that the manufacturers of these devices publish better flow data. One parting shot. Some catalog flow data for valves and similar devices is given as flow to atmosphere instead of Cv. Should you find a 1/4” ported valve that lists 68 SCFM at 100 PSIG as the flow capability, think it through. A 1/4” ported valve should have about a Cv of 1 (.5 to 1.5). If we use the rule of thumb 90 PSIG at 10 PSID = 30SCFM, at 90 PSIG with a 10 PSI delta P a device with a Cv of 1 will flow 30 SCFM, it appears that the valve in question is rated abnormally high or with flow to atmosphere. Write this in your crib notes: For a Cv of 1 flow to atmosphere, QSCFM = .4874 x P1 PSIA. Q = .4874 x 114.7 = 55.9SCFM. Then 68/55.9 = 1.22 Cv for the mystery valve if its flow is rated to atmosphere. I suggest that all pneumatic valves be rated at the flow to atmosphere with the inlet pressure, P1 PSIA specified. All the valves would be rated in the same, simple way. They could be checked in five minutes with a flowmeter. Remember that with pneumatics, bigger is always better. Use generously sized plumbing and accessories. For More Information: Thomas Kreher started Applied Pneumatic Controls, Inc. with his wife, Gloria, in 1995. He can be reached at tom@applied-pneumatic.com. 6 | May/June 2007 www.ifps.org | www.fluidpowerjournal.com FPJMJ07_1-20.indd 6 4/25/07 9:47:58 AM