To choose the right type of valve, is often a question of experience with certain types of valves in certain applications.

Some types of valves, like the needle valves are more typically used for regulating and control applications.

Others, like the ball valves are mostly used for discrete open / close applications.

The choice of a valve is a combination of the compatibility of all the valve parts with fluid pressure, temperature, corrosion resistance, deposition on seals etc. Valve availability and cost are usually of much interest, too. So, once you have found your valve from catalogs or other resources, you need to check if it’s available at your expected time and cost.

			How to dimension the valve step by step:
			If Fluid = gas	1. Application Choose valve type Then		If Fluid = liquid
	2. collect information to calculate cV value: SG Specific gravity of gas p1 Gas Inlet Pressure p2 Gas Outlet Pressure T absolute Temperature Q Gas Flow Rate in SCFH				2. collect information to calculate cV value: Q = flow rate in gpm (US gallons) G = specific gravity of liquid DP = pressure drop = (inlet pressure – outlet pressure) (in psi)
		3. Determine cV  for gases				3. Determine cV  for liquids
	4. Find out if the cV is sized right for your application, usually from valve manufacturers catalog This example is based on Swagelok®’s Integral Bonnet Needle Valve Series 18 For the SS-18RM8 valve, the cV in the catalog text table is given as cV = 1.8. This is usually the highest possible cV, with the valve fully open and this is usually sufficient as a selection criteria. If you want to find out your cV related to the number of turns open, you have to use the graph as shown here. the cV is approx = 0.75 when the valve is open at 2 turns The text form is usually the only indication you will find on ball valves or other discrete type valves