Share. LinkedIn. Facebook. Twitter1I’m going to discuss here some very basic principles around control valve sizing, this will cover liquids. When you’re looking to have a control valve installed in your process you are looking to control some aspect of the liquid flowing through the pipe, typically you want to control either the amount of flow through the pipe or the downstream pressure; in order to control this variable you would install a control valve.
Valve Sizing. Features of the SAMSON Valve Sizing Program. Software for calculating and sizing control valves. The TROVIS-VIEW Configuration.
The basic equipment would consist of a valve body, actuator, and controller; there are many different models, configurations, and accessories that can be changed and they will all depend on the application that the valve will be in.For this example, let’s assume we’re trying to control the flow of 60F water through a 4in pipe where we have an inlet pressure to the valve of 500psig and a consistent downstream pressure of 250psig. Now let’s assume that we know the pump output at 500psig will give out 250, 500, 1,000gpm of flow. When we input this into our calculations to calculate the Cv of the valve, we get Cv values of 15.826/31.7/63.798.Looking at a standard sliding stem valve, we can see that a 3in valve with Equal Percent trim that has a Max Cv of 136 would be a good fit. Our Cv is calculated using the simple calculation of Cv = Q √(G/dP)Where Q = FlowG = Specific Gravity of the FluiddP = Pressure differential in psi.
This fits well on the curve where our control range is between 37% open up to 68% open.Now, something else to look at would be what our dP ratio is, the dP ratio is simply our pressure differential divided my P1 = 250/500. The higher the dP ratio the higher the likelihood that the fluid will cavitate or flash.Cavitation is when a liquid falls below the vapor pressure and turns into a gas and then later the pressure recovers above the vapor pressure and when it returns to a liquid the gaseous bubbles collapse and release a large amount of energy in the form of small shockwaves that can significantly damage the valve body, trim, and pipe walls. Similarly, flashing is when the liquid never rises above the vapor pressure and remains in a gaseous state; this can also damage the valve, trim, and pipe.According the Bernoulli equation - when a fluid passes a valve seat and the fluid velocity increase - the fluid pressure decreaseCAVITATIONIf the speed through the valve is increases enough, the pressure in the fluid drops to a level where the fluid may start to boil, bubble or flash. And when the pressure recovers sufficiently the bubbles will collapse upon themselves. This collapse causes cavitation. Cavitation may be noisy but is usually of low intensity and low frequency.
This situation is extremely destructive and may wear out the trim and body parts of a valve in short time.So for this application we need to look at the dP of the system (250psig) and through calculations given to me by my sizing program I know that the dP Cavitation number, that is the dP across the valve that is allowed before cavitation is present, is between 305 and 309psig (this number changes due to the changes in flow rates). So since our dP is below the 305psi we know that cavitation damage is not a concern for this valve. What this also means is as the water flows through the trim and the velocity increases through the orifice (restriction) and the pressure drops at that moment, it is not falling below the vapor pressure of the water.Some other calculated numbers that are important when sizing a control valve are the Kc (Cavitation coefficient) and Ar (application ratio).Since my company represents Fisher controls, the Kc number is an Emerson Process Management valve parameter dependent on valve style, trim, size, and material; process fluid; and pressure drop. This number predicts the beginning of cavitation related damage for a particular valve/trim style.Kc = dP Cavitation / (P1-Pv)The application ratio (Ar) – is a cavitation index that is depending on the actual service conditions and it indicates the presence of flashing or potentially cavitation services.Ar – (dP Flow) / (P1-Pv)If Av is greater than or equal to 1 then the fluid is flashingIf Av is less than 1 the service might cavitate. What is important is to ensure that Kc is greater than Ar.So how do we ensure that Kc is greater than Ar?