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Technical Articles
January 19, 2016

Valve Pressure Loss & Flow – Q & A and Equations

Valve Pressure Loss & Flow – Q&A and Equations

1. What does Cv factor mean? – The definition of Cv factor is the number of U.S. gallons per minute that will pass through a valve with a pressure drop of one (1) PSI.This ‘factor’ is determined by physically counting the number of gallons that pass through a valve with one (1) PSI applied pressure to the valve inlet and zero (0) pressure at the outlet. Cv is a mathematical constant. For a pressure drop other than one (1)PSI, use the formula in answer number 10 below.

2. Does every valve have a Cv factor? – No. Cv factors typically apply to full open/full closed shut-off valves such as solenoid valves, ball valves, etc.; Valves that are held open without aid of liquid pressure in the pipeline.

3. Which valves do not have a Cv factor? – Cv factors typically do not apply to modulating or regulating valves, spring loaded check valves, etc., that incorporate a control spring since more than one (1) PSI is required just to begin to position such valves.

4. What is Delta P? – A commonly used term, Delta P or its symbol deltap usually refers to the pressure drop across a piping component such as a valve or filter.

delta” (from the Greek Delta) is the ‘change’ in something; in this case a change, or drop, in pressure. To determine the Delta P across a valve, simply subtract the outlet pressure (P2) from the inlet pressure(P1).

The equation is P1-P2 = deltap

smdelta_p

5. Why is pressure drop important? – Pressure drop is a critical element in valve sizing and valve application. Pressure drop must be known by the engineer designing the system to ensure proper valve selection.

6. What factors determine pressure drop across a valve? – The most critical factors are the orifice size and internal flow path. An example would be a full port/full open 1″ ball valve with a typical Cv of 40 versus a full open 1″ diaphragm valve with a typical Cv of 15.

7. What is Backpressure? – Backpressure is simply defined as the pressure found at the outlet or “back” of a valve. It is caused by downstream restrictions.

8. What creates backpressure & why is it important to know? – Resistance to flow in piping systems creates backpressure. Piping components such as spray nozzles, filters and reducing fittings can all contribute to both backpressure and pressure loss. It is important to know the backpressure present (or potential) in a piping system when installing or specifying a valve since many valve designs can be adversely affected if their maximun ratings are exceeded.

9. What is the relationship between the flow rate (GPM) and pressure drop? – Pressure drop and flow rate are dependent on each other. The higher the flow rate through a restriction, such as a valve, the greater the pressure drop. Conversely, the lower the flow rate, the lower the pressure drop.

10. How do the GPM, Cv factor, and deltap work together to size a valve? – At least two of these elements are necessary to properly specify a valve. Here are the flow formulas.

formulas

Where G = Specific Gravity of the Fluid