Discuss about Valves Pressure Loss and questions and answers on valves. Valve Technician interview questions and answers.
When flow goes through a valve or any other restricting device it loses some energy.
The flow coefficient is a designing factor which relates head drop (Δh) or pressure drop (ΔP) across the valve with the flow rate (Q).
Each valve has its own flow coefficient. This depends on how the valve has been designed to let the flow going through the valve. Therefore, the main differences between different flow coefficients come from the type of valve, and of course the opening position of the valve.
Flow coefficient is important in order to select the best valve for a specific application. If the valve is going to be most of the time opened, probably there should be selected a valve with low head loss in order to save energy.
Or if it is needed a control valve, the range of coefficients for the different opening positions of the valve should fit the requirements of the application.
At same flow rate, higher flow coefficient means lower drop pressure across the valve.
Depending of manufacturer, type of valve, application the flow coefficient can be expressed in several ways. The coefficient can be non-dimensional or with units if parameters such as diameter or density are considered inside the coefficient or just in the equation.
Most of valve industry have standardized the flow coefficient (K). It is referenced for water at a specific temperature, and flow rate and drop pressure units. Same model valve has different coefficient for each diameter.
Kv is the flow coefficient in metric units. It is defined as the flow rate in cubic meters per hour [m3/h] of water at a temperature of 16º celsius with a pressure drop across the valve of 1 bar.
Cv is the flow coefficient in imperial units. It is defined as the flow rate in US Gallons per minute [gpm] of water at a temperature of 60º fahrenheit with a pressure drop across the valve of 1 psi.
Kv = 0.865 · Cv
Cv = 1,156 · Kv
At valves which discharge the flow directly into the environment it is used the non-dimensional Discharge Coefficient(C).
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.
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.
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.
A commonly used term, Delta P or its symbol
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.
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.
Backpressure is simply defined as the pressure found at the outlet or “back” of a valve. It is caused by downstream restrictions.
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.
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.
At least two of these elements are necessary to properly specify a valve. Here are the flow formulas.
Also Read : Solenoid Valve Troubleshooting Practices
Actuators for valves can be classified up several characteristics:
Also Read: Questions & Answers on Valves
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