Control valve safety requirements


#1

Control valves are flow control instruments which deals with the high pressure in process industry. Thus valves should be fail-safe to make the entire process safe.

In principle, fail-safe will be needed when the temperature / pressure both side upstream and downstream processes are tried to not change when the control valve is inactive.

The safety requirements for a valve is listed below:

Capacity Coefficients (CV) for the control valve

Cv or valve flow coefficient. This CV depends on the internal dimensions of the valve, and the smoothness of the surface inside the valve. The age of the automatic control valve also affects the character of the Cv.

Cvd can be interpreted as an index that indicates how much volumetric flow-rate (gpm) can be generated when 600F of temperature passes through a control valve which causes a pressure drop of 1psi. Besides CV, there is also Cvc.

Cvc is calculated using normal design flowrate in gpm. From here you can find the appropriate valve, usually a valve is taken with Cv above Cvc. For a good range of controls, Cv is taken from 1.25 to 2 times Cvc

Usually the range is used for plug types which are equal percentage and linear flow. But there are also valves that have a wider range. There is also Cf, critical flow factor. For liquid flow it can be considered subcritical if vapor pressure from liquid will not exceed the lowest pressure when passing through the control valve.

Main operation conditions for a Control valve:

  • Control valve usually has a size below the size of the upstream pipe or the same maximum. Never taken bigger.

  • The size of the control valve can be made much smaller than the size of the upstream if you have to absorb a large pressure drop.

  • The control valve can accommodate a large range of capacities and pressure differences. Flowrate and process conditions are usually predetermined to move piping and other things. So when sizing the control valve it is better to give data of more than one pressure capacity.

  • Sometimes if the system has a large capacity range, it needs up to 2 valve controls in parallel, one for large flowrates, and the other for small ones.

Butterfly and ball valves:

  • Butterfly valve can operate with a small pressure drop (1 point psi). Usually suitable for discharge compressors and line cooling water supply. But trotling, this valve coefficient can go down regularly.

  • Butterfly / ball valve has a side mounted actuator because the actuator stem can rotate the axle valve. The plug characteristics for this can be affected by the connection between the actuator stem and valve axle.

  • Control valve other than butterfly type can only regulate flow by absorbing or giving a pressure drop to the system.

  • If the flow through the valve is a critical flow, then the bypass and downstream control valve sizing must be very careful because vaporization along the control valve causes a pressure drop to rise.

  • To avoid vaporization in the control valve can also by adding static pressure at upstream.

  • At high pressures, high temperatures, or large press differences, the control valve should not be operated to close. Because of the large velocity, it will hit the control valve and will cause flow control to be inaccurate, and cause leakage when the valve is shut off.

Bypass:

  • Bypass is usually used for control valves that are smaller than 2in, or for high viscosity and lethal, or for liquids containing abrasive solids, in boiler feed water services, or for steam systems that require killing high pressure (more than 100psi) but also often bypass is provided for the control valve maintenance without system shutdown.

  • For consistency in the piping design, the coefficeient flow for the bypass valve should be made the same as in the control valve and the pressure drop.

Installation:

  • Usually diplomatic, the control valve is placed on a grade / platform elevation. and is easily accessible except for valves that must be stored in self-draining pipelines. This is intended to facilitate maintenance / quick response if there is a problem with the process.

  • Before and after the control valve is usually given a gate valve for dangerous fluids, usually given a drain at the low point for a fail-open drain one, if fail-close is given 2, upstream and downstream. for maintenance, they close the two gate valves, continue to drain, and take out control valves. For other features, the fluid passing through the control valve saturated steam flow, is usually given steam trap at the lowest point.

  • For maintenance control valve, there needs to be space up and down the right and left to take the control valve. That is the reason why floating over the control valve is given a distance of around 12in.