Index
- Control Valve Stations (link) (download pdf)
CONTROL VALVE STATIONS
R. Van Hudson/July2008
Note: New graphical representation on Process and Instrumentation Diagrams
(We endorse the use of PIP Standards wherever possible. Engineering documents from a global perspective are easier to interpret. )
CONTROL VALVES
Control valves are used to regulate flow, temperature and pressure of various mediums conveyed through pipe. Control valve stations are normally characterized by a control valve with actuator, two block valves, drain valves, a bypass with a manually operated control valve and a temperature or pressure gauge. Control valves are normally one to two line sizes smaller than main line size. Reductions in line size result in better control characteristics. Control valves are normally globe valves due to better control characteristics.
Control valve stations must be located where operators and maintenance have unfettered access. CV stations are always located at grade or main operating levels. The CV is normally placed in the horizontal with reducers on either side of the CV, and block valves in the verticals.
In some cases such as two phase flow (liquid/gas) like steam, eccentric reducers are sometimes installed FOB (flat on bottom).
Where the main line size is 8” or smaller there are economic advantages in using reducing elbows leading into the control valve, instead of reducers. Installing reducing elbows saves the cost of extra fittings and the labor to weld them.
The horizontal run with the CV is located 2’-0” to 2’6” above grade or platform.
When the control valve is a wafer valve it is necessary to install a pup (short spool piece) between the control valve and the reducer on the upstream side to allow installation and removal of stud bolts.
Block Valves
Block valves are generally line size and placed in the vertical legs on either side of the Control Valve.
Bypass
A Control Valve bypass has two functions, To bypass the CV for maintenance, and to provide a way to manually control flow.
CV’s often need maintenance or replacing. A bypass, with a valve that can be manually operated is generally a requirement. For this reason the manually operated bypass valve is normally a globe valve and of the same size as the control valve. The bypass line should be the same size as the bypass valve with the valve positioned as close to the upstream side as possible eliminating accumulation of process material and as well, places the bypass valve out of the way of the actuator.
The bypass line and valve are horizontally located above the CV and the block valves. Care must be taken to assure that the CV actuator can be removed. This usually requires an additional 6” to 10” above the top of the actuator. Exact dimensions can be found on the CV data sheet.
Drain Valves
Drain valves are required on either side of the control valve. Drain valves are normally ¾”, installed on the large side of the reducer on either side of the control valve.
Supports
All control valve stations are supported on either side of the control valve, usually with base ell supports. Refer to client or company engineering standards for correct sizes and callouts.
STEAM CONTROL VALVE STATIONS
Steam control valve stations require more attention. Every steam valve station gets a drip leg and a steam trap station upstream of the control valve. Follow client or company specifications for sizing drip legs. Often the Process and Instrumentation diagram does not reflect this. It is left to the piping designer to make sure they get installed.
Some clients prefer the upstream block valve to be place in the horizontal run with the control valve to facilitate removal of condensate. Eccentric reducers (FOB) help eliminate accumulation of condensate and minimize water hammer.
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