CONTROL VALVE STATIONS
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 CV.
Bypass
The key here is that CV bypasses serve two purposes. One, as a bypass around
the CV and second, it serves as a way for operators 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. This eliminates accumulation of material in the
line and places the 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 which
usually require 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.
References
Pipe Drafting and Design by Roy A. Parisher and Robert A. Rhea
Process Plant Layout and Piping Design by Ed Bausbacher and Roger Hunt
The Piping Guide by David R. Sherwood and Dennis J. Whistance
Process Piping Design Vol 1. by Rip Weaver
