TM 5-810-15
(4) Control valve stations. Control valve sta-
surface will be made to assure tight seating.
Diaphragms will be molded rubber and diaphragm
tions are used to install control valves in piping
housing will be pressed steel. Piston operators will
systems and to provide a means of isolating and
use cast pistons and cylinders with 0-ring seals.
bypassing the control valve for maintenance pur-
Each valve operator will have an air supply
poses. Control valve stations will conform to the
pressure filter regulator. Valve operators for mod-
recommendations of ISA RP 75.06. Control valve
ulating service valves in fast response control
stations consist of a control valve, isolating valves,
loops, such as flow control or pressure control, will
bypass valve, and bypass line. Since control valves
have electropneumatic valve positioners. Limit
are normally smaller than the line size, reducers are
switches will be provided if needed for remote
required and can be integral to the control valve on
indication or control logic.
valves with butt weld end connections. Isolation
(3) Control valve sizing. Proper control valve
valves are required to isolate the control valve for
sizing requires careful analysis of the process and
repair, removal, or calibration and will be installed
piping system in which each valve is to be used. It
on the inlet and outlet sides of the control valve.
is necessary to calculate the required valve flow
Isolation valves will be gate valves or other non-
coefficients based upon flow, valve inlet pressure,
throttling type valves. A bypass valve is necessary
valve outlet pressure, and process fluid conditions.
to provide a means of controlling the process when
Calculations will be based on ISA S75.01. Valve
the control valve is not operable. The bypass valve
flow coefficients will be calculated at the maximum,
will be identical to the control valve except it will
intermediate, and minimum process flow
be manually operated. Using an identical valve on
conditions. The control valves will be selected such
the bypass provides better control during manual
that the maximum flow coefficient occurs at a valve
operation since the valve will have the proper flow
travel between 70 and 80 percent. The minimum
coefficient and special valve trim. The bypass line
flow coefficient will occur at a valve travel between
which contains the bypass valve must be smaller
10 and 20 percent. Control valves will be selected
than the main line size. The bypass line may be the
with a flow characteristic which provides uniform
same size as the bypass valve but in no case will the
control loop stability over the range of process
bypass line be smaller than one half the main line
operating conditions. A quick opening flow
size.
e. Piping instrumentation.
characteristic provides large changes in flow at
(1) Pressure switches. Pressure switches are
small valve travels and should primarily be used for
on-off service applications. With a linear flow
used to monitor pressures for remote indications,
characteristic, the flow rate is directly proportional
interlocking functions, and alarm conditions. Pres-
to valve travel. Valves with linear flow
sure switches may have snap acting switch contacts
characteristics will be used for liquid level control
or mercury switch contacts. Shutoff valves of the
where the ratio of the maximum valve pressure
same pressure and temperature rating as the
differential to the minimum valve pressure
process piping will be provided on each switch for
differential is less than five to one. Linear flow
isolation purposes. Snubbers will be provided on
characteristic valves will also be used for pressure
switches when the pressure connection is located
control of compressible fluids and for flow control
within 15 pipe diameters of a pump or compressor
when the flow rate varies but the valve pressure
discharge.
(2) Pressure gauges. Pressure gauges are used
differential is constant. With an equal percentage
flow characteristic, equal increments of valve travel
to provide local and remote indication of process
produce equal percentage changes in the existing
pressures. Scale ranges will be selected such that
flow rate. Equal percentage flow characteristic
the normal operating pressure is at approximately
valves will be used for liquid level control when the
mid-scale. Shutoff valves of suitable rating will be
ratio of the maximum valve pressure differential to
provided on each gauge for isolation purposes.
the minimum valve pressure differential is greater
Snubbers will be provided on gauges when the
than or equal to five to one. Equal percentage flow
pressure connection is located within 15 pipe
characteristic valves will also be used for pressure
diameters of a pump or compressor discharge.
control of liquids and for flow control when the
Siphons will be provided on pressure gauges for
valve pressure differential varies but the flow rate
steam service. Pressure gauges will be provided on
is constant. Special inner valve trim characteristics
the discharge of all pumps and compressors, all
are required on applications where flashing or
boiler drums, all main process headers, and other
cavitation exist in liquid service and for noise
locations as required to monitor equipment and
control in steam or gas service.
process operation.
9-24
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