Page 102 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.52 CHAPTER TWO

Unless an emergency power source is available, electric motors should not be used
where cycling to a fully opened or fully closed position is a requirement in the event of a
power failure. Motor operators should be limited to moderate cycling functions. They are
not recommended for service where severe cycling is necessary. Generally, 2-in (50-mm)
valves do not have sufficient strength for a standard electric motor actuator. Electric motor
actuators have been extensively tested in seismic and harsh ambient environments.

Pneumatic Actuators. Depending on the system selected, these air-driven devices gener-
ally operate in a range of between 30 and 120 psig (210 and 830 kPa), with 30 to 60 psig
(210 to 415 kPa) being the most common. The compressed air supply should be a dedicated
one, conforming to ISA requirements, and preferably supplied from a control air compressor
assembly. An alternative, but less desirable means, would be to obtain the air supply from
the facility system, using additional compressed air-conditioning filters, dryers, and PRVs
as required.
Pneumatic actuators are well suited for frequent operation and fast response times.
There are two types of actuators, piston and diaphragm. The piston actuator is generally
used for ON/OFF operation. The piston stroke can be long, making it suitable for large
valves. The diaphragm actuator is appropriate for modulating service but because the travel
is short, the valve size on which it can be used is limited.
The failsafe mode is usually accomplished by using either an internal spring or a sec-
ondary accumulator tank to provide the necessary power to cycle to an opened or closed
position. The internal spring may cause the assembly to be flexible, which may be a problem
for seismic installation. The accumulator tank is externally mounted, often on a nearby wall
or column.
Pneumatic actuators are large in size and require frequent maintenance because of air
leakage over time (particularly piston types) that also makes response time longer. There is
a limitation on maximum valve differential pressure.
Hydraulic Actuators. These devices produce torque by using an electric pump to push
fluid to a piston. They are capable of providing fast actuation and are suitable for modulating
service. They allow operation on large valves with high pressure differentials and are appro-
priate for frequent cycling. They have no failsafe mode unless emergency electrical power
is available. The stroke is easily adjustable in service.
Typically, these actuators operate slowly. Their cost is between the electric and pneu-
matic systems.

OPERATING AND TORQUE CONSIDERATIONS

The following factors require consideration in the selection of an actuator:

1. Valve type and size. The valve-operating torque results from the inherent size and char-
acteristics of the valve itself and the type of seat. The amount of torque necessary to
overcome static imbalance must be obtained from the manufacturer.
2. Pressure drop. The operating torque increases with an increase in pressure drop across
the valve. A valve operating at full-rated pressure will require significantly more oper-
ating torque than one operating at a low-pressure drop. Depending on the source of
pressure, it is probable that the pressure differential will vary throughout the valve’s
entire stroke. This condition is important if the actuator torque output must be care-
fully matched with that of the valve.
3. Service-operating conditions. Will the valve be required to be only opened or closed
or will it also be required for throttling flow? Actuators for ON/OFF service will
be selected only on breakaway torque. For quarter-turn valves requiring throttling,

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