Pilgrim MULTIFUNCTION PRESSURE AND TEMPERATURE SWITCHES - Manual (Page 8)

8
PRESSURE, TEMPERATURE AND
DIFFERENTIAL PRESSURE SWITCH
SELECTION
Before making your selection, please consider
the following:
1. Actuator
The actuator responds to changes in pressure,
temperature or differential pressure and oper-
ates the switch element in response to these
changes.
The actuator is normally exposed to process
fluid and must therefore be chemically com-
patible with it. The following may be used to
help select actuator type:
For nominal pressure ranges 0-15 psi through
0-3000 psi, Dresser’s standard actuator is a
diaphragm-sealed piston. In this actuator, pro-
cess pressure acting on the piston causes it to
overcome the adjustment spring force and ac-
tuate a snap-action switch. A diaphragm and
O-ring seal the process media from this
mechanism. These are available in Buna-N,
Teflon and Viton. The standard process con-
nection is stainless steel. Optional all-welded
diaphragms and pressure connections are
available in 316 SS and Monel.
For "H
2
0 Pressure and Differential Pressure
Ranges, a diaphragm actuator is used. In this
design, the standard pressure connections are
carbon steel. Diaphragms are available in
Viton, Buna N and Teflon. Always review pro-
cess temperature limits before making seal se-
lections. Optional stainless steel pressure con-
nections are available (option XTA).
For High Differential Pressure Actuator
Ranges, 3-15 to 60-600 psid, a dual dia-
phragm sealed piston actuator is used. This
actuator is designed for high static-pressure
applications. The standard pressure connec-
tions are nickel-plated brass. Diaphragms are
available in Viton, Buna N and Teflon. Always
review process temperature limits before mak-
ing seal selections. Optional stainless steel
pressure connections are available (option
XUD).
For all temperature ranges, the standard Ash-
croft temperature actuator operates on the va-
por pressure principle: The vapor pressure in a
sealed thermal system is applied to a sensing
element, which in turn actuates a switch. This
is known as a SAMA Class II system. Various
filling materials are used, including Propane,
Butane, Methyl Alcohol, N Propyl Alcohol and
Xylene. High overtemperature capability is
possible with this type of system. The interface
between liquid and vapor is the point at which
sensing occurs. This is the “sensitive” portion
of the bulb. Bulb extensions and capillary are
normally filled with vapor and have little effect
on the setpoint, regardless of ambient tem-
perature variations; therefore, no ambient
compensation is required. For best results, the
bulb should be mounted within 60 degrees of
vertical to assure the liquid remain in the bulb.
2. Enclosure
The enclosure protects the switch element and
mechanism from the environment and has
provisions for mounting and wiring. Ashcroft
switch enclosures are epoxy-coated aluminum
or stainless steel for maximum corrosion re-
sistance. Choose between watertight NEMA 4,
4X for most industrial applications and 316 SS
for more corrosive environments.
Ashcroft enclosures include watertight cover
gaskets, external mounting holes and one or
two 3/4 NPT electrical conduit holes for ease of
installation. Pressure switches may also be
mounted directly to the process by means of
the standard 1/4 NPTF or optional 1/2 NPTM
pressure connection.
Note: When installing Ashcroft switches, refer
to instruction sheets included with each switch,
the National Electrical Code, and any other lo-
cal codes or requirements to assure safety.
3. The Switching Function
Next, consider the switching function. Most
applications for alarm and shutdown are satis-
fied by single setpoint, fixed deadband models.
For high/low or alarm and shutdown, the dual
setpoint models may be selected. For pump,
compressor, level and other control applica-
tions, an adjustable deadband model is often
the best choice.
4. The Switch Element
Finally, the electrical switching element must
be compatible with the electrical load being
switched. For ease of selection, all electrical
switching elements are snap-acting, SPDT
(single pole-double throw), or 2 (SPDT). Refer
to catalog pages for switch element choices.
Select a switch element with electrical rating
that exceeds the electrical rating of the device
being controlled by the switch. For better reli-
ability and safety, optional hermetically sealed
switching elements may be specified.
Enclosure
Cover
Enclosure
Piston Assembly
Diaphragm
O-Ring
Pressure
Connection
Cover Gasket
Switching Element(s)
Switching Function
(Mechanism)
SWITCH SELECTION INFORMATION