美国ASHCROFT B系列开关产品选型.pdf.pdf
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Consult factory for guidance in product selection
Phone (203) 385-0217, Fax (203) 385-0602 or
visit our web site at www.ashcroft.com
Product Selection Information
PRESSURE, TEMPERATURE AND
DIFFERENTIAL PRESSURE SWITCH
SELECTION
Before making your selection, consider the
following:
1. Actuator
The actuator responds to changes in pressure,
temperature or differential pressure and operates
the switch element in response to these changes.
The actuator is normally exposed to process fl uid
and must therefore be chemically compatible
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 dia-
phragm-sealed piston. In this actuator, process
pressure acting on the piston area causes it to
overcome the adjustment spring force and actu-
ate a snap-action switch. A diaphragm and
O-ring seal the process media from this mecha-
nism. These are available in various materials,
i.e.: Buna N, Tefl on and Viton. The standard pro-
cess connection is stainless steel. Optional
monel pressure connection is available.
For ˝ H
2
O 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 Tefl on. Always review process tem-
perature limits before making seal selections.
Optional stainless steel pressure connections are
available (option XTA).
For High Differential Pressure Actuator Ranges,
3-15 to 60-600 psid, a Dual Diaphragm-Sealed
Piston Actuator is used. This actuator is designed
to for high static-pressure applications. The stan-
dard pressure connections are nickel-plated
brass. Diaphragms are available in Viton, Buna N
and Tefl on. Always review process temperature
limits before making seal selections. Optional
stainless steel pressure connections are available
(option XUD).
For all temperature ranges the standard
Ashcroft
®
temperature actuator operates on the
®
temperature actuator operates on the
®
vapor 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 fi lling
materials are used, including Propane, Butane,
Methyl Alcohol, N Propyl Alcohol and Xylene.
High overtem perature 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 fi lled with
vapor, and have little effect on the setpoint,
regardless of ambient temperature variations;
therefore, no ambient compensation is required.
For best results, the bulb should be mounted
within 60 degrees of vertical to assure the liquid
remains in the bulb.
2. Enclosure
The enclosure protects the switch element and
mechanism from the environment and has provi-
sions for mounting and wiring. All Ashcroft
switch enclosures are epoxy-coated aluminum or
stainless steel for maximum corrosion resistance.
Choose between watertight NEMA 4, 4X for most
industrial applications and explosion-proof NEMA
7/9 for most process applications.
Ashcroft enclosures include watertight cover gas-
kets, external mounting holes and one or two
3
/
4
/4/
NPT electrical conduit holes for ease of installa-
tion. Pressure switches may also be mounted
directly to the process by means of the standard
1
/
4
/4/
NPTF or optional
1
/
2
/2/
NPT pressure connection.
Note:
When installing Ashcroft switches, refer to
instruction sheets included with each switch, the
National Electrical Code, and any other local
codes or requirements to assure safety.
3. The Switching Function
Next, consider the switching function. Most appli-
cations for alarm and shutdown are satisfi ed by
single setpoint, fi xed deadband models. For high/
low or alarm and shutdown, the dual setpoint
models may be selected. For pump, compressor,
level and other control applications, 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
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