Pilgrim Pressure Switches B-SERIES - Manual (Page 9)

9
PRODUCT SELECTION INFORMATION
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 fluid 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 diaphragm-sealed piston. In
this actuator, process pressure acting on the piston area causes
it to overcome the adjustment spring force and actuate a snap-
action switch. A diaphragm and O-ring seal the process media
from this mechanism. These are available in various materials,
i.e.: Buna N, Teflon and Viton. The standard process 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 pres-
sure connections are carbon steel. Diaphragms are available in
Viton, Buna N and Teflon. Always review process temperature
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 standard pressure connections are nickel-plated brass.
Diaphragms are available in Viton, Buna N and Teflon. Always
review process temperature limits before making seal selec-
tions. Optional stainless steel pressure connections are avail-
able (option XUD).
For all temperature ranges the standard Ashcroft ® temperature
actuator operates on the vapor pressure principle: the vapor
pressure in a sealed thermal system is applied to a sensing ele-
ment, 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 ambi-
ent 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 provisions for mounting and wiring. All
Ashcroft switch enclosures are epoxy-coated aluminum or stain-
less steel for maximum corrosion resistance. Choose between
watertight NEMA 4, 4X for most industrial applications and explo-
sion-proof NEMA 7/9 for most process applications.
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 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 applications for alarm
and shutdown are satisfied 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 applications, an adjustable deadband model is often the
best choice. Consult your Ashcroft representative for dual setpoint
or adjustable-deadband pressure and temperature switches.
4. The Switch Element
Finally, the electrical switching element must be compatible with
the electrical load being switched. For ease of selection, all elec-
trical 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 reliability and safety, optional
Hermetically Sealed switching elements may be specified.
ADDITIONAL SWITCH TERMINOLOGY
Accuracy – (see repeatability) Accuracy normally refers to con-
formity of an indicated value to an accepted standard value.
There is no indication in switch products; thus, instead, the
term repeatability is used as the key performance measure.
Ashcroft
® pressure and temperature switch accuracy is 1%
of nominal range.
Automatic Reset Switch – Switch which returns to normal state
when actuating variable (pressure or temperature) is reduced.
Adjustable or Operating Range – That part of the nominal range
over which the switch setpoint may be adjusted. Normally about
15% to 100% of the nominal range for pressure and differential
pressure switches and the full span for temperature switches.
Burst Pressure – The maximum pressure that may be applied
to a pressure switch without causing leakage or rupture. This is
normally at least 400% of nominal range for Ashcroft switches.
Switches subjected to pressures above the nominal range can
be permanently damaged. Consult factory for switches that
must operate at pressure above nominal range or reference cali-
bration temperature (70°F).
Deadband – The difference between the setpoint and the reset
point, normally expressed in units of the actuating variable.
Sometimes referred to as differential.
Division 1 – A National Electrical Code Classification of haz-
ardous locations. In Division 1 locations, hazardous concentra-
tions of flammable gases or vapors exist continuously, intermit-
tently or periodically under normal conditions; frequently because
of repair or maintenance operation/leakage or due to breakdown
or faulty operation of equipment or processes which might also
cause simultaneous failure of electrical equipment. Explosion-
proof NEMA 7/9 enclosures are required in Division 1 locations.
Division 2 – A National Electrical Code Classification of Haz-
ardous locations. In Division 2 hazardous locations, flammable
or volatile liquid or flammable gases are handled, processed or
used, but will normally be confined within closed containers or
closed systems from which they can escape only in case of
accidental rupture or breakdown or in case of abnormal opera-
tion of equipment. Either Nema 7/9 explosion-proof enclosures
or any enclosure with hermetically sealed switch contacts may
be used in Division 2 locations.
Explosion Proof – A term commonly used in industry referring
to enclosures capable of withstanding an internal explosion of a
specified gas without igniting surrounding gases. Strict installa-
tion practices in accordance with the national electrical code are
also required for safety.
Fixed Deadband – The difference between the setpoint and the
reset point of a pressure or temperature switch. It further signi-
fies that this deadband is a fixed function of the pressure switch
and not adjustable.
Hermetically Sealed Switch – A switch element whose con-
tacts are completely sealed from the environment to provide
additional safety and reliability. Contact arc cannot cause an
explosion, and atmospheric corrosive elements cannot affect
the contacts.
Manual Reset Switch – Pressure or Temp-erature switch in
which contacts remain actuated even after the actuating vari-
able returns to normal. On Ashcroft manual reset switches, a
button must be pushed to reset the contacts.
National Electrical Manufacturers Association (NEMA) – This
group has defined several categories of enclosures, usually
referred to as “types.” Further, they designate certain features
and capabilities each type must include. For example, among
other features, a NEMA 4 enclosure must include a threaded
conduit connector, external mounting provision and cover gas-
kets. When selecting a NEMA 4 enclosure from any manufac-
turer, a buyer is assured of receiving these features.
NEMA 4 – Watertight and dusttight enclosures intended for use
indoors or outdoors to protect the equipment against splash-
ing, falling or hose-directed water, external condensation and
water seepage. They are also sleet-resistant.
NEMA 4X – Watertight, dusttight and corrosion-resistant enclo-
sures with same qualifications as NEMA 4, but with added cor-
rosion resistance.
NEMA 7 – Enclosures for indoor Class I, Division 1 Hazardous
locations with gas or vapor atmospheres.
NEMA 9 – Enclosures for indoor Class II, Division 1 Hazardous
locations with combustible dust atmospheres.
Normal Switch Position – Contact position before actuating
pressure (or variable) is applied. Normally closed contacts
open when the switch is actuated. Normally open contacts
close when the switch is actuated.
Normally Closed – Refers to switch contacts that are closed in
the normal switch state or position (unactuated). A pressure
change opens the contacts.
Normally Open Switch – Refers to the contacts that are open in
the normal switch state or position (unactuated). A pressure
change closes the contacts.
Overpressure Rating(s) – A nonspecific term that could refer
to either burst or proof pressure, or both.
Proof Pressure – The maximum pressure which may be
applied without causing damage. This is determined under
strict laboratory conditions including controlled rate of change
and temperature: This value is for reference only. Consult facto-
ry for applications where switch must operate at pressures
above nominal range, or reference calibration temperature (70°F).
Repeatability (Accuracy) – The closeness of agreement among
a number of consecutive measurements of the output setpoint
for the same value of the input under the same operating condi-
tions, approaching from the same direction, for full-range traverses.
Ashcroft
® pressure and temperature switch repeatability is 1%
of nominal range.
Note: It is usually measured as nonrepeatability and expressed
as repeatability in percent of span or nominal range. It does not
include hysteresis or deadband.
Reset Point – The reset point is the Pressure, Temperature or
Differential Pressure Value where the electrical switch contacts
will return to their original or normal position after the switch
has activated.
Setpoint – The setpoint is the Pressure, Temperature or Differ-
ential Pressure value at which the electrical circuit of a switch
will change state or actuate. It should be specified either on
increase or decrease of that variable. (See also reset point.)
Single-Pole Double Throw (SPDT) Switching Element – A
SPDT switching element has one normally open, one normally
closed, and one common terminal. The switch can be wired
with the circuit either normally open (N/O) or normally closed
(N/C). SPDT is standard with most Ashcroft pressure and tem-
perature switches.
Snap Action – In switch terminology, snap action generally
refers to the action of contacts in the switch element. These
contacts open and close quickly and snap closed with sufficient
pressure to firmly establish an electrical circuit. The term distin-
guishes products from mercury bottle types that were subject
to vibration problems.
Static Pressure – For differential pressure switches, static pressure
refers to the lower of the two pressures applied to the actuator.