- Pressure Switches
SELECTING A PRESSURE SWITCH
Selecting the appropriate pressure switch for your application should take into account many factors. Following the guidelines below will assist you in choosing the right pressure switch for your specific application.
There are various types of pressure switches. Generally, a pressure switch is composed of a sensing element and electrical snap-action switch. The switch opens or closes a contact at a given level of pressure (set point). A differential pressure switch senses the pressure from two sources and trips a contact when the difference reaches a predetermined level. A vacuum switch actuates at a given level of vacuum.
HOW A PRESSURE SWITCH WORKS
When high pressure enters the pressure port, a chain of events takes place. The diaphragm seal begins to flex and push against the pressure plate. If the pressure force applied to the plate is high enough to compress the spring, the pressure plate will begin to rise. With enough force, the pressure plate will rise until the contacts connect, at which time the electrical circuit becomes closed and power is supplied throughout the circuit.
The movement from the sensor actuates an electrical signal by changing the position of electrical contacts in the switch.
The diaphragm is a deflecting mechanism that is installed with the pressure plate. When pressure is low, there is not enough force to coil or compress the spring, and the electrical circuit remains open.
The electronic snap switch (provides on-off action) is the most important component required for a simple pressure switch.
Winters uses a Single Pole Double Throw (SPDT) switch as a standard. These switches can be wired to make or break an electrical circuit, or normally open and normally close when the switch is not actuated.
Electric snap switches consist of a snap-action mechanism built into a molded case.
LIFE OF A PRESSURE SWITCH
How often a pressure switch is activated will have a direct impact on its service life, system downtime and the overhaul schedule. In general, diaphragm switches will have the highest cycle life, followed by the piston type and bellows switch.
The actual life of a pressure switch will depend on various factors including cycle speed (diaphragm switches acts very much like a spring, so high speed cycles should be avoided), minimum and maximum pressures, set point, rate of pressure change, deadband setting, hydraulic shock, and current (amp) load on the electric switch.
Further note on deadband
The deadband is the difference between the actuation point and the re-actuation point in a pressure switch. If this deadband setting is too small, then the switch will continually open and close for only minimal changes in the process input pressure. This is known as switch “chatter” and is a major cause of switch contact burnout.
TYPES OF SENSING ELEMENTS
A number of types of sensing elements exist. Winters utilizes diaphragm and bellows sensing elements.
When selecting a pressure switch, knowing the process or media is required so that the wetted parts may be determined. Diaphragm operated switches use elastomer diaphragms (Buna N, Viton, Mylar, etc.) while bellows sensing switches generally use stainless steel with silver brazing. The piston device employs a carbide piston and brass trim. Diaphragm seals may be added to all bellows and some diaphragm operated devices.
CONSIDER PRESSURE RANGES
The minimum and maximum set points of the switch, as well as the maximum system operating pressure and maximum system design pressure, must be determined. Adjustable deadbands can be set at 10-50% of the range. For differential pressure switches, the static or “working” pressure is required.
It is normal that only one switch point is required, however, not unusual for a system to require two or even four switch points (e.g. high, low, high-high, low-low) to be monitored, controlled or alarmed. In choosing a pressure switch, one could select a single switch for each switch point or a single switch capable of handling as many as 4 separate switch points.
SET POINT ADJUSTMENT
Adjustment may be required as the applications change. Electromechanical switches have models that are factory set, blind adjustment capability or offer field calibrated adjustment.
TYPES OF HOUSING
Housed switches are the most common type of housing for a pressure switch as they avoid possible hazards from loose wires in an exposed location. They are normally available in a variety of ratings with the most popular industrial switch housing being NEMA 4 & NEMA 4X for corrosive environments. Explosion proof pressure switches are designed with heavy housings and built to conform to UL, CSA and NEMA standards in preventing units from igniting in a hazardous environment. Some pressure switches consist of no housing (i.e. installation into a panel). This type of housing is most common in an OEM application or where cost-saving measures are paramount.
Note: When selecting a pressure switch, always refer to CAN/CSA-C22.2 NO. 61010-1-12 – Safety requirements for electrical equipment for measurement, control, and laboratory use – Part 1: General requirements (Tri-national standard), with UL 61010-1 and ANSI/ISA-61010-1 (82.02.01)
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- Pressure Switches