Ningfeng Engineering - Selecting a Pressure Switch

Cixi Ningfeng Pressure Gauge Factory

Selecting a Pressure Switch

Selecting a Pressure Switch
Selecting the appropriate pressure switch for your application should take into account many factors. Following the guidelines below can assist you in choosing the right pressure switch for your specific application; The most common electromechanical pressure switches are composed of a sensing element and electrical snap-action switch. A number of different types of sensing elements can be used. (Ningfeng utilizes Diaphragm, Bellows or Piston Sensing elements). No matter the type of sensing element used they all have the same thing in common; “the sensing element moves in response to changes in the system pressure.” Through their movement, the sensing element directly acts on the opening and closing of the snap-action switch’s contacts. Understanding the system dynamic is crucial to the pressure switch selection.

Type of Device
There are various different types of pressure switches. Generally, a pressure switch opens or closes a contact(s) at a given level of pressure (set point). A differential pressure switch senses the pressure from two sources and trips a contact(s) when the difference reaches a predetermined level. A vacuum switch actuates at a given level of vacuum.

Wetted Parts
When selecting a pressure switch, the process or media is required so the wetted parts may be determined. Diaphragm operated switches use elastomer diaphragms (Buna N, Viton, Mylar, etc) Bellows operated switches are generally stainless steel with a silver braze. The Piston device employs a carbide piston and brass trim. Diaphragm seals may be added to all Bellows and some diaphragm operated devices.

Pressure Range Considerations
When considering a pressure switch the pressure range is a factor in your selection. You must know the minimum and maximum set points of the switch. Also, what is the maximum system operating pressure and what is the maximum system design pressure. Adjustable deadbands can be set at 10% to 50% of the range. For differential pressure switches, the static or “working” pressure is required.

Pressure Switch Activation
How often a pressure switch is activated will have a direct impact on pressure switch life, system downtime and the overhaul schedule In general, diaphragm switches will have the highest cycle life, followed by the piston type and the bellows switch.

Cycle Speed
The metal or diaphragm switch acts very much as a spring and high-speed cycles should be avoided. The actual life of a pressure switch will depend on various factors including; cycle speed, minimum and maximum pressures, the setpoint(s), rate of pressure change, hydraulic shock, load on the electric switch, etc.

Switch Points
When sensing pressure at one point, it is normal that only one switch point is required. However, it is 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 pressure switch capable of handling as many as 4 separate switch points.

Setpoint Adjustment
In some applications, one doesn’t want to have a changing set point. In other situations, adjustment is required as the application may change. Electromechanical switches have models that are factory set, have 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, built to conform to accepted 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 costs are paramount.