Pressure switchA mechanical or electronic device that is activated by the pressure of a fluid, air, or gas when it reaches a threshold or set point. Pressure switches are designed to include Bourdon tubes, pistons, diaphragms, or films that move or deform in response to the pressure force applied by the system.

The components of the pressure switch are connected to one or more contacts in the switch. Under sufficient force, the contact will close or open the switch according to its configuration. Although pressure switches have a variety of methods for detecting pressure, they can mainly be classified as electromechanical or electronic. Pressure switches are used in most industries with compressed gas systems, HVAC, instrumentation systems, pumping systems, etc.

How it works:

A typical pressure switch has a piston with fluid pressure on one side. The other side is usually under atmospheric pressure. The force applied by the fluid pressure is counteracted by the force of the preloaded spring. The surface area in contact with the fluid and the spring constant are carefully designed so that the piston moves only when it reaches a certain pressure. The spring is preloaded by the setting screw. Adjust the set point screw to set the activation pressure higher or lower.

Cut in and out:

Pressure switches usually have two operating points: cut in and cut off pressure. In pump and compressor systems, the switch is activated when the fluid pressure falls below a set level. This will start the motor of the pump or compressor, bringing the system back to normal levels. The switch is not immediately deactivated when the pressure exceeds the set point. There is a form of hysteresis or differential that prevents sudden tripping. This allows the pressure to increase until the upper end of the pressure range is reached. The switch is deactivated when a higher set point is reached or switched off.

Process (entry) port:

The inlet port is the part that connects the pressure switch assembly to the processing unit. The pressure switch is mounted on the nozzle connected to the tank or pipe. Typical connections are threaded fittings. In rare cases, bolted or welded connections are used. It is important that the joint type and its pressure rating are compatible with fluid pressure.

Pressure sensing element:

Mechanical pressure switches are classified according to their pressure sensing elements. This is the main part of the switch, which mechanically activates the switch through fluid pressure. The piston or diaphragm area on the fluid side is designed to transfer sufficient force from the expected fluid pressure. The larger the area, the greater the driving force and spring force required. Note that only a small amount of force is required to activate the switch. The spring counteracts most of the pressure.

Spring:

The spring resists the force from the fluid. It is preloaded to match the working pressure of the fluid. The switch is activated only when the force from the fluid pressure exceeds the force applied by the spring.

Set point adjustment screws:

Integrated with the spring are set point adjustment screws. Set point adjustment screws are used to increase or decrease starting pressure.

Differential:

This is used to expand or reduce the operating pressure range of the switch. The most common common design found in pump systems is a set of springs and adjustment screws that are significantly smaller than the set point adjustment. Tightening or loosening this screw will only change one end of the pressure range (higher or lower end), while the other end remains the same.

Diaphragms (diaphragm piston assembly), seals and O-rings:

The diaphragm, along with other sealing components, protects the inside of the switch from process fluids. It is a flexible material, usually made of polymers, elastomers, or metal alloys. The type of diaphragm material is selected according to the type of fluid and its temperature. Common diaphragm and sealing materials are:

Nitrile or nitrile butadiene rubber (Buna-N) :

These materials are highly resistant to oil or petroleum-based liquids, but degrade in the presence of ozone and ketones. NBR diaphragms and seals strike a good balance between cost and physical properties, making them suitable for most neutral fluids. Its operating temperature range is -30°C to 100°C.

Ethylene propylene diene monomer or EPDM rubber:

This is another elastomer widely used in high-temperature water and steam services. It operates at temperatures up to 482°F (250°C). It is resistant to ozone, ketones, weak acids, bases and other oxidizing chemicals. They are not used in oil services because EPDM can absorb oil and fuel, which causes them to expand.

Fluorocarbon or FKM (Viton) :

Viton is a proprietary material with properties similar to NBR. The material is resistant to petroleum-based liquids and solvents. They are also not suitable for fluids containing ketones. Viton has excellent operating temperatures up to 200°C.

Teflon:

PTFE is rarely used as a diaphragm for previous materials due to its polymeric chain structure. It is not as elastic as elastomers and tends to creep. They are only considered for very high temperatures (up to 500°C) and corrosive or high wear services. The popular PTFE diaphragm is made of polytetrafluoroethylene (PTFE) and Kapton layer (polyimide).

Switch housing:

The switch housing protects switches and other internal components from the external environment. An important specification of a switch housing is its level of protection. Typical case specifications are IP, NEMA, and ATEX grades. The IP and NEMA levels describe the level of protection against the entry of solid and liquid foreign objects. The ATEX rating is suitable for environments with fire and explosion risks.

Contacts:

The contact is one of the conductive parts of the switch. Separating or connecting contact points will power down or power up the circuit. Switch contacts are made of materials with high corrosion resistance and electrical conductivity, such as copper, silver, gold or brass. As far as its connection is concerned, the contacts can be NO, NC or CO. NO Initial power off circuit used to cut in at set point. NC performs the opposite operation by initially energizing. CO switches serve two connections or circuits, one open and one closed, and are often used to control interlocks or more complex circuits. For simple control activation, NO or NC is sufficient.

Terminal:

Terminals that connect control or instrument circuits. Most pressure switches have markings on the nameplate regarding the terminal configuration relative to the contact. The nameplate includes schematics or diagrams for determining the correct terminal connections in the circuit. As with contacts, terminals must be corrosion-resistant and highly conductive.

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