Pressure Switches

Chapter 1: What is a Pressure Switch?

A pressure switch is a mechanical or electronic device activated by the pressure of the process fluid upon reaching a certain threshold or setpoint. A pressure switch can have a bourdon tube, piston, diaphragm, or membrane that moves or deforms according to the amount of pressure exerted by the system. These components are connected to one or more contacts within the switch. With enough force, a contact closes or opens the switch depending on its configuration. Pressure switches are used in most industries employing compressed gas systems, HVAC, instrumentation systems, pumping systems, and so on.

Working Principle

A typical pressure switch has a piston with one side subjected to the fluid pressure. The other side is usually in atmospheric pressure. The force exerted by the fluid pressure is countered by a force from a preloaded spring. The surface area in contact with the fluid and the spring constant is carefully designed so that the piston only moves when a certain pressure is reached. The spring is pre-compressed by the setpoint screw. The setpoint screw is adjusted to set the activation pressure higher or lower.

Cut-in and Cut-out

Pressure switches generally have two operating points: the cut-in and the cut-out pressure. In pump and compressor systems, the switch is activated when the fluid pressure goes below a set level. This starts the motor of the pump or compressor which returns the system to normal levels. The switch does not deactivate instantly when the pressure goes above the set point. There is a form of hysteresis or differential that prevents sudden tripping. This allows pressure to build up until the higher end of the pressure range is reached. When the higher setpoint or cut-out is reached, the switch deactivates.

Chapter 2: Parts of a Pressure Switch

This chapter discusses the main parts of a pressure switch. Note that each type or proprietary design may include additional components. The parts mentioned below are only applicable to mechanical pressure switches.

• Process (Inlet) Port:

  The inlet port is the part that connects the pressure switch assembly into the process unit. Pressure switches are installed on nozzles connected to a tank or pipe. The typical connection is threaded fittings. In rare cases, bolted or welded connections are used. It is important that the type of fitting and its pressure rating is compatible with the fluid pressure.

  

  
  

• Pressure Sensing Element:

  Mechanical pressure switches are classified according to their pressure sensing element. This is the main part of the switch which mechanically actuates the switch from the pressure of the fluid. The area of the piston or diaphragm on the fluid side is designed to transfer sufficient force from the expected fluid pressure. The larger the area, the larger the actuating force, and so is the spring force required. Note that only a small force is needed to actuate the switch. Much of the pressure is countered by the spring.

• Spring:

  The spring counters the force from the fluid. It is preloaded to match the operating pressure of the fluid. The switch only activates when the force from the fluid pressure exceeds the force applied by the spring.

  

  
  

• Setpoint Adjustment Screw:

  Integrated with the spring is the setpoint adjustment screw. The setpoint adjustment screw is used to increase or decrease the activation pressure.

• Differential:

  This is used to widen or narrow down the operating pressure range of the switch. The common design mostly seen in pumping systems is a set of spring and adjustment screws which are visibly smaller than the setpoint adjustment. Tightening or loosening this screw modifies only one end (higher or lower end) of the pressure range while the other end remains the same.

  

  
  

• Diaphragm (Diaphragm-piston Assembly), Seals, and O-rings:

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

  

  
  

  • Nitrile or NBR (Buna-N):

    These materials are highly resistant to oils or petroleum-based fluids but can degrade in the presence of ozone and ketones. Nitrile diaphragms and seals have a good balance of cost and physical properties making them suitable for most neutral fluids. Its operating temperatures can range from -30°C to 100°C.

  • Ethylene Propylene Diene Monomer or EPDM:

    This is another elastomer that is widely used for high-temperature water and steam service. Its operating temperatures can reach up to 250°C. It is resistant to ozone, ketones, mild acids, alkalis, and other oxidizing chemicals. They are not used in petroleum service since EPDM can absorb oils and fuels which causes them to swell.

  • Fluorocarbon or FKM (Viton):

    Viton is a proprietary material with properties similar to NBR. This material is resistant to petroleum-based fluids and solvents. They are not suitable for fluids containing ketones as well. Viton has superior operating temperatures that can reach up to 200°C.

  • PTFE:

    PTFE is rarely used as a diaphragm membrane than the previous materials due to its polymeric chain structure. It is not as elastic as elastomers and is prone to creep. They are only considered for very high temperatures (up to 500°C) and corrosive or high abrasion service. Popular PTFE diaphragm is made of Teflon (PTFE) with a Kapton Layer (polyimide).

• Switch Housing:

  The switch housing protects the switch and other internal parts from the external environment. An important specification of the switch housing is its protection rating. Typical enclosure specifications are IP, NEMA, and ATEX ratings. IP and NEMA ratings describe the protection level against the ingress of solid and liquid foreign objects. ATEX rating is for environments with risks of fire and explosion.

• Contacts:

  The contacts are one of the conductive parts of the switch. Separating or linking the contacts will de-energize or energize the electrical circuit. Switch contacts are made of materials with high corrosion resistance and high electrical conductivity such as copper, silver, gold, or brass. In terms of its connections, contacts can be NO, NC or CO. NO is for initially de-energized circuits which cut-in at the setpoint. NC performs the opposite by being initially energized. CO switches serve two connections or circuits, one open and one closed which is commonly used for control interlocking or more complicated circuits. For simple control activation, NO or NC is enough.

  

  
  

• Terminals:

  This is where the control or instrumentation circuit is connected. Most pressure switches have markings on their nameplate about the configuration of the terminals with respect to the contacts. The nameplate includes schematics or diagrams to determine the correct terminal connection in the circuit. Like the contacts, the terminals must be resistant to corrosion and highly conductive.

Chapter 3: Types of Pressure Switches

There are two main types of pressure switches: mechanical pressure switches and electronic pressure switches. Mechanical pressure switches are further divided according to the form and construction of the pressure sensing component. Electronic pressure switches are solid-state switches that do not require actuation from the pressure sensing element to operate the switch. They operate indirectly by using other properties such as resistance and capacitance.

Mechanical (Electromechanical) Pressure Switches

The previous chapters mostly describe mechanical pressure switches. They are more widely used than electronic switches due to their simplicity and lower cost. All mechanical pressure switches have a mechanical pressure sensing part that deforms according to the fluid pressure. They are classified according to the type of pressure sensing component.

• Piston Pressure Switch:

  This is the most popular and widely used pressure switch. As the fluid pressure changes, it causes the piston to move axially which activates the switch. It can sense the fluid pressure directly or indirectly. Direct sensing involves seals such as O-rings to prevent the fluid from getting into the electrical components. Indirect sensing involves an elastic diaphragm that separates the piston from the fluid.

  

  
  

• Diaphragm Pressure Switch:

  This type consists of a metal membrane joined or welded directly into the wetted part of the pressure switch. Instead of having a piston, the diaphragm directly actuates the switch.

  

  
  

• Bourdon Tube Pressure Switch:

  A bourdon tube is a flexible metallic or elastomeric tube fixed at one end while the other is free to move. When pressure is increased inside the tube, it tends to straighten. This movement is then used to actuate the switch.

  

  
  

• Differential Pressure Switch:

  This is a special type of pressure switch used to compare the pressures between two points in a system. These points are connected to two process ports. These can be upstream or downstream of equipment, or the top-side or bottom-side of a vessel. If the difference in pressures between the two-sides exceeds a certain threshold, the switch is activated. These are useful in interlocking controls for monitoring pressure drop across filters and screens and tank level.

Electronic (Solid-state) Pressure Switch

An electronic pressure switch has a pressure transducer, typically a strain gauge, with additional proprietary designed electronics that amplify and convert signals into a readable display. Most electronic pressure switches have analog capabilities. This means they are not limited to an open or closed position but are capable of sending a continuous, variable signal that is used for more accurate monitoring. Thus, electronic pressure switches are not switches but transmitters or measuring instruments. Additional features available for electronic pressure switches are on-site programmability of time delay, switching function, setpoint, hysteresis, etc.

Chapter 4: Pressure Switch Selection Criteria

Like any other measuring or monitoring device, pressure switches have several selection criteria that need to be considered. Selecting the right pressure switch for a given application leads to lower costs and longer service life of the device.

• Process Fluid:

  The chemical properties of the process fluid determine the type of material required for the wetted parts. The wetted parts are the ports, seals, and the pressurized side of the pressure sensing component. These parts must be capable of withstanding any chemical or physical attack from the process fluid. Mechanisms of part degradation can be through corrosion, oxidation, or erosion. The most commonly used materials for the rigid parts are steel, brass, stainless steel, PTFE, or PP; while the elastic pressure sensing parts and seals use NBR, EPDM, and FKM.

• Operating Temperature:

  The operating temperature also influences the material used. Certain materials degrade at high temperatures. Materials suitable for high-temperature service are FKM and stainless steel 316.

• Pressure Range:

  The pressure range defines the limits for adjusting the cut-in and cut-out pressures. This is often termed as the working range of the pressure switch. It is recommended to have the setpoint at 40 to 60% of the pressure range for the anticipation of any adjustment or field changes.

• Type of Pressure:

  Pressure switches are often used in positive pressure systems. But there are also cases where they are used in vacuum applications. For negative pressure systems, pressure switches specified for vacuum and compound pressure must be used.

• Switching Function:

  Switches can be characterized according to the number of poles and throws. Pole refers to the number of circuits a switch can control while the throw is the number of connections the switch can make. Both pole and throw can be single or double. The switching function classifications are:

  

  
  

  • Single Pole, Single Throw (SPST):

    This is the basic on and off switch. It can only be either NO or NC.

  • Single Pole, Double Throw (SPDT):

    This is the most common due to its versatility. It can be used as a NO, NC, or CO switch. It can also have three positions with the center being an off position for a CO switch. This is referred to as single pole, triple throw, and is rarely used for pressure switches which typically have only two positions.

  • Double Pole, Single Throw (DPST):

    This is the same as two SPST switches connected in a common actuator.

  • Double Pole, Double Throw (DPDT):

    This is the same as two SPDT switches controlled by a common actuator.

• Differential, Deadband, or Hysteresis:

  This is the difference between the cut-in and cut-out pressures. Pressure switches can have either adjustable or fixed deadbands. Adjustable deadbands are widely used for water pumping services. Fixed deadbands, on the other hand, are seen in packaged equipment and alarm systems where modifications are not necessary or avoided to prevent any inadvertent modifications of the system. Diaphragm and bourdon tube pressure sensing elements generally have a narrower deadband compared to pistons.

• Proof Pressure:

  Proof pressure is the maximum pressure the switch can withstand without causing any change to its properties or performance. This is also known as the over range capacity or maximum system pressure. Identifying the proof pressure considers any pressure spikes or surges occurring in the system.

• Accuracy:

  This is the maximum positive or negative deviation from the setpoint or specified characteristic curve under specific conditions and operation. Accuracy is a more important factor in selecting analog pressure sensors and electronic pressure switches. For these devices, having a higher accuracy significantly increases the cost of the pressure switch. Accuracy is specified in terms of a percentage of the full scale (FS) value. Typical pressure switch accuracies for diaphragm and bourdon tube is ±0.5% while piston pressure switches have an accuracy of ±2%. Electronic pressure switches, on the other hand, have better accuracies of ±0.2 to 0.5% depending on the manufacturer.

  

  
  

• Repeatability:

  Repeatability is the deviation between measurements or activations at the same pressure. This is different from accuracy such that a device can have high repeatability but with low accuracy. A pressure switch can repeatedly activate at a certain pressure, but the activations are far from the setpoint. Like accuracy, repeatability is specified in terms of full-scale percentage.

• Cycling:

  This is the expected period between two activations. This factor must be considered since continuous deformation of the pressure sensing element subjects it to constant fatigue, lowering its service life. Piston and bourdon tubes operate on the principle of deformation and are suitable for low cycling applications. For high cycling, piston and electronic pressure switches are used. A piston pressure switch experiences less fatigue since the actuation relies only on the movement of the piston or plunger. An electronic pressure switch also experiences the same since deformations in a strain gauge are much smaller than mechanical sensing elements.

• Service Life:

  This is directly influenced by the speed of cycling. The service life is the expected number of times the switch can activate and deactivate before failure. Since electronic pressure switches are solid state devices which means they have no moving parts, they have better service life which is expected to be above a million cycles. Among the mechanical pressure switches, piston switches have better service life than bourdon tube and diaphragm switches.

• Control System Voltage:

  This specifies the electrical characteristics of the control circuit. The power switch must be rated to the same current, voltage, and frequency. Otherwise, the switch, particularly electronic switches, may not activate or may have poor accuracy. Control circuits that use pressure switches are usually in DC. However, in some instances, AC voltages are also used. Common DC voltages are 8, 12, 24, and 30 volts; while AC voltages at 60Hz are 24, 120, 240, and 480 volts.

• Fittings:

  The type of fitting connection on the pressure switch must match with the process stub connection or pressure port. The widely used connection in mounting pressure switches is male and female threaded connections. Fitting connection sizes can range from 1/8 to 1/2 inches. Aside from the size and type, the material of the fittings is also specified according to the type of environment and matching connection. The main reason is preventing corrosion either from the atmosphere or from galvanic processes.

• Enclosure Protection Rating:

  This determines the environment the switch housing can withstand. Since pressure switches are widely used in almost all industries, there are various enclosure designs to balance robustness and cost. Enclosure protection ratings are specified through NEMA and IP numbers. Generally, a higher NEMA number indicates a better protection level. IP numbers, on the other hand, have two digits. The first indicates the solids or particulates protection rating while the second is for liquids. For general purpose indoor use, NEMA 1 to 2 or IP 10 to 11 is used where only protection from personnel contact is required. For outdoor, NEMA 3S to NEMA 4X or IP 54 to IP 64 are sufficient which protects against dust, rain, and snow. In the case of occasional washdown and immersion, NEMA 6 and IP 68 are commonly used.

  

  
  

  Aside from solid and liquid protection, enclosures are also rated according to their compatibility with an explosive environment. ATEX and IECEX markings are used to specify the hazardous applications of pressure switches and other electronic devices. Before requiring an ATEX rating, first look into the type of hazardous area where the pressure switch will be used. It is important to specify this accurately since having higher protection ratings greatly increases the cost of the device. Also, having a higher rating does not mean a higher level of protection for a particular application.

• Other Certifications:

  Certifications assure that the product conforms with the general safety standards mandated by national and international organizations. This is especially significant for pressure switches used in applications that directly affect consumer health and safety such as food manufacturing, fire protection, flammable gas handling, and so forth. Widely accepted certifications are Underwriter Laboratories (UL Listed or Recognized), CSA, FM, and CE.

  

  
  

Chapter 5: Applications

There are two main functions of a pressure switch. One is to maintain the pressure or reservoir levels of the system. The other is to protect equipment from damage or from running at low efficiency. Below are some of the common applications of pressure switches.

• Water Pumping Systems:

  This may be the most common use of pressure switches. Pressure switches are used in water pumps to cut-in power into the motor which drives the pump in case of low level or low line pressure. Upon reaching the set pressure, power is cut-out.

  

  
  

• Compressed Air Systems:

  This is similar to water pumping systems. Pressure switches are used to cut-in power to the compressor motor when low pressure is detected. This maintains the pressure of the compressed air system.

• Pneumatic and Hydraulic Systems:

  These are control systems that use pneumatic and hydraulic actuators. Pumps and compressors maintain reservoir pressure and level through pressure switches.

• Air Conditioning and Refrigeration:

  In a refrigeration system, the thermostat provides the controlling feedback signal. However, in case there is a problem in the system, the thermostat will only sense the temperature in the cooled space but not the state of the equipment. A pressure switch serves as a safeguard that trips the compressor motor in case of overpressure. Another use of a pressure switch in a refrigeration system is protection on the low-pressure side which indicates a possible refrigerant leak.

• Furnace and Boiler Systems:

  The pressure switch in a furnace or boiler serves as a safety interlock to prevent the igniter from operating in case there is a problem with the draft system. This prevents the combustion chamber from operating which can result in incomplete combustion.

  

  
  

• Filtering and Screening Equipment:

  A differential pressure switch is used to measure or monitor the pressure drop across filters and screens. The pressure switch triggers an alarm or notification to indicate that the filter is blocked or clogged and is due for maintenance, cleaning, or replacement.

Conclusion:

• A pressure switch is a type of switch activated by the pressure of the process fluid upon reaching a certain threshold or set point. A pressure switch can have a bourdon tube, piston, diaphragm, or membrane that moves or deforms according to the amount of pressure exerted by the system.
• There are two main types of pressure switches: mechanical pressure switch and electronic pressure switch. A mechanical pressure switch has a mechanical pressure sensing part that deforms according to the fluid pressure.
• Electronic pressure switches are solid-state switches that do not require actuation from the pressure sensing element to operate the switch. They operate indirectly by using other properties such as resistance and capacitance.
• There are two main functions of a pressure switch. One is to maintain the pressure or reservoir levels of the system. The other is to protect equipment from damage or from running at low efficiency.