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A pressure switch is a mechanical or electronic device activated by the pressure of fluids, air, or gas when the fluids, air, or gasses reach a threshold or setpoint. The designs of pressure switches include bourdon tubes, pistons, diaphragms, or membranes that move or deform with the amount of pressure exerted by the system.
The components of a pressure switch are connected to one or more contacts in the switch. With enough force, a contact closes or opens the switch depending on its configuration. Although pressure switches have a variety of methods used to detect pressure, they can be primarily categorized as electromechanical or electronic.
Pressure switches are used in most industries employing compressed gas systems, HVAC, instrumentation systems, pumping systems, and so on.
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 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.
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.
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.
The inlet port is the part that connects the pressure switch assembly to 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 fitting type and its pressure rating are compatible with the fluid pressure.
Mechanical pressure switches are classified according to their pressure sensing element. This is the main part of the switch that 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 spring force required. Note that only a small force is needed to actuate the switch. Much of the pressure is countered by the 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.
Integrated with the spring is the setpoint adjustment screw. The setpoint adjustment screw is used to increase or decrease the activation pressure.
This is used to widen or narrow 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.
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:
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.
This is another elastomer widely used for high-temperature water and steam service. Its operating temperatures can reach up to 482 ° F (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.
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 is rarely used as a diaphragm membrane over 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. A popular PTFE diaphragm is made of Teflon (PTFE) with a Kapton Layer (polyimide).
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.
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 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 are commonly used for control interlocking or more complicated circuits. For simple control activation, NO or NC is enough.
The terminal 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.
There are two main types of pressure switches: electronic pressure switches and mechanical pressure switches. 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 pressure switches are further divided according to the form and construction of the pressure-sensing component.
An electronic pressure switch has a pressure transducer, typically a strain gauge, with additional proprietary electronics that amplify and convert signals into a readable display. Some electronic pressure switches have analog capabilities, which means they have switching capabilities and can transmit continuous, variable signals that represent the pressure reading. Additional features of electronic pressure switches are on-site programmability of time delay, switching function, setpoint, and hysteresis.
High pressure switches have high pressure-proof pressure limits and can operate from 1 psig up to over 10,000 psig, with 4500 psig and 7500 psig being the average. They can be actuated using diaphragms, pistons, or piezoelectric crystals. The most common form of high pressure switch is diaphragm activated, which is actuated by pressure changes. As with all diaphragm pressure switches, actuation is triggered when the flow exceeds the set point.
Although most pressure switches will fail under certain conditions, high pressure switches continue operating and maintain pressure control, regardless. Due to their ability to provide continuous pressure control, high pressure switches are used as explosion-proof and waterproof pressure switches under intense pressure conditions.
High pressure switches have high durability and tensile strength and are made of aluminum, stainless steel, Monel, Hastelloy, or steel. Depending on the type of alloyed metal, some high pressure switches are corrosion-resistant.
Light or low pressure switches are designed to respond to reduced or small fluctuations in pressure. They are a protection method that prevents loss of pressure in a line that could damage or harm a system. If flow or pressure is absent in a line, low pressure switches will turn off equipment, activate an alarm, or provide a pressure reading.
Much like high pressure switches, low pressure switches come in several different operational methods, including diaphragms, pistons, and piezoelectric crystals. They are commonly used with hydraulic and pneumatic systems, where constant pressure is necessary. The exceptional sensitivity of low pressure switches allows them to react in correlation to pressure changes in a system.
Differential low pressure switches work by measuring the pressure between two points with different pressures and actuate in accordance with their set point. Positive low pressure switches work by converting a positive pressure signal into electrical output in response to changes in positive pressure. Conversely, negative low pressure switches convert negative pressure signals into electrical output when there is a change in negative pressure.
The previous chapters mostly describe mechanical pressure switches, which 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.
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.
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.
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 the 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.
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 entering the electrical components. Indirect sensing involves an elastic diaphragm that separates the piston from the fluid.
A snap disc pressure switch is a mechanical pressure switch that operates by the expansion and contraction of two metal discs that snap from a convex to a concave shape at a preset temperature. When the pressure switch snaps, it will complete a circuit or interrupt it.
In the design of a snap disc pressure switch, a thin diaphragm is positioned to isolate the pressure chamber from the disc chamber. Pressure forces act against the surface of the disc. The disc is positioned in a disc seat and held in place by the diaphragm. They control pressure with high current capacity and are used for non-hazardous applications.
Snap disc pressure switches are exceptionally consistent, reliable, and accurate. They have been used for many years as switches for the United States space program to deploy reentry parachutes. Their most common use is monitoring process temperatures.
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.
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.
The operating temperature influences the material used. Certain materials degrade at high temperatures. Materials suitable for high-temperature service are FKM and stainless steel 316. The temperature of the media being measured must be within the manufacturer’s specified temperature range for the switch.
The effect of temperature on accuracy must also be considered. When a pressure switch is configured at room temperature, the setpoint may need to be readjusted if the process is at a higher temperature. Fitting connection sizes can range from 1/8 to 1/2 NPT.
The pressure range defines the limits for adjusting the cut-in and cut-out pressures. This is often termed the working range of the pressure switch. It is recommended to have the setpoint at 40 to 60% of the pressure range to anticipate of any adjustment or field changes.
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.
Switches can be characterized according to the number of poles and throws. The 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:
This is the basic on and off switch. It can only be either NO or NC.
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.
This is the same as two SPST switches connected in a common actuator.
This is the same as two SPDT switches controlled by a common actuator.
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 than pistons.
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.
This is the maximum positive or negative deviation from the setpoint or specified characteristic curve under specific conditions and operations. 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%. On the other hand, electronic pressure switches have better accuracies of ±0.2 to 0.5% depending on the manufacturer.
Repeatability is the deviation between measurements or activations at the same pressure. This is different from accuracy because a device can have high repeatability but 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.
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.
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 that 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.
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.
The type of fitting connection on the pressure switch must match with the process stub connection or pressure port. Male and female threaded connections are widely used in mounting pressure switches. 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 to prevent corrosion, either from the atmosphere or from galvanic processes.
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. NEMA 3S to NEMA 4X or IP 54 to IP 64 is sufficient outdoors to 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 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.
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. Underwrite Laboratories (UL Listed or Recognized), CSA, FM, and CE are widely accepted certifications.
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.
This may be the most common use of pressure switches.Water pumps use pressure switches 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.
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.
These are control systems that use pneumatic and hydraulic actuators. Pumps and compressors maintain reservoir pressure and level through pressure switches.
In a refrigeration system, the thermostat provides the controlling feedback signal. However, if 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, indicating a possible refrigerant leak.
The pressure switch in a furnace or boiler serves as a safety interlock to prevent the igniter from operating in case of a problem with the draft system. This prevents the combustion chamber from operating, which can result in incomplete combustion.
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.
Scientists used these building blocks to invent and develop pressure switches, starting in the 1800s. In 1843, French scientist Lucien Vidie invented and assembled the first aneroid barometer, which used a spring balance to measure atmospheric pressure. While under pressure, the spring extension would mechanically amplify on an indicator system. Based on Vidie‘s methods, Eugene Bourdon patented the Bourdon tube pressure gauge in 1849. This was the first well-known mechanical pressure measurement device, which is still used today.
The Bourdon tube was then combined with a mercury switch, facilitating the creation of one of the first pressure switches. From this originated the basic concept of all electromechanical pressure switches, which use a sensing element like the Bourdon tube and a switch.
While the Bourdon tube pressure switch was a revolutionary invention, it was not without its drawbacks. Due to the tube being a tracing-type sensing element, they experience shorter service life. Additionally, the design did not perform well in applications with pump ripple or surge pressure, vibration, or ambient temperature changes. These influences could be lessened by using a higher quality tube; however, they are expensive to manufacture. This led others to seek out a better pressure switch design.
In 1956, Roy Dunlap became aware of some oil workers that needed an accurate pressure switch to prevent their oil tanks from overflowing. For help, Roy contacted Ben Brown, a physics professor at the University of Kansas, and together they created the Static "O" Ring® pressure switch. The design‘s sensing element used a force balanced piston-actuated assembly sealed by a flexible diaphragm and a static o-ring. Fluid pressure against the diaphragm counteracts the force of the range spring, moving the piston shaft only a few thousandths of an inch to directly actuate the electrical snap-action switching mechanism. Their simple design worked flawlessly, and because the o-ring was static, wear and tear were virtually eliminated. Roy started manufacturing and selling Static "O" Ring® pressure switches and changed the company name to Static "O" Ring®, which later became SOR Inc.
Mechanical pressure switches were the only type available until 1930 when engineers began experimenting with transduction mechanisms with sensing device movements as part of an electrical quantity. These were the first pressure transducers. Then, in 1938, engineers at the Massachusetts Institute of Technology and California Institute of Technology independently developed bonded strain gauges. They raced to the finish line, and E.E. Simmons of Caltech was the first to apply for a patent. The development of strain gauges was an important step in the right direction for solid-state pressure switches, which were widely introduced in 1980 by Barksdale Inc. At the time, they featured a bonded strain gauge sensor combined with a triac switch.
Today, solid-state pressure sensors are very popular and feature digital displays, digital and analog outputs, full programmability, and between one and upwards of four switch points.
Although the enhanced features of electronic pressure switches might lead one to believe that mechanical pressure switches serve no purpose, this could not be further from the truth. Electronic pressure switches require a power source to operate, and if power to the device is lost, it will no longer actuate when fluid pressure reaches the setpoint. This could result in monetary damages or harm to human life. Electromechanical pressure switches do not require power to operate and are simply acting as a pair of contacts to make or break a circuit.
Many industries use pressure switches as a redundant safety measure. If power is lost in the primary instrument, such as a pressure transmitter, the pressure switch is a backup to actuate when the setpoint is reached. Their lower instrument cost and lack of power supply give mechanical pressure switches a much lower cost of ownership than their electronic counterparts and are one of the many reasons they are still in use today.
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