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A pressure switch is a mechanical or electronic device triggered by the pressure of fluids, air, or gas when they reach a specific threshold or setpoint. The designs of pressure switches typically include bourdon tubes, pistons, diaphragms, or membranes, which move or deform in response to the pressure exerted by the system.
The components of a pressure switch are linked to one or more contacts within the switch. When sufficient force is applied, a contact either closes or opens the switch, depending on its configuration. Despite the variety of methods used to detect pressure, pressure switches can be primarily categorized as either electromechanical or electronic.
Pressure switches are used in various industries, including those employing compressed gas systems, HVAC, instrumentation systems, and pumping systems.
A typical pressure switch features a piston with one side exposed to fluid pressure and the other side to atmospheric pressure. The fluid pressure's force is countered by a preloaded spring. The surface area in contact with the fluid and the spring constant are carefully designed so the piston moves only when a certain pressure is reached. The spring is pre-compressed by the setpoint screw, which can be 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 activates when the fluid pressure falls below a set level, starting the motor of the pump or compressor to return the system to normal levels. The switch does not deactivate instantly when the pressure exceeds 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 apply only to mechanical pressure switches.
The inlet port connects the pressure switch assembly to the process unit. Pressure switches are typically installed on nozzles connected to a tank or pipe, with threaded fittings being the most common connection. 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 element is the main part of the switch that mechanically actuates in response to the fluid pressure. 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 greater the actuating force and spring force required. Note that only a small force is needed to actuate the switch, as much of the pressure is countered by the spring.
The spring counters the force from the fluid and is preloaded to match the operating pressure. The switch activates only when the force from the fluid pressure exceeds the force applied by the spring.
Integrated with the spring is the setpoint adjustment screw, which is used to increase or decrease the activation pressure.
This adjustment allows for widening or narrowing the operating pressure range of the switch. In many pumping systems, a common design includes a set of springs and adjustment screws that are visibly smaller than the main setpoint adjustment screw. Tightening or loosening this smaller screw modifies only one end (either the higher or lower end) of the pressure range, while the other end remains unchanged.
The diaphragm, along with other sealing components, protects the internal parts of the switch from the process fluid. It is made from flexible materials such as polymers, elastomers, or metal alloys. The choice of diaphragm material depends on the type of fluid and its temperature. Common diaphragm and sealing materials include:
These materials are highly resistant to oils and petroleum-based fluids but can degrade when exposed to ozone and ketones. Nitrile diaphragms and seals offer a good balance of cost and physical properties, making them suitable for most neutral fluids. Their operating temperatures can range from -30°C to 100°C.
Another elastomer widely used for high-temperature water and steam service is EPDM (ethylene propylene diene monomer). It can withstand operating temperatures up to 482°F (250°C) and is resistant to ozone, ketones, mild acids, alkalis, and other oxidizing chemicals. However, EPDM is not suitable for petroleum service, as it can absorb oils and fuels, leading to swelling.
Viton is a proprietary material with properties similar to NBR (nitrile butadiene rubber). It is resistant to petroleum-based fluids and solvents but is not suitable for fluids containing ketones. Viton offers superior performance at higher temperatures, with an operating range that can reach up to 200°C.
PTFE (polytetrafluoroethylene) is less commonly used as a diaphragm membrane compared to elastomers due to its polymeric chain structure, which makes it less elastic and more prone to creep. It is typically chosen for very high temperatures (up to 500°C) and in corrosive or high-abrasion environments. A popular PTFE diaphragm often features a combination of Teflon (PTFE) with a Kapton layer (polyimide) for enhanced performance.
The switch housing protects the switch and its internal components from external environmental conditions. A crucial specification of the switch housing is its protection rating. Common enclosure ratings include IP, NEMA, and ATEX. IP and NEMA ratings describe the level of protection against the ingress of solid and liquid foreign objects, while the ATEX rating is used for environments where there is a risk of fire and explosion.
The contacts are the conductive parts of the switch responsible for either energizing or de-energizing the electrical circuit by separating or linking. They are made from materials with high corrosion resistance and electrical conductivity, such as copper, silver, gold, or brass. Contacts can be normally open (NO), normally closed (NC), or changeover (CO). NO contacts are initially de-energized and close at the setpoint, while NC contacts are initially energized and open at the setpoint. CO switches have two connections or circuits one normally open and one normally closed and are used for control interlocking or more complex circuits. For simple control activation, NO or NC contacts are typically sufficient.
The terminal is where the control or instrumentation circuit connects to the pressure switch. Most pressure switches have markings on their nameplate that indicate the terminal configuration relative to the contacts. The nameplate often includes schematics or diagrams to guide correct terminal connections in the circuit. Like the contacts, terminals must be resistant to corrosion and highly conductive.
There are two main types of pressure switches: mechanical and electronic. Mechanical pressure switches are further categorized based on the form and construction of their pressure-sensing components. Electronic pressure switches, on the other hand, are solid-state devices that do not require direct actuation from the pressure-sensing element. Instead, they operate indirectly by utilizing other properties, such as resistance and capacitance.
The previous chapters primarily describe mechanical pressure switches, which are more commonly used than electronic switches due to their simplicity and lower cost. All mechanical pressure switches feature a mechanical pressure-sensing component that deforms in response to fluid pressure. These switches are classified based on the type of pressure-sensing component they use.
This is the most popular and widely used type of pressure switch. As the fluid pressure changes, it causes the piston to move axially, activating the switch. The switch can sense fluid pressure either directly or indirectly. Direct sensing uses seals, such as O-rings, to prevent the fluid from reaching the electrical components. Indirect sensing employs an elastic diaphragm to separate the piston from the fluid.
This type features a metal membrane that is joined or welded directly into the wetted part of the pressure switch. Instead of a piston, the diaphragm itself directly actuates the switch.
A bourdon tube is a flexible metallic or elastomeric tube fixed at one end, with the other end free to move. As pressure increases inside the tube, it tends to straighten. This movement is then used to actuate the switch.
This type of pressure switch is designed to compare the pressures between two points in a system. These points are connected to two process ports, which can be located upstream or downstream of equipment, or on the top-side or bottom-side of a vessel. The switch is activated if the pressure differential between the two sides exceeds a specified threshold. Such switches are useful for interlocking controls, monitoring pressure drops across filters and screens, and managing tank levels.
A snap disc pressure switch is a mechanical pressure switch that operates based on the expansion and contraction of two metal discs. These discs snap between a convex and concave shape at a preset pressure. When the switch activates, it either completes or interrupts the circuit.
In the design of a snap disc pressure switch, a thin diaphragm separates the pressure chamber from the disc chamber. Pressure forces act against the surface of the disc, which is positioned in a disc seat and held in place by the diaphragm. This setup allows the switch to control pressure with high current capacity and is typically used in non-hazardous applications.
Snap disc pressure switches are known for their exceptional consistency, reliability, and accuracy. They have been used for many years in the United States space program for deploying reentry parachutes. Their most common application, however, is in monitoring process temperatures.
An electronic pressure switch includes a pressure transducer, usually a strain gauge, along with proprietary electronics that amplify and convert signals into a readable display. Some electronic pressure switches also have analog capabilities, allowing them to provide continuous, variable signals representing the pressure reading in addition to switching capabilities. Additional features of electronic pressure switches often include on-site programmability for time delays, switching functions, setpoints, and hysteresis.
High pressure switches are designed to handle very high pressure limits, operating from 1 psig up to over 10,000 psig, with typical values around 4,500 psig and 7,500 psig. They can be actuated using diaphragms, pistons, or piezoelectric crystals. The most common type of high pressure switch is diaphragm-activated, which responds to pressure changes. As with all diaphragm pressure switches, actuation occurs when the pressure exceeds the set point.
Although most pressure switches may fail under extreme conditions, high pressure switches are designed to continue operating and maintaining pressure control even under intense conditions. Their ability to provide continuous pressure control makes them suitable for use as explosion-proof and waterproof switches in high-pressure environments.
High pressure switches are built with high durability and tensile strength, often made from materials such as aluminum, stainless steel, Monel, Hastelloy, or steel. Some of these switches are corrosion-resistant, depending on the type of alloy used in their construction.
Light or low pressure switches are designed to respond to small fluctuations or reduced pressure levels. They serve as a protective measure to prevent pressure loss in a line that could potentially damage or harm a system. When flow or pressure is absent, low pressure switches can turn off equipment, activate an alarm, or provide a pressure reading to indicate the issue.
Similar to high pressure switches, low pressure switches operate using various methods, including diaphragms, pistons, and piezoelectric crystals. They are commonly used in hydraulic and pneumatic systems where maintaining constant pressure is crucial. The exceptional sensitivity of low pressure switches enables them to respond precisely to pressure changes within a system.
Differential low pressure switches measure the pressure difference between two points and actuate based on their set point. Positive low pressure switches convert positive pressure signals into an electrical output in response to changes in positive pressure. Conversely, negative low pressure switches convert negative pressure signals into an electrical output when there is a change in negative pressure.
There are numerous types of pressure switches, each designed for specific functions to support or protect a process. In addition to the classifications of mechanical and electronic pressure switches, there are also specialized pressure switches tailored for particular applications.
Adjustable Pressure Switches: These switches allow users to set the pressure level at which the switch will activate. They are commonly used in applications where pressure levels vary, such as in air compressors, hydraulic systems, irrigation systems, and HVAC systems.
Air Pressure Switches: These switches are used to control air pressure in pneumatic systems, air compressors, HVAC systems, power tools, and various types of machinery.
Gas Pressure Switches: Similar to air pressure switches, gas pressure switches monitor and control gas pressure in home appliances like furnaces, boilers, and hot water heaters. They are also used in industrial applications to regulate pressure in pipelines.
Oil Pressure Switches: These switches are used in engines, compressors, and hydraulic systems to monitor and control oil pressure. They perform a similar function to gas and air pressure switches but are specifically designed for oil. Oil pressure switches are crucial in hydraulic systems, as oil is the driving force behind the hydraulic process.
Hydraulic Pressure Switches: These switches act as a safety measure for hydraulic systems operating under high pressure. They help prevent equipment damage and protect workers. Hydraulic pressure switches are commonly used in industries that rely on hydraulic power.
Vacuum Switches: Vacuum switches measure negative pressure and monitor the status of a vacuum in either open or closed systems. They come in various types, including electromechanical, solid-state, and pneumatic versions. Vacuum switches can have different configurations and designs, such as normally open or closed, single-pole or double-pole, and various throw types, each tailored to specific applications.
Well Pressure Switch: This switch is designed to control a well pump based on the pressure in the well. It has specific cut-on and cut-off pressure settings that determine when the pump should activate or deactivate. Well pressure switches serve as a safety device and control method to ensure that the well maintains the correct water level, preventing it from becoming too empty or too full.
As with any measuring or monitoring device, selecting a pressure switch involves several criteria that must be considered. Choosing the appropriate pressure switch for a specific application can lead to reduced costs and extended service life of the device.
The chemical properties of the process fluid dictate the choice of materials for the wetted parts, which include the ports, seals, and the pressurized side of the pressure-sensing component. These parts must be able to withstand any chemical or physical damage from the process fluid. Potential degradation mechanisms include corrosion, oxidation, or erosion. Commonly used materials for the rigid parts are steel, brass, stainless steel, PTFE, and polypropylene (PP), while elastic pressure-sensing components and seals typically use NBR, EPDM, and FKM.
The operating temperature affects the choice of materials for pressure switches, as certain materials can degrade at high temperatures. Materials suitable for high-temperature service include FKM and stainless steel 316. It is crucial to ensure that the temperature of the media being measured is within the manufacturer’s specified temperature range for the switch.
The impact of temperature on accuracy should also be taken into account. If a pressure switch is calibrated at room temperature, the setpoint might need to be readjusted when the process operates at a higher temperature. Additionally, fitting connection sizes for pressure switches typically range from 1/8 to 1/2 NPT.
The pressure range defines the limits within which the cut-in and cut-out pressures can be adjusted, commonly referred to as the working range of the pressure switch. It is advisable to set the switch at 40 to 60% of the pressure range to allow for potential adjustments or field changes.
Pressure switches are commonly used in positive pressure systems, but they can also be applied in vacuum environments. For negative pressure systems, it is essential to use pressure switches specifically designed for vacuum and compound pressure applications.
Switches can be characterized by the number of poles and throws. The pole refers to the number of circuits a switch can control, while the throw indicates the number of connections the switch can make. Both poles and throws can be single or double. The classifications for switching functions are as follows:
This is the basic on/off switch, which can be either Normally Open (NO) or Normally Closed (NC).
This is the most versatile switch, capable of functioning as Normally Open (NO), Normally Closed (NC), or Changeover (CO). It can also have three positions, with the center position being off for a CO switch. This configuration is known as single pole, triple throw, though it is rarely used for pressure switches, which typically have only two positions.
This configuration is similar to having two Single Pole Single Throw (SPST) switches connected to a common actuator.
This configuration is equivalent to having two Single Pole Double Throw (SPDT) switches controlled by a common actuator.
This refers to the difference between the cut-in and cut-out pressures of a pressure switch. Pressure switches can have either adjustable or fixed deadbands. Adjustable deadbands are commonly used in water pumping services, allowing for flexibility in setting the pressure range. Fixed deadbands are often found in packaged equipment and alarm systems, where modifications are either not needed or are avoided to prevent accidental changes. Generally, diaphragm and bourdon tube pressure-sensing elements have a narrower deadband compared to piston-based switches.
Proof pressure is the maximum pressure a switch can endure without altering its properties or performance. It is also referred to as over-range capacity or maximum system pressure. Determining the proof pressure takes into account potential pressure spikes or surges within the system.
Accuracy refers to the maximum positive or negative deviation from the setpoint or specified characteristic curve under specific conditions and operations. It is a crucial factor when selecting analog pressure sensors and electronic pressure switches. Higher accuracy often results in increased costs for these devices. Accuracy is typically expressed as a percentage of the full scale (FS) value. For diaphragm and bourdon tube pressure switches, accuracy is usually ±0.5%, while piston pressure switches have an accuracy of ±2%. Electronic pressure switches generally offer better accuracy, ranging from ±0.2% to ±0.5%, depending on the manufacturer.
Repeatability refers to the deviation between measurements or activations at the same pressure. Unlike accuracy, which measures how close a device's performance is to the setpoint, repeatability indicates how consistently a device performs at the same pressure. A pressure switch can exhibit high repeatability by activating at the same pressure repeatedly, even if those activations are not close to the setpoint. Repeatability, like accuracy, is specified as a percentage of the full scale (FS) value.
This refers to the expected period between two activations of a pressure switch. It's important to consider this factor because continuous deformation of the pressure-sensing element can lead to fatigue, reducing its service life. Piston and bourdon tube pressure switches, which operate on deformation principles, are more suited for low-cycling applications. For high-cycling applications, piston and electronic pressure switches are preferable. Piston pressure switches experience less fatigue as their actuation relies primarily on the movement of the piston or plunger. Electronic pressure switches also have lower fatigue since the deformations in a strain gauge are minimal compared to mechanical sensing elements.
The service life of a pressure switch is directly influenced by the speed of cycling and refers to the expected number of times the switch can activate and deactivate before failure. Electronic pressure switches, being solid-state devices with no moving parts, generally offer superior service life, often exceeding one million cycles. Among mechanical pressure switches, piston switches tend to have a longer service life compared to bourdon tube and diaphragm switches.
This specifies the electrical characteristics of the control circuit. The pressure switch must be rated for the same current, voltage, and frequency to ensure proper activation and accuracy. Mismatched ratings can lead to malfunction or reduced performance, particularly in electronic switches. Control circuits that use pressure switches are typically DC, but AC voltages are also used in some cases. Common DC voltages include 8, 12, 24, and 30 volts, while AC voltages at 60Hz commonly include 24, 120, 240, and 480 volts.
The fitting connection on the pressure switch must match the process stub connection or pressure port. Male and female threaded connections are commonly used for mounting pressure switches, with sizes ranging from 1/8 to 1/2 inches. In addition to size and type, the material of the fittings should be selected based on the environment and compatibility with the connection to prevent corrosion, whether from atmospheric conditions or galvanic processes.
This determines the environmental protection the switch housing can provide. Pressure switches are used across various industries, necessitating different enclosure designs to balance durability and cost. Protection ratings are specified through NEMA and IP numbers. Generally, higher NEMA numbers indicate better protection levels, while IP numbers consist of two digits: the first digit represents protection against solids or particulates, and the second digit represents protection against liquids. For general indoor use, NEMA 1 to 2 or IP 10 to 11 offer adequate protection from personnel contact. For outdoor use, NEMA 3S to 4X or IP 54 to 64 protect against dust, rain, and snow. For environments requiring occasional washdown or immersion, NEMA 6 and IP 68 are commonly used.
In addition to protection from solids and liquids, enclosures are also rated for compatibility with explosive environments. ATEX and IECEX markings indicate the suitability of pressure switches and other electronic devices for hazardous applications. Before seeking an ATEX rating, it is crucial to accurately determine the type of hazardous area where the pressure switch will be used. Higher protection ratings can significantly increase the cost of the device, and a higher rating does not necessarily equate to better protection for a specific application.
Certifications ensure that a product meets the safety standards set by national and international organizations. This is particularly important for pressure switches used in applications impacting consumer health and safety, such as food manufacturing, fire protection, and flammable gas handling. Common certifications include Underwriters Laboratories (UL Listed or Recognized), CSA, FM, and CE, which are widely accepted and signify adherence to safety and quality standards.
Pressure switches serve two primary functions: maintaining the pressure or reservoir levels of a system, and protecting equipment from damage or inefficient operation.
This is a common application for pressure switches. In water pumps, pressure switches control the power to the motor. They activate the pump when the pressure drops below a set level and turn off the power once the desired pressure is reached.
Similar to water pumping systems, pressure switches in compressed air systems cut in power to the compressor motor when low pressure is detected. This helps maintain the proper pressure within the compressed air system.
In control systems utilizing pneumatic and hydraulic actuators, pressure switches regulate pumps and compressors to maintain reservoir pressure and levels.
In a refrigeration system, while the thermostat provides the control feedback signal by sensing the temperature in the cooled space, it does not monitor the equipment's state. A pressure switch acts as a safeguard, tripping the compressor motor in case of overpressure. It also protects against low-pressure conditions, which may indicate a refrigerant leak.
In a furnace or boiler, the pressure switch functions as a safety interlock, ensuring that the igniter does not operate if there is a problem with the draft system. This prevents the combustion chamber from running, avoiding incomplete combustion and potential safety hazards.
A differential pressure switch monitors the pressure drop across filters and screens. It triggers an alarm or notification when the pressure drop indicates that the filter is blocked or clogged, signaling that maintenance, cleaning, or replacement is needed.
Pressure switches have their roots in the 1800s, starting with the invention of the aneroid barometer by French scientist Lucien Vidie in 1843. Vidie's device used a spring balance to measure atmospheric pressure, with the spring's extension mechanically amplifying the measurement on an indicator system. Building on Vidie's work, Eugene Bourdon patented the Bourdon tube pressure gauge in 1849. This device, which remains in use today, was the first widely recognized mechanical pressure measurement instrument.
The Bourdon tube was later combined with a mercury switch, leading to the development of one of the earliest pressure switches. This innovation established the fundamental concept for electromechanical pressure switches, which integrate a sensing element like the Bourdon tube with a switching mechanism.
While the Bourdon tube pressure switch was groundbreaking, it had limitations. The Bourdon tube, being a tracing-type sensing element, had a shorter service life and struggled with pump ripple, surge pressure, vibration, and temperature fluctuations. Although using higher quality tubes could mitigate these issues, the cost of manufacturing remained high. This prompted the search for improved pressure switch designs.
In 1956, Roy Dunlap, recognizing the need for a reliable pressure switch for oil tanks, reached out to Ben Brown, a physics professor at the University of Kansas. Together, they developed the Static "O" Ring® pressure switch. This innovative design featured a force-balanced piston-actuated assembly sealed by a flexible diaphragm and a static o-ring. The diaphragm's response to fluid pressure counteracted the range spring's force, causing the piston shaft to move slightly and directly actuate the snap-action switching mechanism. The simplicity and durability of this design, combined with the static o-ring reducing wear and tear, led to successful manufacturing and sales. Roy Dunlap renamed the company to Static "O" Ring®, which eventually became SOR Inc.
Before 1930, mechanical pressure switches were the only available type. In that year, engineers began experimenting with transduction mechanisms, incorporating sensing device movements as part of electrical quantities. This marked the inception of the first pressure transducers. By 1938, engineers at the Massachusetts Institute of Technology and the California Institute of Technology independently developed bonded strain gauges. E.E. Simmons of Caltech was first to apply for a patent. The development of strain gauges was a pivotal step toward solid-state pressure switches, which were introduced widely in 1980 by Barksdale Inc. These early solid-state switches featured a bonded strain gauge sensor paired with a triac switch.
Today, solid-state pressure sensors are highly popular and advanced. They feature digital displays, offer both digital and analog outputs, and come with full programmability. Modern solid-state sensors can have one to four or more switch points, providing versatile control and monitoring options for various applications.
Although electronic pressure switches offer advanced features and greater flexibility, mechanical pressure switches still play a crucial role. One significant advantage of mechanical switches is that they do not require an external power source to operate. If power is lost, mechanical switches can still function by acting as a pair of contacts to make or break a circuit. This ensures reliability and safety in critical applications where power interruptions could otherwise lead to significant damage or harm.
Pressure switches are widely used as redundant safety measures across various industries. In situations where the primary instrument, such as a pressure transmitter, might fail or lose power, a mechanical pressure switch can act as a backup. By actuating when the setpoint is reached, mechanical switches provide an additional layer of safety. Their lower cost and independence from an external power supply contribute to a lower overall cost of ownership, which is a key reason for their continued use despite the advancements in electronic pressure switches.
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