Pressure Switches Are Unsung Heroes in Many Process Control Applications

In today's highly connected world of process control instrumentation, the unacknowledged and uncelebrated pressure switch quietly does it's job protecting people and equipment from unsafe conditions and ensuring consistent, efficient operation.

A pressure switch is an electromechanical device that senses fluid pressure and responds by opening or closing an electrical or pneumatic circuit. That's it. That's their job.

However, pressure switch applications exist in almost every water treatment plant, power plant, refinery, chemical plant, paper mill, steel mill, or other manufacturing plant. Pressure switches are everywhere.

Pressure switches are easy to use and consist of four essential components: a pressure port or connection; a sensor that moves in response to changing pressures; an electrical or pneumatic switch that opens or closes in response to movement; and finally, a housing that protects the pressure switch's internals from the environment. 

SOR pressure switches, the recipient of a 2021 CONTROL Readers Choice Award, have been field-proven in many industries for primary and redundant pressure applications. SOR continues to broaden its product ranges to meet evolving market needs, even though the technology of the initial Static "O" Ring mechanical switches set industry standards more than 50 years ago.  In any situation, there's a SOR switch designed to meet or exceed the minimum requirements for any hazardous area or agency approval. 

SOR pressure switches are designed and offered with various agency permits, housing enclosures, sensor materials, and switching configurations. The wide range of product components allows for the best instrument configurations to handle even the most demanding applications.

For more information about SOR pressure switches in Florida, contact Instrument Specialties. Call them at 407-324-7800 or visit this web page.

Pressure Switch Design Details

Industrial process control applications present dynamic and varied requirements for measuring, monitoring and control. Each point calls for specific evaluation of the information needed from the process point for use in monitoring process performance, or control to be applied at the process point to regulate an outcome. Sometimes, a continuous analog signal is needed to provide indication across a range of values. Other times, it is only necessary to have notification of, or take action when, a certain temperature or pressure related event occurs. In those cases, a simple and reliable device can adequately meet the project requirements.

Pressure and differential pressure switches connect to a process and change their switch position when a setpoint condition is reached. The are simple to understand, easy to install, low in cost, and require little maintenance of attention. The switches are available in an extensive array of configurations, with options to fill out almost any application requirement.

SOR, Inc., globally recognized manufacturer of temperature and pressure switches, has produced this video outlining some of the distinctive features of their pressure switches for industrial process control applications. Share your process measurement and control requirements and challenges with product application specialists, and leverage your own process knowledge and experience with their product expertise to develop effective solutions.

New Electronic Pressure Switch With Continuous Output Signal

The new 805QS electronic pressure switch
Courtesy SOR, Inc.

SOR now has an enhanced version of their popular pressure transmitter, model 805PT. The new 805QS provides a continuous output signal corresponding to pressure, but also incorporates a failsafe adjustable pressure switch into the same device. The switch can be configured in the factory or field to operate in one of three ways:

• Switch is closed when pressure is above a low setpoint and below a high setpoint. This is referred to as a "window" setting.
• Switch is closed when pressure is above a setpoint.
• Switch is closed when pressure is below a setpoint.

In all variants, the switch will open on loss of power. The loop powered unit is designed so that the switch and output signal operate independently. Other features include a five millisecond response time, full range temperature compensation, and global certifications.

Learn more from the video, and share your process instrumentation and control challenges with application experts. Combine your own process knowledge and experience with their product application expertise to develop an effective solution.

Rotork Instruments Brands Join ISI Technical Group Product Portfolio

Midland brand air line filter regulator
Courtesy Rotork Instruments

ISI Tech Group now handles a large complement of the Rotork Instruments brand family throughout the Florida and South Georgia area. A number of well recognized brand names have been incorporated into the Rotork Instruments group of products in recent years and ISI now handles four of them.

Midland is a specialist manufacturer of stainless steel large flow air service equipment. Purpose specific, manufactured solutions for filtration and regulation of compressed air and gases is the specialty of the brand. Products include filters and regulators, poppet valves, spool valves, air pressure switches, flow regulators, and a host of specialized ancillaries that fill out a complete system installation.

The Fairchild brand, which has been on the roster of ISI Tech Group products for some time, offers a wide selection of electro-pneumatic transducers, pressure regulators, pneumatic relays, volume boosters, filters and accessories for process machine tool, robotic and other industrial applications. Products are available with designs to suit all environments, including hazardous areas.

M&M industrial solenoid valves
Courtesy Rotork Instruments

M&M International is also a Rotork Instruments brand. M&M solenoid valves are manufactured with high quality materials and to rigorous standards to deliver durable performance and high reliability. Robust construction makes these valves well suited for industrial applications. A choice of seal materials provides wide fluid compatibility. Products include general purpose solenoid valves, air actuated piston valves, automatic drain valves, and accessory products

Block and bleed needle valve assembly
Courtesy Rotork Instruments

Bifold, another Rotork Instruments brand, manufactures medium pressure instrumentation ball valves, needle valves, fittings, manifolds, and relief valves for applications with pressure up to 20,000 psi.

This expanded product offering empowers the process measurement and control specialists at ISI Tech Group to better serve the diverse requirements of their customer base. Share your automation and process control challenges with experts, combining your own knowledge and experience with their product application expertise to develop effective solutions.

Electromechanical Industrial Pressure Switches

Industrial pressure switch
(courtesy of SOR)

Most industrial applications require the monitoring of pressure. Pressure measurement can be accomplished by transmitters, gauges or by switches. This post will provide a quick introduction of industrial electromechanical pressure switches.

An industrial pressure switch is made up of the three main components: 1) the sensor, 2) the housing and 3) the switching element.

Sensor

The sensor is located above the pressure port and process connection. For pressure and differential pressure switches, there are several varieties of pressure sensors to choose.  The most common types of pressure sensors are:

• Metal Bellows - an accordion-like device that provides linear expansion and contraction based upon the application of pressure or vacuum. Bellows are excellent sensors because they provide good overall pressure range and are fairly sensitive to small changes in pressure.
• Piston - A rod and o-ring combination that moves linearly in direct response to applied pressure. Piston sensors are normally only applied to only very high pressure ranges. They have very small surface areas and wide deadbands (the change in pressure required to change the position of the switch output).
• Diaphragm - A thin, elastomer or metallic membrane, often with a rolled lip that allows for greater movement. The diaphragm has a large surface area and provides the most sensitivity to pressure change, making it ideal for low to mid-range pressure sensing.

Housing

Housings are classified and selected based on the atmosphere in which they’ll be used. Housing ratings are classified by several national and international agencies such as NEMA and CENELEC. Very generally put, housings can be rated as general purpose, dust & water resistant, water tight, corrosion resistant and hazardous (explosive) environments. Proper selection of the housing is important to the operation and life expectancy of the device. In hazardous environments, proper selection is absolutely critical. If unsure about the housing classification, consultation with an applications expert is required.

Switching Element

The switching element refers to the signaling device inside the enclosure that responds to the movement of the sensor. It can be either electrical or pneumatic, and provides an on-off signal (as opposed to an analog, or proportional signal produced by transmitters).

The switching element is most times a “micro” type single pole, double throw (SPDT) electrical switch. These microswitches come in many configurations and electrical ratings, such as double pole, double throw (DPDT), 120/240 VAC, 12VDC, 24VDC, and hermetically sealed.

For the switching element and the sensor, it is very important to know the cycling rate (number of on vs. off times over a period of time) the instrument will see. Since both of these elements are mechanical, they will eventually wear out and need to be replaced. Switches are an economical and strong performing choice for low to medium cycle rates. For extremely high cycle rates, the use of solid state transmitters are a better choice.

The video below, courtesy of SOR Inc., provides a look inside how a typical pressure switch works.

Helpful Hazardous Area Definitions for Process Control Instrumentation

Courtesy of SOR Pressure, Temperature, and Level Instruments

SOR Explosion Proof
Pressure Switch

Hazardous Area Classification

Hazardous Areas are locations where the potential for fire or explosion exists because of gases, dust, or easily ignitable fibers or flyings in the atmosphere.

In North America, hazardous area classification is separated by classes, divisions, and groups to define the level of safety required for equipment installed in these locations. Classes define the general form of the flammable materials in the atmosphere. Divisions define the probability of the presence of flammable materials. Groups classify the exact flammable nature of the material.

In Europe and countries outside of North America, classification of hazardous areas is accomplished differently. Zones are used to define the probability of the presence of flammable materials. Protection Types denote the level of safety for the device. Groups classify the exact flammable nature of the material. These groups are separated differently than North American Groups. Temperature Identifications convey the maximum surface temperature of the apparatus based on 104° F (40° C) ambient. These temperature codes are selected carefully not to exceed the ignition temperature of the specific gas or vapor to be encountered in the application.

Some hazardous area classifications are not shown here. For further detailed information, see specific standards published by approval organizations.

Classifications Inside North America

Classes
Class I	Flammable gases or vapors are present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Class II	Combustible or conductive dusts are present.
Class III	Ignitable fibers or flyings are present, but not likely to be in suspension in sufficient quantities to produce ignitable mixtures. (Group classifications are not applied to this class.)

Divisions
Division 1	The substance referred to by class is present during normal conditions.
Division 2	The substance referred to by class is present only in abnormal conditions, such as a container failure or system breakdown.

Groups
Group A	Acetylene
Group B	Hydrogen (or gases of equivalent hazard)
Group C	Ethylene (or gases of equivalent hazard)
Group D	Gasoline (or gases of equivalent hazard)
Group E	Metal Dust
Group F	Coal Dust
Group G	Grain Dust

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Classification Outside North America

Zones
Zone 0	Area in which an explosive gas-air mixture is continuously present or present for long periods.
Zone 1	Combustible or conductive dusts are present.Area in which an explosive gas-air mixture is likely to occur in normal operation.
Zone 2	Area in which an explosive gas-air mixture is not likely to occur, and if it occurs it will only exist for a short time.

Protection Types
		Zone
d	Flameproof (Explosion proof) Enclosure	1,2
e	Increased Safety	1,2
ia	Intrinsic Safety	0,1,2
ib	Intrinsic Safety	1,2
o	Oil Immersion	2
p	Pressurized Apparatus (Purged Apparatus)	1,2
q	Powder Filling (Sand Filling)	2
m	Encapsulation	1,2
n	Normally Nonsparking and/or Nonincendive Circuits)	2

Temperature Codes
	°F	°C
T1	842	450
T2	572	300
T3	392	200
T4	275	135
T5	212	100
T6	185	85

Groups
Group I	For application in below ground installations (mines) where methane (firedamp) and coal dust may be present.
Group IIA	For application in above ground installation where hazards due to propane may exist. This group most closely matches the North American Group D.
Group IIB	For application in above ground installations where hazards due to ethylene may exist. This group most closely matches the North American Group C.
Group IIC	For application in above ground installations where hazards due to hydrogen or acetylene may exist. This group most closely matches the North American Groups A and B.

A Compact, Rugged and Explosion Proof Industrial Pressure Switch

Mini Hermet

Need a robust, explosion proof pressure switch with a small footprint? The SOR "Mini Hermet" is your answer. This pressure switch is ideally suited for a variety of process applications in hazardous locations and hostile environments where stainless steel exterior parts are required and where space is limited. Basic models with standard wetted parts are normally suitable for air, oil, water and non-corrosive process fluids.

The pressure sensing assembly is similar to a conventional pressure switch. The main difference is that the switching element assembly is hermetically sealed in an explosion proof steel capsule. Switching elements are SPDT or DPDT.

The pressure sensing element is a force-balance, piston-actuated assembly. The sensing element is sealed by a flexible diaphragm and a static o-ring. There are only three wetted parts in this arrangement: a pressure port, a diaphragm and an o-ring. A wide selection of wetted parts materials for media compatibility and containment are available.

For more information contact:

Instrument Specialties Inc.
3885 St. Johns Parkway
Sanford, FL 32771
phone 407.324.7800
fax 407.324.1104
e-mail: offices@isisales.com