When Old School is Best: The Continued Relevance of Electro-Mechanical Switches in Modern Process Control

Simple solutions, like electro-mechanical pressure and temperature switches, are often favored in specific industrial process control applications due to their inherent reliability and straightforward functionality. In the bustling environment of an industrial plant, complexity can introduce potential points of failure. While offering a broader range of functionalities and detailed diagnostics, sophisticated digital transmitters can be susceptible to software glitches, require periodic updates, and may be sensitive to electromagnetic interference from surrounding equipment. Furthermore, the cost of purchasing, installing, and maintaining these sophisticated instruments can be significantly higher than that of basic switches. On the other hand, electro-mechanical switches provide immediate, direct responses with minimal chances of malfunction, making them particularly suitable for applications that demand high reliability and quick actions. Their simplicity also means reduced training for operators, easy maintenance, and straightforward troubleshooting. In scenarios where continuous monitoring or data logging isn't essential but reliable actuation is, these traditional switches often emerge as the more pragmatic choice.

Electro-mechanical pressure and temperature switches have been a staple in the process control industry for many decades. Even with the advent of newer digital technologies, these traditional switches remain essential in many factory and plant automation instrumentation schemes. Here's a detailed breakdown of why these devices are still considered vital:

• Simplicity and Reliability: Electro-mechanical devices function on straightforward principles, such as the expansion of a bimetallic strip in response to temperature changes or the deformation of a diaphragm in response to pressure variations. These switches, having few moving parts, are inherently reliable. They don't suffer from software bugs or require firmware updates.
• Direct Control: Electro-mechanical switches can be wired directly to the control elements like solenoids, relays, or alarms, providing immediate response without needing an intermediary control system or software processing.
• Fail-Safe Operations: In safety-critical applications, it's sometimes preferable to have a direct-acting, mechanically-driven switch that can actuate an emergency shutdown or trigger alarms. These switches' inherent "on/off" nature ensures a definite state.
• Cost-effective: Electro-mechanical switches are generally less expensive than their electronic counterparts, especially for applications that don't require continuous monitoring or advanced functionalities.
• Low Power Requirements: Unlike electronic sensors that may need continuous power, these switches only consume energy when they change state, making them energy-efficient for specific applications.
• Resilience to Electromagnetic Interference (EMI): Electro-mechanical devices, being fundamentally mechanical in operation, are less susceptible to EMI, which can be a concern in industrial environments with heavy machinery and electrical equipment.
• Long Lifespan: Many electro-mechanical switches are known for their durability and long operational lifespan, mainly used within their design specifications.
• Versatility: They work in environments where advanced electronics, such as high-temperature areas or corrosive materials, might fail.
• Independent Operation: These switches do not necessarily rely on centralized control systems. This decentralization can be an advantage in scenarios where a failure in the central system shouldn't impact local controls.
• Ease of Integration: The infrastructure is built around electro-mechanical devices for many older plants and factories. Integrating newer electronic devices might require significant overhauls while continuing with electro-mechanical solutions offers seamless integration.

While electronic sensors and digital systems offer advanced functionalities and integration capabilities, there's still a strong case for the simplicity, reliability, and directness of electro-mechanical pressure and temperature switches. As with many technologies in the process control industry, the "best" choice often depends on the specific needs of the application and the context in which the device is applied.

Miller Energy, Inc.
https://millerenergy.com
800-631-5454

New Product Alert: The United Electric Controls Excela™ Electronic Switch

United Electric's Excela™ is the first switch of its type. Excela™ provides plant operators with an affordable way to upgrade to electronic performance. It has only two wires and is simple to place into the existing mechanical switch infrastructure. This unique, high-quality electronic pressure and temperature switch provide everything operators require for improved performance at an affordable price.

There are benefits and drawbacks to using advanced transmitters and old electromechanical switches at a facility. In terms of maintenance, an operator may spend at least ten times trying to maintain a switch over a transmitter. A transmitter, on the other hand, can be expensive and excessive for a modest application. There is a market void for economical, easy-to-install, drop-in-upgrade instrumentation for a facility's old mechanical switch infrastructure. To address the maintenance and upgrade cost concerns, the Excela™ was designed from the ground up to combine the benefits of the electromechanical switch (e.g., simplicity) and the transmitter (e.g., precision) at an inexpensive price point.

The Excela™ electronic switch is for plant upgrades by replacing mechanical switches with cutting-edge digital switch technology. It makes use of the existing mechanical switch wire as well as the attached discrete input power supply. In most cases, Excela™ is a direct drop-in replacement for existing mechanical pressure, differential pressure, and temperature instrumentation, making upgrading instrumentation within a plant cost-effective and straightforward. Typical uses are monitoring pressure and temperature for alarm and emergency shutdown in lubricating oil, boiler, furnace feed pumps, cooling, chiller water injection pumps, compressors, and many others.

For more information about the Excela™ electronic switch contact Miller Energy. Call 800-631-5454 or visit https://millerenergy.com.

How to Select a Pressure Switch

UEC One Series Switch Transmitter Hybrid

Reprinted with permission from United Electric Controls

Pressure switches are widely used by many industries and within many applications. The basic function of a pressure switch is to detect a pressure change and convert it into an electrical signal function – typically on/off or off/on. Pressure switches may be of electro-mechanical or electronic/solid-state design (see our ONE Series); and while each may have its advantages, arriving at the correct pressure switch for your application is the same.

Set Point & Deadband

Application set point (sp) is the desired value reached at rising or falling pressure at which the micro-switch changes electrical states. Depending upon the pressure switch function, the micro-switch could be wired to open (turn something OFF)  or close (turn something ON) when set point is achieved, thereby triggering an event such as an alarm, equipment shutdown, or powering up secondary equipment. Ideally, the set point should fall into the mid 50% of the pressure switch range for best performance including repeatability and long life. On an electro-mechanical pressure switch, set point may be adjusted internally or externally either through blind adjustment or reference dial. An electronic pressure switch would have internal or external adjustment via a key pad and digital display or a handheld programmer.

Deadband (DB) is the on-off differential required to reset the micro-switch. This value may be fixed or adjustable with an electro-mechanical switch and may be up to 100% adjustable on an electronic switch.

Deadband may be an important factor to consider depending upon the application requirements.

System Pressure

Knowing your normal and maximum system pressures will help in selecting a pressure switch with appropriate minimum and maximum operating parameters. Once your set point is established, other factors to consider are vacuum and/or surge pressure that could affect switch operation. This would involve maximum working pressure, over range pressure, and proof pressure specifications of a pressure switch. The relationship between set point and system pressure has a direct effect on switch performance and life.

Electrical Considerations

UEC 100 Series Pressure Switch

Micro-switches are available in a variety of configurations. The most common for electro-mechanical pressure switches is 15A, 480VAC SPDT (single pole, double throw). The advantage of a SPDT micro-switch is that it offers wiring flexibility to either open or close on pressure change. Other micro-switches available include 1A with gold contacts, useful when working with a PLC, or at the upper end, 30A when switching certain high voltage heaters or motors. Adjustable micro-switches help with deadband adjustment. A DPDT (double pole, double throw) micro-switch would provide two simultaneous functions off of one micro-switch. If a low-high limit alarm and shutdown were required, there are pressure switches that include two SPDT micro-switches that are independently settable.

An electronic pressure switch would use solid-state relays to change states. Like an electro-mechanical switch, the electronic switch can be programmed to open or close on rising or falling pressure. There are different capacities for switching voltage and current depending upon the application requirements.

Process Media and Wetted Parts

The pressure connection and sensor are known as wetted parts since they come into direct contact with the process media. Sensor material is either elastomer (i.e. Buna-N, Teflon®) or metallic (i.e. Brass, Stainless Steel) with metallic or composite pressure connections. The process media must be compatible with the wetted parts material. Process media temperature should also be considered as each of the different wetted materials would have differing operating properties.

Pressure Switch Mounting

If the unit is to be installed directly onto the process, there are many methods of installation.
Typically a 1/8”, 1/4”, or 1/2” NPT (national pipe thread taper) connection is used with a mating
fitting to secure the pressure switch to the process. There are also straight threaded (SAE, BSPT)
connections, flush mount connections, and sanitary connections. The pressure switch may be
mounted directly in the process line using the threaded connection, a manifold, or flange; or the
enclosure could be bolted to a mounting plate or other plane to secure it. If heavy vibration is
present, you may choose to use a remote diaphragm seal with the pressure switch. The diaphragm
seal mates with the process connection while the pressure switch enclosure is mounted securely
away from the vibration. 

Process Environment

It is important to know what type of environment the pressure switch would be installed in – hazardous or ordinary location; indoors or outdoors; exposed to salt air; inside a control panel; in high ambient temperature. These are just some of the factors to consider so the right enclosure type is chosen. Enclosure types come in many shapes, sizes, and materials. They also conform to various industry and third-party approval standards. Electronic switches can be used to replace electro-mechanical switches when SIL is needed for safety applications.  There are also electro-mechanical pressure switches without enclosures; typically used in OEM, non-hazardous locations where the environment is benign.

With careful consideration of all the factors listed above, choosing a pressure switch is a snap. If you are at all unsure, please contact your local United Electric Distributor or visit the UE Product Selector to find your pressure switch.

Pressure Switches - The Stalwart of Pressure Instrumentation

Pressure switch in an
explosion-proof housing.
(United Electric)

A pressure switch is an electromechanical device that detects the presence of fluid pressure and responds by opening or closing an electrical or pneumatic circuit.

In heavy industry, pressure switches are used in virtually every power plant, refinery, chemical plant, paper mill, steel mill, or other manufacturing plant that blends ingredients.

Pressure switches are simple devices. They can be broken down to their major parts: a pressure port or connection; a sensor that moves in relationship to changing pressures; an electrical or pneumatic switch that opens or closes upon movement; and finally a housing that protect the internals of the pressure switch from the ambient conditions.

Differential pressure switch.
(United Electric)

Pressure switches use a variety of sensing elements such as diaphragms, bellows, bourdon tubes, or pistons. In most cases, the movement of these sensors, caused by pressure fluctuation, is transferred to a set of electrical contacts to open or close a circuit. Normal status of a switch is the resting state. A pressure switch will be in its “normal” status when it senses low or minimum pressure. For a pressure switch, “normal” status is any fluid pressure below the trip threshold of the switch.

One of the earliest and most common designs of pressure switch was the bourdon tube pressure sensor accompanied by a mercury switch. A mercury switch is a position sensitive glass bulb containing mercury that flows over, or away from, the electrical contacts. When pressure is applied, the bourdon tube attempts to straighten, and moves enough to slightly tilt the mercury switch. Many of these kind of pressure switches were sold on steam boilers, and while they became a de facto standard, they were sensitive to vibration and breakage of the mercury bulb.

NO vs. NC electrical switch contacts.

The most common electrical switch used in pressure switches are "microswitch" type. These are also called "snap switches" because they are actuated by very little physical force, through the use of a tipping-point mechanism. These type of switches offer reliability and repeatability. They also are available in many different voltages and current ratings

One of the criteria of any pressure switch is the deadband or (reset pressure differential). This setting determines the amount of pressure change required to reset the switch to its normal state after it has tripped.  The “differential” pressure of a pressure switch should not to be confused with differential pressure switch, which actually measures the difference in pressure between two separate pressure ports.

When selecting pressure switches you must consider the electrical requirements (volts, amps, AC or DC), the area classification (hazardous, non-hazardous, general purpose, water-tight), pressure sensing range, body materials that will be exposed to ambient contaminants, and wetted materials (parts that are exposed to the process media).

It's always a good idea to discuss your application with an expert before specifying or installing a pressure switch. You'll end up saving time and money, and ensure long, safe operation.

For more information on pressure switches, contact Miller Energy by visiting https://millerenergy.com or by calling one of these numbers: In New Jersey 908-755-6700. In Pennsylvania 610-363-6200.

Bulb and Capillary Temperature Switches

General purpose temperature switch with bottom connection
for capillary and bulb specific for each application.
Image courtesy United Electric Controls

Not all processes or operations require the use of state of the art technology to get the desired results. Part of good process design is matching up the most appropriate methods and technology to the operation.

One method of changing the state of a switch in response to a process temperature change is a bulb and capillary temperature switch. The switch operation produces a state change in the mechanical switch when the temperature of a process control operation crosses a certain threshold. Bulb and capillary switches have the advantage of operating without electricity, simplifying their application.

The physical operating principle behind the capillary thermostat relies on the use of a fluid. The fluid inside the thermostat expands or contracts in response to the temperature at the sensing bulb. The change in fluid volume produces a force upon a diaphragm or other mechanical transfer device. The diaphragm is connected to, and changes the status of, an adjoining circuit using a snap action switch. For example, a main use of the operating principle in action is when a commercial food company relies on the capillary switch to control temperature related to processing and distribution. Each individual use of a bulb and capillary thermostat is specifically designed based on manufacturer and industry specifications, all of which apply the same physical principle of fluid based physics.

Because of their simplicity and comparatively modest cost, commercial versions of bulb and capillary switches find application throughout residential and commercial settings. Some common applications include warming ovens, deep fat fryers, and water heaters. The HVAC industry uses capillary and bulb switches because the rate of temperature change found in their applications fits the adjoining range offered by the bulb and capillary type switches. Operation of the temperature switches is subject to a few limitations. The switching point is often fixed, so the application must be without a requirement for an adjustable setpoint. The temperature range over which the switches are suitable is comparatively limited, with a matching of the bulb and capillary fluid system to the application temperature range a necessary task in product selection. Within its proper sphere of use, though, bulb and capillary temperature switches offer simple, reliable operation, with little requirement for maintenance.

Bulb and capillary switches are typically used to evaluate average temperature and are especially useful for applications where the temperature is to be maintained at a well-known, consistent value. The bulb portion can be configured to accommodate mounting within the media to be controlled. The devices can be applied effectively to liquid and gaseous media when the proper bulb is used.

Industrial versions of bulb and capillary switches are fitted with appropriate housings for the installation environment. Hazardous location installation can be accommodated, as well as high current ratings and auxiliary functions. There are almost countless variants of bulb and capillary temperature switches available. Don’t overlook these simple mechanical devices as candidates for application in any temperature control process. Share your application requirements and challenges with product specialists for useful recommendations.

Temperature, Pressure, Vacuum, and Differential Switches from Miller Energy, Inc.

Myths About Process Switches

Example of a general purpose switch for industrial use
Courtesy United Electric Controls

We may have developed an obsession with data and information. Understatement, right? Whether a process variable is high or low no longer is satisfactory. We want to know how high, how low, how long, how fast is it changing, and more. In many cases, this is useful information that can be applied toward goals of safety and efficiency. Let us not, however, overlook the possibility that some facets of an operation are best served by that old soldier, the process switch.

Process switches are readily available for temperature, pressure, and differential pressure in ranges to suit almost any application. They are rugged time and field proven devices designed to do one thing extremely well. Process switches will reliably and instantly change the state of their mechanical switch when the process value reaches their setpoint. Once properly installed and set, process switches require little, if any, maintenance and can provide extended periods of reliable service.

United Electric Controls, globally recognized leader in the manufacture of process switches for temperature, pressure, and differential pressure, authored an informative piece that debunks some myths about process switches. The piece is included below and makes interesting reading.

Establishing the best instrumentation and control layout for a process benefits from careful consideration of many factors. Share your requirements and challenges with process measurement and control specialists, combining you own process knowledge and experience with their current product application expertise to develop effective solutions.

Myths About Process Switches from Miller Energy, Inc.