7 Switch Myths Busted | Automation.com

7 Switch Myths Busted

7 Switch Myths Busted

By John Sestito, Product Manager, United Electric Controls

It’s been more than 80 years since the introduction and evolution of switch technology.  In that time there has been significant technological breakthroughs that are proving common misconceptions about switches shortcomings in industry to no longer be true.  Seven common myths surrounding switches are analyzed in this paper in conjunction with new high-tech electronic switch designs that are proven to solve historically problematic implementation in the industrial process industries, from the plant to OEM manufacturers.  Readers will acquire a better understanding of these new technologies available to improve safety, process efficiency and control.


Myth 1:  Switches are blind dumb devices

Prior generations of switches were incapable of displaying process measurements locally, which forced installation of local gauges resulting in a more complex design, increased leak paths and overall cost.  Additionally, operators were unaware when installed switches stopped functioning in the field, due to mechanical failure of moving parts within the switch.  Switches required removal from service and manual testing for functionality, which often resulted in the control or safety function foregoing protection for days, weeks or even longer.  In today’s modern workforce, this is becoming less and less acceptable.
These industry-wide problems inspired manufactures to innovate the next generation of electronic switches that incorporate integrated LCDs for local display of process variable measurements, along with internal diagnostics for monitoring the health of the device. The addition of LCDs and device diagnostics increases uptime and improves overall plant safety. For plant operators, knowing the health of the switch allows technicians to repair devices using modern best practices of a predictive maintenance program in favor of traditional preventative techniques. Plant operators can better plan their maintenance program schedules while original equipment manufacturers (OEM) benefit from a reduction in installed components, fittings and leak paths, and a more dependable turnkey product for their customers.

Myth 2: Switches require difficult and time consuming mechanical adjustments

Set point and dead band adjustments are a nuisance for operators and technicians. Traditional switches require removal from service and calibration on a bench in the maintenance shop. Installation instructions are not always available for older devices, leading to wasted time searching for documentation. Delicate adjustments were required to achieve desired set points as the deadbands, (the dead time where no action happens) varied by manufacturer based on the microswitch inside the device.  More often than not, instruments were mishandled leading to premature failure due to inexperienced technicians. 
Today’s generation of switches can offer electronic platforms that reduce setup and programming to a matter of seconds. A user interface on the local LCD provides simple prompts that allow users to program switch set points instantly without the need to remove the instrument from the process.  Deadband and set point are now 100 percent adjustable over the range, allowing operators to choose the desired range based on application requirements.


Myth 3: Switches are unsafe in Critical Applications - Not Appropriate for SIS

Industrial process plants are pushing processes pressure and temperature limits to new boundaries in an effort to stay competitive in a global market. Many of the systems designed 20 years ago were not intended to run at these process extremes. It is only a matter of time before these systems fail.
Safety instrumented systems (SIS) are an added layer of protection put in place in order to mitigate and reduce operational risks that effect the process, people and the environment.  These systems require devices that have been rigorously tested by third party agencies to verify the level of reliable performance.  Based on the strict performance requirements of SIS, some electronic hybrid safety systems have been developed that integrate the functionality of a switch, transmitter, logic solver/trip alarm and relay in a single enclosure have been certified to meet these needs. The switch portion of the device provides a direct digital output (relay output) to a final element that will promptly bring a process to a safe state in the event of a process upset. The embedded analog transmitter output signal can be used for trending to determine the health of the device and the process. With SIL 2 and 3 certification, this offers operators a product that matches the demanding performance requirements for safety instrumented systems. Unlike the past, it is not necessary to replace switches with transmitters when upgrading legacy systems or designing new functional safety loops for additional layers of protection.  


Myth 4:  Switches are problematic in tough environments

Whether installed on plant rotating equipment, such as turbines, or on demanding OEM auxiliary equipment, such as pumps or compressors, switches are required to function in tough environments that include shock, vibration, heat and pressure. Vibration is one of the leading causes of electromechanical switch failure. Most switches are mechanical in design and utilize a plunger to activate a micro-switch. In areas of high shock and vibration, the plunger position can fluctuate and lead to false trips.

For today’s solid-state electronic switches, some have provided a solution to the common problems with mechanical switches installed in high vibration applications. With no moving parts, these electronic switches can be mounted directly to the equipment or process without connecting impulse lines to isolate them from equipment vibration. Several turbine manufacturers and end users operating large compressors in petrochemical plants are experiencing increased reliability and fewer false trips with new electronic switches, compared to its predecessor, the electromechanical switch.


Myth 5:  You can only deploy electromechanical switches when line power is unavailable

Most pressure switches sold over the past 80 years were designed to operate without line power by incorporating a sensor that measures the force on a plunger that would actuate a microswitch to perform an operation. 

The first generation of digital switches required the plant to run additional wires (line power) to power the electronics.  This was not met with open arms by industry for mechanical switch upgrades because of the costly price of wiring, labor, time and permits needed. 

However, the next generation of solid state electronic switches have developed a wire for wire, direct drop in replacement (same 2 wires) for older mechanical switches.  The two wire electronic switches operate on an energy harvesting principle that powers itself from the host device (i.e. Programmable Logic Controller PLC) over the same two wires. Today, we have the ability to replace a blind dumb mechanical switch with a solid-state electronic switch that offers a digital gauge, switch and transmitter in one instrument without adding additional wiring or hardware.


Myth 6: Switches are an antiquated technology

Today’s process plants run their processes faster and hotter than originally designed to optimize profit. Ultimately, these plants will have to initiate modernization projects to support new market demands. Old switches provided users with digital, on-off signals that were either wired to control a piece of equipment directly or sent to a PLC for alarm functionality. As plants go through modernization projects, they restructure control system input/outputs (I/O) to support more analog signals than digital signals used in the past. Transmitters are commonly chosen and recommended over switches, however transmitters alone do not provide internal control functionality like the new generation of solid state electronic switches because transmitters must be connected in conjunction to a separate trip alarm.

These modernization projects are costly requiring new equipment such as updated wiring, I/O, and costly redesign, and result in significant downtime. The average process transmitter can cost upwards of $2,000 compared to the average process switch costing around $500. Process plants often have 100 to 1,000 switches installed. To upgrade all switches to transmitters could cost a plant up to $1.5 million. Consequently, switch manufacturers researched and developed new electronic switches that are capable of producing both digital and analog signals required for these new modernization projects, while keeping a similar price point to the original mechanical switches currently in service.


Myth 7: The response time of transmitters is faster than switches

Without question, electromechanical switches are faster than any transmitter on the market. With transmitters, large amounts of conversions, computations, compensation, and other work must be done to get an accurate signal. Even with embedding today’s high-speed processor technologies in transmitters, it cannot match the speed of the instantaneous reaction that new generation solid state electronic switches or traditional electromechanical devices can deliver. The fastest of these devices can be better than 50 milliseconds while process transmitters can range from 200-500 milliseconds or more. Newer electronic transmitter-switch combinations that are housed in the same enclosure can react in 100 milliseconds or less due to the benefit of computational measurement and switching being performed at the point of measurement. If your application requires fast response, such as in positive displacement (PD) pumps and turbine trip for over-speed protection, consider new solid-state transmitter-switches over process transmitters.


In summary:

  • Electronic pressure and temperature switches with embedded sensors, logic solver/trip alarms, and relays provide the functionality of three devices in one enclosure.
  • Today, solid state electronic switches are designed as a wire for wire (2 wire) drop in replacement for older electromechanical switches that upgrades users with diagnostics, 100% adjustable dead band and reduced inventory.
  • Built-in digital and analog communication can provide users the best of both worlds for controlling a piece of equipment locally and/or sending information back to a central control system for process trending and health.

About the Author

John Sestito is a product manager at United Electric Controls supporting their Pressure and Temperature safety instrumentation approved for hazardous location.  John earned his Bachelors of Science degree from Wentworth Institute of Technology in Electromechanical engineering with a minor concentration in Biomedical devices. With more than 10 plus years of experience in manufacturing, R&D and customer driven product development, for the past 5 years he has been focused on providing safety automated solutions to the industrial process industries, and is a contributing member to several ISA-84 process safety technical reporting committees.

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