Eight problems with outdated, electromechanical switches and eight solutions that will come with a digital upgrade to your plant.
The Problem: Unless tested on a regular basis, there is no way to determine when a problem exists. With mechanical switches, the only way to diagnose a problem is to remove the switch, leaving the control or safety function
Solution: Newer digital switches have an LCD screen that shows local process variable measurements and integrated internal diagnostics directly on the screen. You can easily monitor the health of the device at a glance, without having to remove the switch from operation.
The Problem: Mechanical switches require careful adjustments for reaching desired setpoints. Additionally, once these adjustments are made, settings
are likely to drift with ambient and process temperature changes. Maintaining these instruments can be tedious and can also create an unsafe environment for your plant since the device must be removed to be recalibrated.
Solution: Today’s switches offer digital adjustability, reducing setup and programming time to seconds. The LCD and local keypad offer an easy-to-operate interface for programming setpoints without a calibrated pressure source and without having to remove the device.
Protection in Hazardous Environments
The Problem: Many control switches are installed in tough environments where there are high levels of mechanical shock, vibration, heat and pressure. Vibration is the leading cause of electromechanical switch failure. The vibration often alters the mechanical switch’s plunger position, causing a false trip.
Solution: Digital switches are solid state and have no moving parts. So, they are unaffected by high levels of vibration, heat or pressure. Digital switches can be mounted directly to the equipment or process with better reliability and fewer false trips than mechanical switches.
The Problem: In efforts to stay competitive, industrial plants are pushing pressure and temperature boundaries to new limits. Some of the older, mechanical switches are simply not built to withstand these extremes, making the process unsafe.
Solution: A safety instrumented system (SIS) can be implemented to keep your process safe. An SIS uses sensors to detect dangerous changes in process variables, logic solvers to interpret the level of danger, and switches to activate valves and blowers.
You could also use a hybrid (safety) transmitter as an alternative solution. A safety transmitter combines the functionality of a switch, transmitter, and logic solver into a single unit. Using a safety transmitter is a cost-effective alternative as it doesn’t require having to connect a multiple-element SIS.
The Problem: Mechanical switches are frequently used in critical control applications that may not have safety requirements; however, safety integrity level (SIL) reliability is still valuable for maintaining uptime.
Solution: Since a safety transmitter combines the functions of a sensor, logic solver and switch into one pretested unit, it is likely more reliable and less susceptible to human error. This can have a positive effect on uptime as system error is less likely to occur and prevent a process from being run.
The Problem: Comparatively, process transmitters are rather slow in terms of response time. They have to compute a series of conversions and calculations that slow down a process that is demanding increasingly faster response times.
Solution: A safety transmitter can react in 100 milliseconds or less, where a standard process transmitter would react in 300 to 500 milliseconds. Upgrading to a safety transmitter would be extremely beneficial in applications that require fast response time such as in positive displacement pumps and turbine trip for over-speed protection.
The Problem: Most pressure switches sold over the past 80 years were designed to operate without electric power. When they installed, plant wiring did not need to connect to power. The older, mechanical switches simply required two wires that connected to a distributed control system, PLC or annunciator panel.
Solution: Digital switching technology has been manufactured to use the same two wires by using a small amount of leakage current obtained from a host device, like a PLC. This makes replacing your old mechanical switches easy and cost-effective.
The Problem: Many process plants have replaced mechanical switches with conventional process transmitters in order to meet higher performance demands. However, process transmitters do not provide the switching function directly. A PLC must be installed and programmed to interpret the transmitter’s analog signal before it can activate the switching function.
This replacement can also be expensive as the average process transmitter can cost more than $2000, and upgrading to transmitters requires extensive and time-consuming, rewiring and reprogramming.
Solution: As previously mentioned, a safety transmitter does not require any reprogramming and can be installed using the previous wiring from the mechanical switch. This reduces the overall downtime required for upgrading your plant. Additionally, the average cost for a hybrid transmitter is typically around $800, significantly less than a process transmitter.
Download the full source article, written by Wil Chin and Rick Frauton of United Electric Controls. Time to Switch Switches
Learn more about United Electric One Series SIL-rated safety transmitters