A typical control system design is comprised of a logic controller (such as a PLC) which distinguishes inputs and produces outputs connected to hundreds of peripheral devices. Many of the outputs energize solenoid valves designed to be in a normally-off state. At a specified point, the control system produces a voltage thus energizing the respective solenoid. This is the first opportunity for the control system to distinguish whether the desired solenoid functioned properly. In other words, a typical electrical control system makes no distinction to whether a solenoid is operable prior to energization. At that point, the system must either shut-down or relay on back-up operations thus affecting the overall reliability and safety of the process.
Solenoid valves can fail in a number of ways but generally, failures are a result of excess heat and/or current. The solenoid coil itself energizes to operate a mechanical plunger. To move this plunger, an initial inrush current is produced in the coil which is very high. If the plunger movement is restricted, the length of time that the coil has to handle the inrush current becomes much longer. This is one cause of coil overheating which will result in failure.
In other cases, an elevated ambient temperature will cause excessive heat to build up in the coil leading to failure. The final common cause of solenoid failures is related to duty cycle. If a solenoid is operated repeatedly without sufficient time for cooling overheating can occur. This overheating is caused by the repeating inrush current seen during each separate actuation.
Solenoid failures generally do not directly cause damage. This is the likely reason why so few systems have protection against this condition. However, the indirect consequences can be severe.
Redundancy is the most common solution to vital operations. A control system is designed to energize a back-up system in the event a solenoid does not actuate. Back-up systems offer the best solution but usually come with the greatest cost. Back-up systems should be used in systems where actuation of the solenoid is absolutely required for safety and equipment protection.
In other cases, systems are designed to shutdown to protect equipment or personnel. This is the most common situation with vital control systems and offers excellent protection against the potential down-stream effects of the compromised control system. However, each shut-down comes with great costs and troubleshooting expenses. No solenoid manufacturer can guarantee 100% reliability of the device itself so the key is to focus on overall system reliability. Obviously, back-up systems provide the highest system reliability but at the greatest cost. The answer to achieving high system reliability without a disproportionate cost escalation is to monitor key solenoids for problems prior to attempted actuation. This approach does not increase the reliability of the solenoid itself but allows preventative actions to be made prior to full system failure. Monitoring can be added to the solenoid circuit which detects whether the coil is intact. In the event that a coil has failed, an early warning signal can be provided back to the system well before the solenoid is actually required to operate. Replacement can be scheduled before any system failures or shutdowns occur. This effectively increases overall system reliability in substantial ways.
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