In my (process) plant, a temperature controller is used to regulate the temperature of a heater. The controller sends signal to a relay and once the relay is energised, a signal is sent to a solenoid valve which opens and supplies gas to the main burner of the heater.
Now recently operators reported that the heater was firing erratically and the relay goes on and off intermittently.
In the course of our troubleshooting, we discovered that a control cable inside the solenoid valve was almost completely disengaged. We terminated the cable properly. Also the temperature controller was displaying well when we started troubleshooting, but as we powered it again after initially removing source of power from it, the controller was not displaying well again. Hence we had to change the controller and the system responded perfectly well.
My question is:

Was the partial contact in the solenoid valve responsible for the on-and-off observed in the relay. If yes, is there a feedback mechanism that communicates the status of the solenoid valve to the relay or is it a one-way communication pattern whereby the solenoid valve can cut off on its own without affecting the relay.

Kindly assist

 

Not sure what "controller not displaying well" means, but there is likely some type of feedback. Gas burner systems like this can be very dangerous and therefore are designed failsafe in many ways. There's probably a monitor for the pilot and a timer that will shut everything off if flame isn't detected after x seconds of opening the valve, to name a couple of likely interlocks. These controllers are often programmed for the application, and by you swapping in a new one, you may be no longer utilizing vital safety features. You should really be intimately familiar with these controllers and processes before attempting to service them; which you obviously are not.

 

A poor contact at the solenoid valve could result in arcing and generation of large back-EMFs, which in turn could have caused erratic behaviour of the controller and intermittent operation of the relay.

 

@ strantor, thanks for your reply.

By 'not displaying well', I meant the digital display of the (ERO) Temperature controller was flashing occasionally and the digits were faint making it difficult to reprogram the controller. I fully understand the operational philosophy of the gas heater and we already have documented parameters for programming the temperature controllers to suit our application.

@Alec_t thanks for your response; I really appreciate. I also suspected a back emf as a result of the partial contact and the 'wrong' signal is fed back to the relay causing the on-and-off. Thanks a million.

Guys keep 'em coming!