It would appear that you have a cyclic upset somewhere in your plant.
You need to do some detective work. Find the PRV (Pressure Relief Valve) that is popping off.
Then do a complete check of that system. Start with what should be happening and compare it to what is actually happening.
What is causing the cyclic increase in pressure?
Is it a control valve that is sticking open?
Is it an exothermic reaction that is causing a heat build-up (thus pressure) ?
Some cyclic controls can be improved with cascade loops if the cyclic loop is slow but can form the setpoint of a faster loop. Examples include a temperature or gas pressure loop setting a flow control loop.
