Internal By Pass Relief Valve in Pumps can be a pain during Comissioning

By: admin

When pumping liquid at high temperatures, be careful on type of Relief Valve to be Used.


Most of the Positive Displacement Pumps are provided with internal by-pass safety relief valve to protect the pump from overloading or to protect the pump against high discharge pressure. But sometimes, they can be a real pain during commissioning or for new applications.


Recently we had to face a similar situation, wherein we provided an Internal Lobe Pumps with Mechanical Seal and Internal Bypass relief valve for pumping thermic fluid at 200 LPM and 4 Bar discharge pressure and working temperature of 160 degrees C. Our quotation mistake – “Internal By-Pass Relief Valve”


As soon as they commissioned the pump, calls started flowing in from commissioning engineer, saying that the 5 hp motor is over-drawing the current up to 18 amps in just 2 minutes and tripping off and pump is faulty.

Unable to digest the fact, we depute our service engineer to inspect the pump at the site. He confirms that the issue is same, but when the motor trips, the pump is rotating freely…

I asked him, are you sure that the motor is taking 18 amps? And he says yes… I asked him did you check motor winding resistance and motor performance at no load? And he says “yes”, everything on motor side is perfect and smooth…

I ask him are the valves all open and he says as per commissioning engineer all. Are open and plus the pump is with a built-in relief valve set at 6 Bar from the factory, so this should not be of real concern…

I am still trying to figure out what could be the issue, but clueless as every other option in troubleshooting chart was looked into… Including ruling out the problems due to Bush bearing…  However, I tell. My engineer to retire for the day and wait for my arrival next day… With very little tools available on site and no pressure gauges installed in the line and modern electronic systems and electrically operates valves, I was sure the pump is not at fault. It could be the system or one of its valves…. But least known fact to me was how could it be the relief valve too… Along with their valves in the line…


Next day I visit and yes, the same observations were repeated… Well boom… What I observed was that it seemed to be one of its valves in the line that was blocked… And yes I studied it and found out that a three-way valve was closed. I opened up that valve and bang… The pump is performing normally… But why was the pump Tripping though we had safety relief valve… The question still remains unanswered in my head.


Well when they commissioned, the pump relief valve opened up due to line pressure being more than relief valve pressure and then the discharge was Internally by-passed, as a result of churning of oil at high temperature, the rotor expanded, thereby overloading the pump & motor… Simple reason…


Why was the three-way valve not working as per requirement? Well, they forgot to provide compressed air for its functioning.


Lesson learned:

  • Always give an external bypass relief valve or a line mounted relief valve when Pumping high-temperature fluids.
  • Consider worst case temperature and accordingly design the clearances for rotors.
  • Always have a pressure Guage installed in line or pump for easy troubleshooting.
  • Service engineers must study the customers piping layout correctly.

Should you have any questions, please feel to write it to me on

Chintan Turakhia.
+91 80000 78677
+91 90049 67867
Pumpsquare Systems LLP
Manufacturers of Positive Displacement Pumps !

Best is to Call +91 80000 78677 and get assistance in placing your order. Dismiss