Pump trips on low suction pressure

[dreve]

I'm looking for technical guidance on why slowly opening the discharge valve prevents low suction pressure pump trips. I understand the theory. Doing so allows the discharge line to develop the needed back pressure to prevent a run out condition. I need factual documentation to support my statement.

 

[TD2K]

I suspect it has to do with accelerating the suction volume of fluid into the pump. If you simply had the discharge valve fully open and started the pump, it would quickly accelerate to wherever it is going to sit on its curve, set by the instantaneous hydraulics. That suction line of fluid needs to start moving and I suspect that could result in a low suction pressure trip. If you had a long enough delay on the trip, then the pump would get to its normal operating point and the suction would pressure would be at its expected value. However, I don't have any references to point you to.

Another advantage of starting with a partially closed discharge valve is reducing the load on the motor.

 

[stanier]

The suction acceleration is not usually taken into account for centrifugal pumps but comes into the NPSH calculation for reciprocating pumps. The book by Miller on Reciprocating Pumps is a good reference.

Why not add a timer to the suction pressure switch so that on start up it doesnt trip the pump. This is common practice on flow switches on the upstream side so that the steady state is monitored not the start up transient.

A transient analysis of the start up would possibly reveal the suction transient on start up. From this one could justify the timer solution. Otherwise you may need to add a suction bottle to the system to alleviate the transient.

By doing the transient analysis you would generate your own references. Software packages such as AFT's Impulse has verifiaction models from NRC so the QA aspects are covered. Refer

http://www.aft.com

 

[quark]

Dreve!

Your first question comes into picture when the pump is started. One formula relates to both the questions.
P=WQH/75

At starting rotor of the motor requires more torque and if you load the motor simultaneously the motor will trip. If you open valve slowly Q will be low and accordingly power consumption and torque to be generated by motor will be low.

Secondly the head developed by the pump is proportional to the weight of the fluid. In initial condition if the casing is not full and if you open full valve totally, will reduce the head developing capacity

Regards,