Question 1 One of the key safety issues with pumps is the maximum pressure that can be created by the pump. Excessive pressure can exceed the mechanical integrity of the rest of the system, causing a spill (and the subsequent consequences of a spill). Some methods of limiting maximum pressure work in all circumstances, while others work in only some circumstances.
This is a list of proposed methods to limit maximum discharge pressure in all circumstances. Unless noted, it is assumed the density of the pumped liquid is constant. It can also be assumed that the fluid is such that it will not cause an obstruction in the piping system at all. Finally, it can be assumed that unless stated, the pump runs at constant speed. Your task is to determine which methods will work in all circumstances, and which will not. If it will not, you will need to describe the circumstance where the safeguard will not work.
(NOTE: This question requires you to understand the actual methods of how the pump works).
Shaving the impeller of a centrifugal pump to limit pressure rise
Installing a pressure relief valve on the inlet (not the outlet) of a centrifugal pump
Installing a pressure relief valve on the inlet (not the outlet) of a reciprocating positive displacement pump
Installing a recycle line from pump discharge to pump suction
Installing a pressure relief valve on the compress air supply of an air-operated-double-diaphragm pump
Installing a pressure relief valve on the outlet of a reciprocating positive displacement pump, with the discharge sent to the inlet of a pump
Installing a pressure switch high on the outlet of the pump which, when activated, will shut of the power to the pump motor
Installing a pipe on the outlet of a centrifugal pump. The pipe points upward, is 20 metres high, is large in diameter, and is open ended. Any discharge will overflow into a containment pit.
Question 2 We have a small centrifugal pump that has worked well enough (not great, but good enough) for a few years. It has negative suction (fluid is below the pump). Suction pressure is atmospheric pressure, and the pump discharges into an atmospheric tank. It has flushed packing (not mechanical seals).
Our site begins a water conservation program to reduce water usage. Pump flushing water is one target. We change the packing to a packing that does not require flushing. We then shut off the flushing water.
Within hours, the pump is cavitating. The level in the supply sump is well within normal levels, and flowratres are normal. Power consumption is up but that could be due to the cavitation. All valves on the suction and discharge are checked and found to be unmoved from the last position. We check the sump contents and it is normal – nothing that would suggest caviation.
Our first reaction is to wait to see if it goes away, but the cavitation continues for days.
What happned?