Some years ago, I had been invited to review the horrendous failure rate of pumps and seals in an old chemical process facility in Europe. As we were walking through his plant, we passed a pump where he paused briefly to start up the driving motor, and immediately resumed walking. Knowing that automated controls were unlikely to be connected to this particular installation, I stopped and asked him if he was going to open a valve or two. His response stunned me. He said, “Oh, we’ll just let the pump run for a little while to warm it up, then we’ll put it on line.”
It turned out his “little while” could be anything up to 5 or even 10 minutes and had been regular practice with all pump startups in that plant for many years.
We then reviewed some of the major problems of running a pump at the shutoff condition; challenges such as high temperature build-up and excessive radial loading both of which result in reduced pump and seal reliability. As a high incidence in seal and bearing failure was a normal occurrence in that plant, we then agreed that their traditional startup process should be changed.
To start a pump is relatively simple, we prime it, start the motor and immediately open the discharge valve. We’ll discuss the priming at another time, but the real question is, which comes first, the pump or the valve?
This decision is based on two factors; the pressure in the system and the driver horsepower. With most process pumps, the driver is usually sized to accommodate the run-out condition that allows the pump to be started against an open valve. However, if the pressure in the system beyond the discharge valve needs to be maintained at an operating pressure, then the pump needs to be started against a closed valve, with the valve being slowly opened as the pump comes up to speed.
The major exception to this is with pumps with a high specific speed impeller such as the axial flow design found in vertical propeller pumps. With this type of impeller, the highest power draw is at the shutoff condition and the driving motor horsepower does not usually cover the demand at that point. In view of this, the pump must be started up with an open discharge valve thus allowing the pressure resistance in the system to build gradually up to the required operating head.
Make sure you know which you need to start first, but do them in quick succession, or you could damage the pump.
Ross Mackay is an internationally renowned expert in pumping reliability based in Canada. He specializes in helping companies increase their pump asset reliability and reduce operating and maintenance costs through pump training programs. He is the author of “The Practical Pumping Handbook”, and “The Mackay Self-Directed Pump Reliability Training System”. For more information and to register to possibly attend a Dynapumps and Ross Mackay pump school in 2013 please click here.