As I write this article in early January, many of us are still trying to get started after a relaxing sojourn over the festive season. I’m reminded of an experience I had with a “Start-up” condition some years ago.
I was walking through a client’s plant when he paused briefly and pressed the green button on the driving motor starter, and 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 it run for a little while to let it warm up, then we’ll put it on line.”
It turned out his “little while” could be anything up to 5 or even 10 minutes. When I asked if this was a regular practice he assured me that it was standard with all pump startups, and had been practiced for all the years he had been employed in that facility (over 10 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 a high incidence in seal and bearing failure and reduced pump reliability.
As frequent seal and bearing failure was the reason I had been called into that plant, we then agreed that their traditional startup process should be changed.
In general terms, there are only three things we need to do to start a pump.
Prime the pump.
To do this with a flooded suction, the suction valves are opened and the liquid is allowed to flow into the pump. It may also be necessary to open an air release valve at the highest point in the pump casing to ensure that all the air/vapor is bled from the pump as it fills with the pumpage. When operating on a suction lift, the same effects must be achieved manually and, if a self-priming pump is used, this step is unnecessary.
Start the pump and open the discharge valve.
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 which 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.
Author of “The Practical Pumping Handbook” and a specialist in Pumping Reliability, Ross Mackay can be reached at www.practicalpumping.com or at 1-800-465-6260