THE CENTRIFUGAL PUMP LOSES ITS PRIME PT012
The centrifugal pump works for a while and then looses suction. Cavitation is a main cause of loosing pump suction, but in this section we will be looking at only the intermittent loss of suction fluid. You will be looking at several possibilities:
- A recurring restriction in the suction piping that may or may not be causing a cavitation problem within the pump.
- Intermittent cavitation problems as opposed to a design or operation problem that causes a constant cavitation condition.
- A repetitive need for an increase in the pump’s capacity.
Now we will take a look at each of these possibilities in detail:
A re-occurring restriction in the suction piping that may or may not be causing a cavitation problem within the pump.
- A foot valve or any valve in the suction piping is sticking.
- Something is occasionally plugging up the suction piping. If the pump suction is coming from a river, pond or the ocean, grass is a strong possibility.
- A loose rag is another common cause.
- A collapsed pipe liner will restrict the piping at higher velocities.
- The pump suction is being throttled instead of the discharge to prevent heating of the process fluid. This can happen with some volatile fuel applications.
- A filter or strainer is gradually clogging up.
- Air is being introduced into the suction side of the pump to reduce the capacity. This is sometimes done with low specific gravity fluids to avoid throttling the discharge that might overheat and flash the product.
Intermittent incidents that cause cavitation problems
- The suction tank vent partially freezes in cold weather.
- The sun is heating the suction piping; raising the product temperature close to its vapor point.
- The level in the suction tank increases, decreasing the differential head across the pump. This will increase the pump capacity until the level in the tank drops.
- The level in the open suction tank decreases causing vortex problems that allow air into the pump suction.
- Several pumps in the same sump are running, lowering the level too much.
- The suction tank float is stuck. It will sometimes show a higher level than you really have.
- A discharge recirculation line, piped to the pump suction, opens and heats the incoming liquid.
- Sometimes the suction lift is too high. The increase in pipe friction at higher capacities will reduce the suction head.
- The vapor pressure of the product is very close to atmospheric pressure. The pump cavitates every time it rains because of a drop in atmospheric pressure.
- The tank is being heated to de-aerate the fluid. Sometimes it is being heated too much.
- The process fluid specific gravity is changing. This can happen with a change in product operating temperature or if a cleaner or solvent is being flushed through the lines.
- A booster pump is malfunctioning or leaking excessively.
- The source tank is changing from a positive pressure to a vacuum due to the process. If the pump is taking its suction on a mixer this becomes a common problem because many mixers alternate between a positive and negative pressure.
- A packed valve in the suction piping is at a negative pressure and air is leaking in through the packing.
- The tank is being pumped dry.
A repetitive need for an increase in the pump’s capacity.
- A bypass line, or relief valve opens decreasing the discharge resistance and increasing the capacity.
- A break or leak in the line down stream of the pump will increase the capacity of the pump as the head drops.
- The pump is supplying many sources and too many valves are open at one time.
- The pump discharge is being directed to several different tank farm locations. The changing piping resistance is changing the pump’s head and capacity.
- You are trying to maintain a boiler level and the steam demand is changing. A ship answering bells is a good example of this problem.