SUBJECT: The pump works for a while and then loses suction 10-12
A couple of things you must keep in mind when troubleshooting centrifugal pump problems:
- The centrifugal pump always pumps the difference between the suction and discharge heads. If the suction head increases, the pump head will decrease to meet the system requirements. If the suction head decreases the pump head will increase to meet the system requirements.
- A centrifugal pump always pumps a combination of head and capacity. These two numbers multiplied together must remain a constant. In other words, if the head increases the capacity must decrease. Likewise if the head decreases, the capacity must increase.
- The pump will pump where the pump curve intersects the system curve.
- If the pump is not meeting the system curve requirements the problem could be in the pump, the suction side including the piping and source tank, or somewhere in the discharge system.
- Most pumps are oversized because of safety factors that were added at the time the pump was sized. This means that throttling is a normal condition in most plants, causing the pump to run on the left hand side of its curve.
Cavitation is a main cause of losing pump suction, but remember that there are several different types of cavitation:
- Vaporization of the liquid within the pump caused by a loss of suction head or an increase in suction temperature.
- The “vane passing syndrome” caused by too small an impeller to cutwater clearance.
- Too high a suction specific speed number will cause internal recirculation problems resulting in cavitation. The suction specific speed number is obtained from a formula that can be found in paper 9-12 of this series.
- Air ingestion on the suction side of the pump allows air and bubbles into the suction of the pump.
- Turbulence of the fluid that releases entrained gases into the suction piping.
Each of these cavitations has been addressed in other papers in this site In this paper 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 suction is being throttled 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 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, decreasing 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 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.
- 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 pumps capacity.
- A bypass line, or relief valve opens, decreasing the discharge resistance, 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.