What kind of problems would you anticipate when the suction

What kind of problems would you anticipate when the suction and delivery piping connections are not as per standard? (in centrifugal pump system)

Solution

It is understood that question is about suction and delivery piping arrangement not being as per good engineering practice.

The problems could be as follows:

1. Suction piping

a) If the pressure drop across suction piping is too high, static pressure at suction may reach saturated vapour pressure and hence there could be cavitation. This can be prevented by selecting proper suction pipe size.

b) If suction lift is too high (due to pump being located at higher elevation than liquid level) again cavitation may occur due to reasons mentioned in a) above. This can be prevented by locating the pump at a suitable level wrt the liquid level.

c) If bends or fittings are located very close to pump suction nozzle, it creates flow disturbance and may give rise to a vortex motion to the liquid at suction. This may affect the performance of the pump(flow, head etc) as the pump inlet velocity triangle gets affected due to the vortex motion. This may also give rise to cavitation due to possibility of local higher velocities, eddies, etc.

d) If the suction piping is not properly restrained this may give rise to undesirable piping vibration.

e) If the suction piping is not properly supported, additional loads may be transferred to pump nozzle, affecting the internal alignment and hence possibility of vibration and bearing damage. It may also affect internal clearances and pump performance may be affected. Rubbing damage may also occur between pump casing and impeller.

f) The natural frequence yo the suction piping system should be away from the pump running frequency to prevent resonant vibration.

g) The thickness of piping needs to be calcutated and selected as per code(ANSI) to ensure structural integrity of the piping.

h) For hot piping, stress analysis needs to be done to ensure safe thermal nozzle load on pump. The effect is similar to item e) above.

2. Discharge Piping

a) If the discharge piping is not properly designed and there are too many bends and fittings(resistances) then the required pump head goes up according to head-capacity curve and pump flow may be reduced. On the contrary if there are too less resistance in the discharge line, the required head goes down, increasing the pump flow rate. This may result in higher pump power consumption and motor overloading.

b) The points in 1 d), e), f) g) and h) are applicable to discharge piping also.

c) Since the velocity in pump discharge is kept higher than suction to reduce the pipe size for economic reasons, it is to be ensured that the maximum velocity is within the permissible limit from erosion and erosion-corrosion points of view. The life of piping may become limited due to corrosion. Also the flow acclerated corrosion(FAC) needs to be looked into depending on operating temperature.