Pumps Suction Piping – Eccentric Reducers & Straight Lengths

The design of pump suction / inlet piping defines the resulting hydraulic conditions experienced at the pump inlet / impeller. If the design fails to produce a uniform velocity distribution profile at the pump inlet many pump problems and failures can be traced. For example,

• Noisy operation, turbulence and friction losses.
• Random axial load oscillations.
• Premature bearing or seal failure.
• Insufficient fluid pressure leading to cavitation.
• Occasional damage on the discharge side due to liquid separation.

Any of these issues could lead to pump failure. Pump suction / inlet piping design includes the selection of reducer fitting type and minimum straight length requirement.

• For more details on Pumps, check out Introduction to Pumps
• For more details on Cavitation in Pump, check out Introduction to Cavitation
• For more details on Reducer, check out Reducers in Process Piping

Pump Suction Pipe Size

It’s standard practice to employ suction-side piping one or two sizes bigger than the pump inlet. A reducer fitting is typically used in pump suction piping to reduce the size of the suction pipe to match the size of the pump suction end flange. A reducer is a constriction and requires careful design to avoid both turbulence and the creation of pockets where air or vapour might collect. The best solution is to use an eccentric reducer orientated to eliminate the possibility of air pockets.

Then why don’t we use a pipe of same size as that of suction nozzle??

Because we want minimum frictional losses in pipe and full bore flow to the impeller eye. For the same flow rate, if we increase the line size, fluid velocity as well as frictional losses decreases. Greater the area of pipe, lower is the velocity, lower frictional loses (higher NPSHa), lower is differential pressure generated by the pump and there by lower power required by motor to drive the pump.

Suction pipe size must never be smaller than the pump’s inlet nozzle size.

Suction pipe size smaller than the pump’s inlet size will increase the frictional losses that will further increase the power required by motor to drive the pump. Also the flow being reached at the pump inlet or impeller will not be of uniform velocity profile causing various issues discussed above.

The right pump suction pipe size is a compromise between cost (bigger pipes are more expensive) and excessive friction loss (small pipes cause high friction loss and will affect the pump performance).

Eccentric Reducers Installation

Eccentric reducer is recommended for horizontal flow to the pump. This configuration prevents air pocket accumulation at the upstream end of the reducer. Concentric reducer is recommended for vertical inlet (suction) piping or horizontal installations where there is no potential for air vapor accumulation.

When the source of supply is above the pump, then the eccentric reducers must be placed with the flat side down. When the source of supply is below the pump, then the eccentric reducers must be placed with the flat side up.

In case of long horizontal pipe runs, air pockets are avoided by installing the eccentric reducer with the flat side up.

Straight Length Requirement

Pumps, and especially centrifugal pumps, work most smoothly and efficiently when the fluid is delivered in a surge-free, smooth, laminar flow. Any form of turbulence reduces efficiency and increases wear and tear on the pump’s bearings, seals and other components.

ANSI/HI 9.8 American National Standard for Pump Intake Design (P21, 1998) states, “There shall be no flow disturbing fittings (such as partially open valves, tees, short radius elbows, etc.) closer than five suction pipe diameters from the pump. Fully open, non-flow disturbing valves, vaned elbows and reducers are not considered flow disturbing fittings.” This standard eliminates any reference to the possible flow distribution that could be generated by the reducer.

The concept is simple though, ensure stable and uniform flow onto the impeller eye. This results in fewer pump failures over the life of the pump due to vibration caused by flow induced turbulence.

In case several improperly specified parameters come into the equation (e.g. viscosity changes etc), then it would be prudent to install as many as ten suction pipe diameters of straight piping next to the reducer inlet flange. A number ranging between five (5) to ten (10) suction pipe diameters of straight pipe run is typically the recommended value in published technical literature.

Sometimes due to space constraints, it’s just not possible to make provision for a sufficient settling distance in the pipework before the pump. In these cases, use an inline flow conditioner or straightener.

More Points to be Taken Care while Designing Pump Suction Piping

• Feed pipe must be fully submerged in the tank or vessel. If it’s too close to the surface of the fluid, the suction creates a vortex, drawing air (or other vapors) into the liquid and through the pumping system.
• Feed pipe must not be too close to the bottom of the tank or vessel. If it’s too close to the bottom of fluid, the suction may draw up solids or sludge. This situation can be improved by use of strainer in pump suction piping. As a drawback strainers can create a large pressure drop and be responsible for cavitation and friction-loss.
• All piping, valves and associated fittings should be independently supported, so as to place no strain on the pump housing. Also, forces and moments imposed on the pump nozzles do not exceed the allowable values specified by the vendor.
• Pipe connecting to the pump’s inlet flange must be aligned precisely with it before the bolts are tightened.
• Pump shall be located as close to the source of suction in order to minimize pressure drop in the system.
• Pump shall be placed in such a manner that the suction nozzle elevation is always below the vessel or tank nozzle elevation and suction pipe shall be routed to prevent any pockets in the line.
• As per OSID- 118 (Oil industries Safety Directorate stipulation) there shall be minimum 1 meter spacing between pumps e. g a minimum space of 1 meter must be provided in between the pumps and any potential obstructions (large block valves, steam turbine piping and tee type support from grade.)

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