Multiple Pumps Multiple Pumps The HDM and FHWA

Multiple Pumps

Multiple Pumps • The HDM and FHWA recommend more than one pump • Redundancy • Flexibility • Ability to manage flows changing with time

Multiple Pumps • Hydraulic reasons for multiple pumps • Single pump may not produce enough added head – pumps in series add head • Single pump may not produce enough discharge – pumps in parallel add discharge

Multiple Pumps • Two different kinds of pumps • Pump A cannot meet the needs of the system at any flow rate while Pump B supplies enough head over part of the system curve.

Multiple Pumps • Pump B and A in series • Add head at given Q Added operational range

Multiple Pumps • Pump B and A in parallel (common discharge line) • Add Q at given head Added discharge range

Multiple Pumps • Utility of parallel and series depends on the system curve and the discharge needs. • The parallel example is not useful for the given system curves, but a pair of parallel-series (4 pumps) might be!

Multiple Pumps • Pump B and A in series, 2 pair in parallel • Full range for system Added operational range

Multiple Pumps • Multiple pumps are probably typical • Extend hydraulic range • Provide redundancy • Switching to turn pumps on/off based on stagestorage considerations • Details of pump selection are beyond scope of this class, call DES for guidance. • Pump selection also must consider suction conditions

Suction Conditions • Pump curves report • Added head versus discharge. • Wire-to-water efficiency versus discharge. • Mechanical power versus discharge. • Net Positive Suction Head required versus discharge.

Suction Conditions • Net Positive Suction Head (NPSH-A) Available is the total head available to the impeller of the pump. • Comprised of head provided by the depth of water above/below the impeller and the absolute head, less the • Vapor head • Head losses on suction side of pump • Manufacturer provides a value called NPSH-R; the required minimum NPSH that must be supplied to the pump for it [the pump] to function.

Suction Conditions • NPSH computed from: Absolute pressure at liquid surface in suction pit Suction lift elevation of the liquid to the pump inlet eye (<0 if impeller submerged) Frictional head loss in inlet piping Absolute vapor pressure at liquid pumping temperature

Suction Conditions • The most common cause of pumping failure is poor suction conditions. • A centrifugal pump cannot lift water unless it is primed, or the first stage impellers are located below the static hydraulic grade line in the suction pit at pump start-up. • Liquid must enter the pump eye under pressure; this pressure is called the Net Positive Suction Head available (NPSHa).

Suction Conditions • The manufacturer supplies a value for the minimum pressure the pump needs to operate. • This pressure is the Net Positive Suction Head required (NPSHr). • For a system to work: NPSHa> NPSHr over all operating conditions, including start-up and shutdown. • While these considerations are important to a designer, the final selection should use the pump supplier’s knowledge base.

Suction Conditions • Illustrative Example

Suction Conditions • Illustrative Example

Suction Conditions • Illustrative Example

Suction Conditions • Illustrative Example

Suction Conditions • Illustrative Example

Summary (Lift Stations, Pumps) • Used to lift water from low points into outfalls where gravity flow can continue • Require pumps, and hence available power supply • Frictional losses are shown on system curves, usually for several assumptions of frictional loss coefficients. • Suction conditions matter, NPSH is important in the station design. • Storage matters, used to start and stop pumps • Multiple pumps typical