Urban Storm Drain Design: 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 stage-

storage 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:

€

NPSHa = Habs −Hs −H f −Hvp

Absolute pressure at liquid surface in suction pit

Absolute vapor pressure at liquid pumping temperature

Frictional head loss in inlet pipingSuction lift elevation of

the liquid to the pump inlet eye (<0 if impeller submerged)

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 shut-down. • 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