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PUMP HEADUnderstanding pump head and NPSH

201022503

200722120

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Contents 1. Pump head Head Suction head, Discharge head

Total head

Friction head

Operating point

2. NPSH

Cavitation

NPSH

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Head

The height at which a pump can raise water up.

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Head

The height at which a pump can raise water up.

Head is affected by suction condition.

: Head is proportional to suction level

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Head

Capacity to do a job. (Compared in the same suction level)

Independent of the type of fluid.

(When viscosity is relatively low and similar to water)

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Head

H = 2.31 p SG

H : head(ft)

P : pressure(psi)

SG : Specific gravity of fluid being pumped

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Suction head, Discharge head

Suction head

: vertical distance between the pump datum and the

liquid surface in suction well.

Discharge head

: vertical distance between the pump datum point and

the liquid surface in the receiving tank. (or Discharge

pipe)

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Suction head, Discharge head

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Total head

Vertical distance between the level of the source of the

supply and the surface of the discharge liquid.

Manufacturers use this term to show the performance of

their product.

Total head = Discharge headSuction head

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Total head

When the liquid level is below the pump datum, then

suction head has a negative value.

In the case, suction head is also called suction lift.

Total head = Hd + Hs

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Friction headloss of head that occurs in pipe flow due to viscous effect.

No flow With flow

Total head = HdHs + Friction head

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Operating point

Performance curve: total head vs flow

System curve: total of friction head losses vs flow

Operating point: The point where two curves intersect.

: Pump cannot work without adjusting

system.

: Pump capacity is bigger than system

requirement

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Cavitation

Formation of gas bubbles

in a flowing liquid.

Occurs when the pressurein the pump inlet drops

below the vapor pressure

of the liquid.

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Cavitation

Loud noise

Loss of capacity

Damage

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NPSH

Net Positive Suction Head.

Must be calculated before using pump to prevent

cavitation.

NPSHa and NPSHr

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NPSHr and NPSHa

NPSHr

Required

Minimum pressurerequired at the suction

port of the pump to keep

the pump from cavitating.

Depends on the pump.

Given.

NPSHa

Available

Absolute pressure at thesuction port of the pump.

Depends on the system.

CALCULATE.

NPSHa > NPSHr 1.3

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NPSHa : calculation

NPSHa = Ha HzHfHvp

Term Definition

Ha Absolute pressure on the surface of the liquid in the supply tank

Hz Vertical distance between the surface of the liquid in the supply tank

and the centerline of the pump

Hf Friction losses in the suction piping(Values can be found at the friction chart)

Hvp Absolute vapor pressure of the liquid at the pumping temperature

(Liquid types and temperature of the system should be considered)

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NPSHa : example & selection of pump

GivenHa : 10m

Hz : 3m

Hvp : 0.2m

Hf : 0.5m

NPSHa = 10+3-0.2-0.5

= 6.3

6.3 / 1.3 = 4.85

Thus, we need a pump withthe NPSHr smaller than 4.85