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Pump Terminology & Affinity Laws

Most common pump terminology and abbreviations

QCapacity, actual pump delivery in gallons per minute or m³/hour
HHead, vertical height of a static column of liquid in feet or mwc
nPump speed in revolutions per minute (rpm)
η Efficiency in percentage
PPower required in kW or Hp
HpHorsepower(1 Hp= 0,745 kW kilowatts)
TDHTotal Dynamic Head = Sum of Static Height, Static Suction Lift and Friction Loss
Static HeightThe maximum height reached by the pipe after the pump - measured from the center line
Static Suction LiftTotal Dynamic Head = Sum of Static Height, Static Suction Lift and Friction Loss
Friction LossThe loss of pressure due to the flow of the liquid through pipes and fittings (resistance)
NPSHrNet Positive Suction Head Required in mwc or ft
NPSHaNet Positive Suction Head Available in mwc or ft
CavitationProcess in which small bubbles are formed and implode violently; occurs when NPSHa < NPSHr
BEPBest Efficiency Point
BSFCBrake Specific Fuel Consumption in g/kWh or lb/Hph
dB(A)Sound level in decibels (A) stands for the A-scale
VVelocity in ft/s or m/s
ρDensity in kg/m³ is the mass per unit volume of a substance

Pump affinity laws

The affinity laws for pumps express the relationship between the several variables involved in pump performance. They apply to all types of centrifugal pumps and axial flow pumps.

With the pump impeller diameter held constant:
  1. Capacity is proportional to the pump speed: Q1/Q2 = n1/n2
  2. Head is proportional to the square of the pump speed: H1/H2 = (n1/n2)²
  3. Power is proportional to the cube of the pump speed:  P1/P2 = (n1/n2)³
Pump affinity laws example
BBA Pumps model:BA150E D285
Reference pump speed: 1900 rpm
Duty point Q1 x H1:

 1600 gpm @ 90 ft

Proposed operational pump speed:  1500 rpm
Calculated ratio:

1500/1900 = 0.79

Q1 x 0.79 = Q2:

1600 gpm x 0.79 = 1264 gpm

H1 x (0.79)2 = H2:

 90 ft. x 0.62 = 56 ft.

P1 x (0.79)3 = P2

48 hp x 0.49 = 23.5 hp

Calculated duty point at 1500 rpm:

Q2 x H2 = 1264 gpm @ 56 ft.