Pump Definitions and
Nomenclature:
o
Head
(h) [H]– Head is the expression of the
energy content of a liquid in reference to an arbitrary datum. It is
expressed in units of energy per unit weight of liquid. The measuring
unit for head is meters (feet) of liquid.
H=
P/Density×g
H
:m, P: Pa, Density: kg/m3
o
Total
head (H) [Htx]– This is the measure of energy
increase, per unit weight of liquid, imparted to the liquid by the pump, and is
the difference between total discharge head and total suction head. This
is the head normally specified for pumping applications because the complete
characteristics of a system determine the total head required.
o
Rate
of flow [Q]– The rate of flow of a pump is the
total volume throughput per unit of time at suction conditions. The term
capacity is also used.
o
Best
Efficiency Point (BEP)– The rate
of flow and total head at which the pump efficiency is maximum at a given speed
and impeller diameter.
o
Displacement
(D)– For a positive displacement pump
it is the theoretical volume per revolution of the pump shaft.
Calculation methods and terminology may differ between different types of
positive displacement pumps.
o
Net
Positive Suction Head Available (NPSHA)–
NPSHA is determined by the conditions of the installation and is the total
suction head of liquid absolute, determined at the first-stage impeller datum
minus the absolute vapor pressure in meters (feet) of the liquid at a specific
rate of flow expressed in meters (feet) of liquid. Note that for positive
displacement pumps the term Net Positive Inlet Pressure Available (NPIPA) is
used and is expressed in pressure absolute kPa (psi).
o
Net
Positive Suction Head Required (NPSHR)–
NPSHR is the minimum NPSH given by the manufacturer/supplier for a pump
achieving a specified performance at the specified capacity, speed, and pumped
liquid. Note that occurrence of visible cavitation, increase of noise and
vibration due to cavitation, beginning of head or efficiency drop, and
cavitation erosion can occur when margin above NPSHr is present. Note
that for positive displacement pumps the term Net Positive Inlet Pressure
Required (NPIPR) is expressed in pressure absolute kPa (psi).
o
Net
Positive Suction Head 3% (NPSH3)–
For rotodynamic pumps NPSH3 is defined as the value of NPSHR at which the
first-stage total head drops by 3% due to cavitation. This is determined
by the vendor by testing with water
o
Suction
specific speed (S)– Suction specific speed is an index
of pump suction operating characteristics. It is determined at the BEP rate of
flow with the maximum diameter impeller. Suction specific speed is an indicator
of the net positive suction head required [NPSH3] for given values of capacity
and also provides an assessment of a pump's susceptibility to internal
recirculation. Suction specific speed is expressed by the following
equation:
Source: API 610 11th Edition, Annex A
o
Impeller - The bladed member of a rotating assembly of the pump
which imparts the prinicpal force to the liquid pumped.
o
Casing - The portion of the pump
that includes the impeller chamber and volute diffuser
o
Volute - The pump casing for a centrifugal type of pump,
typically spiral or circular in shape.
o
Diffuser - A piece, adjacent to the impeller exit, which has
multiple passages of increasing area for converting velocity to pressure
Source: http://www.pumps.org/Pump_Fundamentals/Pump_Terms_and_Definitions.aspx
Some misconceptions about desirable construction features of centrifugal pumps
