Centrifugal pumps are not ideal machines - they are not 100% energy efficient.
The energy wasted due to pump inefficiency is converted into heat most of
which is absorbed by the pumped liquid. As a result the liquid outlet temperature
will be higher than its inlet temperature.

The temperature rise can be estimated by using one of the following equations
depending on the available data:

Equation 1:

T = [H / (778xC)] x [(1/E) - 1]

where:

T = temperature rise, in degrees Fahrenheit
H = total differential head, in Feet of liquid
C = specific heat of liquid
E = pump efficiency at the given flow rate, in decimal point
The number 778 is a unit conversion factor.

Or use, Equation 2:

T = [2545 x (BHP-WHP)] / [(QxC)]

where:

T = temperature rise, in degrees F
BHP = brake horsepower
WHP = water horsepower (*)
Q = flow rate, in pounds per hour
C = specific heat of liquid
The number 2545 is a unit conversion factor.

(*) The term water horsepower is used but Equation 2 applies to any type of
liquid provided its specific gravity is applied in the formula.

Use C=1.0 for water, C=0.5 for hydrocarbons if the actual value is unknown

Equation 1 is used when head is known and flow rate is unknown.
Equation 2 is used when flow rate is known and head is unknown.

These equations apply to a pump with an opened discharge nozzle and the
liquid is flowing out - they do not apply if the discharge nozzle is fully closed or
deadheaded. (This is discussed in another article.)

The calculated temperature rise is conservative because the equations are
based on simplistic assumption that all the wasted energy is absorbed by the
liquid in the form of heat. In actuality some heat is dissipated into the
atmosphere by convection or radiation through the pump casing. And the
frictional heat generated at the thrust bearings is not absorbed by the liquid.

[In an actual field study.....]

Pumps with an operating low flow recirculation by-pass line may have higher
liquid temperature rise if the by-pass flow is piped back directly into the pump
suction nozzle because heat is re-absorbed during each return cycle. This is
analogous to the rise in temperature of well water in a test well when a pump is
being tested.

Effects of liquid temperature rise

In most instances the rise in liquid temperature is minimal and will not affect the
pump performance, or the liquid properties. But in some instances - such as in
pumps with high energy density, pumps handling liquids with low specific heat, or
pumps operating at drastically reduced flow rates - the temperature rise can be
significant and can cause harmful effects, such as:

The outlet temperature may exceed the allowable maximum temperature for the
casing, or its flange rating.

It may increase the liquid vapor pressure and reduce the net positive suction
head available (NPSHA).

The increase in vapor pressure may require a higher vapor suppression
pressure at the mechanical seal chamber.

The increase in vapor pressure may make the pump more sensitive to internal
low flow recirculation.

Higher temperature may alter the liquid properties such as its viscosity, or its
tendency to emulsify.

General rules-of-thumb:

It is typical to limit the temperature rise from [ ] degrees Fahrenheit for safe
operation. This guideline was first established for boiler feed service - it may be
a conservative value for cold water application but may be excessive for other
service such as in cryogenic, or in application where there is low vapor
suppression pressure in the seal chamber and the NPSH margin is tight.

This recommended temperature rise limit is normally used to determine the
thermal minimum flow for the pump.

In an upset low flow condition, allow a minimum flow of [ ] GPM for every [ ] BHP
at shut-off to prevent excessive liquid temperature rise.

A low flow recirculation line should be piped back farther upstream of the pump,
or through a holding tank if feasible, so that heat can dissipate before the liquid
returns to the suction nozzle.

The liquid outlet temperature should not exceed the maximum allowable
temperature of the casing or nozzle flanges for a given pressure. Be aware that
the nozzle flanges may have standard pressure-temperature ratings that may be
lower than that of the casing - always check the ratings of both the casing and
the flanges separately.

It is a good practice to check that the pressure-temperature (PT) rating of the
case gasket is not exceeded due to any temperature rise of the liquid.

Reducing the temperature rise

In most applications the liquid temperature rise at normal pump operation is
acceptable. In the rare instance where it is deemed excessive the following
actions can be taken to reduce the temperature rise:

Select a new pump, or hydraulically re-rate an existing pump, to get the highest
efficiency at the operating flow rate.

Consider cooling the pump casing by adding cooling jackets around the suction
and discharge nozzles, or at some convenient areas around the casing, and
circulating some cooling water, or liquid.

Install the pump indoor, or provide protective shelter around the pump station.
(Direct sun exposure, especially during severe summer season, contributes to
the temperature rise of the pumped liquid.)


Example calculations


Example 1:  A multistage water injection pump has a total differential head of
5000 FT and a rated efficiency of 80%. What will be the temperature rise of the
liquid at the pump discharge nozzle?

Solution:

T = [5000 / (778x1)] x [(1/0.80) - 1] = 1.6 degrees F


Example 2:  A boiler-feed unit is pumping 50,000 pounds per hour of feed water.
The pump has 70% efficiency at rated flow and requires 20 BHP. What is the
estimated temperature rise of the feed water?

Solution: The WHP is BHP x Efficiency, or WHP = 20 x 0.70 = 14 HP

T = [(2545x{20-14}) / (50,000x1)] = 0.31 degrees F
CENTRIFUGALPUMP.COM
Temperature rise in pumped liquid

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