closed discharge control valve can be calculated from:

Tr = [ (42.4 x HP) / (W x C) ]

where:

Tr = rate of temperature rise, deg F/min

HP= BHP at shut-off

W = weight of liquid in pump case, lbs

C = specific heat of liquid; use 1.0 for water, 0.5 for hydrocarbons if exact value is

unknown

This equation neglects the heat loss through the pump case so its result is

conservative. The weight of liquid can be calculated by estimating the case

volume or requesting for the case volume data from the pump manufacturer;

some manufacturers collect this data from hydrotest or from hydraulic drawings.

Based on the above rate of temperature rise the duration of safe operation at

shut-off can then be calculated by dividing the allowable temperature rise with the

rate of temperature rise.

limited to 50 degrees F maximum. A lower value of allowable temperature rise

should be considered if the 50 degree limit would result in exceeding the case

design temperature, or to an increase in vapor pressure that could result in critical

NPSH or to potentially damaging vaporization at mechanical seal face.

Example: For how long can a multi-stage pump be operated safely with the

discharge valve closed if the pump required 500 BHP at shut-off and the pump

case contained 200 lbs of water?

Solution: Calculate the rate of temperature rise

Tr = [(42.4 x 500)/(200x1)] = 106 degree/min

Calculate time required to limit temperature rise to 50 degrees F

T = 50/106 = 0.47 minute or 28 seconds

(75 PSIG), with 80 feet shaft length. We want to know its maximum safe operating

time with the discharge valve closed. The operator feel it is safe to run the unit for

five minutes but during this time the pump and motor experienced high vibration.

operating with the discharge valve closed. Based on the limited information given,

this pump is most likely an axial flow or propeller type, in which case, the required

BHP is usually highest at shut-off condition. Check that the motor is not being

over-loaded.

In many horizontal pumps the safe time would calculate to be around one minute.

The estimated 5-minute safe operating time seems high but then again we

assume that this is a vertical pump with the bowl assembly fully submerged and

the long drop column completely filled with liquid - so there is more dissipation of

heat. We have very limited information on vertical pumps - do our readers have

additional information they can share?

Temperature rise at shut-off operation

Warm-up procedure in high temperature service

Thermal shock

Conversion factors

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