SUBJECT : Heat, how it affects the pump and mechanical seal. 1-4

Every day salesmen call on customers and make claims that their pump, or mechanical seal can take more heat than the other guys. Before we rush out to purchase these wonder products, we should take a closer look at the heat problem.

The heat comes from several sources:

The heat will affect you in several ways. It can :

We'll look at each of these areas in detail, and at the end of this paper make some recommendations to improve both your pump and seal life.

WHERE THE HEAT COMES FROM:

HEAT GENERATED AT THE SEAL FACES :

The following numbers are typical of the conditions in a stuffing box when you are sealing with a conventional original equipment, unbalanced seal.

OPERATING CONDITION
INCH SIZE
METRIC SIZE

Stuffing box pressure

100 psi
10 kg/cm2

Seal face diameter

2 inches
50 mm

Seal face area

1 inch2
6,5 mm2

Seal spring load

30 psi

2,0 kg/cm2

Face load from the spring

30 lbs.
13 kg

Shaft speed

3600 rpm
2900 rpm

Stuffing box volume

1 pint of water
500 cc of water

Face coefficient of friction

0.2 average
0,2 average

We will make the first calculation in the inch size:

 

 

Hydraulic closing force = 100 lbs/in2 * 1 in2 = 100 lbs

Hydraulic Opening force = An average of 50 psi on the faces * 1 in2 = 50 lbs.

100 lbs closing + 30 lbs Spring force - 50 lbs opening = 80 lbs closing

80 lbs * 0.2 * 1885 F.P.M. = 30160 Ft lbs./ min

778 ft lbs. / min. = 1 Btu..

30160 / 778 = 38.8 Btu../min.

38.8 Btu../ min would raise 1 pint of water 38.8 degrees Fahrenheit each minute, so we would have to flush in 38.8 pints (4.84 gallons per minute) of cooling water if we did not want the product to get hot.

Metric looks like this :

A Newton Meter is a Joule so we have 690 Joules/ sec.

690 Joules/Sec.* 60 Sec./Min. = 41,400 Joules per minute.

41,400 * 0.239 joules per calorie = 9,895 calories (9,9 Kilo Calories) per minute.

9.9 Kilo calories per minute would raise 9,9 liters of water one degree Centigrade per minute.

Since we have only one half a liter (500 cc ) in the stuffing box, we would have to flush in 9,9 * 2 or 19,8 liters / minute to prevent a temperature rise in the stuffing box.

The amount of heat generated by a properly installed balanced mechanical seal is insignificant.

The amount of heat generated by packing varies with the type of packing and the individual packing the pump. On the average you will find that packing generates six times the heat of a balanced mechanical seal.

 

HEAT GENERATED BY FRICTION WITHIN THE PUMP

No pump is 100% efficient. If a pump is rated 60% efficient, that means that 40% of the power is being converted to heat. In a normal temperature stabilized pump, running at its best efficiency point, (B.E.P.) the temperature rise within the pump is calculated from the following formulas :

A temperature rise of 18° F across the pump or 10° Centigrade is considered excessive. This can occur if the pump is run with a shut or excessively throttled discharged valve.

If you would like to calculate the temperature rise of the liquid in a running pump when the discharge is shut, use the following formula:

HEAT FROM THE AMBIENT CONDITIONS

HEAT IN THE PRODUCT ITS SELF

HEAT GENERATED BY PARTS RUBBING TOGETHER

HEAT GENERATED BY THE BEARING SEALS

WHAT AFFECT CAN ADDITIONAL HEAT HAVE ON THE LIQUID IN THE PUMP?

THE CORROSION RATE OF THE LIQUID WILL INCREASE :

CRITICAL TOLERANCES WILL CHANGE.

SOME MECHANICAL SEAL FACES CAN BE DESTROYED.

ELASTOMER (THE RUBBER PART) LIFE CAN BE DRASTICALLY SHORTENED

THE PRODUCT CAN CHANGE FROM A LIQUID TO EITHER A SOLID OR A GAS.

PIPE STRAIN

THE WASTING OF COSTLY ENERGY.

CAVITATION

CHANGING THE VISCOSITY OF THE BEARING OIL

RECOMMENDATIONS TO LOWER THE AMOUNT OF HEAT BEING GENERATED WITHIN THE PUMP.

PUMP SHAFT PACKING

THE MECHANICAL SEAL.

BEARINGS

AMBIENT HEAT

OTHER HEAT SOURCES

PUMP MODIFICATIONS THAT WILL EITHER LOWER THE AMOUNT OF HEAT BEING GENERATED OR LESSEN THE AFFECT OF THIS HEAT.

CONCLUSIONS

Excessive heat causes seal and bearing problems. Since the heat can increase corrosion, destroy seal faces, vaporize the fluid, coke the oil, solidify some liquids and crystallize others, change critical tolerances, attack the elastomers, increase the bearing squeeze, cause misalignment and pipe strain, etc, it would be ridiculous to try to build a mechanical seal, or bearing capable of operating in excessive heat.

Most claims for high temperature seals address the problem of elastomers and ignore those other factors that we have discussed in detail. This explains the popularity of the high temperature bellows seal that must be cooled in all high temperature petroleum applications. There is no magic, but there is a sensible approach.

Do as many of those things we have discussed in the above paragraphs and if you find that you still have trouble, try to find some logical method of getting additional cooling to the seal and bearing oil. We discussed a lot of those options in the above paragraphs.

Heat is always a problem, but now you have the tools to fight back.

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