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H P In ReliabilityHeinz P. Bloch, Reliability/Equipment Editor

Eliminating cooling water from pumps

HYDROCARBON PROCESSING / AUGUST 1996 17

Extensive experimentation with removing

cooling water from pumps and general-purpose

turbine drivers in large petrochemical plants

indicates that machinery reliabil-

ity may increase. The obvious sav-

ings in capital expenditures for

piping and water-treatment facili-

ties, and savings in operating cost

alone, provide good incentives to

take a closer look at this topic.

Discontinue pedestal cooling. It has been shown

conclusively over many years that pedestal cool-

ing is not required for any centrifugal pump gen-

erally found in pertochemical plants. Pumping

services with fluid temperatures as high as 740F

(393C) require nothing more than hot alignment

verification between driver and pump. Risk of

pump fires due to cooling water-induced corro-

sion, and subsequent pedestal collapse has been

eliminated by numerous companies that accepted

this experience-based recommendation decades

ago.

Mechanical seal cooling alternatives. Pump

stuffing-box jacket cooling, while reducing heat

migration from the pump casing toward the bear-

ing housing, will not lower the temperature in

the seal environment. A changeover to high-tem-

perature mechanical seals may be possible and is

preferred by U.S. plants. If mechanical seals need

cooling because the flush liquid has a low boiling

point, the least troublesome way to control seal

temperatures may be to circulate a coolant such

a water, steam, or cool flushing oil through an

external seal flush cooler. However, many hotservices may be ideally suited for a maintenance-

free dead-ended flush arrangement. This option

may become entirely feasible if narrow-face seals

or suitable seal housing internal geometries are

selected.

Bearing cooling not usually needed. Cooling

water can be deleted from many sleeve bearings

on centrifugal pumps and on small turbine drivers

after experimentally verifying that oil sump tem-

peratures do not exceed 180F (82C). This limit

was found to be extremely conservative from a

bearing-life point of view. If it is exceeded by a

few degrees, more frequent oil sampling or syn-

thetic lubricants will help.

Since most general-purpose machinery is

equipped with antifriction bearings, significant

gnitanimilemorftlusernacstidercecnanetniam

cooling water from antifriction bearings on pumps

and small steam turbines. Experience shows

that equipment life can actually be extended by

removing cooling water from bearings. Cooling

bearing oil sumps invites moisture condensation,

and bearings will fail much more readily if the oil

is contaminated by water. Laboratory tests show

that even trace amounts of water in the lube oil

are highly detrimental.

Hydrogen embrittlement on the steel granular

structure can reduce expected bearing life to less

than one-fifth normal or rated values. Another

reason for not cooling the bearing housing of

pumps and drivers is to maintain proper bear-

ing internal clearances. Hot-service pump bear-

ings have often failed immediately after startup

when the bearing housings were cooled by water.When it was recognized that high temperature

gradients were responsible for reducing bearing

clearances to unacceptably low values, a heating

medium was introduced into the bearing bracket

to heat the housing. The problem was solved.

Parameters influencing bearing cooling. Mini-

mum permissible viscosity of ball-bearing lube

oils at the bearing operating temperature is a

function of bearing size and speed. As a rule of

thumb and valid for most bearings operating in

typical centrifugal pumps, rated bearing life will

be obtained if metal temperatures of operatingbearings remain low enough to ensure minimum

viscosities of 150 SUS (32.1cSt) for spherical

roller bearings in thrust-loaded services, 100

SUS (20.6 cSt) for radially loaded spherical roller

bearings, and 70 SUS (13.1 cSt) for ball and

cylindrical roller bearings. If the viscosities drop

below the given values, the oil film may have

insufficient adhesion or strength, and metal-to-

metal contact could result.

It is safe to assume that standard antifriction

bearings will show no loss of life as long as

metal temperatures do not exceed 250F (121C).

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H P In Reliability. . .

Maintaining oil temperatures within given limits is thus

aimed at satisfying only two requirements:

1. Oil viscosities must remain sufciently high to ade-

quately coat the rolling elements under the most adverse

operating temperature.

2. Oil additives, such as oxidation inhibitors, must not

be boiled off, i.e., adequate service life of the lubricant

must be maintained.

A properly formulated diester or PAO-based synthetic

lubricant would be ideally suited in this case.

BIBLIOGRAPHY

Excerpted from the authors text Improving Machinery Reliability, Second Edi-

tion, Gulf Publishing Company, Houston, Texas, 1988.

The author is a consulting engineer in Montgomery, Texas.

He advises modern process plants worldwide on reliability

improvement and maintenance cost reduction opportuni-

ties.