Failure of mechanical seals in centrifugal pumps

Failure of mechanical seals in centrifugal pumps This fourth paper in the troubleshooting series deals with mechanical seals. Often a seal failure is diagnosed through visual inspection of the failed seal’s components; this may yield the condition that existed in the seal cavity prior to, or at the time of, failure. The events that led to this condition are usually linked to the real cause of failure, which often requires us to move beyond the vicinity of the seal itself in our search for cause of failure. The extensive nature of this topic does not allow full coverage of all aspects of diagnosis, will deal with the prime areas associated but this first of three discussions by with seal failure. His next article will address some specifics in more detail.

S

ince

we

are

mechanical over

at least part

reasonable

seal

normally

life.

achieved

temperature

here

with

liquid

of the face contact For

single

through

within

parameters, (Figure

dealing

such as those

1), also impact

seal

associated

the barrier

with

fluid, which

assumed

that

compatible

pressure

is and

ratio

of a fluid film at

For the purposes

the inner

the seal faces and components

below 2. The

and

must

fluid

that

in the

it can absorb

heat,

that at which fluid

lates, which

allow

higher liquid which faces cavity

liquid

a clean pressure

can

to

associated with the seal balance ratio.

the

conducted

heat

the

accumulation

and allows

point

(point

requires

that prevents

for adequate

within

seal face

Generally

and less than

while

a range being

damage

speaking,

32 cSt would

that

the

shaft

this heat input the seal cavity

maintained

a temperature

at

temperature

and seal

to as the

at which ‘fluid

the fluid will vaporize.

temperature

temperature

margin

as a general

‘rule of thumb’

a viscosity

start-up greater

viscosity

There

are two

(face

break-

temperature

than

14 cSt

l

Cooling

l

Increasing

principal

the fluid being the

erature

margin

cations,

vaporize,

temp-

of the above

two

the

heat

at the seal faces (as can

the total

generation),

for in non-cooled increased

which

These

of the fluid in the seal cavity

1, where

seal face pressure a compromise

discharge

cooling

flush

in seal cavity

method

should

variables

seal cavity

flush

flow

increase

flush flow or increased

0262 1762/02/$

the fluid

cool and is well

it will

the heat generated

called

on

variables.

the seak cavity. so that

is relatively

at which

the pressure

a moderate

decision

adequate

are necessary.)

where

against

it enters

is not an issue and neither

affects

sometimes

June 2002

margin

is raised.

pumped

temperature

increasing

directly

PUMPS

fluid vary;

temperature

in the seal cavity

be seen from Equation

WORLD

required

of achieving

fluid before

temperature

will also increase

20

a minimum

methods

the pressure

below

Since

I302 - Ri*

The

margin:

approaches

ratio =

the

be maintained.

the flushing

(Where

Balance

margin’.

below

This is referred

will vary as the fluid characteristics

of 15 “C (27 “F) should

vaporization

Ro* - Rb*

fluid must be

substantially

that allows for adequate at a low enough

be ideal.

_I-.I.~._.-__._

when

margin

To accommodate

seal

at the

of the fluid in the seal

during

Shaft sleeve

face damage

face lubrication

of

at a temperature

by the pump

of particu-

with.

generated

that the viscosity

is maintained

prevent

away).

seal faces)

at a temperature

running.

(in hot applications).

seal face lubrication, Figure 1. Parameters

ingress

in the seal cavity,

accommodate

and the heat

at the

at the

be maintained,

It is also imperative cavity

liquid

be and

the seal faces.

Fluid temperature Maintaining

must heat

it will vaporize.

not

may abrade

at start-up

seal cavity conducted

and still remain

3. The fluid must be at a viscosity

it is

are chemically

in contact

as follows:

of the

such

seal face generated

of

of discussion

condition

maintained

geometric

and temperature

fills the space between

with the fluid they come

1. The

this

seal balance

on the maintenance

seal, is important.

to achieve

seals

Other

the seal faces. For dual seals the pressure outer

area

of liquid

cavity.

may be summarized

a liquid film

mechanical

control

the

This

lubricated

seals the seal faces must have

is balanced pressure.

predominate,

pressure,

is appli-

The

cooling

rests with

a few

are:

see front matter 0 2002 Elsevler Science Ltd. All rights reserved

1. Degree

of pressure

increase

required temperature

margin.

required

to achieve

the Discharge flush from pump discharge

2. The specific gravity of the fluid. 3. The specific heat of the fluid. 4. The specific

volume

,.

ratio of the gas versus the liquid

:

)

I,

.,

II

I

r,

\

(volume of 1 g of gas versus 1 g of liquid). 5. The net positive

suction head margin (NPSHM)

of the

fluid at pump suction. 6. The lubricity of the fluid. The approach

may he approximated

by considering

only

items 1 and 5 above. If a high pressure increase is required in the seal cavity to obtain an acceptable temperature margin and the NPSH margin is very low, the preferred approach

API plan

11

\

(PiI

would normally be to increase the rate of discharge flush to the seal cavity. Where NPSH margin is more moderate and

will be pressurized,

an adequate

ended seal cavity). The pump needs to be reviewed for the

temperature

more moderate

margin can be achieved

with a

increase in seal cavity pressure, the preferred

extent

if the seal is left without a flush (dead-

of hydraulic

balance

designed

into the impeller

in

facilities are proviJed

to

approach would normally be to increase the fluid pressure in

single stage pumps, or if adequate

the seal cavity. Where the situation is less clear than this the

relieve the seal cavity from the effect of the pump discharge

other variables help in the overall decision.

pressure in multistage

Increasing normally

the pressure of the fluid in the seal cavity achieved

through

the

installation

is

of a close-

Typical mechanical mechanical

Figure 2. Recirculation from pump discharge to the seal, which usually occurs

through a flow control

orifice.

pumps.

design areas, which affect the choice of

seal and/or the seal flush plan, are:

fitting throat bushing (refer to Figure 2). Seal face pressure = Spring pressure + (Seal cavity pressure - Seal face ID pressure)*(Balance

ratio - Seal face pressure

factor)

The shaft flexibility in single stage overhung pumps, where excessive shaft deflection under low flow conditions in volute pumps (other than concentric

volute)

would create a loss of perpendicularity

the seal

between

faces. Diffuser design pumps do not suffer appreciably [Equation

Pf = Psp + [PO - Pi][B - K]

l]

from excessive radial thrust, regardless of flow rate.

Where: Pi = seal face pressure; Psp = seal face spring pressure;

A single volute pump will experience

PO = seal cavity pressure;

flection under low flow conditions

Pi = seal face inner diameter

pressure; B = seal balance ratio (derived

in Figure 1); and

K = seal face pressure factor.

pump. For this reason pumps of single volute

design,

above 30 HP (22.4 kW), should be reviewed

for the

operating range and consideration A review of Figure 2 shows the locations

of PO and Pi.

greater shaft de-

than a double volute

given to recirculation

flow control to maintain flow at or above a minimum.

To arrive at the true cause of seal failure it is usually necessary

The pump orientation,

to perform one or more of the following tasks, any of which

the need to vent the seal cavity prior to start-up. Vertical

may reveal an important

pumps, particularly after a lengthy period out of service or

concern

that may have been over-

looked when choosing the seal and its configuration.

horizontal or vertical, can impact

after a repair should always have the seal cavity vented before start-up, although

1. Review the pump’s design.

this may also be required for a

horizontal pump, where a throat bushing is installed.

2. Review the piping system and vessels. 3. Define the fluid characteristics. 4. Understand

the pump’s operating

Vertical envelope

and process

transients.

motor bearing

indline pumps through support

are very often coupled

a solid type of coupling, system designed

to the

without

any

into the pump. This

5. Review the pump’s start-up procedure.

requires that the motor bearings carry a major portion of

6. Review the pump’s repair procedures. 7. Review the mechanical seal design.

the radial load and all of the thrust load experienced by the pump. As with horizontal pumps, the shaft will tend

8. Review the mechanical

to deflect under low flow conditions.

seal flushing arrangement.

A brief review of the first three potential

areas of concern

Multistage

pumps should be reviewed for adequate press-

follows.

ure relief at the higher pressure seal cavity, where either a

Review the pump’s design

balance device designed into the pump, the low pressure

This is primarily

this pressure to close to a little above suction pressure, or

mate percentage

necessary

to determine

to what approxi-

of pump discharge pressure the seal cavity

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side of which is returned

to the pump suction, controls

the seal cavity is relieved by other means.

WORLD

PUMPS

June 2002

I2l

suction vessel that contains liquid at

pump operation.

a boiling point may be susceptible

will impact

to

such an occurrence.

While

this pressure

the choice

of seal flush

plan, it will also allow the seal vendor to optimize the seal balance

NPSHA

Pump distortion can result

caused by piping

in misalignment

pump/driver set, which

ratio and

in very high

pressure applications.

will cause

erratic seal face motion, usually cau-

The

sed by loss of perpendicularity

consideration

faces and difficulty

of the

in face tracking

due to increased vibration

NPS/HR

of the

face geometry, particularly

levels.

NPSH

margin

is an important

when reviewing the need

to accommodate

the heat generation

at

the seal faces and/or any heat conducted to a seal flush in hot applications.

FIOW

Suction

\ Flow at onset of cavitation

the Figure

3. The

role

of flow rate and net positive suction head margin in induced cavitation.

A suction has

the

vessel

for

when sufficient the that

a marked

in liquid level may cause

intermittent

or chronic

Wh ere a mechanical

cavitation.

seal shows evi-

dence of dry running

this potential

cause should be considered.

l

Fouling

of process

piping can also

lead to cavitation,

if the fouling is in of the suc-

as well as plugging of

control

This

more

flow rate with the

for cavitation.

probable

resistance

where

curve

This the

holds

suction curve.

is particularly

susceptible

pumps with double suction

where

head margin head

positive

suction

low (typically

under

concept

suction

Corrosiveness consideration

with respect to the choice

will

of materials,

but this topic is not dealt

receive

a

disproportionate

amount of the total flow rate.

if a boiler

feedwater influx

an

in the deaerator. existing

15 feet

[4.6

suction

deaerator

re-

water,

This can result in water

section

piping reaching

at pump suction vaporize,

leading

sometimes

severe,

June 2002

of the seal. The are similar to

still

to

in

of pump a pressure

at which

it will

temporary,

cavitation.

In this first of three discussions analysis and troubleshooting fugal pump mechanical have considered,

focus of the failed While

but also include

Any

on the

of centri-

seal failures we

yet gone beyond, the

temperature

inspection

seal components. of parts of a failed

viscosity. They are:

seal will often reveal some clues as to

Specific

ding the time of failure, the events that

the condition gravity

NPSH

margin

Pressure

of the seal at or prece-

true

are more often cause

the mechanical

of failure.

seal vendor

or

OEM, who may be asked to replace or

increase

temperature

versus

improved

margin

repair the seal, is seldom cognizant the potential

conditions

of

that prevailed

Particulates

preceding

Corrosiveness

them to provide a conclusive

failure, it is asking much of diagnosis

of failure. Much of this analysis, thereLow specific

pressure

of colder

hotter

the long vertical

to

Since

which suddenly reduces the pressure the

is essential

Pressure

is

compati-

A last word

those discussed above when reviewing margin,

serious

bility for the purposes of discussion.

the fluid properties that

selection

be given

with, as we assumed chemical

Define the fluid characteristics

the correct

must

for

net

to

ers, where one side of the impeller

this condition

typical of what can occur

ceives

the particulates

the suction side of the impeller.

in

flow rate. This

vessel

which can evacuate

fluid

device,

impell-

volatile reduction,

of a throat

to the

is shown in Figure 3.

Sudden

insertion

linked

head required)

m]) under normal

or the

created

minus

in the

external flush of clean compatible

Viscosity

(net positive

available

of particulates

Lubricity

the net positive

suction

presence

and

potential

suction

The

fluid will dictate the need for either an

flat. It

is not steep

more

cavitation

PUMPS

positive

is

system

the pump curve is relatively also

can

to meet

(NPSHR)

key fluid characteristics

valve opening

lead to excessive

WORLD

net

required

the seal will encounter

potential

22

pump’s

head

is

even

net positive suction

Understanding

Sudden

pattern

is available

gradual surface fouling tion piping,

l

flow

the suction piping. This can include

any installed suction strainer.

l

inlet

head (NPSH)

or reservoir

potential

reduction

pump

such that

uneven can lead to cavitation,

Review the piping system and vessels l

piping installed

above

gravity

fluids are often

liquids with vapour pressures atmospheric.

They

have poor lubrication

may

qualities,

part to their very low viscosity. very low viscosity lubricants, typically degree

Such

pullout

of part

during

the

viscosity

32 cSt,

of sticking

at start-up.

While

fluids may be poor

higher above

also

due in

may cause a

of the sticking

of the

high

fluids,

seal faces can cause

carbon

start-up

face

torque.

Fluid pressure refers to the pressure to be expected

in the seal cavity during

fore, rests with

the pump user, who

may work with the seal vendor/ repairer in determining discussions

the root cause. In the

to follow we will explore

the remaining

areas that may have to

be

and

reviewed

cite

a couple

examples to illustrate the point.

CONTACT Stan Shiels, PO Box 1198, 445 Gage Street, Niagra-on-the-Lake, Ontario, Canada, LOS 1JO. Tel: +l-905-468-9813 Fax: +l-905-468-9865 E-mail: [email protected]

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