Mech. Seal Guide.docx

7/27/2019 Mech. Seal Guide.docx

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Mechanica l Sea ls

Why use a mechanical seal? After all is said and done, its easy to stuff a few extra bits ofpacking into a leaking stuffing box, and it doesn't require any skilled help to achieve this,does it? In this section we take a look at some of the reasons why you should be usingseals. The Economic case and the Environmental case as well as considering some of the

seal types available for general use.

It takes a lot of skill to pack a pump properly with soft packing.

There are two basic cases to be made out for the use of rotary, fluid sealingtechnologies.

The Environmental Case

We all have a responsibility to conserve and protect.

Conserve scarce commodities and to protect theenvironment from pollution. A major spill is news

because it is dramatic but every day, millions ofglands leak chemicals into the environment. You can

stop those leaks and avoid cleanup costs.

Have you thought about what that gland packing is

doing to the shaft of your pump? It works as a brake,gripping the shaft and causing more power to be

absorbed in the unit. The extra power consumption of

the gland contributes to the "green-house" gaseseffect because more power has to be generated at thepower plant to drive your pump.

As an experienced engineer you will know that theoverall thermal efficiency of the power plant is much

less than 35%, so if you can reduce your take-off byreducing the demand by replacing your glands withseals, less fuel is going to be consumed. Less fuel less

emissions and less overall cost of running your plant.

There is lots more to think about but space is limitedso let's get on.

The Economic Case

If you are not convinced that environmental pollution is

your problem, loss of hard cash from your pocket or that ofyour company should be!

Water is becoming a scarce commodity. Let me re-phrase that - clean water is becoming a scarcecommodity. For example, boiler feed water has to beat a high standard of cleanliness and chemicals are


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added to it to ensure that the water quality remains high. Make-up water isusually cooler than the water circulating in the system so additional fuel is

required to heat the makeup water. It all costs money. Water is paid for onamount used. More cost! But you'll look at the pump in the corner dripping awayand think that doesn't seem too bad. Ever done the mug test? A coffee mug holds300cc. It is a simple matter to collect the leakage and note the time it takes to fill

one mug.

Leak Chart

One drip/second 5,256 Litres / year

3 mm stream 315,360 Litres / year

6 mm stream 630,720 Litres / year

One drip a second is the standard rate for a properly adjusted packed gland : it leaks water,chemicals, and heat. Leaks usually get worse so look at the chart and now tell me if aleaking gland is inconsequential! Let's do another sum - how many leaking glands are therein your plant? Not all packed glands hold back water ... there may be more costly fluidsleaking away. Each leaking gland is contributing to hard cash overhead expense. Packing ischeap, to buy, to fit, but its running cost is hidden and can be very expensive.

A mechanical seal appears expensive to buy when compared with a packing ring, butproperly installed a seal will run for many years. The optimum life of a seal is the periodbetween major overhauls of the pump unit. A seal that fails early by this criteria is in needof investigation. The criteria for a failed seal is one in which the running faces are not worndown to their designed minimum. However, an engineer does not want to spend money ona super seal that will last virtually forever because that will also not prove to be costeffective. When a seal fails it is possible, with experience, or the aid of this web site, to

determine the cause of failure and to rectify that fault. This I promise!

I was asked to select a seal for a water pump working in a quarry. The engineer had beenplagued with seal failures for many years on this pump. His success criteria was that theseal should run from tear down to tear down (12 months). I selected a seal which was tentimes more expensive than the one he had been using. It was fitted over the Easter Holiday

1982. Over a year later actually the week after the Easter holiday 1983 he rang me to saythat the seal had failed. I reminded him of my promise that the seal would run for 12months trouble free. He calmed down and started remembering, I told him that actually hehad gotten an extra week over my promised 12 months! The increased price of the seal wasaround $400 but the saving in cost through not having to replace the seal several times in ayear was over $1,500. The whole plant soon became converted to seals because it ispossible to show a cost benefit analysis for every application.

It is often the thought that seals are expensive that prevents the engineer from opting forthem. The same applies to pump manufacturers. Ever wondered why your plant is fittedwith a particular pump make, each with a packing gland? In a word, competition. In theenlightened 1990's whole life costing is becoming the way to assess a particular project'sinitial cost, but in the real everyday world engineers are facing the consequences of shortsighted least cost solutions to immediate problems. But now you do not have to continueliving with these problems if you look at the situation of your plant leakage in a business-like manner.


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This web site does not represent any one company manufacturing seals. There are goodlogistical and economic arguments for standardizing on one manufacturer so long as theyare major enough to run through all the applications you are likely to need. Whilst workingfor one of these majors in Saudi Arabia I found that it was common to find whole refineriesusing one manufacturers' seal. Long way from home, gutsy job, $millions at stake in oilrevenues every day, it made a lot of sense for the engineers concerned. Only one companyto deal with, lucky for some of them it was mine and my expertise was part of the deal! But

there are many designs of seal and some I would think of as cheap and not so nice couldgive some of you excellent service. So this is not aboutprice, but very much concerns cost.Balancing the cost of the seal installation against the outcome compared with thealternative. There is a wide range of materials to choose from. The range encompassessmall variations in generic materials such as carbon, or o-rings and different metals used tocope with the conditions that faces the seal. I am not encouraging you to experimentblindly but to think the problem through and choose your materials carefully.

We are not going to look at the materials in detail here. For that information popover to seal troubleshooting

I have not listed all seal types, the contact-less gas seals for instance are not

covered here, this is because they fall outside the general seal types I aim to

cover. For details on highly specialized seals of this and other types contact yourfavored manufacturer for details. In the links section of this site you will findhyper links to some manufacturers.

Now go look at the various seal types that are available to you for general use. Inthese sections you will find explanations of seal types and some of the problems

associated with them.

Insta l la t ion Checks

Face the facts, seals fail. They do not wear out. Most often something comes along

to disturb the smooth running of the pump and you are facing a steady leak whichhas already destroyed your seal by damaging the seal faces. But there is another

case. The seal that leaks on startup after maintenance. A seal that lasts a week

without letting go is generally thought to be OK. By the way that's a ROT (Rule ofThumb). Running mechanical seals is an art form. There is a lot of science in it but

either you have the knack or you do not (in which case you need this web sitebad).

A seal that leaks after maintenance has been badly installed. It is very unwise to

ignore the basic checks listed here because without these checks there is no

certainty that your seal will perform at all, let alone give a reasonable running life.I hate having to go over a job again after having fitted it all back together... don'tyou?

Pre-installation checks:1. You have the correct seal and all the parts needed for the

replacement.
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2. You have the pump drawing to hand with installation dimensions orthe seal manufacturer's drawing.

3. The pump stuffing box is clean4. On split casing pumps the gasket does not extend into the stuffing

box.5. The shaft is free of scratches and burrs, threads are taped, and

keyways are filled flush with the shaft surface to prevent sealelastomers from being cut on the keyway edge (a dummy wooden keyinsert is ideal).

6. All the seal parts are in their protective coatings at this stage.Pump ChecksShaft Run-out

Shafts get bent. The spinning impeller has unequal loading on in causing the shaft

to deflect away from the volute throat. Constant deflection causes weakness and

can lead to a permanent offset of the shaft leading to shaft run out. Shaft run out

is bad for seals. It causes them to flex twice on every revolution of the shaft. Athigh enough speeds this can cause a vibration in the seal which allows the seal

faces to OPEN. BANG failed seal.

So, look into the dark recesses of your lockers and pull out the Dial Test Indicator

(DTI) or Clock Gauge that lurks there, unloved & unused and check the shaft ofyour pump for any damaging shaft deflections.

Single stage overhung pumps should be checked near the seal running positionbut multi stage pumps should be checked at suitable intervals along the shaft as

well as at the seal running position.

The run out should not exceed 0.002 inches or 0.05 m.metres.

Shaft Sleeve Concentricity.

You have checked the shaft for run-out and because the seal elastomer has atendency to wear a fret ring on the shaft a shaft sleeve is fitted to protect the

shaft. When a new shaft sleeve is fitted, and this should be with every new seal, it


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is a good idea to re-run the shaft run-out check to ensure that the sleeve isconcentric with the shaft.

The run out should not exceed 0.002 inches or 0.05 m.

A note about shaft sleeves. It is a false economy to omit to change the shaft

sleeve when replacing a mechanical seal.

I was called out to a cooling water pump supplying a 100Mw Power station. The

shaft size was 230mm and it took three men two days to strip and rebuild the seal

box. The shaft sleeve cost $4,000 and the seal cost $10,000. The new seal hadbeen fitted onto the old sleeve and leaked immediately on startup. The seal faceswere intact but having been run for 24 hours in that condition another new seal

assembly was required. On examination it was found that the o-ring contacting

the shaft sleeve surface had worn a groove (Fretting damage) and the new o-ringwas unable to seal against this damaged surface. The extent of the damage was

not immediately obvious to the eye but by carefully measuring the surface the

fault was found. Amount saved on first installation $4,000, total cost of seal

change $25,500, and it should have cost $15,500. Believe me, skimping on the jobis not the same as saving hard cash.A x i a l S h a f t M o v e m e n t

Set up your DTI to measure the amount of axial movement of the shaft. The

amount will vary according to the type of pump, its bearing configuration, and thetype of thrust bearing in use.

Essent ia l ly there are four types o f thrust bear ings

Deep groove ball bearings Roller bearings Michell, Kingsbury, or thrust pad bearings, usually made of white

metal bearing surfaces.

Balance piston thrust absorbing arrangement. This type is often foundon high pressure multi-stage water pumps where the hydraulic forces

are partially balanced by the impellers and controlled leakage past abalance piston provides the final stage of rotating unit positioning.

The basic principle is that the shaft should be set to its running position before

attempting to fit the seal. In the case of cartridge seals, the seal cover plateshould be fixed to the pump casing, the shaft positioned, and then the seal locking

screws tightened to the shaft. Non cartridge types need to have a datum mark

scribed onto the shaft relative to the seal plate position and then the fittingdimension marked from this point.

A note about fitting position. It is not good practice to fit a new seal by looking atthe old set-screw marks and then lining up on them. If you want good seal

performance then start out right ... measure the distance required, don't take

short cuts. The last seal could have been fitted incorrectly, perhaps causing therebuild that is now necessary. You are storing up future trouble if you skimp.
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Seal Housing SquarenessThe seal stationary must be fitted at 90 degrees to the axis of the shaft. Failing toachieve this will cause the seal head to move to take up any mis-alignment. Thismovement offers an opportunity for the seal faces to open and for the ingress of

dirt particles. If you are changing out packing and up-grading your equipment to

a mechanical seal you need to pay close attention to setting the seal housingclosing plate in the correct position. The basic check is as shown in the diagram.

It is also wise to check the bore of the seal housing at this point for concentricity

with the shaft. Put the sensing tip of the Dial Indicator inside the bore on the wallof the seal housing and rotate the shaft. A small amount of misalignment is

permitted but the important thing is to check that the seal body cannot touch theseal housing wall at any point of its rotation.

General ChecksWhile the pump unit is in the shop for maintenance take the opportunity to ensure

that the cooling water jacket is clear of debris, that any other cooling waterarrangement is cleared of any obstruction. Orifice plates controlling the flow of

water to a seal housing should be checked dimensionally correct. A seal starvedof its ration of cooling water will be very unforgiving and cause you lots of grief in

a short time. This kind of fault is very difficult to diagnose for the average

engineer. Even the best have trouble with this one, too! So check it out nowwhile the doing is easy.

Bearings need to be replaced if they have been running with any pump leakage

around. Moisture ingress into a bearing dramatically reduces a bearing's usefullife. If you are changing out soft packing for a mechanical seal replace the

bearings on the unit too. The leakage from the packing gland is more than enoughto damage the bearings.

Check the impeller for cavitation damage indicating a system problem that might

go un-noticed during normal running conditions. Cavitation can cause vibration inthe pump shaft which willaffect the seal 's performance.

I know you will ensure that the impeller sealing rings are replaced or re-bushed to

keep the clearances within design limits. Allowing recirculation within the pump


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volute is no way to keep the efficiency of your plant at the highest level, and it canincrease the pressure inside the seal housing which will cause your seal to wear

out faster!

Why Seals Fa i l

Seals fail for a number of reasons. Your job is to pinpoint the reason and fix it.

Here you are in a situation in which the seal has run for a period well beyond theinstallation period. Its leaking and now you have to make a decision. Has the seal

failed or simply worn out? What you decide now will determine whether you fit areplacement seal or seek out an alternative type. The basics are simple.

A worn out seal will leak when the seal face has worn away completely.

If we extend this criteria to all leaking seals it becomes sadly obvious that the

majority of seals, perhaps 85% of process seals, fail long before they are wornout.

This section is devoted to the three main reasons why seals fail. Only three yousay? Three main reasons and lots of routes to them.

Seals fa i l because . . .

The seal faces open. Heat causes a problem. The chemical environment causes a material failure.

OK so there is another category ... the installation failure, but that's covered in theinstallation section.

Seal Faces Open

The shaft moves for many reasons, those that affect the seal operation are:

Axial

End play Thrust movement

Temperature growth Impeller adjustment

Radial

Bearing wear Bent shaft Shaft whip Shaft deflection (discharge closed)
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Vibration

System NPSH incorrect causing cavitation Harmonic vibration, check the coupling, does it "hum" or "buzz".

Rubber couplings can operate with high degrees of misalignment

without total failure but cause problems for the seal.

Impeller imbalance Slip-stick. Not surprisingly not much is known about what happens

between seal faces in service. There are theories. The faces acquire a

film of liquid that lubricates the seal surfaces, the carbon face wearsslightly depositing a layer of carbon on the stationary face so that thecarbon face runs on carbon , but there is a condition that causes the

faces to vibrate open when pumping non-lubricating fluids. Fluidsnear their vapour point, very hot water, can cause these conditions.The seal faces "chatter " against each other in a slip-stick motion

slipping when the drive lug hits the seal head, bouncing round andmomentarily stopping before being hit by the drive lug again. To be asealman you have to believe.

Poor pump performance. This statement covers a host of sins.Consider running two or three pumps into one discharge line, theodds are that the pump performances will not be perfectly matched.Does it matter? Not really, unless you are concerned about your seal

life, because what is happening here? One or other of the pumps,

because of poor performance now combined with poor system design,will be experiencing discharge throttling, tending to over load the

impeller at the throat, causing turbulent flow and shaft bending. Lookinto other causes of poor pump performance.

Other causes

The seal runs against a stationary component. The stationary is usually fitted intothe seal plate which is bolted to the pump and sealed with a gasket. Now, I do notwant to sound too pedantic here but you have to realize that the seal stationaryhas to be fitted square to the axis of the shaft and in proper alignment with theaxis of the pump shaft. The stationary has to be fitted into the seal-plate square.

None of this is easy to achieve and each error compounds the next. The rotatinghead has to follow any misalignment from square that the stationary carries.Every rotation of the shaft causes the rotating seal head to move back-and-forth

twice. Interfere with that movement and the faces are open.

Difficult as it is to get the stationary fitted correctly, should you achieve it then

other factors come into play to limit the excellence of your work. Stress imposedby pipe strain, coupling misalignment, or plain thermal growth put the pump

casing out of shape just enough to cause the seal to work harder.

All of the items described mean that the shaft and seal are in constant relative

movement. If anything interferes with the free movement of the seal, the facesopen.

When the faces open, dirt in the liquid penetrates the lapped surfaces, embeds in

the soft face which gradually changes to a grinding surface to score and wear


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away the hard face of the stationary ring. Have you noticed this effect? Do youlook at your failed seals? You should, because on those faces lie clues to help you

find the faults opposing long seal life. Well when we have gotten through thissection and onto the tell tale signs I bet you will take a bit more notice of yourfailed seal bodies.

The main reasons why seal faces open are:

The elastomer sticks to the shaft. Spring loaded elastomers will stickto the shaft, O-rings will flex by 0.005" (0.13mm) and then roll. O-

rings will fret a shaft but spring loaded elastomers (teflon wedges,

chevrons, etc.) can cause serious surface damage to your shaft orsleeve leading to early seal failure. A leak under the seal head looksvery much like a face leak.

The shaft is out on machining tolerance. Correct tolerance is +0.000"to -0.002" from nominal. A packing sleeve is not machined to anyclose tolerance, after all it is going to wear against the packing so its

external dimension is not too important. An oversize sleeve or shaft

will cause the seal to hang-up, an under size shaft or sleeve willprejudice the ability of the elastomers to seal the head to theshaft/sleeve.

The surface finish on the shaft/sleeve is too rough. A lathe finish isnot good enough. The finish should be at least 32 RMS and for that aground finish is required.

Have you got a hardened shaft on your pump unit? The seal setscrews will not "bite" into the shaft and could slip causing the settingdimension of the seal to alter.

The pumped fluid changes state. Sea water, brine pumps, sugarysolutions, cause crystallizing when the salts come out of solution orthe sugars become caramelized. Other coking substances, heattransfer oil, tar, cause similar problems. You will see the build up of

material around the leak site. Solids can cause the seal head to stick to the shaft or restrict the o-

ring flexibility. Take a look at the double seal arrangement, back to

back version. Used on some services the O-ring could very quicklybecome clogged preventing the seal head from moving to

accommodate wear of the faces.

Incorrect setting length at installation. You may never figure this oneout. Just make sure that the fitting dimension is correct when

installing the seal. Otherwise sometime in the future the seal will letgo, usually after the pump is stopped, and the faces will look good butonly partly worn. What has happened is that the spring pressure has

reduced to the point where the seal leaks during idle periods. This can

be difficult to spot, unless you know what to look for ... and when.

Fretting. Very small movements between components causes apolishing action. The polishing action removes the surface molecules.

On pump shafts made of stainless materials the surface of the metalconsists of chromium oxide. Elastomers moving very slightly against

this surface wipe away the oxide which immediately reforms. Theoxide is carried into the wiping surface changing its character

completely. A rubber ring coated with chromium oxide becomes moreefficient as a polishing, grinding surface and removes material at a

faster rate. A "fret" ring is characterized by a polish mark on the shaft


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surface at the point where the seal elastomer seals against the shaft.If worn badly enough the fret ring can cause a new seal to fail on

installation because the elastomer cannot seal effectively due to thedamage on the surface.

Distortion of the stationary face. This is not common but thestationary could be badly fitted leading to over tightening, especially

the silicon carbide grades which are designed with a lip to be clampedin the seal plate. Failure under these circumstances may be confusedwith cracking due to heat checking of the component. S.C grades of

99.9% only heat check if they are tightened un-evenly, so check out

your grade and suspect poor fitting if its a high grade material failingby cracking. With other materials such as tungsten carbide, or platedsurfaces, such as stellite, consider the distorting effect of poor

clamping if no other solution presents itself.

Face Mis-centering or run-off. This is not common and is easy todiagnose. The faces are not concentric and the rotating head comesoff the stationary track and picks up dirt. Scoring of the stationary

and an off center running track gives you all you need to know.

Incorrect grade of O-ring material. Lots of things happen toelastomers so check out the ones on your seal, are they swollen,hard, squashed, shiny, cracking?

The seal hits something, it is prevented from moving to accommodaterunout.

o Lots of possibilities here, so I list a few.1. The shaft is bent and hitting the stationary face. You will notice

this pretty quick, but bear in mind that the running clearance ofthe seal components and the shaft may be quite tight, so a

small shaft displacement may not be obvious, the seal willshow you what is happening.

2. Solids in the seal chamber hitting the seal.3. Incorrectly fitted gasket extending into the seal chamber. Split

casing pumps can suffer this problem.4. The shaft is not concentric with the seal chamber.5. Insufficient clearance in the seal chamber. Check this out if you

are changing seal type or intend using different materials tocope with other problems.

6. A seal box recirc line is directed at the seal faces. Most sealchambers have a radial flow insert when most seal

manufacturer's will tell you that a tangential flow insert issafer and causes less disturbance to the seal faces.

Heat Causes Seal Fa i lures .

Heat affects the elastomer. This the part most sensitive to extremesof temperature.

Heat can change the state of the fluid being pumped. Raising the temperature of corrosive liquids increases their potency.

A 16 deg F rise doubles the corrosion rate of most acids.

Differential expansion rates can destroy plated seal surfaces. Lowgrade silicon carbide will crack with sudden changes in temperature.


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Differential expansion of shaft and pump casing can change the faceloading by altering the fitting dimension.

We now have the over-view of heat related failures so let us look in more detail atwhat is happening.

E lastomers .

A wide range of elastomers are in use and many of them are rubber compounds.Teflon materials have a predetermined heat range of up to 226 deg C beyond

which Teflon breaks down and burns making small amounts of phosgene gas.

Teflon should not be used in temperatures close to its ultimate limit because it is aheat insulator and local heat production may cause it to reach its ultimatetemperature.

Rubber compounds are made by baking the material until it is cured to apredetermined hardness or durometer. The various materials formed in this way,

nitrile, viton, buna-n, and others, are commonly found in sealing applications. Lesscommon is Kalrez a specialized compound with a high resistance to chemical

attack. Formed in a heat setting process, these materials continue to be affectedby the heat applied during the life of the seal. At temperatures beyond the range

of the rubber seal the material continues to harden. As it hardens the shape of theseal takes on the shape of the groove if an O-ring or splits appear in rubber

bellows as flexibility is lost. O-rings take on a "compression" set and appear oval

and feel hard to the touch. O-rings are manufactured with a 10% toleranceoversize to allow for some thermo-setting in service. At higher temperatures the

elastomer life to full compression set will depend upon the temperature and time

at this temperature. The point for you is that exceeding the range of the rubberparts of your seal will shorten the working life of the seal and you need to bearthis in mind.

An odd case, in Saudi Arabia I was called to a refinery that had been under

construction for several years and pumps had been installed, but not run, for

varying periods. Pumps under going test runs were leaking along the shafts.Investigation showed that over time in ambient temperatures of 55 deg C the sealelastomers had baked hard and vulcanized to the metal parts. All seals had to bechanged.

Heat is generated from the friction running at the seal faces. Depending upon thetype of face material and the seal box environment a rise of around 25 deg C

above the seal fluid temperature can occur. Look at your seal types, where is the

elastomer in relation to the seal faces. The nearer the elastomer is placed to the

running faces the greater the additional heat it will experience. The use of lowfriction seal face combinations will reduce this effect. The carbon / ceramiccombination has the lowest friction rating with hard faces such as tungsten /tungsten faces the highest.

Unbalanced seals, because the face weight is varying with the system pressure,can experience greater rises in face generated heat creating damage to theelastomer.


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Excessive heat producing a temperature rise of 55 Deg C on a Viton O-ring willreduce its useful life to less than 1000 hours running time. For a seal that is

expected to run for one year that is an 88% reduction in useful life. An 82 deg Crise will reduce the life of the seal by 97%.

Loss of water to a cooling water jacket, loss of any cooling arrangements puts

your seals at risk.

I was called to a split-casing boiler feed pump that was experiencing out-boardseal failure. Normally I would expect more problems with the in-board (coupling

end ) seal due to less opportunity to dissipate the heat soak along the shaft.

Examination of regular temperature recordings made of the cooling water systemand seal box temperatures revealed that the out-board seal was being starved ofcooling water flow. Dismantling the orifice plate controlling the flow to the in-

board seal showed excessive wear enlarging the orifice and allowing through alarger proportion of the flow. Replacing the orifice plate solved the problem. Allcan seem well with your equipment but the seals will always let you know firstwhen problems are arising.

Changing s tate o f the f lu id

Liquid gases and other volatile fluids can vaporize and freeze water out of the airon the outside of the seal restricting movement. Shortly before I took up my post

in Saudi Arabia a liquid propane pump blew its seal open due to a build up of ice

around the seal faces. Liquid released into the atmosphere created a vast cloud ofhighly flammable gas. Fortunately no one was hurt and no explosion occurred but

it was a close thing. It was thought appropriate to fit a double seal with a barrierfluid for future installations.

Liquids changing state to a gas experience enormous volume increases. Water

increases in volume by 1700 times, so a small drop vaporizing across a seal facewill explosively blow apart the faces. Boiler feed pumps and other hot water

pumps can be heard "popping" or "puffing" if the seals are not working correctly.

As the water droplets expand and open the seal faces more water rushes in to coolthe area, collapsing the steam bubble and causing the faces to snap shut. Anothersmall droplet penetrating the faces vaporizes and causes the faces to open again.

Water treatment crystals, entrained oxides, other dirt particles are trappedbetween the faces as they close. Your seal is on its way to the scrap yard.

Some fluids crystallize with additional heat. Sea-water, brine, and similar fluids

leaking past your seal and drying out around the seal plate can build up to affect

the seal head and prevent it from moving. Crystals can also score the runningsurfaces of the seal causing damage leading to failure.

Hydrocarbons form coke as they partially burn or vaporize. Coking causes a hard

solid to form around the seal effectively stopping it from moving freely. A similareffect is seen in food plants handling product containing sugar. Sugar escaping

across a seal face can crystallize, or simply burn and coke. The signs are un-mistakable on the seal face.


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Heat can cause impurities to come out of solution and plate onto seal surfaces,building up hard films or lacquers.

Heat can dest roy seal faces .

I have mentioned some of these effects but I think a defined list will help you.

Plated materials can experience differential expansion. Often materials such as

stellite are plated over stainless steel. The expansion rates are poorly matched sooperating outside of the design limits of the materials will cause strains to appear

in the plating interface, causing cracks to appear. The cracks will cause the carbonface to wear dramatically fast.

The less expensive ceramic material (85%) will crack if cold shocked. Suddenchanges in temperature of 38 deg C or more will destroy the seal face. The higher

quality ceramic (99.9%) will cold shock if it is under distorting stress, properlyfitted and evenly clamped it will survive sudden changes in temperature. Get toknow which materials are being fitted into your seal installations.

Carbon rings using fillers and fitted into high temperature pumps can have thefiller material melt out of the carbon causing them to become porous

Poor carbons with voids can blister and pit as the trapped air or gases expand andblows pieces off the carbon surface.

Lapped seal faces can distort, going out of flat. The effect of touching the lapped

surface with a finger is to coat the surface with dirt and skin oils but also to distortthe surface away from flat by the application of heat from your hand. Distortedseal faces leak.

Heat increases the corros iveness o f most corros ive mater ia ls

The carbon part of the seal will show signs of being attacked. O-ring grooves can be damaged limiting their ability to seal

effectively.

O-rings can become hard or start to crack, or become swollen andexcessively soft.

Metal surfaces can be attacked and appear pitted which will prejudicethe seals ability to work properly.

Springs and other highly stressed parts can fail due to increasedcorrosion.

Expans ion due to heat ing e f fects .

All metals expand when heated. A stainless steel shaft 48" long by 4" dia will grow0.138" in length when heated through 300 deg F. The working limit of most carbon

seal faces is 0.125" . Seal compression is set at about 0.064" to produce the springface weight. A seal mounted on a shaft moving by 0.138" with other expansion

effects happening to the pump casing is in danger of opening. Apart from ensuringthe accurate placing of the seal on the pump shaft there is little to be done to


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compensate for such movement. Tell-tale signs of inaccurate setting of the sealwill be where you need to be looking.

The shaft diameter will expand too, by about 0.010". The seal material will expandalso but under extreme circumstances this expansion can cause the seal to hang-

up on the shaft. Over-compression of the elastomers will limit their effectiveness,

as well as the other effects mentioned earlier.

Mater ia l Fa i lure .

Failure of materials is usually a sign of a mis-match of material to environment.

The substantial construction of seals excludes major failure of some maincomponent, so we concentrate on the effects of environmental attack on sensitivecomponents.

Chemical attack on the elastomer will cause it to swell. The carbon will appear pitted. Acid attack on carbon is directed

against the impurities. The reaction of the impurities to the acidsolution cause holes and pits to form, weakening the structure and

producing a porous carbon. A higher grade of carbon is required.

The springs can break. Stainless steel is known to fail due to chloridestress corrosion. Many single coil spring driven seals fail because thespring breaks. They are usually in-expensive and over-engineered,

but they still fail.

Metals corrode. In seals where metal parts are designed to be thindue to flexibility requirements, metal bellows seals, welding

techniques used in construction and material compatibility with

mating components and pumped fluids are factors that affect the lifeof a seal.

Set screws clamping onto a hardened shaft material will not gripproperly, allowing the seal body to slip, leading to a range of othereffects, but ultimately to a seal failure.

Plated seal faces are not corrosion resistant, so the plating materialcan be removed from the surface.

This list is not exhaustive however comprehensive it may appear. You will find

some new problem and when you do I want to hear all about it. So do all the otherguys visiting this site. Look forward to hearing from you, I just know I will in time!

Single Seals

Single shaft seals are by far the most common configuration to be found in industry.

They consist of a rotating unit with a mating face which butts onto a stationary unit.

The rotating unit seals in two places, at the interface between the rotating face andstationary and between the shaft and the seal body. The stationary seals against the pumpbody.


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The seal body is sprung by either a metal or rubberbellows, a series of small springs, or by a single coilspring. The purpose of the springs is to hold the sealshut when the pump is stopped. In the case of themany less expensive seals the spring is also used todrive the rotating head of the seal. These seals mustbe fitted with the correct coil spring for the rotation

of the shaft. The major cause of failure of this type ofseal is a broken drive spring. This is due to the use of316 SS stainless steel as the coil material. The springhas to maintain the drive of the seal head and to flexwith the seal head as it rotates. Any mis-alignment inthe stationary face will cause the seal head to flextwice in every rotation. Stainless steel suffers from

chloride stress corrosion therefore as the spring is under constant stress and probablyexposed to chlorides, cracking will occur in the spring, leading to complete failure.

The same effect can occur when the seal is fitted with a series of small springs if they areconstructed from 316 SS. A better material to use is Hastelloy C which does not suffer fromchloride stress corrosion.

The purpose of the spring is to keep the seal shut when the pump is stopped. When thepump is running the pressure in the seal chamber, being higher than the outside pressure,acts upon the seal surfaces to increase the closing force. Higher closing force means that

any lubricating effect between the seal faces is marginalised leading to higher friction,more heat generation, and increased wear. Seal design can mitigate this increase in closingforce. The design of a balanced seal incorporates surfaces to reduce the load imposed onthe seal by the chamber pressure. By this means a manufacturer can reduce the pressurebetween the seal faces and produce longer life of the seal.

The seal body has to make an effective seal to the shaft. This is achieved in a number ofways. The single drive spring seal is a variant of the rubber bellows seal. The rubberbellows allows the seal head to flex and to become vulcanized to the shaft producing theshaft seal required. This arrangement is often found on small water pumps. It should not be

used on abrasive water service as the abrasives will wear the bellows causing failure.

The more common types of seal / shaft elastomers are O-rings, U-cups, Chevrons (made of

various materials), and wedges. There may be others but when you find them on your sealconsider carefully how they must work.

O-rings seal by deforming under pressure to effect a seal inside their operating grooves.The pressure also acts to squeeze the ring onto the shaft. Have you noticed that where theo-ring seats a shiny, polished area can be seen when you remove the seal body? This area isvery important to you, it is known as a fret ring and represents the damage that the o-ringhas caused on your shaft or shaft sleeve. As mentioned above, the head of the seal has toflex with any mis-alignment. The shaft may experience harmonic vibration, transientvibrations, the stationary unit may not be true to the axis of the pump, the shaft may bend

slightly, and in order to maintain a proper seal the head has to accommodate all thismovement. Additionally the seal faces are constantly wearing so the seal head has to beable to move down the shaft towards the stationary unit. The consequence of all thismovement is that the O-ring too has to flex. It moves back and forth polishing the surfaceof the metal.

The oxides present on the surface of the metal become embedded in the O-ring. Graduallythere is a build up until the O-ring becomes a very effective polishing tool covered in oxideand grinding away at the surface of your shaft sleeve. Metal removed from this area willprejudice the replacement seal you fit because a leak has developed, unless you change the


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sleeve. It is a good idea to fit a seal which includes a disposable shaft sleeve in its design,or be prepared to replace the whole shaft.

The O-ring account serves to illustrate exactly what is going on with all of the other sealelastomers. The Teflon wedge, the U-cup or the chevron set all produce the same or similareffects. I have worked mainly with O-ring seals.

There may be cases where alternative elastomers are effective, they may well work for youbut my experience is that O-rings are:-

simple and easy to fit. give a high degree of flexibility in material compatibility. survive well in a wide range of temperatures. provide the long term flexibility required by the seal head. do not cause damage to pump shafts if properly fitted.

The rule is always check the shaft or shaft sleeve for damage under the seal's shaft seal.

You'll find it there, I have no doubt, leave it, save a few pennies, and have to change outthe new seal in hours. If, like me you hate having to do the same job twice in the same

shift, do it right first time around and replace all worn parts.

To avoid fretting corrosion from damaging your pump ensure that the seal you fit has adisposable sleeve and change that sleeve whenever you change the seal. Some single sealsincorporate a static O-ring together with a dynamic O-ring mounted onto a sleeve made asan integral part of the seal body, the Chesterton 880 series seal is one example of this type.

Seal Face Combinations

When you have got the backup materials and configuration of the ideal seal for yourapplication its time to look at the seal face combination. The most common facecombination is carbon / ceramic. However there are two types of ceramic and manydifferent types of carbon. For applications beyond water pumps different material

combinations or variations on the basic carbon and ceramic should be considered if youraim is to extend the life of your seals.

Specific pump environments cause readily identifiable seal problems. Coking, chipping onseal ID or OD, hang-up, heat check, chemical attack, all related to the inter-action betweenthe seal materials and the environment.

I was called to a food plant. The company made custard which contained sugar. To keep the

product safe from bacteriological contamination the seal box was flushed with wet steam at1 atmosphere. The seal face combination was carbon / ceramic. The seal was mounted on along shaft operating in a dense and turbulent stream resulting in shaft "bounce" whichprejudiced the seal's ability to remain closed. The escaping product heated by the steamformed a sticky mess of carbonized sugar at the periphery of the seal face. The seal wasrotating at 80 RPM and the sticky material caused the seal faces to momentarily lock

together. This was evidenced by the chipping away of the carbon seal face from the outsidediameter until the seal face failed completely. The carbon face was torn away. Seals oftenlasted a matter of hours, continuous production was disrupted, product was lost. Theanswer was to adopt a hard seal face strategy. Tungsten carbide working against eitherceramic or silicone carbide. Tungsten against tungsten was the ultimate option. Such astrategy would resist the failure of the seal material and cope with the abrasive nature ofthe resulting carbonized sugar product. Seal life would be extended indefinitely andcombining this with a seal repair facility to ensure that replaced seals were returned withthe faces within flat limits, the seals were used effectively for many years.


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Look at the seals you remove from your pump units, keep the parts together and take timeout to think of the conditions which could cause the damage / wear / failure that you areseeing. At that point seal face selection can begin, the seals fitted to your plant in the firstinstance were an informed choice, but if that combination has failed in service for anyreason, your job is to fix it

Double SealsDouble Mechanical seals mean arduous service, an application where leakagecannot be tolerated. It's not just a cost issue but can mean danger for the

environment, fire or explosion, or in the case of the shaft seal on a submarine,death by drowning.

There are two configurations for double seals

Back to Back Seals Tandem Seals

B a c k t o B a c k S e a l s

This seal configuration sites the seals literally back - to - back. The inner seal isplaced with the running faces toward the impeller, the outer seal is facing the

gland plate. The space between the seals can be flushed or statically pressurized(or both) and can act as a warning of leakage taking place on the inner seal.

This type of seal has an inherent fault. You need to know about it even though thismay not mean that the seal is not suitable for some services, and ideal for aparticular application in your plant.

The fault is that the internal seal operates in reverse mode. The chamber between

the seals is often pressurized. above the gland pressure. The excess is limitedbecause in this mode the external seal starts to wear faster than the internal sealbecause the pressure differential becomes greater than the internal sealdifferential. So greater wear takes place. To cap it all the internal seal face can

"blow open" if there a loss of pressure between the two seals. Now consider this,the external seal is wearing faster than the internal seal, it starts to fail and thenleaks enough to reduce the pressure between the seal faces, the differential

pressure gets above the design limit for a reverse acting seal and the internal sealblows open. The seal unit fails completely.


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The internal seal has to move into the stuffing box to accommodate any wear ofthe faces. The problem is that any dirt that accumulates under the seal at the seal

body / shaft interface can lockup the seal head preventing it from moving toaccommodate wear. The seal faces wear, the head locks up, the face pressure

reduces and a leak develops. At this point the leakage can be detected in thebarrier fluid between the two seals, or by a change in pressure. From a

maintenance point of view the seal has failed before its useful life is exhaustedleading to higher cost than is necessary.

T a n d e m S e a l

The tandem seal arrangement sites the seal components in a configuration withthe seal heads operating normally and not in reverse operating mode.

The seal body / shaft interface seal area is kept clean, not exposed to thedirt in the system. The system pressure is acting on the back of the sealtending to close the seal faces. Additionally, the pressure differential across

the seal faces of the internal and external seals ensures that the greatest

differential is across the internal seal allowing in effect the external seal to"idle". At the very least the external seal wears more slowly, ensuring that

when the internal seal lets go there is a second seal ready to take over,preventing any leakage to atmosphere.

This seal has a greater chance of failing safe than the back to backarrangement.


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Documents

Mech. Seal Guide.docx