Mechanical Seal Failures
Why Seals Fail
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 the installation 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 a replacement 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 worn out.
This section is devoted to the three main reasons why seals fail. Only three you say? Three main reasons and lots of routes to them.
Seals fail 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 the installation section.
Seal Faces Open
The shaft moves for many reasons, those that affect the seal operation are:
Shaft deflection (discharge closed)
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.
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 wears slightly depositing a layer of carbon on the stationary face so that the carbon face runs on carbon , but there is a condition that causes the faces to vibrate open when pumping non-lubricating fluids. Fluids near 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 and momentarily stopping before being hit by the drive lug again. To be a sealman you have to believe.
Poor pump performance. This statement covers a host of sins. Consider running two or three pumps into one discharge line, the odds 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. Look into other causes of poor pump performance.
The seal runs against a stationary component. The stationary is usually fitted into the seal plate which is bolted to the pump and sealed with a gasket. Now, I do not want to sound too pedantic here but you have to realize that the seal stationary has to be fitted square to the axis of the shaft and in proper alignment with the axis 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 rotating head 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 imposed by 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 faces open.
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 away the hard face of the stationary ring. Have you noticed this effect? Do you look 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 this section and onto the tell tale signs I bet you will take a bit more notice of your failed seal bodies.
The main reasons why seal faces open are:
The elastomer sticks to the shaft. Spring loaded elastomers will stick to 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 or sleeve leading to early seal failure. A leak under the seal head looks very 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 any close 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 will prejudice the ability of the elastomers to seal the head to the shaft/sleeve.
The surface finish on the shaft/sleeve is too rough. A lathe finish is not good enough. The finish should be at least 32 RMS and for that a ground finish is required.
Have you got a hardened shaft on your pump unit? The seal set screws will not "bite" into the shaft and could slip causing the setting dimension of the seal to alter.
The pumped fluid changes state. Sea water, brine pumps, sugary solutions, cause crystallizing when the salts come out of solution or the sugars become caramelized. Other coking substances, heat transfer 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 quickly become clogged preventing the seal head from moving to accommodate wear of the faces.
Incorrect setting length at installation. You may never figure this one out. Just make sure that the fitting dimension is correct when installing the seal. Otherwise sometime in the future the seal will let go, usually after the pump is stopped, and the faces will look good but only 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 a polishing action. The polishing action removes the surface molecules. On pump shafts made of stainless materials the surface of the metal consists of chromium oxide. Elastomers moving very slightly against this surface wipe away the oxide which immediately reforms. The oxide is carried into the wiping surface changing its character completely. A rubber ring coated with chromium oxide becomes more efficient as a polishing, grinding surface and removes material at a faster rate. A "fret" ring is characterized by a polish mark on the shaft 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 the damage on the surface.
Distortion of the stationary face. This is not common but the stationary could be badly fitted leading to over tightening, especially the silicon carbide grades which are designed with a lip to be clamped in the seal plate. Failure under these circumstances may be confused with 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 failing by cracking. With other materials such as tungsten carbide, or plated surfaces, 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 to diagnose. The faces are not concentric and the rotating head comes off 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 to elastomers 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 accommodate runout.
Lots of possibilities here, so I list a few.
The shaft is bent and hitting the stationary face. You will notice this pretty quick, but bear in mind that the running clearance of the seal components and the shaft may be quite tight, so a small shaft displacement may not be obvious, the seal will show you what is happening.
Solids in the seal chamber hitting the seal.
Incorrectly fitted gasket extending into the seal chamber. Split casing pumps can suffer this problem.
The shaft is not concentric with the seal chamber.
Insufficient clearance in the seal chamber. Check this out if you are changing seal type or intend using different materials to cope with other problems.
A seal box recirc line is directed at the seal faces. Most seal chambers have a radial flow insert when most seal manufacturer's will tell you that a tangential flow insert is safer and causes less disturbance to the seal faces.
Heat Causes Seal Failures
Heat affects the elastomer. This the part most sensitive to extremes of 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. Low grade silicon carbide will crack with sudden changes in temperature.
Differential expansion of shaft and pump casing can change the face loading by altering the fitting dimension.
We now have the over-view of heat related failures so let us look in more detail at what is happening.
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 a heat insulator and local heat production may cause it to reach its ultimate temperature.
Rubber compounds are made by baking the material until it is cured to a predetermined hardness or durometer. The various materials formed in this way, nitrile, viton, buna-n, and others, are commonly found in sealing applications. Less common is Kalrez a specialized compound with a high resistance to chemical attack. Formed in a heat setting process, these materials continue to be affected by 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 the seal 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% tolerance oversize 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 rubber parts of your seal will shorten the working life of the seal and you need to bear this in mind.
Heat is generated from the friction running at the seal faces. Depending upon the type 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 low friction seal face combinations will reduce this effect. The carbon / ceramic combination 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 the elastomer.
Excessive heat producing a temperature rise of 55 Deg C on a Viton O-ring will reduce 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 C rise 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.
Changing state of the fluid
Liquid gases and other volatile fluids can vaporize and freeze water out of the air on 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 of highly 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 barrier fluid 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 face will 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 cool the area, collapsing the steam bubble and causing the faces to snap shut. Another small droplet penetrating the faces vaporizes and causes the faces to open again. Water treatment crystals, entrained oxides, other dirt particles are trapped between 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 running surfaces 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 similar effect 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.
Heat can cause impurities to come out of solution and plate onto seal surfaces, building up hard films or lacquers.
Heat can destroy 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 so operating 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 carbon face to wear dramatically fast.
The less expensive ceramic material (85%) will crack if cold shocked. Sudden changes 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, properly fitted and evenly clamped it will survive sudden changes in temperature. Get to know which materials are being fitted into your seal installations.
Carbon rings using fillers and fitted into high temperature pumps can have the filler 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 and blows 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 distort the surface away from flat by the application of heat from your hand. Distorted seal faces leak.
Heat increases the corrosiveness of most corrosive materials
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 and excessively soft.
Metal surfaces can be attacked and appear pitted which will prejudice the seals ability to work properly.
Springs and other highly stressed parts can fail due to increased corrosion.
Expansion due to heating effects
All metals expand when heated. A stainless steel shaft 48" long by 4" dia will grow 0.138" 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 spring face 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 ensuring the accurate placing of the seal on the pump shaft there is little to be done to compensate for such movement. Tell-tale signs of inaccurate setting of the seal will be where you need to be looking.
The shaft diameter will expand too, by about 0.010". The seal material will expand also 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.
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 main component, so we concentrate on the effects of environmental attack on sensitive components.
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 acid solution 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 chloride stress corrosion. Many single coil spring driven seals fail because the spring breaks. They are usually in-expensive and over-engineered, but they still fail.
Metals corrode. In seals where metal parts are designed to be thin due 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 life of a seal.
Set screws clamping onto a hardened shaft material will not grip properly, allowing the seal body to slip, leading to a range of other effects, but ultimately to a seal failure.
Plated seal faces are not corrosion resistant, so the plating material can be removed from the surface.