Seal leak paths
SEAL LEAK PATHS ST002
- Hydraulically balanced.
- O-ring elastomer seal.
- Rotating configuration.
- Clamped “L” shaped stationary hard face.
- Metallic version.
- Mounted inside the stuffing box.
- Solid seal.
- Non-cartridge design set screwed to the rotating shaft.
- Single seal.
- Multiple springs located outside the sealing fluid.
- Carbon-graphite face inserted into a metal holder.
The lapped seal faces are being held together by multiple springs and the pressure of the fluid in the stuffing box. If we look at the illustration closely we can see six potential leak paths for the fluid we are sealing:
- The O-ring located between the seal sleeve and the pump shaft is called the static O-ring. It is a potential leak path.
- The O-ring between the seal sleeve and the outside barrel is called the dynamic O-ring. It is a potential leak path also.
- In this example the rotating face has been press fit or shrunk into the outer barrel of the seal. This face is usually a form of carbon-graphite. There is a potential leak path between the carbon-graphite and the outer barrel where the carbon has been inserted.
- The stationary face is normally the hard face and in this instance is a clamped “L” shaped design with gaskets on either side. The gasket located between the stationary face and the pump stuffing box is a potential leak path also.
- There is a potential leak path between the carbon rotating face and the stationary hard face.
- Although not shown in the illustration many pump shafts are specified with a shaft sleeve for corrosion resistance or to position the impeller in a double ended design. This shaft sleeve must be sealed to the shaft by a gasket or some other means. This gasket is a leak path also.
Many double-ended pump designs lack gaskets between the impeller and the sleeve used to position and hold the impeller.
In the following paragraphs we will be discussing all of these potential leak paths. As we do keep in mind that in addition to this balanced rotating version, seals are classified into many other categories that include: stationary, unbalanced, outside, non-metallic, dual, metal bellows, rubber bellows, cartridge, split, etc.
These classifications are described in detail in the alphabetical section of this manual. Try to keep these classifications in mind as we investigate the cause of seal failure. I have assumed you have a pretty good knowledge of mechanical seals or otherwise you would not be attempting to trouble shoot them, but if some of the terms I use are not familiar you can look them up in the alphabetical section of this book.
We will start with the causes of leakage between the stationary and rotating seal faces:
One or both of the seal faces is not flat. These faces should be flat to within three helium light bands (0,000033″ or 1 micron). Here are some reasons a once lapped face is no longer flat:
- Mishandling damaged the lapped face.
- Poor packaging. The seal should be able to survive a 39″ (1 meter) drop. To insure this, the seal should be shipped in a reusable box lined with plenty of foam or any other adequate protection.
- The stationary hard face was distorted when you tightened it against an uneven surface. The faces of some pump stuffing boxes are really in bad shape because of packing leakage over the years.
- The clamping is not “equal and opposite” across the stationary face. This is a common problem with “L” shaped and “T” shaped stationary hard faces. It is important that both stationary face gaskets be the same width to prevent this problem. You can see these same width gaskets in the last illustration. Sometimes mechanics are tempted to make the outboard gasket wider to match the wider gland. If this happens the clamping will be opposite but not equal and the hard face can be distorted.
- The “hard” seal face has been installed backwards. You are running on a non- lapped seal surface. Among seal companies it is common practice to lap only one side of a hard face. Sometimes the unlapped face is identified with a marking but not always.
- One or both of the faces is being distorted by a change in temperature. This can happen when you forget to vent a vertical pump or someone runs a water hose on the stuffing box.
- High pressure or surges in system pressure distorted the face. Water hammer would be an example of a surge in system pressure.
- The face never was flat; you have a bad part. Sometimes the automatic lapping equipment gets hot and the seal faces are lapped flat at this elevated temperature. When you try to run the seal at a lower temperature the faces are no longer flat.
- The carbon metal composite was not stress relieved after the carbon was “shrunk in”. The carbon should have been pressed into the metal holder where it could shear to conform to the irregularities in the metal part and avoid a lot of residual stress problems. If it has been “shrunk in” the assembly must be stress relieved or allowed to relax over a period of several weeks to remove residual stress in the lapped face.
One of the lapped faces has been chemically attacked.
- Oxidizing agents and halogens attack all forms and grades of carbon-graphite.
- Some de-ionized water will attack any form of carbon.
- Any number of chemicals can chemically attack filled carbons. It all depends what fillers were put into the carbon-graphite composite.
- Reaction bonded silicon carbide will be attacked by high pH liquids.
- A cleaner or solvent is being flushed through the lines and it is attacking the seal face.
- You are using a poor grade of carbon-graphite. You should go to an unfilled grade such as Pure Carbon Co. grade 658 RC. The substituting of inferior grades of carbon-graphite is a common occurrence if someone other than the original manufacturer is repairing the seal.
- Corrosion increases with any temperature increase. A 10 degree Centigrade (18°F) rise in temperature will double the corrosion rate of most fluids.
The plating or hard coating is coming off of the hard face.
- All coatings are porous. The product is penetrating this porous coating and attacking the bond between the coating and the base material, or the base material its self.
- An inferior plating was originally put on the base material. You should not use hard coatings or facings but if you must, use the D-gun process rather than plasma spraying. The D-gun process is described as a chemical bonding while the plasma process is described as a mechanical bonding.
- Differential expansion of the coating and the base material is causing the materials to separate. This causes a condition known as heat check.
The seal face is cracked, pitted or damaged.
- High temperature at the seal faces is heat checking (cracking) the plated face. This is a common problem with cobalt based tungsten carbide. The nickel base version is less likely to crack because it is not as hard as the cobalt. The nickel version also has better corrosion resistance than cobalt.
- The product is solidifying between the faces and they are breaking at start up. Most face materials have high compressive strength, but tend to be weak in tension or shear.
- Excessive vibration is causing the drive or anti-rotation pins to crack the face. Lower cost seals where the carbon is driven by a metal drive lug experience this problem quite often.
- There is a high temperature differential across the ceramic. Seven to ten temperature cycles can break grade 99.5 ceramics in hot water or hot petroleum products. Grade 85 ceramic can break with a single cycle of temperature differential.
- Air is trapped in the carbon. Heat generated between the faces is causing the air to expand and blow out pieces of the carbon face. The carbon usually blisters prior to blowing out. The solution is to go to a more dense carbon.
- The product is vaporizing and allowing solid material to blow across the lapped face. This is a common occurrence in boiler feed water applications.
- The seal faces have opened; solids penetrated and imbedded into the soft carbon causing rapid wear of the hard face. The same problem occurs if the carbon was relapped using lapping powder. You will see this condition in almost every seal you inspect. It is the most common seal failure.
- Lubricant on the lapped faces is freezing in cryogenic (cold) applications.
- The elastomer is being chemically attacked and swelling up. This can break the face in those seal applications where the elastomer is positioned at the seal inside diameter because most seal faces have poor tensile strength. In some instances the swelling elastomer will open the two lapped faces allowing the solids to penetrate. This can be a problem with boot mounted faces.
- The rotating shaft or sleeve is hitting the stationary face. This happens if the pump is running off of its best efficiency point (BEP) which almost always occurs at start up.
- The seal is being mishandled during installation. Good packaging and proper training can solve many of these installation problems.
- The crack you see may have occurred during disassembly. Check to see if there is discoloration deep in the crack. Discoloration in this area means that the crack occurred during or before operation of the seal.
- Petroleum products can coke at the higher temperature seal faces causing pieces of carbon to be pulled out as the face rotates. You will have to select two hard faces for this application if you want to stop the problem.
- The rotating face is not centered in the stationary face and is running off the edge of the stationary face. Look for rubbing marks around the outside diameter of the rotary unit. A bent shaft or an out of balance rotating shaft is the most common cause.
- You will notice a much wider wear track on the hard face if you are experiencing this problem.
- The seal will appear to “sputter” as lubricant is dragged across the face and off the seal inside diameter.
- Dirt can be dragged or blown across the faces as they separate.
The movable face is not free to follow whip, wobble or run out.
- The rotating face is hitting the inside diameter of the stuffing box.
- The recirculation line from the pump discharge is aimed at the seal faces and interfering with their free movement.
- Dirt or solids are clogging the movable components. Magnetite is a very big problem in most hot water applications.
- The product you are pumping is interfering with the free movement of the components. The fluid is:
- Crystallizing ( like sugar)
- Solidifying (like glue)
- Viscous (like molasses)
- Building a film on the sliding components ( hard water or paint)
- Coking. Oil or any other petroleum product will form a hard carbon film on moving seal parts.
- The elastomer has been chemically attacked causing it to swell up and interfere with free movement of the face.
- Temperature growth of the shaft is interfering with the free movement of the movable face. The metal vibration damper that is part of the carbon face holder used on some metal bellows seals is manufactured from a low expansion metal to prevent the carbon face from falling out. In a high temperature application the differential expansion rate between the shaft and the carbon holder can seize the vibration damper and pull the faces open.
- The shaft or sleeve is the problem.
- It is over size. + 0.00″ – 0.002″ ( 0,00-0,05 mm.) is ideal.
- The shaft or sleeve is too rough. The sleeve should have a finish (polish) of at least 32 R.M.S. (0,8 microns)
- The shaft is fretted, corroded or damaged in some way and this is interfering with the free movement of the dynamic elastomer
- Solids have attached themselves to that portion of the shaft where the dynamic elastomer is located preventing it from moving.
- A gasket or fitting is protruding into the stuffing box and touching seal component. I have seen a gasket extrude into the stuffing box when the top half of a horizontally split pump was tightened to the bottom half. The gasket pushed against the rotating seal outer barrel preventing it from moving.
- Solids from outside the stuffing box are getting under the faces. This is a problem with vertical pumps.
- The elastomer (O-ring) is spring-loaded and the interference on the shaft is restricting the face movement.
- The elastomer has extruded because of high pressure or excessive clearance.
- A foreign object has passed into the seal chamber and is interfering with the free movement of the seal.
- Solids have packed up in front of the inner seal in a “back to back” dual seal design. This is a very common occurrence.
- The dynamic elastomer or O-ring is sealing on a porous carbon surface rather than a hard metal surface. When the shaft rotation stops the elastomer relaxes and flows into the carbon surface irregularities. O-rings and elastomers flex and slide better on polished hard surfaces.
The product had plated or formed on the face and a piece of it has broken off.
- This problem occurs with products that are sensitive to temperature and/ or pressure changes. Coke is typical
- The setscrews have come loose. Most seal set screws are manufactured from corrosion resistant austenitic materials that are soft.
- The shaft has been hardened.
- They have worked loose in a sleeve that is too soft.
- The hardened setscrews have corroded.
- They were not replaced when the seal was rebuilt and as a result are not “digging” into the shaft. Stainless steel set screws should be used only one time.
The face has lost its spring load.
- The initial setting was wrong. The seal experienced a little wear and the spring load is gone.
- Axial temperature growth of the shaft has altered the original setting.
- The impeller has been adjusted towards the wet end of the pump. In most applications this will open the seal faces.
- The sleeve moved when the impeller was tightened to the shaft. Depending upon the design this can open up or over compress seal faces.
- The cartridge seal was pushed on the shaft by pushing on the gland. The sleeve static O-ring is providing an interference on the shaft and the seal is now over compressed. In a dual seal application this can over compress the inner seal and open up, or unload the outer seal.
- Some one has painted the springs of an outside mounted seal or the outer seal of a dual seal application.
- The seal was set-screwed to a hardened shaft or sleeve and has slipped due to vibration.
The product is vaporizing and blowing the faces open.
- This happens in hot applications any time there is water in the product. It can also occur if the pump or seal was hydrostatically tested with a water base fluid.
- Many products can vaporize if subjected to the heat between seal faces or if they experience a pressure drop across the lapped faces. Unbalanced seals present some real problems in this area because of the higher face temperature they generate.
The inner seal of a dual seal application was not balanced in both directions and is opening up with reversing pressure. This is a problem in unbalanced seals that are subject to both vacuum and pressure, or if the barrier fluid pressure varies. Many mixer applications alternate between a positive and negative pressure.
The single spring found in some seal designs was wound in the wrong direction for the shaft rotation. The brand “Flexibox” has this problem with some of their designs.
The next failure we will look at is leakage through the elastomers (O-rings)
Compression set causes the elastomer to change shape if you exceed the curing temperature of the elastomer you are using. The high temperature causes the material to reverts back to its cure state and assume the shape of its container. This is the reason round O-rings sometimes look square when we disassemble the mechanical seal.
- The product is too hot, you are getting poor heat conductivity, or there is too much heat being generated at the seal faces. You must be sure to vent vertical pumps to help prevent this problem.
- Compression set is a common problem with most grades of Dupont’s Kalrez®, or Green Tweed’s Chemraz because they are not true elastomers. Compression set prevents the O-ring from being free to flex and roll.
The elastomer is cracked.
- The shelf life has been exceeded. Buna N (nitrile) has a shelf life of only twelve months because of its sensitivity to ozone attack. Most other elastomers do not have this shelf life problem but you should be aware that buna N is very popular with original equipment manufacturers (OEM), especially in the form of a rubber bellows.
- High heat is the main cause of a hard or cracked elastomer. First the material takes a compression set and then it becomes hard and cracks.
- Chemical attack is another possibility. In most cases the elastomer swells with chemical attack, but cracking and shrinking do occur in isolated cases.
- Cryogenic (cold) temperatures freeze the elastomer and it will crack when hit or dropped.
- In some applications you will find that the rubber bellows did not stick to the shaft because the wrong lubricant was used during the installation process. This caused the seal faces to stick together and the shaft turned inside the bellows. The turning friction caused the high heat that cracked the rubber. You will also see fretting under the elastomer if you are having this problem.
The elastomer is cut or damaged.
- Mishandling is always a problem. Some people try to pry O-rings out of their groove with the tip of a pen knife causing damage to both the elastomer and the metal O-ring groove
- During the installation of the seal the elastomer was slid over a rough spot on the shaft or sleeve. Be careful of old setscrew marks, splined shafts, keyways, etc.
- The O-ring was extruded by high pressure. You may need a backup ring.
- The product you are sealing is penetrating into the elastomer and blowing out the other, low pressure side. This is a problem when you are trying to seal ethylene oxide.
- Teflon® jacketed O-rings can split in the presence of oxidizers and halogenated fluids. The halogens will cause the elastomer to swell up inside of the Teflon® jacket. Halogens can be recognized because they end in the letters “ine” such as bromine, astintine, chlorine, fluorine and iodine.
The elastomer (O-ring) is not seated properly.
- It was twisted during installation.
- High pressure can cause the elastomer to extrude or twist in the O-ring groove.
- Solids have “built up” or penetrated between the elastomer and the shaft.
- The shaft is corroded, damaged, or fretted under the elastomer.
- The shaft is oversized causing too much of an interference fit.
- Excessive travel can cause the elastomer to “snake”. Most O-rings flex and then they can roll up to one half of their diameter before they slide or twist.
- The O-ring groove is damaged or coated with a solid material. If you remove O-rings with a penknife or any sharp instrument you can damage both the O-ring and the O-ring groove.
The elastomer has swollen or changed color.
- Look for product attack. This is the most common cause and usually occurs within five to ten days of exposure to the product.
- The wrong lubricant was used at installation. As an example, you should never put petroleum grease on EPR O-rings.
- Solvents or chemicals used to clean the lines are often not compatible with the elastomer.
- Steam can harm many elastomers including most grades of Viton®.
- Oxidizers can attack the carbon black in O-rings and other elastomers.
The elastomer leaks when pressurized in the opposite direction.
- A common problem with unbalanced, dual seal applications. Two way balanced seals are recommended for these applications.
- Remember that O-rings are the only common elastomers that seal in both directions. Wedges, U cups, and chevrons do not have this ability. This is important if the application changes from vacuum to a positive pressure. We find this problem in condensate pumps and lots of mixer applications
Here are a few more leak paths you should know about
- The leak is between the carbon and its metal holder.
- Some seal companies and most seal repair facilities glue the carbon in place. The glue may not be compatible with the product you are sealing.
- “Pressed in” carbons can leak in a high temperature application because of the differential expansion between the carbon and its metal holder. Low expansion metal is available for the carbon holder in these applications.
- “Shrunk in” carbons do not have a 100% contact on their outside diameter because of the “out of roundness” caused by the tolerace on the carbon outside diameter and the metal holder inside diameter.
- Between the shaft and the sleeve.
- Damaged gasket or gasket surface.
- Distorted sleeve or shaft.
- Many packed, double-ended pumps have this problem because there is no gasket to seal between the impeller and the sleeve that is holding the impeller in place.
- A pipe flange is leaking above the seal and dripping into the seal area.
- I found this one after every other avenue was exhausted.
- A stationary face gasket or elastomer leaking. Please look at (1) in the following illustration of a balanced stationary seal.
- The stationary face elastomer (1) is a dynamic elastomer. This leak path is not always obvious. It often looks like face leakage.
- The gland gasket or gasket surface (5) is a potential leak path. A leak in this location is always visible.
- In this stationary version, The rotating O-ring (2) is static.
At the weld if a seal face holder is welded to a cartridge sleeve. You can see this weld in the following illustration
- In this illustration, the shoulder on the end of the seal sleeve, with the two O-rings mounted in it, has been welded to the sleeve.This weld is a potential leak path also.
- At the pipe connections, ancillary hardware, API (American Petroleum Institute) gland fittings, and recirculation lines.
- A scratch or nick in the O-ring groove will cause a small leak that is very difficult to identify. Remember that up to 100 psi. (6 bar) O-rings seal on the inside and outside diameters, not their sides.
Steam going through a quench connection (Q) is commonly misdiagnosed as a seal leak. Watch out for this one because operators have been known to shut off the quenching steam thinking they have solved the leak problem and caused a premature seal failure.
- Steam is often introduced into port “Q” of an API (American Petroleum Institute) gland to keep the other side of the seal warm, or to prevent ice formation with some low specific gravity products.
- The disaster bushing (DB) restricts the amount of quench leakage into the atmosphere and at the bearing housing.
- On February 18, 2018