19-10 QUICK REFERENCE GUIDE FOR MECHANICAL SEAL FAILURE
Of all the seal related activities, analyzing mechanical seal failure continues to be the single greatest problem for both the consumer and the seal company representative.
If you will take a little bit of time to familiarize yourself with the following outline you should feel a lot more comfortable the next time you are called upon to do some seal troubleshooting.
As you inspect the failed seal components keep in mind that a rebuilt seal may have some rub or damage marks that occurred during a previous failure making them especially difficult to analyze.
Regardless of the design, mechanical seals fail for only two reasons:
- Damage to one of the components
- The lapped seal faces open
Damage to one of the components.
This damage is almost always visible. Look for :
The elastomer swells or the other seal parts become "sponge like" or pitted.
- The product you are sealing is attacking one of the seal components.
- The attack is coming from the cleaner or solvent used to clean the lines between batches or at the end of a "run."
- The attack is coming from lubricants put on the elastomers or seal faces. petroleum grease on Ethylene Propylene O-rings will cause them to "swell up".
- Galvanic corrosion - Happens with dissimilar materials located close to each other and connected by an electrolyte. As an example, stainless steel can attack the nickel binder in a tungsten carbide face.
- Oxidizers attack all forms of carbon including black O-rings.
- Wear or rubbing of a flexible component.
- Thermal shock of some seal face materials. Especially those that are hard coated or plated.
- Thermal expansion of the shaft/sleeve can break a stationary seal face or interfere with the free movement of a dynamic elastomer.
- The rotating seal hits something because of shaft deflection.
- Temperature extremes (both high and cryogenic) will destroy elastomers and some seal face materials.
- Erosion from solids in the product you are pumping.
- Fretting caused by the dynamic elastomer removing the passivated layer from the corrosion resistant shaft or sleeve.
- Fluid abrasion that can weaken materials and destroy critical tolerances.
- A discharge recirculation line can break a metal bellows and injure lapped seal faces, as well as interfere with the free movement of the seal.
- The elastomer can swell because of chemical attack and breaks the lapped face.
- Problems at installation. This includes mishandling, setting at the wrong compression, putting the wrong lubricant on the elastomer etc. Fatigue of the springs caused by misalignment.
The seal faces open prematurely
Scoring or wear of the hard face is the most common symptom of this failure. The scoring occurs because the solids imbed into the softer carbon face after they open. The seal faces must stay in contact, but there are all kinds of conditions that are trying to force or pull them open.
Causes of physical opening:
- Axial shaft movement (end play or thrust). This is normal at start up.
- Radial shaft movement (run out or misalignment)
- Operating off of the pump best efficiency point causes shaft deflection.
- Hysteresis caused by a viscous (thick) product.
- Centrifugal force tries to separate the faces in a rotating seal application because the flexible mounted face attempts to position its self perpendicular to the shaft and the stationary face is not perpendicular because it is referenced to the face of the pup stuffing box
- Hydrodynamic or lifting forces generated between the lapped faces.
- Pressure distortion caused during pressure peaks such as water hammer and cavitation.
- Thermal distortion that can cause the seal face to separate from its holder or "go out of flat."
- A failure to provide equal and opposite clamping across the stationary seal face will cause distortion.
- A hardened sleeve can cause the holding set screws to slip.
- A wrong initial setting of the face load.
- Impeller adjustment after seal load set
- Springs clog if they are located in the product.
- Loose set screws. If the sleeve is too soft they can vibrate out.
- Shaft tolerance and finish is out of specifications.
- The rotating shaft or seal hits something.
- The discharge recirculation line can direct liquid towards the seal faces and restrict their movement.
- Outside springs painted by maintenance.
- A cartridge seal installation method can compress one set of faces and open the other.
- Fretting hang up.
- Cartridge mounted stationary seals move excessively unless they have some type of "built in" self-aligning feature.
Product problems that open the seal faces
With a loss of an environmental control. The fluid can:
- Vaporize between the faces forcing them open and causing a "chipping" of the carbon outside diameter as well as leaving solids between the lapped faces.
- Become viscous preventing the faces from following normal "run out".
- Solidify between the lapped faces or around the faces.
- Crystallize between the faces or around the dynamic portions of the seal.
- Build a film on the sliding components or between the faces causing them to separate.
- Be operating in a slurry and/ or abrasive
- Be operating in a vacuum causing the ingestion of air between the faces of some unbalanced seal designs.
- Swell up the dynamic elastomer locking up the seal .
- Cause slip-stick between the faces if the sealed fluid is a non or poor lubricant
The common causes of shaft displacement:
- Operating off the pump's best efficiency point (B.E.P.).
- Misalignment between the pump and its driver.
- The rotating assembly is out of balance.
- A bent shaft.
- A non concentric sleeve or seal.
- Slip stick
- Passing through, or operating at a critical speed.
- Water hammer in the lines.
- The stuffing box is not square to the shaft, causing misalignment problems.
- Pipe strain.
- An impeller adjustment is made to compensate for normal impeller wear.
- Thermal growth of the shaft in both a radial and axial direction.
- Bad bearings or a poor bearing fit.
- Two direction axial thrust at start up is normal.
- The pump’s motor is finding its magnetic center.
- Cavitation - there are five separate types of damage that can be observed.
- The sleeve moved when the impeller was tightened.
- The unit is pulley driven causing side thrust
- The impeller is positioned too far from the bearings. This is a severe problem in mixer or agitator applications.
Prevent premature seal failure by controlling the environment
Control the temperature in the seal area:
- Use the correct spring or bellows compression.
- Use only hydraulically balanced mechanical seals.
- Select a low friction face combination.
- Avoid "dead ending" the stuffing box.
- Jacket the stuffing box
- Quench behind the seal
- Use a gland jacket
- Utilize two seals with a buffer/ barrier fluid between them
- Use heat tape around the stuffing box
- Use a heat pipe to remove heat from the stuffing box.
- Vent the stuffing box -- especially in a vertical application
- Flush in a cool compatible liquid.
Control the pressure:
- Be sure to use only hydraulically balanced mechanical seals.
- Discharge recirculation will raise the pressure in the stuffing box if you put a restrictive bushing into the bottom of the box.
- Suction recirculation will lower the pressure.
- Use two seals and let the barrier fluid control the pressure between the seals.
- Cross-connect the stuffing boxes to equalize the stuffing box pressures in a multi stage pump.
- Stage the pressure with tandem seals.
- Impeller pump out vanes will lower stuffing box pressure.
Give the seal more radial space:
- Bore out the existing stuffing box if it is possible.
- Make or purchase a new back plate with the large stuffing box cast in.
- Make or buy a large bore stuffing box and attach it to the back plate after you have machined the old one off.
Flush the product away if you are unable to control it:
- Suction recirculation will bring fluid into the stuffing box from behind the impeller, where it is usually cleaner. This works on most closed impeller pump applications and those open impeller pump applications where the impeller adjusts to the volute rather than the back plate.
- Flush with a clean liquid from an outside source.
- A pressurized barrier fluid between two seals can keep solids from penetrating between the faces, if the faces should open. This application will also work if the solid particles are less than one micron in diameter (Kaoline is such a product).
Build the seal to compensate for operating extremes.
Slurry or anti-clogging features that can be part of seal design:
- Springs out of the fluid
- Teflon coating - so particles will not stick to sliding components.
- The elastomer moves to a clean surface as the face wears.
- Rotate the spring loaded face to take advantage of centrifugal force that will throw solids away from the lapped faces.
- Rotate the fluid with the seal to prevent erosion of the seal components. A simple vane arrangement can accomplish this.
- Use two hard faces if you find it impossible to keep the lapped seal faces together.
- Use a pumping ring to keep solids away from the faces.
- Mount the seal closer to the bearings to diminish the affect of shaft deflection.
Design for higher temperature capability
- Eliminate elastomers when ever possible.
- If you cannot eliminate elastomers the O-ring location is important. Try to move the elastomer away from the faces.
- Hydraulically balanced mechanical seals generate less heat.
- Select low friction faces.
- Fool proof, correct installation dimensions are necessary. A cartridge design is your best choice.
- Keep a good product circulation around the components.
- A good lapping technique will keep the faces flat at high and cryogenic temperatures.
- Pumping rings will keep fluid circulating between two seals. If you are using balanced seals a simple convection tank is usually more than adequate. An air operated diaphragm pump can be used in the line to increase the circulation. Try to avoid the use of petroleum based fluids as the barrier fluid between the seals. Petroleum based fluids have a very low specific heat that will increase the temperature between the seals,
- Gland features such as quenching, recirculation, venting and flushing help.
- Choose well designed faces that will resist thermal distortion. The closer you get to a "square block" design, the better off you are going to be.
- Do not insulate pressed in seal faces with an elastomer (rubber part or gasket).
Design for pressure resistance:
- Limit the number of diameters in any single seal component
- Laminated bellows will allow you to keep a low spring rate while maintaining pressure capability, if you are using a welded metal bellows design.
- Finite element analysis of the seal components will prevent pressure and temperature distortion. No long thin sections
- Use more mass to resist hoop stresses.
- Higher modulus materials will resist bending and deformation.
- Use a tandem seal design for pressure break down between two seals.
Design for corrosion resistance:
- Choose good materials - clearly identified.
- Eliminate elastomers when possible. Elastomers are the most corrosion sensitive part of the seal.
- Design non stressed parts when ever possible
- No weldments. If they are necessary monitor the temperature to prevent inter granular corrosion
- Control the temperature. Corrosion increases with temperature.
- Use non metallic materials for non metallic equipment.
- Watch out for galvanic corrosion when using dissimilar materials.
- Do not use stainless steel springs. Stick with Hastelloy "C" if the metal parts of the seal are manufactured from iron, steel, stainless steel, or bronze. If the seal is manufactured from a different metal, use springs manufactured from that material.
- Do not depend upon flushing to provide corrosion resistance. Use the correct materials, keeping in mind that solvents and steam are sometimes used to flush the lines. Any materials that you select must be compatible with these flushing or cleaning fluids also.
If you need cryogenic capability:
- Go to a welded metal bellows configuration to eliminate all elastomers.
- You will need a special carbon/ graphite face that has an organic material impregnated to assist in the release of the graphite.
- Avoid plated or coated hard faces. Differential expansion will cause them to crack.
- Always lap both seal faces at a cryogenic temperature.
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