PREVENTING PREMATURE SEAL FAILURE P035_1
Here are some techniques you can use to prevent product problems that cause premature seal failure.
You can control the environment around the mechanical seal:
- Control the temperature in the seal area
- Use the correct spring or bellows compression.
- Use only balanced 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 barrier or buffer 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 in the seal area
- Be sure to use only hydraulically balanced seals.
- Discharge recirculation will raise the stuffing box pressure if you put a restrictive bushing into the bottom of the stuffing box.
- Suction recirculation will lower the stuffing box 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 or double-ended pump.
- Stage the stuffing box pressure with tandem seals.
- Impeller pump out vanes can lower stuffing box pressure.
Give the seal more radial space
- Bore out the existing stuffing box if it is possible. The wall thickness may not be thick enough.
- Make or buy a new back plate with the large stuffing box cast into it.
- Make or buy a large bore stuffing box and attach it to the back plate after you have machined the old one off.
- Remove the troublesome product from the seal area 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 the stuffing box 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).
Design the seal to compensate for extremes in the operating conditions.
- Slurry 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.
- Mount the seal into a large clearance gland positioned between the stuffing box face and the bearing case.
Design for higher temperature capability:
- Eliminate elastomers when ever possible.
- If you cannot eliminate elastomers, the O-ring location becomes important. Try to move the elastomer away from the hotter seal faces.
- Hydraulically balanced seals generate less heat between the lapped faces.
- Select low friction faces.
- Fool proof, correct installation dimensions are necessary. A cartridge seal design is your best choice.
- Keep a good product circulation around the seal components.
- A good lapping technique will keep the faces flat at both 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 the faces with an elastomer.
- Make sure the stuffing box is vented, especially in vertical applications
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.
- Use more mass to resist compressive hoop stresses.
- Higher modulus materials will resist bending and deformation.
- Use a tandem seal design for a pressure break down between two seals.
Design for corrosion resistance:
- Choose good materials. Make sure they are clearly identified by type and grade.
- Eliminate elastomers when possible. Elastomers are the most corrosion sensitive part of the seal.
- Design non-stressed parts when ever possible
- Try not to weld any of the metal components. If it is necessary, monitor the temperature to prevent inter-granular corrosion
- Control the stuffing box 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 lubricant.
- Avoid plated or coated hard faces. Differential expansion will cause them to crack.
- Always lap the seal faces at the proper cryogenic temperature.