Mechanical seal installation affects reliability

A pump which cavitates, has high vibration, or needs an overhaul rather than a seal change is not going to allow a seal to reach its design life of more than 3 yr of continuous service.

06/01/1999


A pump which cavitates, has high vibration, or needs an overhaul rather than a seal change is not going to allow a seal to reach its design life of more than 3 yr of continuous service. To obtain maximum seal life, a pump and seal support system must be operating properly.

Modern mechanical seal designs can be extremely cost effective and reliable when installed on correctly sized and maintained pumps. While all of the conditions which must be considered to attain a minimal seal service life cannot be covered, there are typical problems that everyone encounters when installing seals.

Mechanical seals are precision devices, with faces lapped within one micron flatness (Fig. 1). Seals can be very costly, and the installation procedure can determine how much of the dollar value spent is actually realized. With the increase in multicraft personnel doing seal installations, correct procedures become even more important to obtain reliability and value. Poor installation, even with newer cartridge seal designs, is still one of the major causes of short seal life and accounts for up to 20% of seal failures.

The old mechanical seal should be disassembled to determine the cause of failure, unless the causes are obvious. A few minutes taken here can dramatically improve the life of the next installation. Many times, old seals can be field repaired with a kit (Fig. 2). New seals may require installation of different O-ring compounds, based on the failure analysis.

O-rings

Proper elastomer material choice is essential to seal operation. Temperature limits and chemical compatibility must be checked. Charts are available from seal and O-ring manufacturers. The O-ring selected must be correct for the products being sealed and any cleaners used to flush the system. Be careful to only use the O-ring lubricant supplied with the new seal.

O-ring damage can easily occur. Cutting or nicking is a common problem, especially when the O-ring is compressed in its groove and must slide over holes, snap-ring grooves, key-ways, slots, or threads. Ensure that all surfaces are properly deburred and feel smooth to the touch. It is sometimes possible to cover obstructions with thin tape or plastic wrap to ease installation.

Be certain that O-rings are in their proper position, in grooves or counterbores. Some seal designs have many steps, and the O-ring location may not be obvious. Refer to the installation instructions and drawings or photographs. Silicone grease can be used to keep the O-ring in place when it serves as a gasket. In these cases, a groove is usually machined into the seal gland.

The surface finish on which the secondary seal or O-ring must seal is very important. For a static surface, where there is no relative movement, a maximum of a 45 rms finish can be used. For dynamic surfaces a maximum of 32 rms should be present, and in applications where there is substantial axial movement a finish of 16 rms is preferable. The harder the O-ring durometer, the finer the finish required. Surfaces must be free from defects such as scratches, nicks, and burrs.

Teflon or Teflon encapsulated O-rings are stiff and easily damaged. Placing these O-rings in hot water softens them and facilitates installation. Lubricate them well before installation. Check the orientation of spring-energized Teflon seals because they are typically unidirectional.

Graphite secondary seals must be handled with care. They are fragile and easily damaged; O-rings can be twisted and stretched but graphite rings break. One factor to carefully control is the compression of graphite rings. Very often they are compressed in arrangements which use screws.

It is imperative that the loading be done uniformly to maintain squareness to the shaft on rotary parts and parallelism to the gland on stationary parts. A torque wrench and a dial indicator should be used to maintain squareness and parallelism within the limits specified by the seal manufacturer.

Installation

Installation instructions contain a wealth of information. Read the instructions before installing the seal and save the instructions for future reference. They can be useful when a problem arises.

When reassembling the seal, the instructions discuss, at length, a number of precautions to be observed; read and follow them. Don't force components together; seal faces are easily damaged and expensive to replace. If a seal face or component is damaged it should not be installed.

Work on clean surfaces, with clean hands. Particulates can destroy a seal face in a few revolutions. Take the time to do the installation at a relaxed pace. Do not hurry, as this often leads to tearing the equipment down again.

Be careful when handling the seal, particularly the faces. If the seal is dropped, do not use it. Have the seal vendor check it out thoroughly. If parts do not go together, don't force them. Measure, check drawings for orientation, and lubricate parts.

Don't overtighten, which is probably one of the most common mistakes. Never use a "leverage enhancement" method. Wrenches and hex keys are designed for the average person to apply the proper amount of torque to a fastener. Two-bolt glands are easily overtorqued, which distorts stationary seal components and results in face leakage. Overtightening can damage the gasket or cause extrusion, which can also result in leakage.

When installing a noncartridge seal, be certain to use the stationary seal centering shims that are included. If the stationary seal face is not centered, the rotary face can run off of it, shortening seal lie.

Unfortunately, when leaks are encountered the natural tendency is to keep tightening. Instead, try loosening; it often works. The preferred method is to use a torque wrench correctly calibrated in in.-lb. Installation instructions give torque values for gland bolts, set screws, and socket head cap screws.

If there is doubt about anything not being correct, or the seal doesn't feel right, it's a good idea to call the manufacturer or the sales representative for help. The seal manufacturer wants to ensure your plant's success with their product. Calling them two days after the seal started leaking doesn't help anybody.

Testing

Once the seal is assembled, it should be tested. Dual seals are often tested with pressurized air and submerged in water. Seals designed for liquids may let some small bubbles escape past the faces. Slowly rising bubbles between faces are not a problem because liquids do not leak under these circumstances. Bubbles coming from O-rings, through a casting, welding, or a bellows, are not acceptable.

Test for face leakage by rotating the seal by hand. This method usually stops the leak. Put a few drops of water between the faces to verify the integrity of a liquid seal. Use distilled water because tap water leaves mineral residues on the faces once it evaporates. This approach is particularly important if the seal will be stored for several weeks prior to installation.

Auxiliary equipment

Although not specifically part of a seal installation, auxiliary equipment installation and placement can have a significant impact on seal performance. Most common problems relate to seal pots/ barrier fluid reservoirs, also referred to as convection tanks.

It is important to place them close to the seal, within 3-5 ft, and at least 1-ft above the seal. Use tubing fit with large radius bends as opposed to piping with a multitude of elbows and other connections. Keep the piping as simple as possible; the more complicated the piping to and from the seal, the poorer the flow of the barrier fluid.

Another common environmental control is a flush, introducing a clean liquid into the seal gland or stuffing box to extend seal life. If installing a flush, determine that it has at least 15-psi more pressure than the stuffing box, and ensure that the flush liquid is properly connected to the correct flush port.

Most balanced seal designs do not require a large volume of flushing; 5-8 gal./hr is normally all that is required for proper heat removal. Unbalanced seals require substantially higher flows. Install a flowmeter/rotometer to visually ensure the flush is entering the seal area (Fig. 3). The flush supply valve should be lockable in the open position.-- Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, j.foszcz@cahners.com

Key concepts

Determine the cause of seal failure to prevent repeating the same mistakes.

Read and follow installation instructions thoroughly and save them for future reference.

Seals are precision components that must be treated carefully in a clean environment.

Ensure that barrier and flush fluids are flowing with enough volume and pressure.

Common seal lubricants

- Silicon grease can be used on most O-rings

- Petroleum jelly (be careful not to use with ethylene propylene seals)

- Glycerin

- Water, sometimes used if there is a need for the elastomer to vulcanize to another part when it is used as a drive mechanism

- Special lubricant, for compatibility with the product pumped

Installation instructions

- Assembly/disassembly procedures to follow when repairing a seal or when changing O-rings.

- Dimensional data that should be checked prior to installation to ensure the seal fits in the equipment before everything is torn apart.

- Operating limits that spell out the pressure, speed, and temperature at which the seal can operate.

More info

The authors are available to answer questions about seal installation. Henri Azibert can be reached at 617-430-7000, ext. 6697. "Doc" Burke can be reached at 708-596-8400.

See the "Fluid handling" channel on www.plantengineering.com for more articles related to this topic.





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