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How to avoid pump packing failures.

While mechanical seals have become a more common way to seal rotating pumping equipment, there are still many pump types--centrifugal, rotary, turbine and reciprocating--found in many industries that use compression packing. But when this type of packing fails, the consequences, from pump damage and pump failure to unscheduled plant shutdowns, can be devastating. So keeping compression packing functioning properly may well be the most critical factor that keeps the pump working. Here's how to do that.

Choose the correct packing

The first key to guard against packing failure is to select the proper packing. Check to see that your packing meets the operating parameters of the system into which it will be installed. Confirm packing temperature ranges, shaft-speed capabilities and fluid compatibility. Next, check packing samples to make sure they are easy to work with and install. Some packings are too rigid, hard to cut or need to be stretched or rolled prior to installation. What you want is a flexible, easy-to-cut, consistently braided packing with a uniform packing width.

Don't overlook ease of removal. Avoid packings that harden, leave residue or tend to score shaft sleeves. Always use a packing that is non-hardening, non-abrasive, and is as easy to remove as it is to install.

Durability is key, too. Make sure the packing you choose is strong and long-lasting. And don't automatically choose the lowest-cost alternative. Remember that the cost of the packing is insignificant compared to the cost of repacking the pump or facing a packing failure.

Compression packings are made from many different materials and combinations of materials, each offering its own set of properties. The performance of any packing is a direct result of these combinations of properties. Because frictional heat buildup is a leading cause of packing failure, it's important to ensure that your packing will run cool, even after extended periods of continuous operation.

To predict how a packing will work, it is not enough to focus on a single performance attribute. Conformability, lubricity, low coefficient of expansion, braid construction, low abrasiveness, and ease of installation all impact performance. Also, be aware that packing manufacturers often mix a number of materials in their braided packing to achieve a particular attribute. While one attribute may be enhanced, another may be diminished. Examine each ingredient to determine the weakest link, and match your application parameters with the packing operating specifications.

Other failure factors

Other factors can defeat even a properly chosen pump packing material. These are improper installation of the packing, and faulty pump equipment. To determine if one of these might be the cause of packing failure, look for clues in the old packing. If the removed packing is thinner in one area of the inside surface, then the shaft is improperly aligned. If there is raggedness or shredding on the inside surface, you'll probably also find chipping or burrs on the shaft sleeve. When old packing has become hard or abrasive, it means that either the wrong packing material was used or the stuffing box is worn, causing gland water to flow outside the packing rings and not cool them properly.

If one or more rings are missing or if there is partial extrusion of the inside ring, there is too much throat clearance. On the other hand, if the outside ring shows evidence of extrusion between the shaft and the follower, there is too much gland follower clearance.

Proper packing installation

To ensure proper packing installation, first measure the packing for consistent width. Don't rely on what's printed on the spool or box. Next, inspect the old rings as discussed above, and look at the stuffing box and sleeve. Don't ignore gouges or burrs, as they will destroy any packing. Also, many packing rings will shrink over time, and misdirected cooling water will seep outside the rings, causing them to become hard and fail. Replace any damaged equipment now.

When repacking, the best way to cut packing is to use a mandrel the same size as the shaft and cut the packing on a 45[degrees] angle. Seat each ring individually, staggering the joints, and install any lantern ring properly. Then tighten the gland follower nuts finger-tight to start. Compress the packing by evenly tightening the follower with a wrench, one full turn. Back off the nuts and tighten again, also only finger tight. Turn on the gland water for a few minutes, then run the pump for 30 minutes. Adjust the gland follower by turning the nuts 1/6 of a turn, one flat at a time, every 15 minutes until an acceptable leak rate is achieved.

By selecting the proper packing, making sure there are no equipment problems, and installing packing correctly, you'll be able to rely on this packing until the next scheduled packing replacement. Normal periodic inspections are all that should be required along the way.

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Title Annotation:Tech Check
Author:Guizzetti, Al
Publication:Industrial Maintenance & Plant Operation
Date:Nov 1, 2004
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