Gene Vogel
EASA Pump and Vibration Specialist

When talking with various pump manufacturers and mechanical seal suppliers, you’re likely to encounter a variety of recommendations. (Some are complementary and some are contradictory.) With the variety of seal types, materials and pump designs, it is inevitable that what works well for one mechanical seal installation may not work for others. We will explain some good general practical considerations that will help service centers make judgments about the best techniques for the pumps they may encounter.

A primary consideration is whether the seal is a pusher or non-pusher type. The difference is not always obvious. The seal head of a pusher type seal must move freely on the shaft during operation. If it sticks to the shaft, the spring cannot apply proper tension to the seal faces, and the seal will leak. Non-pusher seals, however, should ac­tually grip the shaft during operation. In either case, the seal must slide onto the shaft to the proper location during installation without “rolling,” cutting or otherwise damaging the elastomer device that seals it to the shaft.

Non-pusher and pusher type seals
So how can a seal be identified as pusher or non-pusher type? As stated earlier, it is not always obvious. Non-pusher type seals will have provisions for the spring to move the seal face independent of the elastomer that seals it to the shaft. That elastomer is gener­ally referred to as the secondary seal. It can be an o-ring, a boot, a wedge or other shape or construction. 

Reach inside the bore of the seal head and hold the head at the secondary seal. Now push on the seal face assembly and see if it will move slightly toward the secondary seal. A non-pusher type seal will move an eighth of an inch (3 mm) or so; a pusher type seal will not. John Crane Type 1, Type 2 and Type 21, and U.S. Seal Manufacturer Type A through Type Q and Types S, T & U are all non-pusher type seals (see Figure 1).

For a pusher type seal, the shaft should be polished to a 32-16 rms finish. That’s a glassy appearance. For a non-pusher type seal, the shaft should be a 90-62 rms fin­ish, which has a slightly duller appearance. No machining marks, thread­ing or scratches of any kind should be present in either case. 

Some seal manufacturers recom­mend that a glycerin based, water soluble lubricant should be applied to the shaft to allow the seal to slide smoothly into place. Others recom­mend using a light machine oil. Some pump manufacturers recommend a light grease for this purpose. The key is to use a lubricant that is compatible with the elastomer material and with the application (the pumpage). For in­stance, Nitrile (Buna-N) is not compat­ible with brake fluid. Motor oil may not be something customers want in their process fluids. Silicone lubricants such as WD-40^® will not allow non-pusher seals to grip the shaft after installation. The glycerin-based lubricants seem to be satisfactory for most installations.

If the seal head must slide over a keyway or shaft shoulder during installation, the elastomer secondary seal may be damaged. Wrapping that section of shaft with a small piece of Mylar^® or winding slot insulation pa­per will prevent such damage.

Amount of spring tension
Another consideration is the amount of tension the spring applies to the seal faces. This is a factor of the amount the spring is compressed (see Figure 2). Most seal suppliers and pump manufacturers provide a working length for the seal which de­termines the amount of spring tension. But switching seal suppliers can mean different working lengths. The pump manufacturer may specify a certain length, but a third party seal supplier may have a different design, and a dif­ferent working length. 

Any machine work that may have been done to the shaft, bearing hous­ing or seal housing has the potential to alter the seal working length. On occasion, it may be advantageous for a service center to swap parts from vari­ous new or used mechanical seals. In such cases it is important to properly calculate the correct face tension and appropriate seal working length. While springs from different seals may ap­pear similar, their spring constant (K) may be quite different.

Seal faces are lapped to precision smooth and flat specifications, usually to 3 helium light bands. (A discussion of seal faces and the helium light band spec is a topic for a future Cur­rents article.) The faces must be kept absolutely clean and not be scratched in any way. Many suppliers caution against using any type of lubricant on the seal fac­es. However, at least one pump manufacturer has seal installation instruc­tions that tell you to oil the seal faces. Some seal suppliers recommend that a glycerin-based lu­bricant should be applied to the seal faces. 

Seal faces can always be lubricated with either a pure quantity of the fluid that is being pumped (pumpage) or with the barrier fluid used in the seal cavity for dual seals. Most submersible pumps use oil as the barrier fluid.

Work in a clean environment
One issue with apply­ing a lubricant to the seal face is that lubricants trap airborne contamination, which can damage the seal face when the pump is started. Working in a clean environment and minimizing the time that seal faces are exposed will alleviate this risk. Incidentally, some may ask: “Where do you find a clean environ­ment in a service center?” The answer is for you to create one; a clean environ­ment for pump assembly is an impor­tant element in quality pump repair. 

Stationary face ring
The other important component of the seal is the stationary face ring. Often this component mounts in a recessed bore at the bottom of the seal chamber. That can make installa­tion difficult since the size of the seal chamber limits the ability to reach in to install the stationary face ring. Even when the mounting location is acces­sible, applying the necessary force to properly seat the face ring can be a challenge. As with the secondary seal on the rotating head of the seal, the stationary face ring will be mounted in an elastomer cup or boot, or with an o-ring, and the same concerns for using a lubricant exist. 

It is often necessary to press directly on the seal face to seat the stationary face ring (see Figure 3). But some seal suppliers caution against even touch­ing the seal face, since that may leave fingerprints which can trap contamina­tion and damage the face. Either wear clean latex gloves when handling the seal and touching the seal face, or use a piece of the wrapping paper from the seal package to protect the seal face when pressing it into the bore. Some seals have special installing tools avail­able, which may be simply a section of plastic tube that fits the outer edge of the seal face. Sometimes a suitable installing tool can be fashioned from a clean piece of PVC pipe. Any such tool should not contact the seal face in the area where the rotating face will contact.

It is absolutely critical that the in­stalled stationary seal face be square to the shaft rotating axis. The seal ring mounting bore should have been checked for squareness during the repair process. The elastomer cup, boot or o-ring must be in excellent condition, free of any anomalies that would prevent the face ring from seating properly. The edge of the mounting bore should have a slight chamfer to allow the face ring to be pressed in smoothly and fully against the bottom of the bore.

With the wide range of seal and pump types, and applications, there is no single “best way” of installing a mechanical seal. The tips outlined here will help deal with common issues that occur during mechanical seal installation. Seal suppliers and pump manufacturers often have instructions and recommendations available that apply to a specific ap­plication.

Categories: Pumps, Pump seals

Tags: Pump seals

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