Experienced technicians know that seals found in industrial equipment can create both problems and solutions, and that they are critical to the operation of many types of machinery. Failure of a seal can spell big trouble.
Seals serve two main functions in industrial equipment. They are designed to keep lubricants and grease in a contained system and to keep contaminants out. Failure of a seal can result in a loss of lubrication and can ultimately cause component failure as the system loses its lubricant. If a seal fails to keep contaminants out, the contaminants can cause failure as they are mixed with and carried by vital lubricants.
While the purpose of seals is simple enough, the composition of seals and their design has evolved to become very complex. Seals are made from many materials, ranging from rubber O-rings to polyacrylates and silicone substances. Seals can also be of a mechanical design. Each design is equally complex, and each should be carefully selected for a job and properly maintained.
Improper or defective seals can have a huge impact on oil waste. It is estimated that over 10 million gallons of lubricants could be saved each year if external leakage from industrial systems such as pumps, hydraulics, gears and oil sumps were eliminated.
Several years ago, Mobil Oil Corporation did a study demonstrating that many hydraulic systems in the U.S. actually used four times their normal capacity in lubricants every year. It is certain that much of this excessive usage can be attributed to leaks.
Unfortunately, the mindset among many industrial maintenance managers has been to regard leaks as a part of normal operation.
With the current sealing technology, that type of thinking is far from sound. In fact, millions of dollars worth of lost lubricants could be saved with the right approach to lubricant seals. Maintenance mangers who are educated on seal technology find it easy to adopt a position that fluid leaks are intolerable and ultimately reduce plant operating costs significantly in the long term.
Static Type Seals
O-rings and gaskets are some of the most commonly used seals in today’s machinery. These are static-type seals that are used when there is no movement between mating surfaces.
O-rings make effective seals because they can return to their original shape even after they have been deformed. However, due to their elastomer construction, they can also shrink to several times smaller than the metal surfaces they are fixed to when the temperature drops. This causes some leaking and may require the substitution of a larger diameter O-ring.
In some cases, O-rings get worn away by pressure increases, and this can eventually cause leaking and subsequent mechanical failures.
Gaskets are the oldest sealing devices. They are designed to absorb the entire load across their surface. Under this load, the gasket material can eventually creep out of place, and the seal will need to be reestablished by tightening on occasion to prevent any leakage.
Mechanical Seals
Mechanical seals consist of two main parts, a stationary part which is attached to the housing and a second part which rotates on a shaft. The sealing action occurs between the stationary prt and the flat face of the rotating part which is perpendicular to the shaft. This is typically known as the end face seal or axial seal. By contrast, a simple packing seal does not rotate.
A mechanical seal requires mechanisms to insure that this facing occurs. Springs, bellows and glands are all frequently utilized. The center of such seals may even be fluid-filled. Magnets are frequently used to eliminate the need for springs, and positive drives are often employed to allow the seal to rotate with the shaft. Some lubrication of the sel es needed to allow it to properly close. This type of seal generally experiences the most wear across its face.
Mechanical seals have several advantages over packing type seals. These include:
- Elimination of wear on the shaft or sleeve
- Little leakage
- Automatic wear adjustment
- Reduced power consumption due to lower contact area with the shaft
Packing Seals
Packing seals are designed to restrict the leakage between a moving and a stationary part. These seals are commonly referred to as compression seals, automatic seals or floating seals.
Compression seals are made up of woven, twisted, braided or wrapped materials such as flax, asbestos, rubber or some other type of elastomer. These materials are eventually compressed down, which causes lubricant that accumulates in the material’s open spaces to squeeze out over time. These packings are also squeezed into place and pushed forward by a gland plate, so some leakage from all of this pressure is perfectly normal and desirable. However, care should be taken not to tighten these assemblies to the point where there is no lubrication, as some lubrication is always necessary to lubricate the seal itself.
Automatic packing requires no glad adjustment. Instead, this system relies on the pressure of the fluid to form a seal. O-rings and V-rings are both seals that exhibit automatic packing.
Lip seals, also known as flanges or piston cups, are another type of packing seal. These seals use fluid pressure on the inside of the lip to force it against a mating surface and prevent leakage.
Lip seals are among the most common rotating seals. They are often spring supported to add stability and flexibility as well as strength.
Lip seals only form a seal in one direction. If placed inward, they will seal in a lubricant or fluid. Such types of inward lip seals are known as retention seals. If placed outward, they will seal out contaminants. F If both contaminant control and fluid control are required for an application, two lip seals can be placed back-to-back to accomplish both functions simultaneously.
Why Do Seals Fail?
Seals in pump applications can fail for a variety of reasons including improper alignment, dry run, low flow operations, axial shaft movement, cavitation, lack of seal face lubrication, solids in the fluid solution or from other forces and movements. In addition, seal failure is often driven by the wear generated by the actual component.
For example, in a centrifugal pup, seal wear can be caused by what is known as impact wear. This occurs when improper alignment between motor shafts and the pump causes excessive vibration and subsequent stress on any seals, bearings and couplings. The seal may even get chipped and begin leading.
Failure can also occur with axial shaft movement. When the seal is overloaded, it can potentially crack or be damaged by heat. This type of damage can result in fretting damage because the motion is being transferred to the seal. Heat damage or heat checking can be identified as either a single crack through the seal material or a series of surface cracks on the seal material. This is dependent upon the type of seal material utilized.
| Synthetic Seal Recommendations |
Synlube
Bases/Oils |
Recommended
Elastomers |
Not
Recommended |
| Diester |
Viton, Buna* |
Neoprene |
| Polyolester |
Viton, Silicone |
Styrene, Neoprene |
Synthetic Hydrocarbon
(PAO) |
Buna* |
Natural Rubbers |
| Polyalkylglycol (PAG) |
Viton, Buna* |
Natural Rubbers |
| Phosphate Esters |
EPR |
Styrene |
| High Nitrile content, typically above 36 percent |
|
Erosion by abrasive materials is another circumstance that can lead to seal damage, because abrasive particles can wear away the carbon ring found on most seals.
Another form of erosion that can occur on the seal face is known as “wire drawing.” This can happen if a pump is running on idle while the mating seal surface becomes distorted. A similar condition known as “wire brushing” occurs when abrasive contaminants are present in the seal chamber. Such contaminants will cause the seal to operate with a mechanically distorted mating ring.
Corrosive wear typically occurs when high temperatures are present at the seal face. This situation is usually the result of abrasive particles in the liquid coming into contact between the elastomer and the interface.
Acid attack can also cause swelling or shrinkage of seals. This can happen when the seal face is not properly aligned.
There are many causes of seal wear and fatigue. As always, proper damage diagnosis and replacement can prevent eventual seal failure and subsequent machine failure.
Seal Selection Criteria
When selecting and specifying seals, consider the following:
- Shaft Speed
- Temperature – This must not exceed the operating temperature of the elastomer
- Pressure – Most seals are not designed for pressure above 8 psi. Relief systems are needed for higher pressure.
- Shaft surface finish – Tool marks and the type of finish are seal life factors. Polished or ground shafts with concentric finish marks are best.
- Concentricity – Misalignment will cause wear on one side of a seal lip and will shorten seal life.
- Tolerance – The best seal life can be obtained when there is close shaft and bore tolerance. Shaft vibration and end play are also factors in seal life.
- Runout – Bearing wobble or shaft whip should be controlled to keep runout to a minimum. Failure to do so will shorten seal life. Also note that flexible couplings will not fix a misalignment problem.
- Lubricant – seals must be lubricated with compatible lubricants in the right viscosity. Seal compatibility with synthetic lubricants is frequently ignored.
