WHAT DO YOU KNOW ABOUT THE ADVANTAGES AND DISADVANTAGES OF MECHANICAL SEALS AND THEIR DEVELOPMENT HISTORY?

 

 

 

WHAT DO YOU KNOW ABOUT THE ADVANTAGES AND DISADVANTAGES OF mechanical seals AND THEIR DEVELOPMENT HISTORY?

 

 

 

The mechanical seal is a shaft seal device used for rotating fluid machinery. It is used for equipment such as centrifugal pumps, centrifuges, reactors, compressors, etc., there is a circumferential gap between the shaft and the equipment cavity, and the equipment medium leaks from it. Therefore, a leakage blocking device must be equipped. Since the mechanical seal has advantages of less leakage and long life, it has become the main shaft sealing method, also known as end seal.

Definition in relevant national standards: it is a device for preventing fluid leakage, which is consists of at least one pair of end faces perpendicular to the axis of rotation, under the joint action of fluid pressure and elastic force (or magnetic force) of the compensation mechanism, and the cooperation of the auxiliary seal ring, the pair of end faces remain attached and relatively slide.

 

 

 

1. Development History of Mechanical Seals

The machine that promoted the development of the industrial revolution was the steam engine, and its basic movement is reciprocating. It wasn't until the invention of the electric motor that the rotary motion equipment really appeared.

 

The sealing of reciprocating machines and equipment was achieved by various compression packing or improvement of compression tools, such as herringbone rings. The materials are all natural, such as leather, cotton and hemp fibers. Even if artificial materials are used instead of natural materials, the sealing method of reciprocating equipment is basically the same as before.

 

The earliest fluid conveying pipelines were aided by gravity, and many early post-industrial revolution factories were built on some natural slopes. If the drainage ditch is at the bottom of the slope, the conveying will become easier.

 

However, in addition to the scarcity of natural resource advantages, there are many other reasons that make the pump become the main method of liquid delivery gradually. It was initially used in reciprocating machines and then used in rotary motion equipment. At that time, pump seals appeared.

 

The original form of rotary pump packing is compression packing, which is still in use today. However, until the 1930s, pump manufacturers and compression packing manufacturers had a new idea of using a moving ring to rotate against a static ring to make a liquid seal. So the mechanical seal was born.

 

Although from the current perspective, the design of the early mechanical seal is very rough compared to the current new-generation mature product. But the basic principles and components are similar. The material is bronze or brass.

 

In the 1940s and 1950s, with the rapid development of the petroleum, chemical and pharmaceutical industries, the demand for machinery and equipment was increasing, and the demand for sealing products also increased greatly, such as spring seals, elastomer bellow seals, etc. Of course, the development of mechanical seals is also keeping pace with the times, but the development is more limited to structural materials rather than design. Therefore, our mechanical seal today has truly become an extremely important and exquisite mechanical equipment, especially with the consideration of environmental issues, the improvement of function becomes particularly important.

 

 

 

2. Advantages and Disadvantages of Mechanical Seals

 

Advantages:

1) Reliable sealing in long-term operation. The sealing state is very stable, and the leakage is very small. According to rough statistics, the leakage is generally only 1/100 of the soft gland packings.

 

2) Long service life. It is generally up to 1 ~ 2 years or more in oil and water media, and usually more than half a year in chemical media.

 

3) Low friction power consumption. The friction power of the mechanical seal is only 10% to 50% of the soft gland packings.

 

4) The shaft or bushing is basically free from friction.

 

5) Long-term maintenance cycle. It can automatically compensate after the end face is worn. Under normal circumstances, no frequent maintenance is required.

 

6) Good vibration resistance. It is not sensitive to the vibration, deflection of the rotating shaft and the deflection of the shaft to the sealed cavity.

 

7) Wide application range. Mechanical seals can be used for sealing at low temperature, high temperature, vacuum, high pressure, different speeds, and various corrosive media and media containing abrasive particles.

 

 

Disadvantages:

1) The structure is relatively complex, and the requirements for manufacturing and processing are high.

 

2) Installation and replacement are more troublesome and require workers to have a certain level of installation skills.

 

3) When an accident occurs, it is more difficult to handle.

 

4) High one-time investment.

 

The history of Mechanical seals

The history of Mechanical Seals (MS) goes back 100 years. It started in the USA in 1905, the inventor was Mr James Cook. He invented so-called Cooks seal. There was a lot of leaking, but the basic principle of operation and their adoption was invented.
These days, a big part of the market is controlled by the big˝three˝.
Largescale production of mechanical seals started in 1939 when Crane Packing started to produce MS for the automotive industry. The company later joined with John Crane from Great Britain and since then they are among the big three in MS in the world.
The second company among the big three is FLOWSERVE, it was established in 1930. It grew mostly through the acquisition of other producers of MS.
The third one is Eagle Burgmann. Burgmann was established in 1962 in Germany, Eagle in Japan in 1964. In 1987 they merged with nowadays Eagle Burgman.
These three companies control the lions share of the market for MS.
Apart from these three, there are numerous smaller companies that operate mainly within their country or regionally. Vital technology at producing MS is lapping. In the 80s, standards for mechanical seals were adopted. The result of that was that numerous small companies that produced mechanical seals occurred. Many disappeared, many others were eaten by larger ones. But adopted standards allowed companies from another part of the world to compete with their products for their share of the market in Europe and North America. This became obvious when China opened to the world. They entered developed markets with much lower prices, at the beginning also with lower quality. Most of them are from NINGBO industrial area. But with time, the quality improved and today their products are as good as any other. The impact of these companies on European and NA-n markets was enormous. Lots of companies disappeared. They could not compete with them, especially when their quality became high. And many EU and NA companies transferred their production to China, because of low producing costs and potentially huge Chinese market.
As time went on, living standards in China grew, production costs grew, and prices of MS also grew. Today, production costs of some types are very much the same in China as they are in EU. And with growing wealth of Chinese society, this gap will become smaller and smaller. Small MS producers are fighting a hard battle to survive in this environment. Their biggest problem is marketing, how to display and offer their products to a larger number of customers. Because marketing is expensive our website
Compare-offers.com gives them the solution. To compete with stronger and larger companies on an equal basis. This is an opportunity that no producer of MS should miss. If you are in CO, you can grow, otherwise, you might disappear.

 

Why Use Mechanical Seals?

A question that comes up often is, “why should I use mechanical seals instead of packing when packing is cheaper?” There are some advantages that are obvious and easy to understand, and a number of others where the benefits may not be so readily apparent.

Why Use Packing?

First, it’s useful to look at why people use packing and why they might prefer it to a mechanical seal solution.

Pump packing is the earliest form of pump sealing and is still a widely used sealing technique. Originally gland packing was made from old ropes and natural fibre products that were packed around the shaft physically stuffing the gap with material to reduce leakage. This is where the name ‘stuffing box’ comes from.

Modern packing makes use of more sophisticated braiding and materials designed to wear less, reduce friction, and withstand heat better

Maintenance personnel are familiar with the process of maintaining and re-packing packed glands.

When packing fails it simply starts to leak more and so can be readjusted and tightened to reduce the leak with the pump remaining in service. Whereas mechanical seal failure can necessitate the shutdown of the pump. This is why pump packing is the primary source of packing for fire pumps and other applications that can’t risk catastrophic failure.

The initial cost of Gland (Pump) Packing is low when compared to the costs associated with mechanical seal.

 

Why use a Mechanical Seal?

Cost of Product

Correctly adjusted gland packing should be maintained with a leakage rate of approximately 1-drop per minute of sealed product per inch/25mm of outside diameter of shaft. On a 2.000”/50mm diameter shaft this equates to 2 drops/min or 100 gals/month or 1200 gals/year (450 Lt/month or 5400 Lt/year).

These numbers are based on an ideal scenario and, as experience tells us, the leakage rates from packed glands are considerably higher than this, often by as much as 5-10 times.

Correctly designed, specified and fitted mechanical seals have no visible leakage and therefore no product loss. On the figures used above, if the product cost 50p/lt, reduction in leakage alone would pay for even an advanced mechanical seal in around 6 months.

 

Disposal of Effluent and Environmental Issues

Depending on the type of product the waste disposal costs can be significantly higher that the initial cost of manufacture. Just looking at our own domestic water bills illustrates this. Even when only dealing with water sewage, charges are significantly higher than the initial cost of the product.

Companies are also now under extreme pressure and constant observation on environmental issues Governments are constantly under national and global pressure to ensure that manufacturers accept the responsibility for the effects their products have on the environment and companies themselves often have environmental targets to meet. Leakage is becoming less and less acceptable and conservation of resources is increasingly important.

Because correctly fitted mechanical seals have no visible leakage they therefore have no effluent disposal costs and provide substantial benefits to emissions reduction.

Power Consumption

Mechanical seals reduce power consumption when compared to gland packing.

On average the majority of gland packing consumes 6 times more power than a balanced mechanical seal. Running rotating equipment with packing in the stuffing box is like driving your car with the hand brake on. Would this affect your fuel consumption? Of course it would and so packed glands therefore greatly increase power costs. In some cases pumps can be made to run at a lower speed and efficiency, increasing costs still further.

Bearing Life

The single major cause of bearing failure in rotating equipment is contamination of the lubrication. In a study conducted by Mobil Oil it was discovered that water contamination of 0.002% reduces the rated bearing life by almost 50%. This is the equivalent of 2 drops of water in a litre of oil.

 

Packing leaks by design and is in close proximity with the bearings. This subjects the bearings to a constant stream of leakage and contamination. Any reduction in contamination will greatly increase the Mean Time Between Failure (MTBF), and in turn reduce maintenance costs.

No leakage equates to longer bearing life and less downtime costs. Many mechanical seal failures are attributed to excessive shaft movement caused by failed bearings.

Shaft / Sleeve Wear

Correctly designed mechanical seals do not wear expensive shafts or sleeves.

Most rotating equipment is fitted with repairable wear sleeves in the region of the gland packing to adress the fact that packing is designed to wear the shaft in this area. If a replaceable sleeve were not fitted, the shaft would need to be replaced or rebuilt in the worn area. This would be both expensive in terms of spares holding or extended downtime while shafts are repaired.

If a separate wear sleeve is fitted there is the cost of holding a spare in stock but there is an underlying danger that the shaft diameter under the sleeve has been greatly reduced to allow for fitting, considerably reducing the shafts resistance to bending.

Broken shafts are not uncommon and are generally caused by fatigue due to the constant bending of the shaft under operation. How does this happen? Running the pump away from its Best Efficiency Point, against throttled or closed discharge valves, over its designed capacity; long shaft overhang and stop-starting create massive forces within the volute.

It is then all down to the principle of levers and that thin things bend easier than fat things.

Shaft sleeves can also be notoriously difficult to remove. This can be extremely time consuming and may even lead to damage of the shaft itself.

Product Dilution.

Gland packing often requires the supply of a clean flushing medium to the lantern ring. This was originally done to prevent the ingress of air into a system under vacuum and was also found to cool the packing as it generated friction against the shaft.

The costs here are the supply of the flush, the leakage of the flush to atmosphere and the dilution of the product itself. This dilution may be acceptable or costs may be incurred in the removal of the dilutant at a later stage in the process.

No product dilution with mechanical seals means a better quality product and no additional costs to remove the dilutant.

Maintenance Costs

To ensure that packing operates as efficiently as it should it requires constant attention. Too loose and leakage rates are unacceptable, too tight and the packing can burn out on start up, increasing leakage, wear on the shaft or sleeve and increasing power consumption. Packing is not simply just greasy rope but it is often treated as such. Maintenance must be constant and conscientious throughout the life of the packing to ensure acceptable operation.

Mechanical seals require no routine maintenance.

Housekeeping

Eliminating leakage reduces secondary maintenance costs by reducing corrosion, cleaning and painting. Secondary maintenance costs would be greatly reduced.

Safety

Packing leaks, as it is designed to and it leaks the “sealed” product to atmosphere. As we have already agreed this is becoming more and more unacceptable for environmental reasons, especially if the product is corrosive, toxic or explosive

All products classified by fugitive emission or hazardous should be double sealed. Airborne pollution is greatly reduced; double seals can achieve zero product emissions.