Mechanical seals, importance, advantages, disadvantages, classification, basic functions of mechanical seals, seal failures, prevention of failure, factors affecting selection of mechanical seals

MECHANICAL SEALS

INTRODUCTION :

        In process industry, reliability of rotating equipment’s such as pumps, compressors, agitators, mixers etc., is of utmost importance, which depends largely on the ability to avoid leakage through shafts. The industry spent well over a billion dollars annually world over in maintenance of rotating equipment’s and a substantial amount of this expenditure was leakage through shafts related. Mechanical seals play an important role in avoiding leakage.

In general today the process industry is demanding in terms of requirements for safety and component reliability that the industry is increasingly conscious of the need for safer improved performance of mechanical seals.

        It is important that the mechanical seals are properly maintained by users / operators. Therefore it is important that a thorough functional understanding, installation requirement and failures of mechanical seals. Here an attempt has been made to compile the various failures that are encountered with mechanical seals by consolidating failure analysis report on mechanical seal failures in the complex. The study of this report and understanding the reason of seal failures will help to great extent, in avoiding premature failure of mechanical seal and costly shutdown of equipment’s and in case of critical equipment’s, the plant.

     Struggling to meet ever-tightening, stringent emission regulations, hydrocarbon processing plants are evaluating all possible alternatives that can cut or  eliminate fugitive releases. Nearly 70% of centrifugal pump maintenance is due to mechanical seal failures, a leading contributor to fugitive emissions. The new standard-API 682 was developed to set  guidelines that dictate mechanical seal performance and specifications. API-682 defines centrifugal seal-sealing system performance and design criteria that will improve reliability and increase pump-seal life.

BASIC FUNCTIONS OF MECHANICAL SEALS :

Mechanical seal is one of the technique to seal the gap formed between a rotary shaft and a stationary stuffing box. This situation is mostly commonly encountered in centrifugal pumps. Mechanical seals are used to prevent leakage of gases and liquids in rotating shaft applications that exceeds the capabilities of radial lip shaft seals and packing.

 
A rotating face forms a seal with a mating face or mating ring. Successful operation depends on maintaining a thin lubricating film of fluid between the faces. Mechanical seals can withstand high operating pressure, temperature and shaft speeds and give longer life with less leakage than packing and radial lip seals. The initial cost of mechanical seal is high as compared to soft packings. However the power consumed, maintenance and down time spent in renewing or tightening the packings overweigh the initial cost of mechanical seals, which works unattended for a long time. From the ecological point of view also seals are preferred over packings. Mechanical seals functions, statistically and dynamically, can withstand large pressure changes, are compatible with many fluids and will function in applications where shaft rotation changes directions.

 
Mechanical seals of conventional design and material can be selected to function at
pressures up to 200 atmosphere, at speeds up to 50,000 rpm and with a temperature
ranging from -200 deg C to 650 deg C.
 

MECHANICAL SEAL COMPONENTS :

 
The Basic components in a mechanical seals include the following

1. A stationary sealing face.
2. A Rotating sealing face
3. A Static secondary seal for stationary face.
4. A Static secondary seal for rotating face.
5. A spring or a bellow to press the sealing faces together.
6. A system to flush seal area.
7. A method to prevent slippage of sealing faces (Keyways, pins or secondary seal friction).

In a simple mechanical seal the rotary sealing face acts as the primary sealing ring and stationary sealing face as mating ring. The primary seal ring is flexibly mounted in the seal head assembly which rotates with shaft and the mating ring is mounted on the pump gland plate. The seal head assembly consists of the method of driving and the method of pressing the two seal ring faces axially. The secondary seals (elastomers ) is installed in the confined space between rotating seal ring and shaft or sleeve, and in the confined space between stationary ring and gland plate.
 

ADVANTAGES / DISADVANTAGES OF MECHANICAL SEALS :

Some of the advantage and disadvantage of mechanical seals over conventional packings and lip seals are listed below.

Advantages :

1. Handles all types of fluids (Acids, salts and abrasive particles)
2. Handle slightly misaligned / non concentric
3. Handle Bi-directional shaft rotation, large pressure, temperature and speed excursions.
4. Shaft condition is not critical (Finish roughness, roundness, hardness and material)
5. Operation does not cause shaft wear.
6. Long operating life.
7. Positive sealing for food processing, hazardous chemicals and radioactive fluids.

Disadvantages :

1.  Requires more space than radial lip seals.
2.  Cannot handle axial end play.
3. Sealing faces must be finished smooth (0.08 to 0.4 micrometer) and can get easily damaged.
4. High initial cost.

 CLASSIFICATION OF MECHANICAL SEAL DESIGNS :

SINGLE SEAL :

90% of the installations are of single seal type. Single seal is the most economical sealing device among mechanical seals as it has minimum number of parts. Further generally pumping fluid is used for seal lubrication. Generally in single seals the pumped fluid should be cool, non volatile, have good lubricity, not contain abrasive or dissolved solids. If the pumped fluid is not satisfactory as a sealing fluid, an auxiliary fluid can be injected across the seal face.

The limitations with single seals is that, in case seal leaks there is no back up available. Also system upset causes the flashing of liquid and flashing causes damage of faces.
 

a) Inside Seal :
 

Figure 1 illustrates a single inside mechanical seal. The material of construction for the inside seal are selected to withstand corrosive liquids in the stuffing box. Inside seals requires a suitable stuffing box housing for installation and cannot be adjusted without dismantling the equipment unless they are cartridge mounted. In inside mounted seals the fluid under pressure acts with spring load to keep the seal faces in contact. Inside seals are easily modified to accommodate environmental controls and can be balanced to withstand high stuffing box pressures.

There are many advantages of locating seals inside. The liquid pressure acts on the OD of the seal. The pressure acts to force the seal faces together. Solids are thrown away from the seal faces by centrifugal force. Seal force leakage is opposite direction to centrifugal force. The seal is submerged in the liquid making it easier to flush and carry away heat.

b) Outside Seal :
 

If an extremely corrosive liquid has satisfactory lubricating properties, an outside seal offers an economical alternative to the expensive metallurgy’s required for an inside seal to resist corrosion. Seals are mounted outside in little pumps that have no room inside for a seal. Figure.2 shows a typical outside seal arrangement in which only the insert seal ring and secondary seals are exposed to the product. All these components can be non-metallic. The metallic rotary unit parts are exposed to the atmosphere.

The outside seals are easier to access for adjustment and trouble shooting. The disadvantages being, it is difficult to flush this seal. The service must be free of solids which might collect under the seal. The hydraulic pressure in an unbalance outside seal tends to open, rather than to close the seal faces. All outside seals are limited to applications having moderate pressures.

MULTIPLE SEAL :

Some fluids are not compatible with a single mechanical seal. Often these liquids carry abrasive material in suspension that would rapidly wear the seal faces or the liquid may be corrosive and hazardous. There are two solutions to this problem. One is the application of environmental controls and the other is the use of Multiple seals or double seals.

A multiple seal may be mounted as double - Back to back, double - face to face or tandem arrangement.

a) Double - Back to Back seal :

Figure 3 illustrates a typical double- back to back seal arrangement. A  clean and non-corrosive liquid called barrier fluid is injected in the cavity between the inboard and outboard seals at a pressure higher than the product being pumped. In this design the buffer fluid prevents the product from contacting the inner portion of the seal and provides lubrication to both seal faces. The inboard seal prevents the buffer fluid from entering the pump where the outer seal prevents the buffer fluid from escaping to the atmosphere. The double seal is unaffected by the product being pumped. 

          

The life of double back to back seal can be up to five times that of a single seal in certain severe environments. Both inboard and outboard seals can be either balanced or unbalance depending upon the pressure encountered. Back to back seal does not tolerate the pressure reversals. Pressure upsets can result in a seal failure. It can not be bench tested, hence seal performance cannot be verified until it is actually assembled in the pump.

b) Double - Face to Face Seal :

The double face to face seal is usually cartridge mounted with one seal inside the stuffing box and one seal on the outside. Both seals rotate against a common or separate stationary insert. Figure 4 represents a double face to face cartridge seal. Face to face seal can be used as either a tandem seal or a double seal. If the liquid between the seals is at higher pressure than the product in the stuffing box, the inner seal is lubricated by a sealing liquid. If the liquid is circulated between the seals at a lower pressure than the stuffing box pressure, the purpose of the inner seal is same as that of any single seal and the outer seal simply serves as a back up or a tandem seal.

This seal is compact and can be bench tested using air as a buffer fluid before assembly. Pressure reversal will not cause it to blow up. The main limitation is in the exposure of inner seal to the product. Viscous, abrasive, thermosetting or corrosive products can damage the inner seal and cause leakage.

c) Tandem Seal :

The purpose of this seal is not to create an artificial environment as is the case with double seal, but to provide a back up seal in the event of inner seal failure. A typical tandem seal is illustrated in the Figure. 5 . The inner seal functions similar to a conventional single seal. The cavity between inner and outer seal is flooded from a closed reservoir, which provide lubrication to outer seal. The inner seal is lubricated by the product. If the inner seal fails, the resulting pressure rise in the area between seals is sensed at the reservoir, where it can be
either registered on gauge or activate an alarm. In any event, failure of the inner seal can be detected while the outer seal assumes the responsibility of sealing the shaft until pump is taken for seal repair. Hence reliability of tandem is high compared to other seals.
 

BALANCED :

Balancing seal involves a simple design change which reduces the hydraulic forces acting to close the seal faces. As seal faces rub together excessive heat is generated. This heat can be removed by increasing the lubrication between the faces. Lubrication can be increased by reducing the effect of seal cavity hydraulic pressure on the seal faces. This is done usually by building a step in the sleeve which allows the wear nose of stationary insert to be moved toward the centre of the seal. More of the seal face is thus exposed to the seal
chamber pressure which in-turn reduces the forces acting to close the seal faces.

 Because balanced seal enable seal face contact pressure to be kept low, a thicker film of stable liquid can exist between faces, reducing friction and the consequent heat generation. Normally a balanced seal is designed to operate with the lowest face pressure that will effectively prevent leakage between the faces. 

 UNBALANCED :

Unbalanced seals are often the design of choice for inside seals. In unbalanced arrangement, all of the seal face contact area lies outside of the shaft diameter. The amount of seal face leakage is inversely proportional to the amount of seal face loading. Higher the loading, the lower is the leakage. Unbalanced seals are having higher face loading than balanced seals, leak less and are more stable when subjected to vibration, misalignment or cavitation. They are often less expensive and more adaptable to standard stuffing boxes without need for modification of either the shaft sleeve or the stuffing box.

One of the disadvantage of an unbalanced seal is its relatively low pressure limit. If the closing force exerted on the seal face exceeds that limit, the lubricating film between the faces is squeezed out and soon the faces get destroyed. This problem is overcome by balancing seals.

SINGLE AND MULTIPLE SPRING DESIGN :

 The single spring is less critical in its compression setting. This makes it a good choice for pumps which have a lot of thermal expansion, with the impeller between the bearings. The single spring is less prone to clogging in dirty service. Multiple springs are small springs and are not susceptible to distortion at high speeds and exert even closing pressure on the seal ring at all times. The same size spring can be used in a range of seal sized just by varying quantity.

PUSHER AND NON-PUSHER TYPE :

Pusher seals have a dynamic secondary seal under the movable seal ring. The dynamic seal ring can take several forms like., ‘O’ ring, Teflon ‘V’ ring, wedge ring, ‘U’ cup etc.,. For high temperature (upto 500 deg.F) or aggressive chemicals a Teflon wedge ring may be used. Since Teflon is plastic and does not rebound like elastomer, it has to be pushed by spring force into the wedge shaped opening to maintain a seal on the shaft. Non-Pusher type or Bellow seals have no dynamic secondary seal under the movable seal ring. This makes them more flexible and better able to tolerate mating ring misalignment.
 

The bellows can be made of rubber, Teflon and metal. Rubber bellows are used for less critical applications like car water pumps, circulating hot water pumps etc., Teflon bellows are used for low pressure, moderate temperature acid services. Metal bellow seal is its ability to run at a very high temperature (750 degF) if the elastomer ‘o’ rings are replaced with grafoil for the secondary seals. The use of bellow seal is limited to 400 PSI on OD.

CARTRIDGE SEALS :

Nearly all mechanical seals can be provided in a cartridge design. The seal is mounted usually with a gland ring on a sleeve which fits directly over the equipment shaft or shaft sleeve. Major benefit of cartridge seal is that they do not require the usual seal setting measurement for their installation. Cartridge seals are used to reduce installation errors and turn around time for repair. This optimum seal solves five common causes of component seal failure.

FACTORS AFFECTING THE SELECTION OF MECHANICAL SEALS

1. Media Handled :

• Corrosiveness of media decides the material of construction.
• Density of viscosity shows the lubricating properties of the media. This
decides the seal arrangement.
• Abrasives in the media decides the type of flushing plan required.

2. Stuffing box pressure :

• When stuffing box pressure < 10 kg. / cm2, unbalanced seals are used.
• When stuffing box pressure . 10 Kg. / cm2 , balanced seals are used.

3. Shaft Speed :

• With increase in speed the chance of seal running dry increases. So proper
flushing plan has to be decided which can give positive flush between the
faces.

4. PV Factor :

• PV factor is defined as the product of the pressure drop across the seal and
the average rubbing velocity. As a general guide, Seal capabilities can be
classified as, Low , if the PV value is 0.7 MPa m/s ( about 20,000 psi ft/min)
Medium, if the PV value is between 0.7 and 10 MPa m/s ( about 285,000 psi
ft/min) High , if the PV value ranges from 10 to 70 MPa m/s (about 2,000,000 psi
ft/min). Balanced seals reduce the pressure acting on the seal faces, therefore they have higher PV values than unbalanced seals.

 5. Temperature :

• This decides the material of construction so that the seal does not fail at operating temperature.
• This will decides the seal flushing plan.