An Introduction To Mechanical Seal

An Introduction To Mechanical Seal

What are mechanical seals ?

Power machines that have a rotating shaft, such as pumps and compressors, are generally known as "rotating machines." Mechanical seals are a type of packing installed on the power transmitting shaft of a rotating machine. They are used in various applications ranging from automobiles, ships, rockets and industrial plant equipment, to residential devices.

Mechanical seals are used in many pumps to prevent water (or other liquid) leakage. Mechanical

seals might be chosen over packing on a given application for three reasons: 

(1) mechanical seals provide a better fluid seal than packing, 

(2) mechanical seals usually require less maintenance than packing, and 

(3) mechanical seals can withstand higher pressure than stuffing boxes 

Without a seal

If no mechanical seal or gland packing is used, the liquid leaks through the clearance between the shaft and the machine body.

With a gland packing

If the aim is solely to prevent leakage from the machine, it is effective to use a seal material known as gland packing on the shaft. However, a gland packing tightly wound around the shaft hinders the motion of the shaft, resulting in shaft wear and therefore requiring a lubricant during use.

With a mechanical seal

Separate rings are installed on the shaft and on the machine housing to allow minimal leakage of the liquid used by the machine without affecting the rotating force of the shaft.
To ensure this, each part is fabricated according to a precise design. Mechanical seals prevent leakage even with hazardous substances that are difficult to mechanically handle or under harsh conditions of high pressure and high rotating speed.

The essential point is to control leakage and friction

All mechanical seals have three primary sealing points.
1-  The first is the area between the stationary element and the seal housing. This area is sealed with regular gaskets or O-rings.
2-  The second is between the rotating element and the shaft. This is also sealed by O-rings. 
3-  The third is between the polished faces that seal water flow; the very close contact between these faces achieve this seal.
To increase the life of the mechanical seal and to achieve a tight seal, the surfaces of the polished faces are made of dissimilar materials; for example, one face might be made of stainless steel while the other one is a synthetic Teflon® material
The face materials where the stationary ring and the rotary ring rub against each other are the most important portions as a barrier to the fluid. If the clearance is too small, the friction increases, hindering the shaft motion or resulting in seal breakage. Conversely, if the clearance is too large, the liquid will leak. Consequently, it is necessary to control the clearance in the order of micrometers to prevent leakage, but at the same time ensuring lubrication by the fluid, thereby reducing the sliding torque and avoiding hindrance to the machines' rotation.

The seal faces are pushed together using a combination of hydraulic force from the sealed fluid and spring force from the seal design. In this way a seal is formed to prevent process leaking between the rotating (shaft) and stationary areas of the pump.

The surfaces of the seal faces are super-lapped to a high degree of flatness; typically 2-3 Helium light-bands (0.00003” / 0.0008mm).

If the seal faces rotated against each other without some form of lubrication they would wear and quickly fail due to face friction and heat generation. For this reason some form of lubrication is required between the rotary and stationary seal face; this is known as the fluid film

The Fluid Film

In most mechanical seals the faces are kept lubricated by maintaining a thin film of fluid between the seal faces.  This film can either come from the process fluid being pumped or from an external source.

The need for a fluid film between the faces presents a design challenge – allowing sufficient lubricant to flow between the seal faces without the seal leaking an unacceptable amount of process fluid, or allowing contaminants in between the faces that could damage the seal itself.

This is achieved by maintaining a precise gap between the faces that is large enough to allow in a small amounts of clean lubricating liquid but small enough to prevent contaminants from entering the gap between the seal faces.

The gap between the faces on a typical  seal is as little as 1 micron – 75 times narrower than a human hair.  Because the gap is so tiny, particles that would otherwise damage the seal faces are unable to enter, and the amount of liquid that leaks through this space is so small that it appears as vapor – around ½ a teaspoon a day on a typical application.

This micro-gap is maintained using springs and hydraulic force to push the seal faces together, while the pressure of the liquid between the faces (the fluid film) acts to push them apart. 

Without the pressure pushing them apart the two seal faces would be in full contact, this is known as dry running and would lead to rapid seal failure.

Without the process pressure (and the force of the springs) pushing the faces together the seal faces would separate too far, and allow fluid to leak out.

Mechanical seal engineering focuses on increasing the longevity of the primary seal faces by ensuring a high quality of lubricating fluid, and by selecting appropriate seal face materials for the process being pumped. 

STUFFING BOX AND SEALS

Sealing devices are used to prevent water leakage along the pump driving shaft. Shaft sealing

devices must control water leakage without causing wear to the pump shaft. The two systems

available to accomplish this seal are the

 (1) conventional stuffing box/packing assembly and 

 (2) mechanical seal assembly.

Spring types employed

• Central spring, conical or cylindrical, mounted onto shaft as single spring
• Multi-spring arrangement consisting of concentrically arranged multiple springs
• Metal bellows
• Wave springs

The friction losses generated are lower than those of gland packings. Heat is generated in the shaft seal housing due to friction; depending on the amount produced, it can be dissipated either via convection from the seal housing to the atmosphere or via forced circulation through an externally installed heat exchanger.

As with gland packings, mechanical seals are available in various designs and configurations to handle diverse operating conditions.

Pumps are specially designed and manufactured to cater for a whole range of different applications. This process takes into account aspects such as resistance to the fluids handled, temperature and pump pressure. The appropriate seal type for the individual pumping requirements is chosen from a wide variety of different shaft seals.
The design is based on one of the two following principles: sealing by means of a narrow radial gap (parallel to the shaft axis) or a narrow axial gap (at a right angle to the shaft axis). For both sealing principles, the gaps may either employ a contact or non-contact design.

classification

Classification of Seals by Arrangement

Seal arrangement is used to describe the design of a particular seal installation and number of seals used on a pump. Sealing arrangements may be classified into two groups:

• Single seal installations

           * Internally mounted

           * Externally mounted

• Multiple seal installations

         * Double seals

                  1- Back to back

                  2-Face to face

         * Tandem

Single seals are commonly used on most applications. This is the simplest seal arrangement with the least number of parts. An installation mounted inside the stuffing box chamber is referred to as an inside mounted seal . Here, liquid in the stuffing box and under pressure acts with the spring load to keep the faces in contact.

An outside mounted seal refers to a seal mounted outside the stuffing box . Here, if the seal is not balanced for pressure at the inside diameter of the seal face, the pressure will try to open the seal.

Outside mounted seals are considered only for low pressure applications.

The purpose of an external seal installation is to minimize the effects of corrosion that might occur if the metal parts of the seal were directly exposed to the liquid being sealed. Multiple seals are used in applications requiring:

• A neutral liquid for lubrication

• Improved corrosion resistance

• A buffered area for plant safety

Double seals consist of two seal heads mounted back to back with the carbon primary rings facing in opposite directions in the stuffing box chamber . A neutral liquid with good lubricating properties,

at higher pressures than the pumpage, is used to cool and lubricate the seal faces. The inboard seal keeps the liquid being pumped from entering the stuffing box. 

Both the inboard and outboard seals prevent the loss of the neutral lubricating liquid. Circulation for cooling is normally achieved with an external circulation system and smaller pump. An axial flow pumping ring and closed system may be considered on low pressure systems.

Double seals may be used in an opposed arrangement . Two seals are mounted face to face with the primary sealing rings running on mating rings supported by a common end plate. The neutral liquid

is circulated between the seals at the inside diameter of the seal faces,

The circulation pressure is normally less than the process liquid being sealed. The inboard seal is similar to a single inside mounted seal and carries the full differential pressure of the pump stuffing box to the neutral liquid. The outboard seal carries only the pressure of the neutral liquid to atmosphere. Both inboard and outboard primary rings are balanced to handle pressure at either the outside or inside diameter of the seal faces without opening. The purpose of this arrangement is to fit a stuffing box having a smaller confined space than what is possible with back-to-back double  smaller confined space than what is possible with back-to-back double seals. Double seals are normally applied to toxic liquids for plant safety.

Tandem seals are an arrangement of two single seals mounted in the same direction . The inboard seal carries the full pressure differential of the process liquid to atmosphere. The outboard seal contains a neutral liquid and creates a buffered zone between the inboard seal and plant atmosphere. Normally the neutral lubricating liquid is maintained at atmospheric pressure. Developed heat at the inboard seal is removed with a seal flush similar to a single seal installation.

 The liquid in the outboard seal chamber should be circulated with a pumping ring to remove unwanted seal heat. Tandem seals are used on toxic or flammable liquids that require a buffered area or safety zone in the seal installation.

in the case of boiler feed pumps, seals have to cope with high sliding velocities, heat transfer from the fluid handled and the heat generated by friction.

The sealing gap temperature is generally higher than the fluid temperature in the seal housing. The latter can be kept well below 100 °C by circulating the fluid through to an external cooler by means of suitable pumping devices inside the pump. Pumping screws, holes in the shaft protecting sleeve or small pumping discs serve as pumping devices.

Classification of Seals by Design

Certain design features are considered important and may be used to describe a seal.  These descriptions also form four classification groups.

A seal may be referred to as:

• Unbalanced or balanced

• Rotating or stationary seal head

• Single spring or multiple spring construction

• Pusher or non-pusher secondary seal design

The selection of an unbalanced or balanced seal is determined by the pressure in the pump stuffing box and the type of liquid to be sealed. Balance is a way of controlling the contact pressure between the seal faces and power loss at the seal. When the percentage of balance b (ratio of hydraulic closing area to seal face area) is 100 or greater, the seal is referred to as unbalanced. When the percentage of balance for a seal is less than 100, the seal is referred to as balanced. 

The selection of a rotating or stationary seal is determined by the speed of the pump shaft. A seal that rotates with a pump with the shaft is a rotating seal assembly..

When the mating ring rotates with the shaft the seal is stationary. Rotating seal heads are common in industry for normal pump shaft speeds and where stuffing box space is limited. As a rule of thumb,

when the shaft speed exceeds 5,000 ft/min, stationary seals are required, Higher speed applications require a rotating mating ring to keep unbalanced forces that may result in seal vibration to a minimum. Also, for certain applications like vacuum tower bottom pumps, a stationary seal allows a steam quench to be applied to the entire inside diameter of the seal to prevent

The selection of a single spring or multiple seal head construction is determined by the space limits available and the liquid sealed. Single spring construction is most often used with elastomeric bellows seals to load the seal faces . The advantage of this type of construction is that the openness of the design makes the spring a non-clogging component of the seal assembly. The coils are made of a large diameter spring

Multiple spring seals require a shorter axial space. Face loading is accomplished by a combination of springs placed about the circumference of the shaft . Most multiple spring designs are used with

assemblies having O-rings or wedges as secondary seals.

Pusher-type seals are defined as seal assemblies in which the secondary seal is moved along the shaft by the mechanical load of the seal and the hydraulic pressure in the stuffing box. 

This designation applies to seals that use an O-ring, wedge, or V-ring. 

Non-pusher seals are defined as seal assemblies in which the secondary seal is not forced along the shaft by the mechanical load or hydraulic pressure in the stuffing box. Instead, all movement is taken up by the bellows convolution. A non-pusher design   applies to those seals that use half, full, and multiple convolution bellows as a secondary seal.

Materials of Construction

The selection of materials of construction must be based on the operating environment for the seal. The effects of corrosion, temperature, deformation from pressure, and wear from sliding contact must be considered for good life. Each seal must be broken down into component parts for material selection. The effects of corrosion must be known for the secondary seal, primary and mating rings, as well as hardware items.

Design Fundamentals

Seal Balance. This is the ratio of hydraulic closing area to seal face area:

where 

b = seal balance

𝐚c = hydraulic closing area in²

𝐚0 = hydraulic opening area (seal face area), in²

Seal balance is used to reduce power loss at seal faces from sliding contact.

The pressure in any stuffing box acts equally in all directions and forces the primary ring against the mating ring. Pressure acts only on the

annular area  so that the closing force in pounds on the seal face is:

where 

p = stuffing box pressure, lb/in²

Fc = hydraulic closing force, pounds

The pressure in pounds per square inch between the primary and mating rings is:

Face Pressure

This is an important factor in the success or failure of a mechanical seal. Hydraulic pressure develops within the seal faces that tend to separate the primary and mating rings.

where 

Ph = AP(b - k), lb/in²

P = pressure differential across seal face, lb/in²

b = seal balance

k = pressure gradient factor

Pressure-Velocity. The value for a seal installation may be compared with values developed by seal manufacturers as a measure of adhesive wear. As the primary and mating rings move relative to each other, they are affected by the actual face pressure and rotational speed. The product of the two, pressure times velocity, is referred to as PV and is defined as the power N per unit area with a coefficient of friction of unity;

where 

Vm = velocity at the mean face diameter dm ,  ft/min

Power Consumption. The power consumption of a seal system can be estimated using the PV value and the following equation:

where

 f    is the coefficient of friction.

As a general rule, the power to start a seal is five times the running value.

The coefficients of friction for various common seal face materials are given in Table . These coefficients were developed with water as a lubricant at an operating PV value of 100,000 lb/in² ft/min. Wues in oil would be slightly higher as a result of viscous shear of the fluid film at the seal faces.

Seat Leakage

An estimate for seal leakage in cubic centimeters per minute can be made from the following equation:

where

 C3 = 532

h = face gap, in

P2 = pressure at face ID, lb/in²

PI = pressure at face OD, lb/in²

𝜇 = dynamic viscosity, CP

R2 = outer face radius, in,

R1 = inner face radius, in.

MECHANICAL SEAL INSTALLATION PROCEDURE

The routine maintenance for mechanical seals involves inspecting the seals daily, ensuring that the seal water is flowing, and replacing the seal when it no longer prevents leakage. Anyone responsible for maintenance of pumps employing mechanical seals should read the seal manufacturer’s instructions for the operation and maintenance of the seal carefully. Because of the wide variation in seals being used, it is difficult to describe a step-by-step replacement procedure similar to the one for packing systems discussed earlier. The outline that follows points out a few general steps that apply to most seal replacements or installations. Again, the manufacturer’s technical manual (or literature) provided with the mechanical seal is the best source of instructions and should always be used when available.

1. Shut the pump down and lockout/tagout the system.

2. Close the suction and discharge valves and remove the drain plug.

3. Dismantle the pump and inspect the shaft or shaft sleeve. If a mechanical seal is being

installed to replace conventional packing, the shaft sleeve must be replaced. If the

mechanical seal is being replaced with another seal, the shaft or sleeve should be cleaned

with emery cloth.

4. Clean the shaft and/or sleeve to remove any filings. A shaft or sleeve that is pitted or

corroded should be replaced.

5. Check the shaft for end play and runout. End play cannot exceed 0.005 inch, and runout

should be less than 0.001 inch per inch of shaft diameter. If shaft end play or runout is

excessive, the shaft bearings or shaft should be replaced.

6. Spray or brush layout bluing on the shaft around the area of the seal housing.

7. Reinstall the seal housing and mark the location of the top of the housing on the shaft;

remove the housing.

8. Using the manufacturer’s specifications, mark the location of the rotating element on the

shaft.

9. Before installing the rotating element, check the edge of the shaft for burrs that could cut

the O-ring secondary seal.

10. Remove the seal from its container; care must be taken not to damage the primary sealing

faces.

11. Position the rotating element on the shaft at the marked location; fasten it down

temporarily.

12. Place the stationary element into the seal housing and install the housing on the pump.

13. Using the feeler gauge, adjust the rotating element to establish the proper clearance;

fasten the element in place.

14. Reassemble the pump and put it back in service; check seal operation.

MECHANICAL SEAL HAS A SHORT LIFE

Possible causes:

1. Shaft is bent.

2. Shaft sleeve is worn or scored.

3. Seal is improperly installed.

4. The seal is incorrect for operating conditions.

5. Liquid being pumped has abrasive solids.

6. Mechanical seal was run dry.

7. Bearings are worn.

8. Pump shaft is misaligned.

MECHANICAL SEAL LEAKS EXCESSIVELY

Possible cause:

1. Leakage under shaft sleeve is occurring due to gasket or O-ring failure.

Actions or remedies:

1. Determine if the leakage is actually between the shaft sleeve and shaft; replace the gasket

or O-ring.

2. Inspect the motor shaft and replace if it is bent.

3. Remove the gland and packing and inspect the shaft sleeve for wear (this generally has to

be done with the hands or with a packing tool).

4. Remove the seal; consult with the manufacturer’s representative and read instructions

for the seal; reinstall the seal following proper instructions.

5. Consult with the manufacturer’s representative and choose the proper seal for the

application.

6. Provide a separate clean seal water source.

7. Operate the pump only while seal water is flowing.

8. Replace bearings.

9. Inspect the shaft for damage due to misalignment and replace it if it is damaged; correct the problem causing the misalignment (e.g., worn bearings, out of balance impeller, pipe strain).

In Summary - Why Do We Use Mechanical Seals?

• No “visible” leak - seals do leak vapour as the fluid film on the faces reaches the atmospheric side of the seal faces.
  • This would approximate to 1/2 teaspoon a day at normal operating pressures and temperatures, if it were captured and condensed.
• Modern cartridge seal designs do not damage the pump shaft or sleeve. 
• Day to day maintenance is reduced as seals have inboard springs which make them self-adjusting as the faces wear. 
• Seals have lightly loaded faces which consume less power than gland packing. 
• Bearing contamination is reduced in normal operation as the lubricant does not become affected by seal leakage and wash out.
• Plant equipment also suffers less from corrosion if the product is contained in the pump.
• Vacuum can also be sealed with this technology, a problem for packing as air was drawn into the pump.
• Less wasted product will save money, even water is an expensive commodity and less clean up of the area will be needed.