Hear that pop? That's the sound a mechanical seal makes when it's running dry.
Mechanical seals are designed to use a small amount of process fluid between seal faces, which keeps them lubricated and cooled. However, when liquid vaporizes between sealing surfaces, an audible popping sound is produced.
Following are common causes of lubricant/coolant vaporization:
1) Improper axial adjustment;
2) entrained air;
3) Trapped steam in stuffing box;
4) Residual solids in stuffing box;
5) Pump runs dry;
6) Insufficient cooling;
7) There is no or wrong flushing scheme installed;
8) Fluid vapor pressure is too high.

 

Figure 1: Example of severe thermal damage (coking) caused by dry running

 

Figure 2: Damage to sealing surfaces due to dry running and mechanical deformation
How to prevent or fix: To prevent dry running, keep heat away from mechanical seal face by increasing flushing flow rate or re-evaluating structural design of stuffing box, seal flushing system or flushing scheme; Formed hole stuffing box gland releases entrapped steam.

If you see radial cracks (heat cracks) or buildup (coking) on sealing surface, it may be due to excessive temperature. In this case, high temperature faced by seal is caused by heat generated during operation of pump or seal.
Hot Cracks: Hot cracks can be identified by radial cracks that appear in the center of seal face (see Figure 3). These cracks act as cutting edges, causing premature wear of sealing faces as they rub against each other.

 

Figure 3: Radial thermal crack originating from center of sealing surface
Coking: Coking can leave buildup or grinding debris on atmospheric side of a mechanical seal. This buildup can occur when seal is operating at excessively high temperatures, but can also occur due to reasons such as dirty flushing fluid or contamination from external environment (see Figure 4).

 

Figure 4: Example of coking
Pumps running (on performance curve) away from BEP generate excess heat; seals generate excessive heat from running at high speed or exerting excessive pressure on seal faces.
Certain sealing materials are not designed for high temperature applications. Using wrong materials of construction when pumping high temperature liquids can lead to premature mechanical seal failure.
How to prevent or fix it: Keeping seal cool is critical to its long-term reliable operation. If signs of heat cracking or coking can be seen, it is time to take a closer look at seal's flushing schedule (or increase seal flushing schedule if necessary); consider increasing flushing flow, or changing flushing schedule; have you selected seal correctly for application? Proper materials of construction and seal design ensure that seal can withstand high temperatures.

There are several different types of shocks that can cause a mechanical seal to fail, including mechanical shocks and thermal shocks.

 

Figure 5: Example of impact damage
Mechanical shocks are caused by deteriorating equipment operating conditions such as bearing damage, cavitation, excessive torque, uneven loading and shaft misalignment. More commonly, however, mechanical shock is result of poor seal handling and improper assembly.
Thermal shock occurs when a seal is exposed to large temperature fluctuations within a short period of time. Different areas of sealing surface will expand and contract to different degrees, causing excessive stress or strain on sealing surface.
How to prevent or fix:
How to solve this special case? Why it happens needs to be fully understood in the first place. Here are a few things to keep in mind when dealing with shocks:
1) When installing mechanical seal, avoid uneven or excessive tightening of fasteners;
2) Pay attention to ensure that installation matches flushing scheme with seal;
3) checks to ensure that flushing fluid and quench systems are designed to minimize possible momentary interruptions;
4) Increase vibration inspection in daily project inspection;
5) If operating parameters specify a large temperature difference, as a general rule of thumb, limit change to 1°F per minute.

If mechanical seal squeals during operation, reason may be lack of lubrication between sealing surfaces.
Lubrication plays a key role in operation of mechanical seals. In addition to lubrication, its function also has functions of cooling, sealing, cleaning and protecting sealing surface.
Poor lubrication on hard surfaces such as silicon carbide or ceramics can lead to hot cracking. Hot cracks show radial cracks on sealing surface. Height and distance between cracks can vary from very small to very large (see Figure 6).

 

Figure 6: Example of poor lubrication damage
When two rotating surfaces are in contact under heavy loads, localized high temperatures may occur due to excessive frictional heating near surfaces. Heat generation, poor lubrication between sealing surfaces and superposition of mechanical loads lead to cracking of material near contact zone.
How to prevent or repair: Confirm that operating conditions such as seal chamber pressure are within limits of seal design; confirm that seal chamber has or can fully discharge air; provide sufficient continuous (lubrication) flushing for seal.

As is case with almost any rotating equipment, revs are often fatal. High starting or running torques, frequent starts/stops of equipment can damage mechanical seals.

 

 

Figure 7: Example of High Torque Damage
How to prevent or fix it: Check condition of equipment and fix it to proper limits. Select appropriate drive mechanism according to torque or other equipment operating conditions. Use balanced seals to reduce seal face torque and pressure torque.

Some of the worst damage to mechanical seals can be from chemical attack. Incompatible materials can make a huge difference. When seals are chemically attacked, any of following may occur:
1) Serious leakage;
2) Signs of excessive wear;
3) Fragile or damaged parts;
4) Pitting corrosion on sealing surface;
5) Corroded metal parts;
6) Expanded O-rings.

 

Figure 8: Example of chemical attack damage
Chemical attack is caused by improper selection of seals and their materials of construction. It is vital that selection of a mechanical seal is made by someone who has extensive experience with different materials used for mechanical seals and who also understands how certain chemicals react.
How to prevent or fix it: In order to properly select proper material for process fluid, a complete chemical analysis should be completed. When selecting a seal material, please have a detailed understanding of all possible operating conditions for seal, including properties of cleaning chemicals and their operating temperatures. Choose a flushing scheme that will use clean, compatible fluids; in some cases, dual mechanical seals may be required to neutralize or contain corrosive environments.

Mechanical seal overpressure causes gravity contact at outside diameter of dynamic and stationary ring seal faces that tapers inward until there is almost no visible contact. High pressures can cause chipping (nicking) at OD edge of primary seal ring.

 

Figure 9: Example of overvoltage damage
How to prevent or fix it: Reduce seal chamber pressure as much as possible. Changes in seal design or material may be required to reduce deformation caused by high seal chamber pressures.

Solid particles (abrasive) + wrong seal material = seal wear much faster than expected.
Damage to sealing surface by solid particles is indicated by:
1) Sealing surface is severely worn;
2) worn grooves have appearance of a "gramophone record";
3) There may be edge chipping or rounding on sealing surface.

 

Figure 10: Example of solid particle damage
How to prevent or repair: Before applying to a working condition containing solid particles, characteristics of fluid must be determined, including solid content, solid size, and solid type; select appropriate structural materials, or seals specially designed for applications containing solid particles.
Next, look at flushing regimens. Modify flushing arrangement so that flushing fluid flushes on sealing surface, increase flushing speed, and increase seal chamber pressure by installing a throat bushing.

Misalignment is one of the most common causes of damage to mechanical seals. Misalignment can be caused by pipe strain, deflection during hard starts, shaft runout, and countless other conditions. Misalignment can put excessive stress on mechanical seal components, causing them to malfunction, wear prematurely and eventually fail.

 

Figure 11: Example of misalignment damage
How to prevent or fix it: Be sure to follow proper installation guidelines and use tools such as laser alignment to ensure that pump rotor is properly aligned. However, pump may be misaligned during operation, even if it was perfectly centered during installation. Equipment displacement caused by thermal expansion and dynamic load changes can throw pump out of direction.

Vibration can cause problems with almost any type of equipment, from pumps to fans. How to judge whether vibration is culprit of mechanical seal failure?

 

 

Figure 12: Example of Vibration Damage
Just like misalignment, vibration has many sources:
1) unbalanced;
2) Misalignment;
3) run away from BEP;
4) Pump cavitation;
5) entrained air;
6) Pipeline design is unreasonable.
How to prevent or fix it: Proper equipment installation is critical to solving vibration problems. Pump base should be properly installed and grouted against soft ground. Pump shaft should also be accurately aligned with motor shaft, eg laser; OEM spare parts should be used. Vibration problems can arise when components are outside dimensions and tolerances specified by design; finally, proper piping techniques (according to ANSI/HI standards) can have a major impact on minimizing vibration.

Mechanical seals are easily damaged during installation. Following are some common installation errors that occur with mechanical seals:
1) End face of stuffing box installed is not perpendicular to shaft;
2) Install coupling on shaft by hammering;
3) Insufficient or improper lubrication used to mount (slide) mechanical seal onto shaft;
4) Failure to follow installation steps described in mechanical seal manual.

 

Figure 13: Example of damage caused by not following all installation steps
How to prevent or fix:
Mechanical seals need to be handled and installed with care, following are some tips during installation:
1) Before formal installation, do not remove packaging of mechanical seal;
2) Wash your hands before installation. Even tiny particles or oil on skin may cause wear and leakage on sealing mating surface during operation;
3) Do not touch sealing surface with your hands;
4) Put a layer of clean wrapping paper on workbench to prevent contamination of mechanical seal;
5) Before installing mechanical seal on pump, clean sealing surface with a clean soft cloth and an approved solvent;
6) Use lubricant recommended by manufacturer according to installation instructions.

Before installing and/or replacing a mechanical seal, check to make sure pump (parts that work with seal) is in good condition.
A scored shaft or bushing can damage mechanical seal before it operates; a scored shaft and bushing can damage O-rings when installing (sliding in) mechanical seal.
Bent pump shafts, old and out-of-spec bearings, out-of-balance impellers causing vibration, contact friction between moving and static parts inside pump, bearing damage, etc., all of which can shorten life of a mechanical seal.

 

Figure 14: Severely scratched bushing
How to prevent or fix it: All used parts should be thoroughly inspected before being reused to ensure they meet design dimensions and tolerances. If size of used parts exceeds design requirements, replace them with new ones.

There is no doubt that accidents happen from time to time. Whether it's improper priming of a pump, or running a pump with a valve closed, these mishaps can have costly consequences.
Improper priming of pump can cause motor to trip, shaft to twist, and even cause mechanical seal to move abnormally. Dry starting pump means experiencing all of problems presented in above method, which can eventually damage mechanical seal. Operating system with suction valve closed (dry running) or discharge valve closed (dead head) can lead to premature seal failure.
How to prevent or fix it: Make sure all operators are trained in proper procedures for starting pump unit and operating system.

Seal failures are often repeatable. If seals were operating in same manner, same failure rate would be expected.