Workshop on Improving the Reliability of MECHANICAL SEALS Location: Mitsubishi Chemical Corporation (MCPI), Haldia, WB, India Date: 6 th & 7 th May, 2013 Trainer: Md Aminul Islam (Mechanical Engineer) Contents: 1. About Mechanical Seals 2. Basic components 3. Gland Packing Vs Mechanical Seals 4. Type of mechanical seal 5. How a mechanical seal works 6. Mechanical Seal Selection 7. Materials of Construction 8. Guides for the successful installation of mechanical seals 9. Seal Care 10. Seal Life 11. Seal failures & Causes 12. Analysis of failures 13. Mechanical Seal Application Limit 14. Pressure and temperature limits of common seal types 15. Typical PV – Limits of face material 16. Troubleshooting of mechanical seals 17. API plan 18. Standard EN 12756
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Transcript
Workshop on
Improving the Reliability of MECHANICAL SEALS
Location: Mitsubishi Chemical Corporation (MCPI), Haldia, WB, India
Date: 6th & 7th May, 2013
Trainer: Md Aminul Islam (Mechanical Engineer)
Contents:
1. About Mechanical Seals
2. Basic components
3. Gland Packing Vs Mechanical Seals
4. Type of mechanical seal
5. How a mechanical seal works
6. Mechanical Seal Selection
7. Materials of Construction
8. Guides for the successful installation of mechanical seals
9. Seal Care
10. Seal Life
11. Seal failures & Causes
12. Analysis of failures
13. Mechanical Seal Application Limit
14. Pressure and temperature limits of common seal types
15. Typical PV – Limits of face material
16. Troubleshooting of mechanical seals
17. API plan
18. Standard EN 12756
About Mechanical Seals:
A mechanical seal is a sealing device which forms a running seal between rotating and stationary parts.
They were developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level
meeting environmental standards of government regulating agencies and maintenance costs can be lower.
Advantages of mechanical seals over conventional packing are as follows:
1. Zero or limited leakage of product (meet emission regulations.)
2. Reduced friction and power loss.
3. Elimination of shaft or sleeve wear.
4. Reduced maintenance costs.
5. Ability to seal higher pressures and more corrosive environments.
6. The wide variety of designs allows use of mechanical seals in almost all pump applications.
Mechanical seals are designed to prevent leakage between a rotating shaft and its housing under
conditions of extreme pressure, shaft speed and temperature. Mechanical seals can be single acting or double
acting. Single (acting) mechanical seals have one sealing gap. The lubrication film required by the sliding seal
faces is provided by the medium to be sealed. The lubrication film required by the seal faces in double
(acting) mechanical seals is provided by a higher pressure buffer medium (sealant liquid) that is compatible with
the pumped product. The sealant liquid is at a higher-pressure so that any leakage across the seal faces will be the
sealant liquid into the pumped product. This buffer serves to separate the product and the atmosphere. Seal
design choices include pusher, metal bellows, and elastomeric bellows. A pusher mechanical seal utilizes a
dynamic secondary seal or o-ring that is responsible for sealing the fluid path between the pump shaft and the
inside diameter of the rotating seal face. The secondary seals move axially along a shaft or sleeve to maintain
contact at the seal faces, compensating for seal face wear and for any seal wobble due to misalignment. Metal
bellows design is a non-pusher seal design. The secondary seal in a non-pusher design does not have to move
along the shaft or sleeve to maintain seal face contact. The bellows itself provides the necessary spring loading
for seal face contact. Metal bellows provide effective sealing in a wide range of temperatures and use no
elastomers. An elastomer bellows seals is a non-pusher seal design in which a single spring coil fits over the
shaft and bellows.
Other important design parameters to consider for mechanical seals include spring configuration, shaft
mounting, and seal configuration. Spring configuration can be single or multi. Single springs are sometimes
called "monocoil" or "single coil" design. This type of seal uses a large spring cross section that resist corrosion.
Its chief limitations are its tendency to distort at high surface speeds, the large axial and radial space it requires
and the need to stock a different size spring for each seal size. Multiple small springs are not as susceptible to
distortion at high speeds as are single coil springs and they consequently exert an even closing pressure on the
seal ring at all times. Shaft mounting choices include cartridge unit, noncartridge, split seal (fully split), and
semi-split seal. The seal can be tandem, face-to-face, back-to-back, or concentric.
Common applications for mechanical seals include pump, agitators or mixers, marine stern tube
(propeller shaft), gas seal (spiral groove seal), and cryogenic seal. The seal can be internally or externally
mounted. Important shaft size and service limits to consider when searching for mechanical seals include
nominal shaft diameter, shaft speed, alternate shaft or rubbing speed, operating pressure, and operating
temperature. Common features for mechanical seals include balanced or unbalanced construction, dependent on
direction or rotation or independent of direction of rotation, capability to handle slurries, and encased spring
element. The direction of the shaft rotation is important to consider. This is the direction of a shaft's rotation as
seen from the drive. Mechanical seals that are dependant on the direction of rotation are those that transmit
torque using a conical spring or those that are equipped with a pumping screw. The direction can be clockwise or
Chart No. 1 - Categories, Arrangements and Seal Types
Step I Category (see page 28)
Category 1 Category 2 Category 3
ISO 3069 type C, ASME B73.1, ASME B73.2 ISO 13709 / API 610 10th edition
-40 °C … 260 °C, 21 bar g (-40 °F … 500 °F, 300 PSI) -40 °C … 400 °C, 41 bar g (-40 °F … 750 °F, 600 PSI)
Minimal data requirements Rigorous data requirements
Step II Arrangement and configuration
Arrangement 1 1CW-FX
Step III Type and spring position Flexible element
Type A Rotary springs
Single seal cartridge Contacting Wet - FiXed throttle bushing Category 1: carbon throttle bushing Category 2: non-sparking metal throttle bushing Category 3: not applicable
1CW-FL
Contacting Wet - FLoating throttle bushing
Pusher seal Temperature: -40 to 176 °C (-40 to 350 °F) Pressure: 41 bar g (600 PSI) Multiple springs: Alloy C-276 Single spring: SS 316 O-rings: FKM or FFKM
For service conditions outside the operating limits of type A, B and C Temperature: < -40 or > 260 °C (< -40 or > 500 °F) category 1
< -40 or > 400 °C (< -40 or > 750 °F) category 2 and 3 Pressure: > 21 bar g (300 PSI) category 1
> 41 bar g (600 PSI) category 2 and 3 Surface speed: > 23 m/s (75 ft/s) Shaft diameter: below 20 mm (0.75 inch) or above 110 mm (4.3 inch) Medium: highly corrosive fluids for which the specified materials in
API 682/ISO 21049 are not suitable, fluids with absolute vapour pressures > 34 bar a (493 PSI), unstable liquid properties (e.g. multiphase, non-Newtonian), high viscosity or pour point above or within 20 °C (68 °F) of the mini- mum ambient temperature.