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

counter-clockwise.

Basic components:

All mechanical seals are constructed of three basic sets of parts as shown in Fig.

1. A set of primary seal faces: one rotary and one stationary, shown in Fig. as seal ring and insert.

2. A set of secondary seals known as shaft packings and insert mountings such as 0-rings, wedges and V-

rings.

3. Mechanical seal hardware including gland rings, collars, compression rings, pins, springs and bellows.

Gland Packing Vs Mechanical Seals

Conventional sealing is by means of gland packing .

The problems with gland pickings are:

1] PRODUCT LOSS

2] EXCESSIVE WEAR ON SHAFT & SLEEVE

3] FREQUENT ADJUSTMENTS

4] ENERGY LOSS

Product Loss:

• Gland packings work on principle of controlled leakage for proper life .

• Gland packing has to leak to perform. The product loss resulting from this leakage can be quantified as

follows,

Leakage rate Quantum

One drop every five seconds 550 ltrs/year

Two drops per seconds 5500 ltrs/year

Steady stream leakage 40000 ltrs/year

Product loss = above quantities * cost/lt of liquid

Energy loss :

• It is estimated that the energy lost due to gland friction is usually up to 10% of the motor output, as

against this the mechanical seals consumes negligible power.

• Today electricity due to its high cost is one of the raw material for any plant setup, so the savings in this

area would more than offset the cost of maintaining a mechanical seals.

Mechanical Seal Types

Mechanical seals can be classified into several types and arrangements:

PUSHER:

Incorporate secondary seals that move axially along a shaft or sleeve to maintain contact at the seal faces. This

feature compensates for seal face wear and wobble due to misalignment. The pusher seals' advantage is that it's

inexpensive and commercially available in a wide range of sizes and configurations. Its disadvantage is that ft's

prone to secondary seal hang-up and fretting of the shaft or sleeve. Examples are Dura RO and Crane Type 9T.

UNBALANCED:

They are inexpensive, leak less, and are more stable when subjected to vibration, misalignment, and cavitation.

The disadvantage is their relative low pressure limit. If the closing force exerted on the seal faces exceeds the

pressure limit, the lubricating film between the faces is squeezed out and the highly loaded dry running seal fails.

Examples are the Dura RO and Crane 9T.

CONVENTIONAL:

Examples are the Dura RO and Crane Type 1 which require setting and alignment of the seal (single, double,

tandem) on the shaft or sleeve of the pump. Although setting a mechanical seal is relatively simple, today's

emphasis on reducing maintenance costs has increased preference for cartridge seals.

NON-PUSHER:

The non-pusher or bellows seal does not have to move along the shaft or sleeve to maintain seal face contact, The

main advantages are its ability to handle high and low temperature applications, and does not require a secondary

seal (not prone to secondary seal hang-up). A disadvantage of this style seal is that its thin bellows cross sections

must be upgraded for use in corrosive environments Examples are Dura CBR and Crane 215, and Sealol 680.

BALANCED:

Balancing a mechanical seal involves a simple design change, which reduces the hydraulic forces acting to close

the seal faces. Balanced seals have higher-pressure limits, lower seal face loading, and generate less heat. This

makes them well suited to handle liquids with poor lubricity and high vapor pressures such as light hydrocarbons.

Examples are Dura CBR and PBR and Crane 98T and 215.

CARTRIDGE:

Examples are Dura P-SO and Crane 1100 which have the mechanical seal premounted on a sleeve including the

gland and fit directly over the Model 3196 shaft or shaft sleeve (available single, double, tandem). The major

benefit, of course is no requirement for the usual seal setting measurements for their installation. Cartridge seals

lower maintenance costs and reduce seal setting errors

Balanced vs Unbalanced Seals

Balance ratio:

How a mechanical seal work:

The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to

the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all seals, one face is

held stationary in a housing and the other face is fixed to, and rotates with, the shaft. One of the faces is usually a

non-galling material such ascarbon-graphite. The other is usually a relatively hard material like silicon-carbide.

Dissimilar materials are usually used for the stationary insert and the rotating seal ring face in order to prevent

adhesion of the two faces. The softer face usually has the smaller mating surface and is commonly called

the wear nose.

There are four main sealing points within an end face mechanical seal (Fig. 10). The primary seal is at the seal

face, Point A. The leakage path at Point B is blocked by either an 0-ring, a V-ring or a wedge. Leakage paths at

Points C and D are blocked by gaskets or 0-rings.

Sealing Points for Mechanical Seal

The faces in a typical mechanical seal are lubricated with a boundary layer of gas or liquid between the faces. In

designing seals for the desired leakage, seal life, and energy consumption, the designer must consider how the

faces are to be lubricated and select from a number of modes of seal face lubrication.

To select the best seal design, it's necessary to know as much as possible about the operating conditions and the

product to be sealed. Complete information about the product and environment will allow selection of the best

seal for the application.

Mechanical Seal Selection

The proper selection of a mechanical seal can be made only if the full operating conditions are known:

Liquid

Pressure

Temperature

Characteristics of Liquid

Reliability and Emission Concerns

Liquid: Identification of the exact liquid to be handled is the first step in seal selection. The metal parts

must be corrosion resistant, usually steel, bronze, stainless steel, or Hastelloy. The mating faces must also

resist corrosion and wear. Carbon, ceramic, silicon carbide or tungsten carbide may be considered.

Stationary sealing members of Buna, EPR, Viton and Teflon are common.

Pressure: The proper type of seal, balanced or unbalanced, is based on the pressure on the seal and on the

seal size.

Temperature: In part, determines the use of the sealing members. Materials must be selected to handle

liquid temperature.

Characteristics of Liquid: Abrasive liquids create excessive wear and short seal life. Double seals or clear

liquid flushing from an external source allow the use of mechanical seals on these difficult liquids. On light

hydrocarbons balanced seals are often used for longer seal life even though pressures are low.

Reliability and Emission Concerns: The seal type and arrangement selected must meet the desired

reliability and emission standards for the pump application. Double seals and double gas barrier seals are

becoming the seals of choice.

Materials of Construction

Primary seals:

1] Ceramic (metal oxide)[ for corrosive chemicals]

2] Carbon

A] Carbon graphite, resin impregnated,[carbon R]

B] Carbon graphite, antimony impregnated,

[carbon M]

3] Tungsten carbide

* It can be re-lapped & reused

4] Silicon carbide (hardness= 2 x tungsten carbide)

5] Glass filled teflon (externally mounted bellow

Type seals , extremely corrosive services)

Secondary seal materials:

Elastomers Temp range( c )

1] Nitrile -40 to 105

2] Neoprene -40 to 120

3] Butyl -40 to 140

4] Silicone -100 to 200

5] Viton -50 to 200

6] Hyplon -40 to 120

7] Fluorosilicone -70 to 205

Non Elastomers Temp range( c )

1] PTFE -200 TO 260

2] GLASS FILLED PTFE -200 TO 260

(MORE HARDNESS)

3] GRAFOIL -270 TO 450

(VERY GOOD RESISTANCE TO HIGH & LOW TEMPERATURE)

Spring materials

1] SS-304 (Cr,Ni)

2] SS-316 (Cr,Ni,Mo)

3] HASTEALLOY-C

4] ALLOY-20

5] MONEL ALLOY

Common Face Combination

SEAL FACE MATING FACE

CARBON S.STEEL

STELLITE

CHORME OXIDE

COATING TUNGESTAN CARBIDE

CERAMIC

LEAD BRONZE

SILICON CARBIDE

TUNGSTEN TUNGSTEN CARBIDE

(CARBIDE) SILICON CARBIDE

Guides for the successful installation of mechanical seals

Preparation:

(a) Be sure seal chamber is clean and free of all foreign matter.

(b) Shaft of shaft sleeve on which the seal is to operate must be to size, must be smooth, straight, and free of all

burrs, sharp corners, nicks, or excessively deep scratches.

(c) Plug all holes in the stuffing box which are not to be used in the operation of the seal.

(d) Face of stuffing boxes must be smooth, clean, and square with the axis of the shaft.

(e) Halves of boxes on horizontal split case pumps must match perfectly, with the gasket between the halves

extending flush with the surface on which the mechanical seal gland-gasket is to seal. Remove all sharp corners

and burrs from stuffing box face.

(f) Check the shaft for alignment with a dial Indicator. The maximum allowable runout for optimum seal

performance Is .005" TIR. Excessive misalignment may mean faulty bearings or bent shafts.

(g) Keep shaft end-play at a minimum recommended maximum end-play is .005 inches.

(h) Check pump wear rings and impeller for proper clearances. Shaft must turn freely. Vibrations caused by

rubbing and improper clearances can cause seal failure.

(i) Wherever shaft sleeves are used, make certain the sleeve is properly gasketed to the shaft to prevent leakage

under the sleeve.

Installation of the Seal :

(a) Always handle mechanical seals with extreme care. Cleanliness is imperative. Never place faces face down

on bench or floor. Keep seal in shipping containers until ready to install.

(b) Follow seal Installations Instructions carefully, (See Installation Instructions).

(c) Be sure all seal set screws are tight.

(d) Where set screws are used as a drive between seal and shaft, shaft should be counter-sunk to receive cup

point.

(e) Care should be exercised In tightening gland bolts. Tighten evenly and do not spring gland.

(f) Use four equal-spaced gland bolts wherever possible; API-ASME Code for Unfired Pressure Vessels should

be followed wherever possible when selecting gland bolt size and spacing.

(g) When tightening gland bolts, check clearances between shaft and gland with feeler gauges. This is

particularly important when the gland is not piloted on the stuffing box as glands must be accurately cantered.

(h) Test seals statically under pressure before starting pump. Make slight adjustments in gland nuts as necessary

to stop any leakage which may occur through gland-gasket.

(i) Never operate mechanical seals dry. Carefully follow instructions for flushing and cooling connections where

specified. Be sure suction and discharge of pump is open and a positive head of fluid is present before starting

pump. This applies even to that period when checking for proper direction of rotation and adjustment of motor

electrical connections.

Seal Care:

Examine the faces

o Chipped edges on either or both faces

o Flaking or peeling of the hard facing

o Pitting, blistering, corrosion of the carbon face

Check the springs

o Spring(s) or bellows breakage (metal)

o Clogged springs

o Bellows clogged at the inside

o Deep wear in the hard face

o Worn drive lugs or worn drive slots

Check the elastomer

o Swollen, sticky, or disintegrating elastomer

o Hardening of the elastomer

o Charring of the elastomer, cracking, burned appearance

o Elastomer has changed shape, "O" rings square, etc.

Check for accidental rubbing

o Widened wear track

o Broken ceramic

o Worn spot in the stationary ring

o Heat

o Inspect the seal drive

o Hysterisis

o Face separation

o Proper size wear track

o Narrow wear track

o No wear track

o No wear track - shiny spots on the face

o Collect the entire seal

Seal Life:

Short Seal Life

o The major cause of short seal life is excessive abrasives getting between the faces and causing

rapid wear

o The source of these abrasives may either come from slurry condition or they may come from the

super-cooling of a supersaturated solution or it may occur due to flashing across the seal faces,

causing the dissolved solids to crystallize out between said faces and, again, causing wear

o Cooling or heating, as the situation might occur, and/or most assuredly circulation of pumpage

from discharge to the stuffing box or external clear flushing will alleviate these conditions

o Misalignment of equipment

o Pipe strain distortions as mentioned

o Seal shows signs of running too hot when a by-pass flush or recirculation may be necessary

o Check for the possible rubbing of seal components along the shaft

o Throttle bushings and poorly piloted glands can often cause this condition

o Attempt more effective cooling of the seal area by connecting all cooling lines, checking to

ascertain that all cross drilling of flush lines, etc., are clear and unobstructed (remove all scale,

etc., that may accumulate in these lines)

o By increasing the capacity of cooling lines or open the orifice clearances on circulation lines

o Possible improper choice of type of seal

Seal failures & Causes:

1] Mishandling the components:

o Allowing the seal’s components to become chipped, scratched or damaged.

2] Incorrect seal assembly:

o This includes the incorrect setting or misplacing of the seal component in the seal cavity

3] Improper materials or seal design:

o This consists of selecting the wrong materials of construction or an incorrect design for the

combination of pressure, temp, speeds and fluid properties required for a given application

4] Improper start up and operating steps:

o This might be something as a simple as failing to pressurize a double seal before starting a pump,

or inadvertently running a seal dry

5] Fluid contamination:

o This might be the presence of harmful solid particles in the seal cavity fluid.

6] Poor equipment condition:

o Here the problem might be caused by excessive shaft run-out, deflection or vibration.

Analysis of Failures

Failure analysis should follow a systematic approach:

Step 1:

Identify the problems resulting in short seal life. This may not be always be a seal design problems.

Step 2:

Carefully examine possible solution to the problem. Past experience, feed-back from equipment manufactures

and consultation with a knowledgeable seal expert will be valuable in formulating a list of possible answers.

Step 3:

Choose an appropriate remedy and take corrosive action. Selecting the best one will require an analysis of cost,

availability of hardware and future economic benefits.

Step 4:

Follow up on the problem – solving efforts.

Mechanical Seal Application Limits

General application guide per seal type

Seal Type Applications

Non-pusher elastomeric bellows seal A - B - D - E - L

Non-pusher metal bellows seal A - D - E - F - I - J - L

Pusher o-ring secondary seal A - B - G - H - K

Pusher polymer seal A - B - G - K

Pusher stationary slurry seal A - B - C - D - E - F - M

Pusher split seal A - B - K

Pusher dual gas seal A - B - E - F - G - H - L

Fluid – Characteristics A - Clean Lubricating

B - Clean Non-lubricating

C - Viscous

D - Clogging / Scaling / Polymerizing / Fibrous

E - Crystallizing

F - Molten Liquid

G - Corrosive - Acids

H - High Vapor Pressure

I - Cryogenic

J - High Temperature (> 260 ºC / 500 ºF)

K - Solids (< 0.1% by volume and less than 10 micrometers (394 micro inches) in size.

L - Solids (< 2% by volume and less than 10 micrometers (394 micro inches) in size

M - Solids (> 2% by volume).

Typical dynamic pressure and temperature limits of common seal types

Seal Type Pusher Non-pusher Balanced Unbalanced Max. Pressure

(kPag/psig)

Temperature Range

(ºC / ºF)

Elastomeric

bellows x x

2070 / 300 -40 to 205 / -40 to 400

Elastomeric

bellows x x

6900 / 1000 -40 to 205 / -40 to 400

Metal bellows x x 2070 / 300 -75 to 425 / -100 to 800

O-ring

secondary seal x x

1380 / 200 -40 to 260 / -40 to 500

O-ring

secondary seal x x

6900 / 1000 -40 to 260 / -40 to 500

Polymer

secondary seal x x

1380 / 200 -75 to 260 / -100 to 500

Polymer

secondary seal x x

5070 / 500 -75 to 260 / -100 to 500

Stationary slurry x x 2670 / 400 -40 to 205 / -40 to 400

Split seal x x 1380 / 200 -40 to 205 / -40 to 400

Dual gas seal x x 2070 / 300 -40 to 260 / -40 to 500

Dual gas seal x 1725 / 250 -40 to 260 / -40 to 500

Typical PV – Limits of face material combinations in non-lubricating fluids, i.e. watery

substances

PV = face pressure x velocity

Is an indicator for the severity of an application

Is limited in usefulness

For lubricating fluids multiply number by 1.5

Primary Ring Mating Ring PV Limit

(MPa x m/s)

PV Limit

(psi x ft/min)

Glass-Filled PTFE Ceramic / Silicon Carbide 6.13 25,000

Carbon Cast Iron 24.52 100,000

Carbon Ceramic 24.52 100,000

Carbon Tungsten Carbide 122.59 500,000

Carbon Silicon Carbide 147.11 600,000

Tungsten Carbide Tungsten Carbide 24.92 120,000

Silicon Carbide Silicon Carbide 85.81 350,000

Typical angular misalignment limits

Shaft Speed

(rpm)

Pusher & Metal

Bellows

(mm)

Pusher & Metal

Bellows

(in)

Elastomer

Bellows

(mm)

Elastomer

Bellows

(in)

500 0.152 0.006 0.279 0.011

1000 0.127 0.005 0.254 0.010

2000 0.089 0.0035 0.191 0.0075

3000 0.064 0.0025 0.152 0.006

4000 0.051 0.002 0.127 0.005

5000 0.038 0.0015 0.089 0.0035

6000 0.025 0.001 0.151 0.002

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

NBR, HNBR, EPM, EPDM, TFE …

Seal face surface speed < 23 m/s

Stationary springs

Seal face surface speed > 23 m/s

Category 3: carbon throttle bushing Category 1 and 2: carbon throttle bushing

Type B Rotary bellows

Metal bellows seal with O-rings

Arrangement 2 2CW-CW Temperature: -40 to 176 °C (-40 to 350 °F) Pressure: 21 bar g (300 PSI)

Stationary bellows

Dual seal cartridge - pressure between seals

less than seal chamber pressure

- internal reverse balance feature

Contacting Wet - Contacting Wet

2CW-CS

Contacting Wet - Containment Seal (non-contacting or contacting CS)

2NC-CS

Metal bellows: Alloy C-276 O-rings: FKM or FFKM

NBR, HNBR, EPM, EPDM, TFE …

Type C Rotary bellows

Metal bellows seal with flexible graphite

- fixed carbon throttle bushing

Non-Contacting - Containment Seal (non-contacting or contacting CS)

Temperature: -40 to 400 °C (-40 to 750 °F) Pressure: 21 bar g (300 PSI) Metal bellows: Alloy 718

Stationary bellows

Arrangement 3 3CW-FB Sealing element: flexible graphite

Dual seal cartridge - pressure between seals

higher than seal chamber pressure

- internal reverse balance feature

- fixed carbon throttle bushing

Contacting Wet - Face-to-Back

3CW-BB

Contacting Wet - Back-to-Back

3CW-FF

Contacting Wet - Face-to-Face

3NC-BB

Non-Contacting - Back-to-Back

3NC-FF

Non-Contacting - Face-to-Face

3NC-FB

Non-Contacting - Face-to-Back

Totally Engineered Sealing system (ES)

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.

Default configurations Optional configurations Optional design featur