Bearings and Seals in Oil Field Machinery

7/28/2019 Bearings and Seals in Oil Field Machinery

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Bearings and Seals in Oil Field Machinery

Introduction

Bearings are very important parts of rotating machinery. They are installed around the shaft of the

rotating element andservea number of purposes :

1. In conjunction with oil, bearings help to minimise friction.

2. They support the weight of the rotor.

3. Bearings minimise and control shaft movement which takes place in all directions as follows:

1. RADIAL MOVEMENT This is movement of the shaft in all vertical, horizontal and

other directions ACROSS the cross-section of the shaft.

2. AXIAL MOVEMENT In all rotating machinery, there is a tendency for a shaft and

rotor to move horizontally -ALONG THE LENGTH of the shaft. This is due to a force

called ' THRUST ' and is caused by pressure difference across the rotor of the driver-( Turbine ..etc ) or the machine being driven - ( Pump, Compressor ..etc.).

( See Figure : 6 )

SHAFT MOVEMENT

Figure : 6
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In modern industry, large powerful, high speed machines normally have some method of monitoring

shaft movement and vibration.

Sophisticated electronic equipment is used which constantly measure these movements and, a

detection probe, at a pre-set vibration level, will initiate a high vibration alarm.

At some slightly higher setting, a trip device will operate to shut-down the machine automatically and

safely.

Highly expensive machine damage and losses of production can be averted by this early warning of

impending bearing or rotor problems.

RADIAL MOVEMENT ( or Vibration across the shaft ).

This vibration movement must be minimised and controlled in order to prevent serious damage to

shafts, rotors and casings. The control of radial movement can be achieved by a variety of bearings.

1. ' SLEEVE ' or ' JOURNAL ' BEARINGS

2. ' BALL ' BEARINGS

3. ' ROLLER ' BEARINGS

Any, or all of the above bearings, in addition to helping to minimise friction, also support the weight

of the rotating element. (See Figure : 7)

Figure : 7


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Figure : 8

AXIAL MOVEMENT ( Horizontal movement or vibration )

This movement, also known as 'THRUST', is mainly controlled by using 'Thrust Bearings' or 'Thrust

Pads'.

Thrust in many machines can also be reduced by the line-up of the driver and the driven machine in

a way that the thrust of one is counter-balanced by that of the other.


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SEALS Purpose And Types

Seals are used in machines to prevent leakage of fluids to the atmosphere and prevent internal

leakage, particularly between the stages of multi-stage centrifugal machines which will decrease the

machine's efficiency. Leakage to atmosphere is undesirable, especially when toxic or other

dangerous fluids are concerned.

There are many different sealing devices and systems in use. Following is a list of some seals, their

construction and purpose :

A. LABYRINTH SEAL

A ' Labyrinth ' is a twisting, turning passage through which progress is difficult - like a maze.

This type of seal is made of soft, white metal and fitted like ' teeth ' between the shaft and the casing.

The clearance between the inter-meshed teeth is very small. Gas which tends to pass through the

teeth enters cavities where, due to the teeth, it has to change direction and begins to swirl. This

action decreases the gas energy, (velocity and pressure).

The action is repeated as the gas passes through the labyrinth and the leakage of gas is thus

minimised.

( See Figure : 10 )

B. CARBON RING SEALS

These, as their name suggests, are rings made of carbon. They consist of a series of carbon rings

held around the shaft by 'Garter Springs'. The rings are in contact with the shaft giving a good seal

against leakage.

The rings and springs are held together by 'Ring Cups'.

This type of seal is commonly used in the glands of steam and gas turbines to prevent leakage of

very hot steam or gases to atmosphere.

( See Figure : 11 )

LABYRINTH SEAL


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Figure : 10

CARBON RING SEAL

Figure : 11


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C. MECHANICAL SEAL

Mechanical seals consist of two rings very close together (almost in contact). One ring is made of

carbon and the other of metal. The almost-contacting surfaces of the rings are highly polished to

minimise friction. The mechanical seal is installed with the metal ring fixed to the casing (stationary),

while the carbon ring rotates with the shaft.

A slight flow of fluid is allowed to leak from the machine or a fluid is injected. This forms a very fine

film between the two surfaces effectively preventing leakage and providing lubrication and cooling to

the assembly. ( See Figure : 12 )

Figure : 12

D. PACKED GLAND SEALThis type of seal is often called a 'Stuffing Box Seal' and consists of a chamber in the casing of the

machine through which the shaft passes to the outside.

The chamber, or stuffing box, is filled with rings of soft packing material placed around the shaft. The

packing is held in position by a 'Gland and Gland Follower' which is mounted around the shaft in a

way that it can be adjusted.


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The adjustment to the gland is made to compress the packing and give a good seal against the shaft

and housing thus virtually preventing leakage. A slight leakage is desirable in order to lubricate the

shaft. The packing gland should never be tightened down too much as this will cause friction and

heat build-up.

When leakage is excessive and tightening the gland has little effect, the old packing should be

removed and replaced with new. The installation of packing may include a 'Lantern Ring' placed

opposite to a fluid injection point for lubrication, cooling and sealing of the assembly. The injected

fluid may be from the discharge of the machine itself or from some outside source.

STUFFING BOX SEAL (or PACKED GLAND SEAL)


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Figure : 13

FILM SEAL FOR SOUR GAS COMPRESSOR

Figure: 14

The oil-film seal is used where the process gas is sour (contains H2S and/or acid gases). H2S in

particular, is a toxic, foul smelling gas and its release to atmosphere must be minimised.

The sealing faces and 'O' rings are kept in contact with the seal housing by the springs. The

sealing action is enhanced by the introduction of a pressurised seal oil into the assembly.

The seal oil supply is maintained at a pressure about 5 psi higher than the compressor suction gas.

This assists the springs in keeping the tight seal. Because the oil pressure is higher than that of the

gas, the oil will tend to leak: - 1. Inwards towards the compressor, mixing with any leaked gas and

passing out of the sour oil drain to disposal or reclamation. 2. Outwards towards the atmosphere but

passes back to the oil reservoir via the sweet oil line i.e. has not been contaminated by the sour

gas.


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The flow to the sweet oil drain is greater that to the sour oil due to the seal to shaft clearance i.e.

0.008" compared to the 'sour' end at 0.002". This greater flow is desirable for cooling and sealing of

the seal assembly.

(More information on oil-film seals is available.)

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Bearings and Seals in Oil Field Machinery