Environmental controls

Suction bypass

The suction bypass is good for:

■ Removing heat

■ Removing suspended solids

AUTHOR'S NOTE

Be aware that the liquid may vaporize if the fluid's vapor pressure should surpass the absolute pressure in the seal chamber.

The external flush

The external flush is good for:

■ Removing heat

■ Separating the seal environment from the pumped liquid

AUTHOR'S NOTE

This environmental control may also dilute the pumped product. If the external flush is water, it may be necessary to evaporate it later in a costly posterior process.

Figure 14-15 The external flush

Figure 14-15 The external flush

The thermal jacketed seal chamber

The thermal jacket seal chamber is good for removing/controlling heat without introducing additional liquid into the process.

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Figure 14-16 thermal jacketed seal chamber ■ 216

The slurry seal with evacuation line

The slurry seal with the evacuation line is designed to handle/evacuate suspended solids, crystals, sediment, and dirt in the pumpage. The seal s springs are located out of the fluid. The o-rings move and rub across a clean surface as the faces wear. The o-rings are placed away from the heat generated by the faces.

The quench and drain

The Quench and Drain control is good for removing heat without contaminating or diluting the pumped product.

AUTHOR'S NOTE

This method requires a source of water or steam for the quench. The pumped liquid may contaminate the drainage. If so, it should be discarded.

The heat exchanger

The heat exchanger is good for controlling temperature in the seal chamber utilizing the same pumped product. This method doesn't dilute the product.

PUMP SUCTION

Figure 14-19 The heat exchanger

PUMP SUCTION

Figure 14-19 The heat exchanger

The double or dual mechanical seal

The dual seal is good to control the environment inside the seal without diluting the pumped product. It works with re circulated barrier fluid. It establishes a barrier between the pumped fluid and the environment (Figure 14-20).

You've just seen six different concepts to control the environment inside the seal chamber and mechanical seal. Some methods are economical. Others are costly. Some have secondary side effects to contend with. Now let's consider the 14 difficult sealing situations and apply the environmental controls to extend the running time of the seal and pump.

Figure 14-20 The double or dual mechanical seal

1. Crystallization Some liquids form crystals with heat (think of sugar and rock candy), and others with cold (think of ice). To control and prevent crystallization in the seal chamber it is necessary to control the temperature.

■ Suction Bypass

■ External Flush

■ Thermal Jacketed seal chamber

■ Quench and Drain

■ Heat Exchanger

2. Liquids that Solidify To deal with liquids that try to solidify (think of ice, cement, glue and paints) it is necessary to identify the cause and solution for the solidification:

CAUSE OF SOLIDIFICATION SOLUTION

1.

Air (paint, glue)

Quench and Drain or the Double Seal

2.

Change in Temperature (ice)

Control the temperature, either up or down.

See Crystallization.

3.

Agitation (merengue)

Evacuate the seal chamber, suction bypass.

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4. Time (cement)

Evacuate the seal chamber, suction Bypass.

3. Vaporizing Liquids Certain liquids vaporize with heat (think of steam), and other liquids vaporize with a drop in pressure (think of liquid propane or freon). To control vaporizing liquids so they don't change phase in the seal chamber.

CAUSE OF VAPORIZATION SOLUTION

1. Too much heat Cool the seal chamber. See Crystallization.

2. Drop in Pressure Double seal with pressurized barrier fluid

4. Film or Skin Formation Some liquids form a skin (like hot milk) with heat. Others form a film (like paints) on contact with air:

CAUSE OF FILM SOLUTION

1. Temperature Control it See Crystallization

2. Contact with air Quench and Drain or Double Seal

5. Very High Temperature Remove the heat with cooling. See Crystallization.

6. Non-Lubricating Liquids No lubrication or heat dissipation. Remove the heat with a double seal and a barrier tank with forced convection flow.

7. Dry Running Pump Again, no lubrication or dissipation of heat. Remove the heat with a double seal and barrier tank with forced convectivc flow.

8. Dangerous Liquids Use a dual seal with a pressurized barrier tank.

9. Gases and Liquids Tending to Gas Gases cannot lubricate the seal faces. No dissipation of heat. Use a dual seal with forced convectivc flow.

10. Slurries and Suspended Solids Should be purged or flushed with:

■ The filtered or cleaned product

■ A compatible liquid

■ A posterior additive in the system

11. Cryogenics Cryogenic liquids are too cold for most seals. Use a metal bellows mechanical seal with no elastomer parts.

12. Corrosive Liquids Vary with the temperature. Use seal components resistant to corrosion. You should take into consideration the:

■ Elastomer parts and gaskets

■ Chemical compatibility with the face materials

■ Metallurgy: sleeve, shaft, set screws, gland, metallic parts, drive pins, clips, keyways, anti rotation pins, etc.

13. Very High Pressures Use balanced o-ring cartridge seals up to about 500 psi. Above 500 psi use the tandem double seal with the barrier fluid pressurized at /> the seal chamber pressure. Remember as pressure goes up the o-rings will extrude and metal parts will distort. Use a torsion balanced seal.

14. Hard Vacuum Use a balanced o-ring seal for industrial vacuum. For absolute pressure less than 1 kpa (1 kilopascal) use a torsion balanced seal. (Must verify this measurement)

15. Extremely High Velocity Shaft Speed Some pump companies use very high velocity, 30,000 rpm, to improve efficiency and generate high head with small equipment. Use a stationary seal, with the springs in the stationary element.

A big part of the overall problem with adequate mechanical seal life is trying to make a precision mechanical seal run into the same space that previously was occupied by the packing rings. Pump design evolved over time to accommodate the packing rings.

For example, the restriction bushing in the bottom of the packing stuffing box is designed to prevent the gland follower from pushing the packing out of the bottom of the box. With a mechanical seal, the restriction bushing in the bottom of the packing box is a 'dinosaur of design'. It performs no function with the mechanical seal, except to shorten its life by holding heat, preventing clean cooling liquid from arriving to the mechanical seal faces, and trapping abrasives, sediment, crystals, and dirt. If you were to remove the restriction bushing by placing the part on a lathe and machining it away, certain pumping applications could immediately quadruple the service life of the mechanical seal.

The bore of the packing box serves to hold the packing around the shaft so that the pressure from the gland follower can axially compress the rings to affect a shaft seal. With a mechanical seal, the reduced tight bore of the packing box is another dinosaur of design. It could be opened on a lathe in the same function as the removal of the restriction bushing at the bottom of the box. This would immediately triple or quadruple the service life of most mechanical seals.

Many packed pumps have an installed discharge bypass line running from the discharge nozzle of the pump to the packing box. This line damages most seals by blasting the seal with the highest concentration of solids moving through the pump. As a pump is converted from packings to mechanical seals, removing the discharge bypass line, opening the seal chamber bore, and machining the restriction bushing in the bottom of the chamber will go a long way to achieving the desired life with a mechanical seal.

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