Centrifugal Pump Mechanical Seals

Centrifugal Pump Mechanical Seals, mechanical seal have been used for many years to seal any number of liquids at various speeds, pressures, and temperatures. Today plant operators are benefiting from improved seal technologies driven by the US clean air act of 1980, and the American Petroleum Institute (API) Standard 682. These new seal technologies are based on advanced commence testing at simulated refinery conditions required by the API. The results to date indicate not only an improvement in emissions control, but also a major increase in equipment reliability.

Classes of seal technology
Emerging seal technologies are providing clear choices for sealing. Various plant services require the application of these new technologies for emissions control, safety, and reliability. Sealing systems are now available that are based on the preferred method of lubrication to be used.

These classes of seals are as follows:
1. Contacting liquid lubricated scale:

• Normally a single seal arrangement is cooled and lubricated to the liquid being sealed. This is the most cost effective seal installation available to the industry.
• Dual seal are arrange to contain a pressurized or non pressurized barrier or buffed liquid. Normally this arrangement will be used on application where the liquid being seal is not a good lubricating fluid for a seal and for emissions containment. These arrangements require a lubrication system for the circulation of barrier or buffer liquids.

2. Non contacting gas lubricated seals:

• Dual non-contacting gas-lubricated seals are pressurized with an inert gas such as nitrogen.
• Dual non-contacting, gas-lubricated seals are used in a tandem arrangement and pressurized by the process liquid being sealed, which is allowed to flash to a gas at the seal.

A tandem seal arrangement is used on those liquid that represent a danger to the plant environment. For non-hazardous liquids, a single seal can be used.

Gland Packing

For packing to operate properly, the finish on the shaft sleeve must be at least 16 µin (o.4 µm) centerline average (CLA) and the finish in the bore should be 63 µin (1.65 µm) CLA. The sleeve must be harder than the packing and chemically resistant to the liquid being sealed. If the sleeve has a coated material for a hard wear surface, the sleeve must also have good thermal shock resistance.

Lantern Ring (Seal Cages)When an application requires that a lubricant be introduced to the packing, a lantern ring is used to distribute to flow. This ring is used or near the center of the packing installation. For ease of assembly, most lantern rings are axially split. The construction material range from metal to TFE (tetra fluoroethylene). TFE lantern rings are usually filler with glass or with glass and molybdenum disulfide. They are inherently self lubricating and will not score the shaft. A throat brushing at the bottom of the stuffing box can be used to provide a closer clearance with the shaft to prevent packing extrusion.

Staffing Box Gland Plates
All mechanical packing are mechanically loaded in the axial direction by the stuffing box gland. In cases where leakage of the process liquid is dangerous or can vaporize and create a hazard to operating personnel, a smothering gland is used to introduce a neutral liquid at lower temperature. A sufficient quantity of quenching liquid should be used to eliminate the danger from the liquid being pumped. The neutral circulated in the gland mixed with the leakage and carries it to a safe place to disposal. Close clearances to the gland control the leakages of the combined liquid to the atmosphere. This quenches can also be used to protect the packing from any wear through abrasion, because the leakage cannot vaporize and leave behind abrasive crystal.

Gland packing are usually made of bronze, but case iron or steel can be used for all iron pumps. When iron or steel glands are used, they are normally bushed with a non sparking material like bronze.

The size and Number of Packing Ring

The number of packing rings may vary depending on the objective of the sealing system or the requirements of the rotating equipment. The most common packing arrangement for rotating equipment is illustrated in figure. Three rings of packing are used to seal the process liquid from the packing lubricant. Two rings between the lantern and gland are used to restrict the leakage of the lubricant to the atmosphere. The size of packing depends on the size of equipment, typically for rotating shafts, the standard square size packings shown in the table may be considered.

For packing to operate properly the finish on the shaft sleeve must be at least 16 μin (0.4 μm) conterline average (CLA) and the finish in the bore should be 63 μin (1.65 μm) CLA. The sleeve must be harder that the packing and chemically resistant to the liquid being sealed. If the sleeve has a coated material for a hard wear surface, the sleeve must also have good thermal shock resistance.

Fibers	Metal
Mineral	Animal	Vegetable	Synthetic	Lead
Metal Graphite	Wool Hair Leather	Flax Ramie Jute Cotton Paper	Nylon Rayon TFE Carbon Aramid Polyamide	Copper Brass P-Broze