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Tuesday, June 30, 2009

The Design of Packing Rings

Packing is used in the stuffing box of a centrifugal pump to control the leakage of the pumped liquid out, or the leakage of air in, where the shaft passes through the casing. The basic form of a seal can be applied in light-to medium duty services and to those liquids that prove to difficult to mechanic seals.

Packing may be referred to as compression, automatic or floating. Each term describes the type of operation in which the packing will be used.

Automatic and floating packing require no gland adjustment in controlling leakage. Automatic packing are confine to a given space and are activated by the operating pressure. Automatic packing rings are designed in the form of V-rings, U-cups and O-rings. Floating packing include piston rings and segmental rings that may be energize by a spring. These type of packing are commonly used in reciprocating application.

Compression packing is most commonly used on rotating equipment. The seal is formed by the packing being squeeze between the inboard end of the stuffing box and the gland. A static seal is formed at the end of the packing ring and at the inside diameter of the stuffing box. The dynamic seal is formed between the packing and shaft or shaft sleeve. Under a load, the packing deforms down against the shaft, controlling leakage. Some leakage along the shaft is necessary too cool and lubricate the packing. The amount of leakage will depend on the materials of the construction for the packing, the operating condition of the application, and the condition of the equipments.

Packing must be able to withstand equipment variables. The design of the packing ring and the materials of construction must be resilient to follow shaft run out and misalignment, as well as to compensate for thermal growth of the equipment without an appreciable increase in leakage.