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Stem Seals and Packing Configuration
Determining Load on Thermafoil Stem packing
Torque Formula for Grafoil Valve Packing Rings

Stem Seals and Packing Configuration

Several styles and configurations are available using Thermafoil seals and Thermabraid packing.  Many stem seal designs use a single style of material or a combination of styles for a specific set of parameters. We have chosen the most popular styles and their recommended use for your design consideration. The following is the definition of various styles used in stem sealing arrangements.

Die-molded rings: made from Grafoil/Thermafoil ribbon material these are the most widely used as a seal rings in valve applications. .

Die cut laminated rings: made from sheets of Thermafoil laminated and cured, these are the most precision rings for valve applications.

Die cut and coined laminated rings: constructed the same as our standard laminated, these rings are coined for high density, precision applications.

Tri-clad rings: using either die-molded or die cut rings and cladding them with metal foil provides all the features of yarn rings and seal rings in one piece. .

Thermabraid 6000: high-density die-molded Thermabraid for use as anti-extrusion rings. .

Graphite and carbon yarn rings: supplied as end rings for packing sets, these rings provide two benefits. First, they provide anti-extrusion benefits for the flexible graphite and should be used in pressures exceeding 500 psi. Secondly, they provide a wiping action to clean the valve stem and should be used in most rising stem control valves. Caution should be taken when using this type of product in very high pressures.

Determining Load on Thermafoil Stem Packing

Achieving the proper load on valve stem packing takes in many variables and it would be most impractical to try and address all of them here. The following is from UCAR Carbon's technical bulletin and is a good reference.

Torque Formula for Grafoil Valve Packing Rings

The development of Union Carbide's torque formula is based on the valve being packed with Grafoil and graphite or carbon yarn end rings.  The Grafoil rings should be standard 90 lbs/cu. ft. +/-5 lbs/cu. ft. DMRP  packing rings.  The stack height of 5 rings is preferred for developing good sealing characteristics.  A spacer in the stuffing box does not change the results.  If double stack height is needed in the valve, each stack should be torqued to specification before a lantern ring or another stack is installed.

To calculate minimum torque requirements for Grafoil, the following equations apply:

Where: safety factor

1.75 max.

1.25 min.

all well lubricated bolts and nuts

0.12

steel bolts and nuts

0.2

stainless steel bolts and nuts

0.5