Stem Packing

Stem packing performs one or both of the following two functions, depending on the application:

● Prevent leakage of flow medium to the environment

● Prevent outside air from entering the valve in vacuum applications

Stem packing is contained in a part called the stuffing box. Packing rings are packed and compressed by tightening a packing nut or packing gland bolts. Compression must be adequate to achieve a good seal. Sometimes it requires regular inspection and tightening of packing rings, if required, to stop leakage. If this does not stop the leakage, the packing may need to be replaced. Belleville washers are used to maintain live loading, or the required compression of packing to achieve an effective seal against leakage.

A stuffing box may be provided with some or all of the following features as dictated by the valve application:

● Two sets of packing rings separated by an intermediate lantern ring

● A bottom junk ring

● A leak-off connection which detects leakage past the lower set of packing rings and is piped off to a leakage collection tank

● A blow-off connection for removal of packing rings using compressed air

● Belleville washer, live loading

● A steam-seal connection, where an external steam supply is used to prevent leakage from the packing chamber

● As an alternate to steam-seal connection, a grease or sealant-seal connection used to prevent loss of vacuum within the valve

● Inverted-V packing for vacuum service

Figure A10.3a shows a standard graphite packing arrangement; Figure A10.3b depicts the inverted Teflon packing arrangement for vacuum service; Figure A10.3c reflects a lantern-ring packing arrangement; and Figure A10.3d shows a live-loading packing system.

Stem Protector. In the case of outside-screw-and yoke rising-stem gate and globe valves, a portion of the threaded valve stem is exposed to the outside environment when the valve is in the open position. Airborne dirt and other substances may be deposited on the exposed portion of the threaded stem and impair its smooth operation or shorten its stem-bushing life. A stem protector in the form of a clear plastic sleeve, tubing, or a pipe with a cap at the end is installed to protect the stem. The length of the stem protector must be adequate to allow full stem travel.

Back Seat. Back seat is comprised of a shoulder on the stem and a mating surface on the underside of the bonnet. It forms a seal when the stem is in the fully open position. It prevents leakage of flow medium from the valve shell into the packing chamber and consequently to the environment. Back seat enables dismantling of the valve beyond the bonnet, without disrupting the fluid flow through the valve. In addition, it allows the replacing of the stuffing box while the valve is in service.

Yoke Bushings.

An internally threaded nut held in the top of a yoke through which the valve stem passes. In gate and diaphragm valves, the yoke nut is turned and the stem travels up or down depending upon the direction of rotation of the nut. In the case of globe valves, the nut is held fixed and the stem is rotated through it. Usually, the yoke nut or yoke bushing is made of softer material than the stem for valves requiring medium effort to actuate. Valves which require greater effort to open or close are provided with anti-freeze yoke-sleeve bearings that minimize the friction between the hardened stem and the yoke bushing.