Regardless of valve type, all stem-actuated control valves require a seal to allow movement of the stem from an external device (an actuator) while sealing process fluid to prevent leaks between the moving stem and the valve body. Packing is a broad word for this type of sealing mechanism.
This mechanical element is similar to the stuffing box that keeps seawater out.
The essential difficulty for the ship and the control valve is the same: how to allow a moving shaft to pass through an impenetrable barrier to some fluid The solution is to wrap the shaft with a flexible material that ensures a tight fit while allowing the shaft to move freely. Flax rope is a common packing material for ship propeller shafts.
So that this packing material does not impose undue friction on the shaft’s motion, some type of lubrication is normally given. (Self-lubricating packing materials, such as Teflon and graphite, are common.)
Instead of flax, modern marine stuffing boxes employ sophisticated materials like Teflon (PTFE) or graphite, which last longer and leak less water. The conventional packing material for control valves used to be asbestos (formed into rings or ropes, similar to how flax was made for use in stuffing boxes), but it is now more commonly Teflon or graphite.
A small amount of water leaking in the stuffing box of a ship is not an issue because all ships are equipped with bilge pumps to pump out collected water over time. Leakage, on the other hand, is simply unacceptable in many industrial control valve applications where fugitive emissions must be kept to a minimum.
Any undesirable escape of process substance into the surrounding environment is referred to as a “fugitive emission,” which mainly occurs as a result of leakage around pump and valve shafts.
For control valve applications where this is an issue, special “environmental” packing sets are offered. The packing of a sliding-stem valve is housed in the bonnet, which is depicted in this simplified picture of a single-ported.
The packing material is piled on the valve stem like washers on a bolt in the form of numerous concentric rings. The packing flange forces these packing rings down from above to produce a compressive force around the valve stem’s perimeter.
This compressive force is required to cause mechanical stress in the packing material, which allows it to seal snugly against the valve stem and the internal wall of the bonnet.
The packing rings are held in place by two nuts inserted into studs. If these nuts are over-tightened and the packing material is over-compressed, the packing will cause excessive friction on the bearings. This friction will not only obstruct precise valve stem movement, but it will also cause excessive wear on the stem and packing, increasing the risk of future packing leakage.
A deeper examination of the bonnet reveals a slew of components that work together to provide a low-friction, pressure-tight seal for the sliding valve stem:
A metal item called a lantern ring separates two sets of packing rings in this diagram. The lantern ring works as a spacer, allowing lubricant to reach both packing sets from the middle of the bonnet via the lubrication port. The compressive force exerted by the packing follower “loads” the packing depicted below.
The only flexibility in this arrangement comes from the packing material itself. This is referred to as stagnant loading, or jam-packing. The packing follower must be re-compressed when the packing material wears and fatigues over time by carefully tightening the packing nuts.
When torquing the packing nuts on a stationary-loaded packing set, extreme caution is required. Process fluid leakage is caused by insufficient torque (which corresponds to insufficient tension applied to the packing).
High valve stem friction and premature packing breakdown will result from excessive torque (putting excessive stress on the packing). The latter scenario is more common in industrial settings, where well-intentioned but inexperienced staff overtighten valve packing to prevent leakage. If a packing assembly leaks despite being correctly torqued, it should be replaced rather than tightened further.
Inserting a metal spring into the packing assembly as an alternative to “stationary” loading allows the spring’s flexibility to help maintain an adequate amount of packing tension as the packing material wears and ages. This is known as live loading, and here are some examples:
A coil spring inside the bonnet is utilized to live-load the packing in one of these models. The other example is a Belleville spring, which is a series of spring-steel washers. The concave contour of Belleville springs provides resistance to compression along the shaft axis.
Spring washers are always stacked in pairs (concave against concave, convex against convex) so that they can compress. The structure of the packing and accompanying components is revealed in photographs obtained of an actual valve packing assembly removed from the bonnet (left) and rebuilt on the valve stem (right).
This packing system does not have a lantern ring, but it does include a coil spring. As a result, it’s a live-loaded packing rather than a jam packing.
A stem packing lubricator can be attached to the lubrication port on the bonnet in packing applications that require external lubrication.
To use a lubricator, first secure the hand valve in the closed (shut) position, then fully unscrew the bolt until it falls out of the lubricator body. The bolt is threaded back into place until hand-tight, and an adequate lubricating grease is squeezed into the bolt hole in the lubricator body. Tighten the bolt a little more with a wrench or socket.
The bolt is then fully tightened, forcing all of the greases into the packing. Finally, the hand valve is entirely closed to ensure that no process liquid leaks past the bolt threads. Teflon (PTFE) and graphite are the two most prevalent packing materials used nowadays. In terms of fluid sealing, stem friction, and stem wear, Teflon is the superior material.
Teflon is also extremely resistant to a large range of chemical compounds. Unfortunately, its temperature range is limited, and it cannot tolerate severe nuclear radiation (making it unsuitable for use near reactors in nuclear power plants).
Graphite is another self-lubricating packing material that has a much wider temperature range than Teflon and can endure higher temperatures. Due of its electrical conductivity, graphite packing also has the unpleasant trait of allowing galvanic corrosion between the stem and bonnet metals.
To assist alleviate this corrosion, sacrificial zinc washers are occasionally included to graphic packing assemblies, however this simply delays rather than avoids corrosive damage to the stem.
Longer sections of woven graphite (left) and Teflon (right) “rope” packing would generally be bent around valve stems to form seals, as shown in the photos below. The graphite packing is glossy and flakes quickly, whereas the Teflon packing is plain white and maintains its integrity. Both have a slick feel about them:
Hybrid packing materials, such as carbon-reinforced Teflon, have been created to combine the best features of both materials.
Asbestos is a heritage valve packing material that is woven into packing rings similarly to how graphite fibres are woven into current packing rings.
Because asbestos is a mineral, it can be used in high-temperature processes. Its electrical non-conductivity eliminated graphite’s galvanic corrosion problem.
Unfortunately, because it is classified as a dangerous substance, it cannot be used as a packaging material in modern applications.