Valve Packing: Understanding Packing for Maintenance

“Valves and actuators make up a small part of a power station – perhaps only two or three percent by capital cost,” according to Power Engineering International.

Despite this, valves are essential in starting, stopping, and regulating the flow of liquid/gas in a plant. Valves provide a method to isolate components of the plant for maintenance. But just like the rest of the plant, valves also need maintenance. Among required valve, maintenance is packing treatments.

What is packing?

Packing is a set of rings, resilient or semi-resilient material, placed around the shaft or stem to create a seal. These rings push on the stem and the body of the valve, assuring no liquid/gas escapes.

• Resilient packing, when it compresses and releases, can then resume its original shape.

• Semi-resilient packing, when it compresses and releases, does not entirely resume its original shape.

Types of Valve Packing

Valve packings are present in four basic styles:

• Twisted Packing – good general-utility or emergency-type packing where packing space is small
• Square-Braid Packing – suited for high-pressure valve-stem packing and high-speed reciprocating service
• Braid-Over-Braid Packing – good for valve stems where out-of-square cross sections are useful because of their high density
• Interlocking Braid Packing – because of its flexibility, this type of packing is ideal for applications around small diameter valve stems

Materials Found in Valve Packing

Lubricant impregnates most packing materials. This helps reduce friction and packing wear. These lubricants can be mineral oils, waxes, Teflon, and particles of graphite. The graphite (at least 99% pure) is ideal because it is maintenance-free, permanently elastic, non-hardening, non-aging, highly formable, and permanently resilient.

Graphite Packing

Valve packing often contains graphite because it is maintenance-free, permanently elastic, non-hardening, non-aging, highly formable, and permanently resilient. 

• Ribbon Graphite Packing – It can pack almost any type of valve by wrapping a given length of ribbon around the valve stem and compressing it into the stuffing box to produce an endless packing ring

• Braided Filament Graphite Packing – It can be die-formed, or rings can be cut from the rope form. Braided graphite filament packing is lubricated, usually with TFE (Teflon), for braiding purposes. 

Fabric and Rubber Packing

Fabric and rubber packings come in different weight cotton duck fabrics, ceramics, and Kevlar with natural, neoprene, or rubber binders. All of the types are available in die-molded rings, spiral, and coil forms.

• Laminated Construction – It is made up of cotton duck and rubber that are laminated into a slab form and then made into coils, spirals, or rings.
• Rolled Construction, hollow center – It is made with a medium-weight rubber-coated, cotton duck. It also has a cross-section that is square permitting expansion and contraction of the packing without binding.
• Rolled Construction, round rubber core – It is made with a medium-weight, rubber-coated cotton duck rolled around a round rubber core, producing a round cross-section. This type is highly resilient.

Ceramic and Kevlar Packing

Just as the Fabric and Rubber, Ceramic and Kevlar come in two types of packing: Laminated Construction and Rolled Construction.

• Laminated Construction – It is made with ceramic or Kevlar and rubber is laminated into a slab form and then made into coils, spirals, or rings.
• Rolled Construction  – It is made with rolled cloth and rubber or metal in three types: with rubber core, with a lead wedge, or with accordion folded asbestos cloth and a rubber strip.

Plastic Packing

There are three main types of plastic valve packing: a mixture containing soft-bearing metals, Teflon packing, and Teflon-impregnated packing.

• A Mixture of Soft Bearing Metals – The mixture is extruded in a rectangular cross-section, in a spiral form. It is soft, readily formable, and particularly suited to sealing gases and highly mobile liquids.
• Teflon (TFE) Packings – It is produced from a continuous TFE filament in the same way as conventional braided and twisted packings.
• Teflon Impregnated Packings – These consist of cotton fiber impregnated with TFE. In some styles, TFE impregnates each individual strand of cotton fiber prior to braiding.  Other styles are produced by impregnating an already formed braided packing.

Metal in Valve Packing

The metals in valve packing are either crimped, spiral wound, or braided to form packings that can withstand higher temperatures than fibers and produce less friction against moving surfaces.  Metal foil packings are durable, but are not resilient, so the shaft must run fairly true, and lubrication is an important consideration in their application. Brass valve stems or any valve stem where the base metal of the valve stem and packing is of similar material should not contain metallic packings. Additionally, these packings are common and effective for high-temperature and high-pressure applications. 

Want to learn more about Valve Packing and Valve Maintenance?

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