Stem packing is a familiar product. The most common type is braided compression packing. Braided packing is used in a wide range of applications. Depending on the service, construction materials can be as diverse as plants or animal derivatives, mineral fibers or synthetic plastics and even metal. The process of cutting rings from rope packing, inserting them into a stuffing box and torquing them to the right density is common, but it is not always the best choice.
Another widely used manufacturing method is die-molding. It is the process of wrapping a material around a mandrel, placing it in a die and preforming it to make a seal. Using these and other manufacturing technologies, packing is found to work in applications as different as aerospace, heavy trucking and power generation. A review of some unusual applications demonstrates the versatility of compression packing as a sealing solution.
The Origin of Packing
Compression packing is an ancient technology dating back more than 5,000 years.
Boats and ships used a rudder as a steering mechanism. The rudder shaft penetrates the hull of the vessel below the water line, so water can leak into the bilge. Ancient sailors, using the top technology of the day, would take pieces of clothing, sail cloth and rope, cover it with animal fat or wax and stuff it into the gap around the shaft. Eventually, a box was secured around the shaft and a gland, which could be tightened to compress the packing material, was created to improve sealing and longevity.
The terms compression packing, stuffing box and gland come from these early sailors.
Over time, many improvements in packing construction and materials were made. Packing today can be made of flax, Kevlar, polytetrafluoroethylene (PTFE), graphite or metal. It typically has a square cross-section and is sold in precut rings or in large coils, as shown in Image 1.
Synthetic aramid fibers are abrasion-resistant and can handle higher temperatures and shaft speeds. PTFE has excellent lubricity and chemical resistance. Graphite coupled with mica or an aramid fiber can stave off the heat generated by a rotating shaft and provide long life in challenging applications.
Die-formed compression packings are excellent in terms of sealing performance and reliability and offer a wide range of long-term, low-emission and low maintenance products. See Image 2.
Not only are die formed rings easier and quicker to install, but the pre-compression increases the density of each ring and reduces the gland loads necessary to seat and compress multiple rings in the stuffing box. The result is lower friction on the shaft or the spindle, with improved sealing performance and a longer life.
Factor in STAMPS
As mentioned in an article previously published by the Fluid Sealing Association, (Sealing Sense, Pumps & Systems, March 2005), there are several key factors to consider when choosing the right packing. They include:
- size or stuffing box bore
- temperature inside the stuffing box application: whether it’s a pump, valve, mixer, refiner, process, characteristics such as pH level and chemical compatibility
- motion: rotary, helical or reciprocal
- pressure inside the stuffing box
- surface speed expressed in feet per minute or meters per second
Keeping this in mind, here are some applications to consider when you are going way beyond the typical stuffing box.
Graphite and wire mesh for exhaust systems. Die molded configurations of flexible graphite, mica or vermiculite densifies well when loaded in these flange couplings to seal truck and auto emissions before being cleaned by the after-treatment and catalytic scrubbing systems. Due to the high vibration in these applications, wire mesh is used in combination with the sealing material to provide a skeleton, giving the sealing material structure to add durability.
Metal caps for pressure seals. In the typical stem packing in a valve stuffing box, the high axial to radial load transition of a die-molded configuration from the gland load creates the seal around the stem. Bonnet pressure seals were originally designed to be made of solid metal and relied on process pressure to effectuate a seal. The all-metal seal used the angle of the bonnet to amplify the unit load on the toe creating a single point metal to
More recently, engineers found a way to use the high axial to radial load transfer of die molded flexible graphite to create a full cavity seal. However, they had to find a way to keep the graphite from extruding under high unit loads.
The most common way was to place a thin cap on the top and bottom of the seal. As opposed to a full metal ring sealing only at the toe, and having a potential to damage the valve, the packing-style die molded graphitic seal fully compresses and densifies to fill and seal throughout the entire bonnet cavity. See Image 4.