Mechanical and compression packing consists of various combinations of fibers, yarns and lubricants. Three basic yarn types are made from a variety of fibers. They are: 1) gang spun, which is made by spinning short, non-continuous staple fibers (usually natural fibers); 2) continuous filament, which is manmade fibers of any desired length; and 3) a combination of materials such as graphite and wire or other materials to create yarn with characteristics of the combined -components.
Many yarn types are used to make general packing (e.g., graphite, carbon, PTFE, acrylic, aramid, fiber glass, cotton). However, when considering packing for use in valves, the number of materials is more limited.
One of the most common materials is graphite, which is divided into two categories: flexible graphite and PTFE.
Flexible graphite is pitch-based carbon mined from the earth and made from a process called pyrolization. Base fibers of pitch, rayon and acrylic are thermally treated in the presence of oxygen to drive off impurities. The pitch base then receives an additional chemical exfoliation before it goes through pyrolization.
The yarn made from pitch has to be made with either an internal carrier or external carrier because the pitch-based fibers cannot be made continuous without the use of a carrier. Graphite yarn made from rayon or acrylic is manmade and also can be made into a continuous filament by adding a carrier.
PTFE has a very high resistance to chemicals with exceptionally low friction and adhesion to the valve stem. However, its use is limited to lower temperatures than graphite, and it can be affected by thermal growth.
Various types of braiding are employed to create the different braid patterns used in making packing. The machines that manufacture mechanical packing today are inverted, overhead, square, upright and round braiders. The braiding is different for a variety of reasons, some because of the actual packing performance in different applications and others because a certain type offers special ways of applying lubricants and blocking agents needed to effectively seal certain applications. The different braider types also provide the ability to increase or decrease the actual strands of yarn needed for the different cross sections of packing or are made for other, more technical reasons.
The braiders produce various braid types as well. Common braid types include plait, square, round, interbraid, braid-over-core and twisted. The difference in types is related to the material being braided along with the level of sealing needed from a particular packing product. This can be based on pressure, temperature or fugitive emission sealing. For example, many of the products manufactured from flexible graphite yarn that will be used for low-emission sealing are braided using a plait braid and lubricants and blocking agents. This arrangement provides long-term sealability below 100 ppm as required by EPA.
REVIEW
It is critical when understanding the basics of packing to consider how packing is viewed as part of the larger picture of valves and flow control. In too many instances, packing is an afterthought. For example, when valve repair work is to be performed, rushed decisions are too often made causing installation of the wrong material or bad installations of the right material.
On the other hand, the right packing style used with the right valve design and proper installation is taking on increased significance for industry as EPA places more aggressive regulations on end users. In the end, it is clear that packing will continue to become more and more effective.
Copyright © 2019 Teenma Thermal Insulation Material Manufactory. All Rights Reserved.
