As the name suggests, composite-type expansion joints are used in situations where elevated temperatures or unusual accumulation problems prevent standard materials from being employed. Traditionally, metal joints have been used in these applications, but modern insulation techniques have allowed fabric composite joints to be specified in virtually any service.
Composite joints are divided into two basic groups: corrosive service and non-corrosive service. It is important to remember that flue gas from any fossil-fired source will contain elements which can create acid at the proper temperature. It is important to include a barrier at the appropriate point to prevent the acid from accumulating in a manner which will destroy the joint from the inside.
This is the outside “jacket” which holds the assembly together and serves to protect the insulation and sealing element. Bi-State Rubber uses metallic meshes and screens, fiberglass and purified silica cloths for this purpose.
This is the key item in bringing the temperature down to levels which can be borne by the sealing ply. The newer types of insulation allow fabric joints to be used up to 2000º F. The most common types are mineral wool, various grades of fiberglass, and ceramic insulation for the higher temperatures. It is important to remember that ceramic tends to shrink, up to 10%, in high temperatures. No insulation has much “memory”. Consequently, extreme care in design and installation has to be observed to overcome these problems.
As illustrated above, this is a crucial element in fighting potentially destructive vapors and condensates. A variety of TFE/Glass materials is used, depending upon the conditions. Sometimes the sealing element can be used for this purpose.
A wide variety of polymers can be used here, depending upon the conditions at the outside of the joint. The most common items are flouroelastomer, EPDM, and TFE/Glass products.