Squeeze type seals

Alternatively, squeeze type seals operate by distorting under compressive load, and the hardness specified for such an application must be sufficient to ensure adequate retention of the sealing pressure. The squeeze resistance can be enhaneed by incorporating one or more plies of fabrics in the rubber section rather than increasing the hardness of the compound as this might adversely affeet other properties. Dimensional variations due to contact with fluids can be adjusted to achieve a small positive swell which can maintain seal efficiency, by compensating for wear and compression set. The choice of eompound will depend on the effects of the fluids with which the seal is in contact, the operating temperature, and mechanical conditions such as pressure, relative veloeity and abrasion. 

An organic chemistry laboratory should be equipped with a carbon dioxide or dry chemical (monoammonium phosphate) /- extinguisher (see Fig. 3). To use this type of extinguisher, lift it from its support, pull the ring to break the seal, raise the horn, aim it at the base of the fire, and squeeze the handle. Do not hold onto the horn because it will become extremely cold. Do not replace the extinguisher report the incident so the extinguisher can be refilled. 

Compressible materials rarely have a good bond to the sides of the joint but, if installed under pressure, will expand to release that pressure if the joint opens, or squeeze together if it closes. Without a good bond to the joint sides, this type of filler is rather easily penetrated by contained liquids. It is, therefore, common practice to fill the joint to within V4 in. to Vz in. of the top with the compressible foam, then seal the top space with the deformable sealant, applied in putty or fluid form and bonded to the sides of the joint. It is important that the sides of the joint are truly vertical., at 90° from the floor surface, so that as the the joint opens or closes, the movement is at right angles with the sides. 

The reaction represented by Equation 5.26 is an example of an exothermic (heat out) reaction, in which heat is released as the reaction takes place. In endothermic (heat in) reactions, heat is absorbed. A reaction of this type is utilized in emergency cold packs, which consist of a small plastic pouch of liquid sealed inside a larger pouch that also contains a solid substance. When the large pouch is squeezed firmly, the small pouch of liquid breaks, the liquid and solid react, heat is absorbed, and the liquid contents of the larger pouch become cold. This reaction is described in more detail in Chapter 7. 

---