Plant Services - August 2008 - (Page 44) Safety Valves Easy in, easy out Ball valve assembled Ball valve disassembled Figure 1. Valves that have a flange-type body seal consist of three discrete parts that are joined together with flanges, seals and long bolts. Such valves come apart for easy repair in situ. The manufacturer’s standard sealing material isn’t always the answer. System designers should research sealing materials in conjunction with their system operating conditions, considering the full range of options including metal gaskets, the many different types of elastomer O-rings, and Grafoil packing, which might offer a more robust valve design. Beyond sealing materials, an advantage of the flange-type design is ease of maintenance. Once the bolts are removed, the valve’s body swings out for of polytetrafluoroethylene (PTFE), is crushed, filling the space between the stem and the body housing. Unfortunately, PTFE and similar packing materials are subject to cold flow, which is the tendency for certain materials to change shape over time. Cold flow can be exacerbated by pressure and temperature. In some cases, the material might extrude into areas where it wasn’t intended to go, undermining its effectiveness and leading to leakage. To compensate for cold flow, the packing bolt might need to be tight- The manufacturer’s standard sealing material isn’t always the answer. easy repair without removing the entire valve from the system. A ball valve requires some means of ensuring that the system medium, whether liquid or gas, doesn’t leak from the stem and body interface. This is the role of the stem seal. With sufficient cycling frequency, stem seals are subjected to wear, and wear can lead to leakage. However, some seals are more effective than others in certain applications. ened frequently to maintain a constant compression load on the stem seal. With enough retightening, the packing bolt might bottom on the valve body, at which point the packing will need to be replaced. This basic packing technology requires frequent inspection and adjustment; otherwise, leakage might occur. Unfortunately, to the untrained operator, it’s not always clear when adjustment is required. One-piece stem packing The most basic technology is a onepiece gasket that encircles the stem. As the packing bolt is tightened down on the stem, the gasket, usually made 44 www.PLANTSERVICES.com Two-piece chevron stem packing A two-piece chevron stem packing is an improvement on the one-piece design and allows for wider temperature and August 2008 http://www.mactekcorp.com http://www.PLANTSERVICES.com
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