Manual stop valves are required throughout a circuit to permit isolation during partial operation, service or maintenance (see Figures 9.8 and 9.9).
Figure 9.9 Ball valve (Henry)
Small valves which are to be operated frequently have a packless gland, either a diaphragm or bellows, and a handwheel.
Valves of all sizes which are only used occasionally will be sealed with ‘O’ rings. As a safeguard against leakage, they have no handwheel fitted and the stem is provided with a covering cap which is only removed when the valve is to be operated. The stem will have flats for operation by a spanner. Most such valves can be back-seated to permit changing the ‘O’ rings.
Valves should not be installed with the stem downwards, as any internal dirt will fall into the spindle thread.
Under low-temperature conditions, ice will form on the spindle and will be forced into the gland if the valve is operated quickly. Under such circumstances, the spindle should be well greased, or the ice melted off first.
Service stop valves on small compressors will usually carry a connection for a pressure cut-out or gauge, or for the temporary fitting of gauges or charging lines when servicing. The valve backseats to close off this port while gauges are being fitted. Valve seats are commonly of soft metal or of a resistant plastic such as PTFE.
Ball valves, Figure 9.9, are sometimes used for secure isolation of sections of circuits and they give less pressure drop when open.
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