| HVAC CPD PROGRAMME disc is rotated against a seat in the body of the valve. Butterfly valves typically take up little more room than a pair of pipe flanges, and are therefore an attractive alternative to the ball valve where space is limited. With an appropriate combination of the elastomer (elastic polymer) liner and disc materials (for example aluminium-bronze, stainless steel and ductile iron), these are suitable for use on CHW, LTHW, and domestic applications. They are primarily used as on-off valves but can also be used for non-critical throttling applications. Butterfly valves are particularly useful where increased pressure and elevated temperature specifications are outside the normal operating parameters of other isolation valves. Gate and globe valves are better suited to more arduous applications, such as the chemical and pharmaceutical industry, and are also used commonly on mediumtemperature hot water (MTHW) systems or where higher-pressure rating may be required, such as PN 40. They are often employed as replacements for currently installed systems where the face-to-face dimension needs to match the valve that is being replaced, but they are less frequently used in new projects. 1 THE COST OF PUMPING THROUGH VALVES AND FITTINGS CIBSE Guide C3 Q/ = ) 3 = 36Q / s ) and )2 s s m-3 s m-3 2 2 m ) 3 s -3 2 Gate valves employ a gate-like disc, actuated by a stem screw, that moves up and down perpendicular to the water flow. They are available in rising stem and nonrising stem variants, as illustrated in the examples in Figure 4. For rising stem valves, rotation of the stem drives the stem up and down as the threaded section of the stem turns in the threaded section of the body. The disc is engaged onto the end of the stem (below the threaded section) and rises/falls as the stem unscrews/screws. The threaded stem in a non-rising stem valve screws directly into a threaded hole, into the core of the disc. As the stem is rotated clockwise, the screw draws the disc upwards, going deeper into the disc core, and so opening the valve. The most significant difference between rising and non-rising is that the opening position of a rising stem valve can be readily observed (since the stem has risen). This will require more space than an equivalent non-rising stem valve (and the stem may be exposed to the elements). Materials and manufacturing methods employed in modern valves mean that both types have similar operating lives and maintenance needs for HVAC applications. Gate valves can provide dependable service, particularly where a pressure drop is important. They can allow fluid flow in either direction and the straightthrough design offers little resistance to flow, and pressure drop is low. An example 50mm gate valve has a Kv of approximately 280. They should not be used for throttling, since they are not designed to provide a specific control characteristic, and subsequent damage, because of erosion, may prevent the valve subsequently providing an effective shut off. Globe valves may be used to provide isolation, but can also provide an effective throttling device because seat and disc designs may be selected to provide flow characteristics with proportional relationships between valve lift and flowrate. Globe valve bodies are normally of a spherical shape, ensuring maximum strength against line pressures and pipeline strains. A wide choice of disc and seat materials are available to suit the application. The change in direction of the water flow combined with the restriction in the flow area mean that these valves will impose a large resistance to flow, and hence globe valves have a low Kv value for example, around 40 for a 50mm valve and so are not a popular choice as an isolation valve. In most cases, IVs are likely to be hand actuated; however, most types may be readily adapted to accept motor actuation. They are typically available with stem extensions to allow, for example, access to a handwheel beyond the thickness of insulation. IVs are normally connected to the pipework by taper thread (such as in Figure 2), flange (such as in Figure 3), or by using a compression or press-fit fitting. Threaded connections may be to UK/European standard BS EN 10226-2 or US-based ANSI B1.20.1 although these appear similar, they employ a different taper angle and are not interchangeable. All isolation valves include sealing glands and packing to prevent the leakage of water around the stem. Although modern materials, such as PTFE, can provide a long-lasting, robust seal these will still 74 October 2021 www.cibsejournal.com CIBSE Oct21 pp73-76 CPD 185.indd 74 24/09/2021 14:28