The choice of end connections for connecting a valve to its associated pipework is dependent upon the pressure and temperature of the working fluid and the frequency of dismantling the pipeline or removing the valve from the line. The types of end connection in general use are as follows:
1. Screwed Valve End Connections. - Male threads of various forms may be used for special purposes, but as a rule screwed end valves have female pipe threads, wither tapered for assembly to taper threaded pipe, or parallel for assembly to taper or parallel threaded pipe. In taper-to-taper and in taper-to-parallel connections, the pressure-tight joint is made on the threads. In parallel-to-parallel connections, the pressure tight joint is made by compressing a grummet or gasket against the end face of a valve. Screwed ends, usually confined to pipe sizes of 150mm and smaller, are widely used for bronze valves and to a lesser extent in iron and steel valves.
2. Flanged Valve End Connections. - Flanged-end valves are easy to install or remove from a pipeline, being bolted to the mating pipe flanges. To ensure a tight seal, a gasket is usually fitted between the machined facing of the flanges. The type of gasket, which can be non-metallic, metallic or a combination of both, depends upon service conditions and upon the type of flange facing employed. Bronze and iron valves are normally supplied with plain (flat) facings, and steel valves with plain (flat), raised or male facings, although female, tongue and groove, or ring-joint types are available. Flanged end valves are made in sizes from 15mm upwards.
3. Socket Weld Valve End Connections. - In this type, the ends of a valve are socketed to receive plain-end pipe. A circumferential weld is made on the outside of the pipe so that 'icicles' and weld spatter are unable to enter the pipeline. Socket-weld ends are used only on steel valves, and as a rule they are limited to sizes of 50mm and smaller for higher pressure/temperature applications in pipelines not requiring frequent dismantling.
4. Butt Weld Valve End Connections. - In this case, the ends of the valve are bevelled to match wall thickness and machined bevel at the end of a mating pipe. A circumferential weld is made at the abutted mating bevels. 'Backing rings' which are basically sleeves fitting inside the pipe, are sometimes used to align the pipe and valve bores also to prevent 'icicles' and weld spatter from entering the pipeline. Butt-weld ends are used only on steel valves, normally in sizes 50mm and upwards, for the higher pressure/temperature applications in pipelines which do not require frequent dismantling.
5. Compression Valve End Connections. - This type of valve end has a socket to receive the pipe and is fitted with a screwed union nut. The joint is made by the compression of a ring or sleeve on to the outside of a plain-end pipe, or by compressing a preformed portion of the pipe end. As a rule compression ends are used with copper tubing and steel tubing up to 65mm diameter and are used for low pressures or where pipes may require frequent dismantling.
6. Capillary Valve End Connections. - These valves are soldered to the mating pipe. The ends of the valve have a socket, machined to close tolerances to receive the plain-end pipe. The joint is made by the flow of solder by capillarity along the annular space between the socket and the outside of the pipe. Capillary ends are commonly used with copper tubing and confined to sizes 65mm and smaller. The high temperature use of capillary end valves is limited due to the comparatively low melting point of the solder.
7. Socket Valve End Connections - The ends of the valve are socketed to receive the plain spigot end of the pipe, the seal being made by the insertion of a yarn ring joint, corked with lead. Other forms of socket ends use a rubber scaling ring. These ends are either in the style of flange and socket adaptors for bolting to flanged end valves, or incorporated in the valves. Socket ends are normally associated with cast iron valves for water services in sizes 50mm and upwards.
8. Spigot Valve End Connections - The type of socket used in the coupling or on the pipe end determines the form of the spigot ends. For cast iron pipes with lead joints the spigot end is provided with a raised band, for screwed and bolted gland and other forms of mechanical joint the spigot end is prepared to suit the joint. For asbestos cement connections, the spigot end is finished plain in the same way as the pipe. These spigot ends are normally associated with cast iron valves for water services in sizes 50mm and upwards.
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