Pipeline ancillaries

Rotary movement valves, often called quarter-turn valves, include ball valves and butterfly valves. Regardless of the type of rotary movement valve, the obturator rotates about an axis perpendicular to the direction of flow. Fluid may flow through the obturator, as is the case with ball valves, or around it, as with butterfly valves. Rotary movement valves tend to have a simple operating mechanism and are therefore easy to automate and maintain.

Ball valves are available as reduced bore or full bore. Full bore valves have an orifice that is the same size as the diameter of the pipe, whereas in reduced bore valves, the orifice diameter is less than that of the pipe. Full bore valves cost more than reduced bore valves, and they should be used where the pressure drop across the valve is critical or where ball valves are used upstream of flowmeters.

Full bore valves can be used in flowmeter applications to minimise fluid turbulence upstream of the measuring device.

In order to insert the ball into the body, three different types of assembly exist. Not only does the type affect the ease of assembly, but it also influences the maintainability of the valve.

• Two and three piece valves - The body of the valve is split in one or two places in the same plane as the valve flange, and these pieces are bolted together. This has the advantage of simplified, in-line maintenance.
• Top entry valves - The ball is inserted through a bonnet in the top of the valve. This facilitates in-line maintenance.
• Single piece valves - The ball is enclosed in the body by an insert fitted along the valve’s axis. This eliminates the possibility of body joint leakage and any chance of disconnection whilst in service, but when maintenance is required, the whole valve has to be removed from the pipeline.

The choice of seat material determines the conditions for which a particular ball valve is most suited. Although new seat materials are continually being developed, Table 12.2.1 lists some of the more common materials in use today.

In addition to the inherent safety of the seating mechanism, the stem seal must also be capable of preventing leakage to atmosphere under ‘fire’ conditions. This can be achieved by using high temperature seals made from flexible graphite or Grafoil®; alternatively, a bellows sealed arrangement can be used (see Figure 12.2.4).

• Clean steam valves - A number of applications exist that require the valve to be of a ‘clean’ design; these include steam applications where there is direct injection of steam into the product and process fluid lines in the biotechnology, food and electronics industries. The main area of concern in such applications is the space between the body and the ball; process fluid may accumulate in these spaces leading to contamination and corrosion. This can be overcome by inserting cavity fillers in these spaces. The cavity filler may be an integral part of the seat or a separate component in the valve assembly. Furthermore, ball valves used in clean steam applications should be made from stainless steel with a good surface finish (less than 81 microns Ra is recommended).
• Throttling applications - When ball valves are used in throttling applications, high velocity flow can impinge against a localised area of the ball and seals, causing premature deterioration of the seating material. Modifications to the standard design are required for ball valves to be used for throttling; these include the use of metal seats, hard coatings and, sometimes, modifications to the ball, to give a characterised flow pattern.

Butterfly valves

Although there are many different designs of butterfly valve, they all consist of a disc that rotates on a shaft at right angles to the fluid flow. When open, the disc is edge-on to the flow and the fluid passes around it, offering limited resistance. In the closed position, the disc is rotated against a seat in the body of the valve.

Butterfly valves usually 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. In fact, some butterfly valves are designed specifically for insertion between pipe flanges, these are known as wafer butterfly valves.

The main disadvantage of butterfly valves is that the shut-off is not as tight as that achieved by other valve types. This can be alleviated to an extent by offsetting the axis of rotation of the discand using pressure assisted seats. By using an offset axis of rotation, a ‘camming’ action is generated, which means that the disc creates a tight seal with the seat during the last few degrees of shut-off. These high performance or eccentric-type butterfly valves have improved shut-off capabilities and their design enables them to be used for throttling.

For steam applications, butterfly valves have largely been superseded by ball valves. Butterfly valves are more commonly used in liquid systems or where space is limited. The compactness of butterfly valves means less material is required and they are therefore ideal where the application specifies the use of costly materials, for example, in seawater applications where nickel is specified.

This formula forms the basis of the chart shown in Figure 12.2.7, which was first introduced in Block 3, Module 21.

If the isolating valve is to be used in a liquid system, the pressure drop across the valve is determined using the following equation: