Isolation Valve

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An isolation valve ia a valve designed to separate or "isolate" the fluid media from the inner workings of the valve. In a solenoid valve this is typically accomplished using a diaphragm. Also known as a "diaphragm valve", the only materials wetted by the fluid media are the valve body and the diaphragm itself. The plunger, spring, etc. are isolated from the fluid media. Isolation valves are typically desireable in sensitive applications such as scientific and medical instrumentation, where engineers want to maintain the chemical integrity of a fluid sample.

Isolation valve

Definition:-

Isolation valves are a key component in any fluid system as they are used to stop the flow of fluid into a particular area of the system. They are also sometimes used to manually control the flow of the fluid.

Application:-

    Flow isolation to: 

• Facilitate maintenance • Allow the removal of equipment • Allow the shut down of plant Types:- Valves are commonly classified into two groups according to the operating motion of the closure device 1- Linear movement valves

The obturator moves in a straight line. Included in this category are gate valves, globe valves, diaphragm valves and pinch valves. 

2- Rotary movement valves

The obturator rotates about an axis at right angles to the direction of flow. Ball valves and butterfly valves are the two most important rotary valves associated with steam applications 

Table Obturator motion in the basic valve types

1- Linear movement valves

1-Gate valve Gate valves are probably the most common valves in use today due to their widespread use in domestic water systems. Gate valves are specifically intended for use in isolation applications. A gate valve consists of four main components, the body, bonnet (or cover), gate and stem. A typical gate valve is shown in Figure

Fig. Typical wedge gate valve

2-Globe valve Globe valves constitute a major class of linear movement valves; they have become more popular than gate valves as there is a wide variety of configurations available to suit most applications. The movement of fluid through the valve seat is longitudinal to the operating motion of the obturator; this means that for a valve in which the inlet and outlet are horizontally opposed, the fluid must follow a changing course. The main advantage of this arrangement is that a globe valve opens more rapidly than a gate valve as the disc only needs to move a small distance from its seat to allow full flow. This is an advantage when there is frequent operation of the valve.

The Disadvantage is that the fluid has to change course, increasing the resistance to flow and generating turbulence. This results in a higher pressure drop across a globe valve than a gate valve.

Fig. 12.1.2 A conventional globe valve

3-Piston valve One of the main disadvantages of linear movement valves is the fact that their seats are prone to damage from dirt and wiredrawing, and therefore, depending on the application may require regular maintenance. Although these seats are replaceable in theory, it usually involves significant time and cost, and it is often more advantageous to replace the entire valve. To overcome this problem, piston valves have been developed. The piston valve is a variant of the conventional globe valve, with the traditional seat and cone replaced by a piston and lantern bush. The piston is connected to the valve stem and handwheel, and passes through two sealing rings that are separated by a lantern bush. When assembled, the two sets of sealing rings are compressed around the piston by the load exerted along the stem. The upper set of sealing rings acts as conventional gland packing, and the lower set acts as the seat. Furthermore, the large sealing area between the piston and rings assures a high level of shut-off tightness. The piston valve is not designed for throttling duties and must be used in the fully open or closed positions. When the valve is fully opened, only the bottom face of the piston is exposed to the fluid as the rest of the body is protected by the upper sealing rings. This means that the sealing surfaces (the sides of the piston) are protected from erosion by the fluid flow.

Fig. 12.1.4 A piston valve If the valve requires maintenance, all the internals can be easily removed by undoing the cover nuts and withdrawing the piston. The rings and the lantern bush can then be removed using an extractor tool. This operation is simple and can be undertaken without having to remove the valve from the pipeline. In general, the piston should never have to be replaced, but the sealing rings may wear over a long period with frequent operation. 4-Diaphragm valve Diaphragm valves constitute the third major type of linear movement valves. The stem of the valve is used to push down a flexible diaphragm, which in turn blocks the path of the fluid. There are two different classifications of diaphragm valve based on the geometry of the valve body: • Weir type

   A weir is cast into the body, and when closed, the diaphragm rests on the weir, restricting the flow (see Figure 12.1.5 (a)).

• Straight-through type

The bore runs laterally through the body and a wedge shaped diaphragm is used to make the closure (see Figure 12.1.5 (b)).

Fig. 12.1.5 The weir type (a) and straight-through type (b) diaphragm valves The main advantage of a diaphragm valve is the fact that the diaphragm isolates the moving parts of the valve from the process fluid. They are therefore suitable for handling aggressive fluids and for those containing suspended solids. In addition, as the bonnet assembly is not exposed to the fluid, it can be made from inexpensive materials such as cast iron, thereby reducing the overall cost. The development of new diaphragm materials enables diaphragms to be used on most fluids. Their application is however limited by the temperature that the diaphragm can withstand - typically less than 175°C. Diaphragm valves are generally used on process fluid applications.

5-Stem sealing In order to prevent leakage of the process media from around the stem of a valve, a barrier must be placed between the fluid and the environment. Stem sealing is usually achieved by one of two methods, namely gland packing and bellows sealing. Gland packing consists of a polymeric material, typically PTFE, packed tightly between the stem and the bonnet of the valve, thereby preventing any process media escaping.

Fig. 12.1.8 Bellows sealed valve In bellows sealed valves, a flexible metallic bellows is used. It is connected on one end to the stem and the other end is connected to the bonnet, effectively producing a barrier between the fluid and the environment. This bellows extends and contracts as the stem moves up and down. The bellows is so effective, it produces a ‘zero emissions’ seal. Fitted to the bellows is an anti-torsion device, which prevents the bellows from rotating with the stem. Such a device is essential; otherwise the repeated twisting of the bellows would lead to the failure of the seal Although less costly than the bellows sealed valves, the gland packed valve does not produce such a tight seal as the bellows. If a gland packed valve is not used for a significant period, the gland packing can stiffen, and leakage will occur the next time the valve is used. The bellows sealed valve does not suffer from this problem. Furthermore, gland packed valves requires regular re-packing of the gland, whereas a typical bellows requires no maintenance for over 10 000 cycles. 2- Rotary movement valves 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. 1- Ball valve Ball valves were developed during World War II and were initially intended for use in aircraft fuel systems, where weight and space are at a premium. They consist of a body which houses a rotating ball which has an orifice or bore machined directly through it. The ball is located in the body by two sealing rings.

Rotation of the ball through 90° opens and closes the valve and allows fluid to flow directly through the orifice. In the closed position, the blank sides of the ball block the inlet and the outlet preventing any flow. There are two basic designs of ball valves – the floating ball design, which relies on the valve seats to support the ball, and the trunnion mounted ball, which uses a trunnion to support the ball. Trunnion mounting is used on larger valves, as it can reduce the operating torque to about two-thirds of that provided by a floating ball. Conventionally, the handle that is attached to the ball is in-line with the axis of the pipe when the valve is open; conversely, if it is at right angles to the pipe axis, this indicates that the valve is closed.

Fig. 12.2.1 Ball valve (shown in its open position) 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.

2- Butterfly valve 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.

Fig. 12.2.5 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 disc and 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.