Basically cavitation is a phenomenon that occurs during flow of a liquid in a closed channel such as a pipe. Normally when a liquid flows over any distance, there a drop in pressure – which is essential to maintain the flow.At some point this pressure drops much below even atmospheric pressure – particularly in case of water flowing in a pipe. When this happens water in the pipeline tends to evaporate. This water vapor forms bubbles within water body and these bubbles keep on popping up as well as collapsing. Any solid surface facilitates this process. as such surface of the pipe suffers maximum variation in pressure.

 

The needle valve is another type of globe valve which is used for accurate throttling in high-pressure and/or high temperature service. Needle valves are designed for small diameter lines requiring fine throttling of gases, steam, low-viscosity oil, water,
or any other light liquid. The valve consists of a sharp, pointed (needle-shaped) stem that controls flow through the seat.

Needle valves open and close an orifice like valve opening with a tapered needle shaped disc that raises and lowers with the spin of a hand-wheel or with the help of any automatic control mechanism.

Air release valves, also called single function air valves, are designed only to release air pockets accumulated during working conditions. Something important we must take into account about this model is that they don’t have any capacity of air inflow during pipe draining, and outflow during filling, due to the limited passage through the nozzle, therefore they won’t provide any protection for the pipeline against negative pressure conditions.

Plunger valve is composed of valve body, valve cover, valve stem, plunger, hole rack, sealing ring, handwheel and other parts. When the handwheel rotates, the valve opens and closes by driving the plunger up and down in the middle of the hole rack by the valve stem. The interference fit between the plunger and the sealing ring is adopted in the valve. By adjusting the flange bolt in the pressure cap, the lateral force produced by the compression of the sealing ring is sealed with the orifice in the valve body and the outer circle of the plunger, 

It is simply an industrial version of the pinch cock used in the laboratory to control the flow of fluids through rubber tubing. A pinch valve offers a cost-effective and practical solution for controlling the flow of media through a pipe line or any other system. It comprises of a rubber sleeve, which is the only wetted part of the entire valve, and prevents the other components from contamination.

Cryogenic valves refer to those working under -29℃. Some of them can even deliver liquefied gas ( medium like oxygen, nitrogen, natural gas) at -196℃.

As the temperature of working medium drops, the material and structure of cryogenic valve, especially ultra-cryogenic valve, should be differed with regular valves.

Cryogenic valve testing standard and Cryogenic Testing Procedure Cryogenic valve  design standards and performance standards for valves in cryogenic services.

Cryogenic valve standards that are used in the valve industry include:

  1. BS 6364: 1984: Valves for Cryogenic service
  2. MSS SP-134: 2012: Valves for Cryogenic      Service,   including   Requirements for Body/Bon net Extensions.

Pneumatically-operated control valve actuators are the most popular type in use, but electric, hydraulic, and manual actuators are also widely used. The spring-and-diaphragm pneumatic actuator is most commonly specified due to its dependability and simplicity of design. Pneumatically-operated piston actuators provide high stem force output for demanding service conditions. Adaptations of both spring-and-diaphragm and pneumatic piston actuators are available for direct installation on rotary control valves.

Why name is “Globe valves” are named for their spherical body shape. The two halves of the valve body are separated by an internal baffle which has an opening forming a seat onto which a movable disc can be screwed in to close (or shut) the valve. In globe valves, the disc is connected to a stem which is operated by screw action. A globe valve is a linear motion valve used to stop, start, and regulate the fluid flow. The globe valve disk can be removed entirely from the flow path, or it can completely close the flow path.

There are different Types of valves used in used in process industry or plants based on the needs.

 Valves can be grouped into two main types:

  • Shut off valves: They are ON/OFF valves either allow full flow or shut off completely. Shut off valves can be operated by hand or automatically. They are not used for controlling the flow.
    eg: Gate valves

  • Control valves: These valves act as the correcting element of a control loop. Control valves are normally operated automatically, there are also hand operated valves.

  • Body
    It is a type of a pressure vessel containing an orifice or an opening. The controlled liquid is allowed to flow through the body of the valve. It helps to monitor the flow regulation behaviour.
  • Trim
    Besides the body, trim is one such part of the valve that comes directly in contact with the fluid. It consists of the seat, disc, plug, and stem.
  • Actuator
    It consists of electric or pneumatic mediums to provide the force required to operate  the control valve.

Why name is “Globe valves” are named for their spherical body shape. The two halves of the valve body are separated by an internal baffle which has an opening forming a seat onto which a movable disc can be screwed in to close (or shut) the valve. In globe valves, the disc is connected to a stem which is operated by screw action.

 

Plug valves (also known as cock valves) consist of a tapered or parallel side plug which can be rotated a quarter turn within a valve body. The quarter turn gives full closure or fully open operation of the valve.

Plug valve is a Quarter-turn rotary motion Valve that uses a tapered or cylindrical plug to stop or start the flow. The disk is in plug shape, which has a passage to pass the flow.

 Butterfly valves consist of a disc attached to a shaft with bearings used to facilitate rotation. These are considered high recovery valves, since only the disc obstructs the valve flow path. The flow capacity is relatively high and the pressure drop across the valve is relatively low. The butterfly valve offers a simple  design that is lightweight, compact, and inexpensive, particularly in larger valve sizes. Because of the advances in seating material, butterfly valves have found general acceptance in the oil, gas, chemical, water, and process fields. The valve is often used in place of a gate valve, but has the added advantage of flow regulation. 

The swing check valve design consists of a hinged disk that swings open when flow starts in the desired direction and swings closed in flow reversal situations. Because of this swinging action of the disk, it is important to install all swing check valves so that the disk closes positively by gravity. When fully open, the swing check offers less flow resistance than the lift check valve design.In flow situations where shock closure and/or disk chatter are encountered.

Diaphragm valves (sometimes referred to as saunders valves) are designed to control flow in corrosive services where line content could adversely affect valve components. Other applications for diaphragm valves are in services where contamination from outside sources cannot be tolerated; for example, the pharmaceutical and food industries.Diaphragm valves differ from other valves in that the body of the valve and line content is sealed off from all moving parts of the valve by a flexible diaphragm.

gate valve is the most widely used in industrial valves for pipelines. its wide range application. The gate valves operated with a handwheel or with the help of electric actuators are popular. A pneumatic or a hydraulic actuator can be applied as well.

Types of gate valves are there

  • Conduct gate valve

  • Knife gate valve

  • High pressure gate valve

  • Wedge gate valve

  • Parallel gate valve

Diaphragm valves (sometimes referred to as saunders valves) are designed to control flow in corrosive services where line content could adversely affect valve components. Other applications for diaphragm valves are in services where contamination from outside sources cannot be tolerated; for example, the pharmaceutical and food industries.Diaphragm valves differ from other valves in that the body of the valve and line content is sealed off from all moving parts of the valve by a flexible diaphragm. This flexible diaphragm seal prevents stem packing leakage of line content and flow contamination by packing lubricants.

Control Valve Leakage –

This is basically the fluid which passes through the valve when the valve is fully closed. This value however should not be considered as the valve Cv at NIL Opening.

So this leakage shall depend on the contact of valve plug & seat with the seating force applied for holding the plug over the seat.