Introduction to Gate Valves

Gate valves are primarily designed to start or stop flow, this is done by inserting a rectangular or wedge into the path of a flowing fluid. In service, these valves generally are either fully open or fully closed. Gate valves require very less space for fitting along the pipe axis and hardly restrict the flow of fluid when the gate is fully opened enabling gate valves to offer straightway flow with very little pressure drop. 

Working Principle

Gate valve consists of three major components: body, bonnet & trim. The body is generally connected to the piping by means of flanged, screwed or welded connections. The bonnet, containing the moving parts, is joined to the body generally with bolts, to aid in cleaning and maintenance.  The valve trim consists of the stem, the gate, the wedge, or disc, and the seat rings.

The main operation mechanism is as follows: when the hand-wheel is turned, it rotates the stem, which in turn moves the gate vertically via threads. They are considered multi-turn valves as it takes more than one 360 degree turn to fully open or close the valve. When the gate is lifted from the path of flow, the valve opens and when it returns to its closed position, it seals the bore resulting in a full closure of the valve.

Gate Valve Closing Member Types 

• Parallel Gate Valve
• Wedge Gate Valve
• Knife Gate Valve
• Split Wedge Gate Valve
• Flexible Wedge Gate Valve

Use of Gate Valves

Gate valves are majorly used in refineries, petrochemical, gas processing plants where pressure remains relatively low, but temperatures may be very high. They are suitable for most fluids including steam, water, oil, air and gas.

Gate valves are designed for fully open or closed service. They are installed in pipelines as isolation valves and should not be used as flow control or regulating valves. Gate valves are often used when minimum pressure loss and a free bore is needed. A gate valve is a multi-turn valve meaning that the operation of the valve is done by means of a threaded stem. As the valve has to turn  multiple times to go from open to closed position, the slow operation also prevents water hammer effects. 

Gate valves should not be used for regulation or throttling of flow because, 

• The flow rate of the fluid is not proportional to the amount that the valve is open thus accurate control of fluid flow is not possible 
• Shearing of high-velocity flow will cause a partially open gate valve from vibration which will damage the seating surfaces and prevent a tight seal
• Gate and seat may be subject to excessive wear if the valve is partially open

Advantages of Gate Valve

• Gate valve provides good shut off features
• Pressure drop during operation is very less
• Gate valve are bi-directional valves and can provide shut-off in either flow directions
• They are suitable for high pressure and temperature application and required less maintenance
• gate valves tend to be slightly cheaper than ball valves of the same size and quality

Disadvantages of Gate Valve

• It can not be used to regulate or throttle the flow
• A gate valve is slow in operation and can not be quickly  opened or closed 
• It is prone to vibration and noise i the partially open state
• It is more subject to seat and disc wear
• Gate valves require large space for installation, operation and maintenance

What is a valve

A valve is a simple mechanism that opens and closes to control the flow & pressure of fluids and in scientific context, fluids include both liquids or gases - any substance that can flow freely. Valves are essential components of a piping system that conveys liquids, gases, vapours, slurries etc.

The simplest valves are the check valves i.e. one way flap mechanisms that allow flow in one direction & shut-off with back-flow pressure. In addition to running automatically, a valve can be operated through pressure, temperature, electrically or manually.

Manual valves are usually operated by levers, pedals, hand knobs or wheels to turn a screwed system. Large valves usually have a geared mechanism inside that actuate the disc against its seat. A valve may control fluid flow to one or often, as many as four ports according to the application. Valves come in different sizes from 0.1 mm to up to 5 metres for special applications. Valves are available in different set of materials too according to the requirement. 

Types of valve available:

1. Gate valve

2. Globe valve

3. Plug valve

4. Ball valve

5. Butterfly valve

6. Check valve

7. Diaphragm valve

8. Pressure relief valve

9. Control valve

Each of these type has a number of models, each with different features and functional capabilities. Some valves are self operated while others manually or with an actuator or pneumatic or hydraulic is operated.

Functions of valves are:

1. Starting & stopping flow

2. Reduce or increase flow

3. Controlling the direction of flow

4. Regulating a flow or process pressure

5. Relieve a pipe system of a certain pressure

Indian Navy submarines to be equipped with leathal 'AIP technology'

Indian Navy is all set to receive a boost through Air Independent Power (AIP) for its submarines. This system is bound to be installed by Defence Research & Development Organisation (DRDO) aboard all Kalvari Class submarines. India's neighbours, Pakistan & China already possess this technology.

The AIP technology allows a non-nuclear submarine to stay underwater for extended period of times which in turn makes them harder to detect and allows patrolling in high risk areas. This locally designed system is expected to increase the endurance of submarines under water by two weeks.

This system burns ethanol with stored oxygen to produce steam, which turns a turbine just like a nuclear plant.

Pakistan is also buying eight Type 093B submarines from China that will come with another type of AIP called a stirling generator, which uses a closed cycle diesel engine. The Stirling generator is famous because of its use by Swedish as well as Japanese Navy.