What is a gate valve?
A gate valve can be defined as a type of valve that uses a gate or wedge that moves perpendicular to the flow to start or stop the flow of fluid in the pipeline.
Gate valves are one of the most commonly used types in any process plant. It is a linear motion valve that is used to start or stop the flow of fluid. In service, these valves are either in the fully open or fully closed position. When the gate valve is fully open, the disc of a gate valve is completely hidden from the flow path. Therefore, there is virtually no resistance to flow, its head loss is very low.
To achieve a proper seal, when the valve is fully closed, a 360° surface contact between the disc and seats is required.
Gate valves should not be used to regulate or throttle flow, because precise control is not possible. The high flow velocity in the partially open valve can cause erosion of the wedge and seating surfaces, and also creates vibration and noise.
PARTS OF A GATE VALVE
TYPES OF GATE VALVES
There are three ways to classify a gate valve.
TYPES OF WEDGE
SOLID WEDGE VALVE
The solid wedge type is the most common and widely used type of seal due to its simplicity and strength. A solid wedge valve can be installed in any position and is suitable for almost all fluids. It can also be used in turbulent flow.
However, it does not compensate for changes in seat alignment due to pipe loads or thermal expansion. Therefore, this type of disc design is more susceptible to leakage. The solid wedge is subject to thermal locking if used in high temperature service.
Thermal locking is a phenomenon in which the wedge is jammed between the seats due to metal expansion. Solid wedge gate valves are generally used in moderate to lower pressure-temperature applications.
FLEXIBLE WEDGE VALVE
The flexible wedge is a one-piece solid disc with a cut around the perimeter. These cuts vary in size, shape and depth. A narrow, shallow cut around the perimeter of the wedge provides less flexibility but retains strength. A recessed recess or a deeper, wider cut around the perimeter of the wedge provides more flexibility but compromises strength.
This type of design improves seat alignment and provides better sealing. It also improves performance in situations where thermal expansion is possible. Gate valves of this type are widely used in steam systems.
Thermal expansion of the steam line sometimes causes distortion of the valve bodies, which can lead to thermal blockage. The flexible damper allows the wedge to flex as the valve seat compresses due to thermal expansion of the steam line and prevents thermal blockage.
The disadvantage of flexible dampers is that line fluid tends to accumulate on the disc, causing corrosion and weakening the disc.
PARALLEL DISC SPLIT WEDGE VALVE
The split wedge valve consists of two solid parts and is clamped with the help of a special mechanism. In case the disc half is misaligned, the disc can be freely adjusted to the seating surface. The split disc can be either wedge-shaped or parallel disc-shaped.
The parallel discs are spring loaded, so they are always in contact with the seats and provide bi-directional sealing. The split wedge is suitable for handling gases and liquids without condensation at normal and high temperatures.
The freedom of movement of the disc prevents thermal binding even if the valve is closed when a line is cold. This means that when a line is heated by a fluid and expands, it does not create thermal locking.
TYPES OF DAMPERS BASED ON BODY COVER CONNECTION
THREADED COVER: This is the simplest design available, used for economy valves.
SCREWED COVER: This is the most popular design and is used on a large number of gate valves. This requires a gasket to seal the joint between the body and bonnet.
WELDED LID: This is a design used when disassembly is not necessary. It is lighter and prevents leakage through the lid.
PRESSURE SEAL: This type is widely used for high pressure and high temperature applications. The higher the body cavity pressure, the greater the force on the gasket in a pressure seal valve.
TYPES OF COVER
RISING SPINDLE GATE VALVE
In a rising stem valve, the shaft will rise when the valve opens and move downwards when the valve closes. This circumstance means that the shaft itself serves as a visual position indicator.
This type of valve is also known as OS & Y valve.
FIXED SPINDLE GATE VALVE
This type of valve has no up and down movement of the shaft. The valve disc is internally threaded. The wedge travels along the shaft like a nut when the stem is turned. In this type of valve, the stem threads are exposed to the flow medium. Therefore, this design is used when space is limited to allow linear movement of the stem and the flow medium does not cause erosion, corrosion or wear and tear of the stem material. This type of valve is also known as an internal screw valve.
