what is gate valve?

Gate valves are used to shut off the flow of liquids rather than for flow regulation. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low flow resistance.[1] The size of the open flow path generally varies in a nonlinear manner as the gate is moved. This means that the flow rate does not change evenly with stem travel. Depending on the construction, a partially open gate can vibrate from the fluid flow.[1]
Gate valves are mostly used with larger pipe diameters (from 2″ to the largest pipelines) since they are less complex to construct than other types of valves in large sizes.
At high pressures, friction can become a problem. As the gate is pushed against its guiding rail by the pressure of the medium, it becomes harder to operate the valve. Large gate valves are sometimes fitted with a bypass controlled by a smaller valve to be able to reduce the pressure before operating the gate valve itself.
Gate valves without an extra sealing ring on the gate or the seat are used in applications where minor leaking of the valve is not an issue, such as heating circuits or sewer pipes.

A gate valve can be defined as a type of valve that used a gate or wedge type disk and the disk moves perpendicular to flow to start or stop the fluid flow in piping.
A gate valve is the most common type of valve used in any process plant. It is a linear motion valve used to start or stop fluid flow. In service, these valves are either in a fully open or fully closed position.
When the gate valve is fully open, the disk of a gate valve is completely removed from the flow. Therefore virtually no resistance to flow. Due to this very little pressure drops when fluid passes through a gate valve.
To achieve proper sealing, when the valve is fully closed, 360° surface contact is required between disk and seats.
Gate valves should not be used for regulation or throttling of flow because accurate control is not possible. The high velocity of the flow in the partially open valve may cause erosion of the disc and seating surfaces and also creates vibration and noise.

The gate valves are used where minimum pressure drop and / or bi-directional on-off service are required. Typical applications are:
§ water
§ chemicals
§ petrochemicals
§ steam
§ gases
§ liquid gases (cryogenic service)

CONSTRUCTION DETAILS

Body

The body geometry is designed as the result of stress calculations to achieve the most regular distribution of the internal forces dues to pressure action. The body material is high quality cast steel. The seats surface is covered by a wear resistance stainless steel deposited by welding overlay with a hardness difference of +50 HB in comparison with the wedge seats. On request the seat surface can be covered also with stellite or other special material overlays.

Bonnet

The bonnet is the bolted type, or pressure seal type (for higher pressures), designed to achieve the minimum turbulence and flow resistance. The bonnet material is the same of the body material. Beside, the bonnet is designed and manufactured in order to ensure a perfect seal, as well as to allow an easy reassembly and reassembly work.

Wedge

All our standard gate valves are “full bore” and in the open position their wedge is fully retracted to ensure the lowest pressure drop. The gate valves can be provided with the following obturators:
§ Split wedge (double disk wedge): this type is the best solution for small valves with frequent operation due to his durability guaranteed also by the construction with the inner parts in stainless steel. The normal wear is easily recovered by the automatic adjustment of two disk and a perfect tight can be obtained with low torque. In high and low temperature services, compensating the body deformations, this wedge provide the best performances. The split wedge is always required in case of valve installation with horizontal stem.
§ Flexible wedge: this is the most used and well known type of wedge. Owing to his low weight and the flexibility due to his particular construction can be employed in medium and large size valves with very good result also in high and low temperature services.
§ Solid wedge: this type is generally used in larger size where due to small space between the two body seat other types can’t be used (typically in short face to face valves). This execution is required also when the valves are used for fluid that can be solidified due to temperature variations. Please refer to technical sheets for the execution provided by standard on each size and type.

Yoke and handwheel

The standard valves are provided with yoke sleeve and handwheel for manual operation. On request the yoke can be provided with a connection flange according to ISO 5210 that allow the mounting of other operation devices (actuator, gearbox, etc.). In high-pressure valves or in the larger sizes the yoke sleeve is supported by two thrust roll bearings. The yoke sleeve is treated to prevent the seizure and the corrosion. A feather key transmits the torque from the handwheel to the yoke sleeve and permits a quick and easy disassembling. The handwheel is retained by a clamping ring (seeger) and it is not rising.

Stem

In all outside screw valves the stem is provided with integral backseat as emergency device in case of packing failure. The stem is produced whit special turning machinery for a high resistance and durability. The stem thread is metric type according to ISO 2901 with trapezoidal section. The thread is left hand type in the way that the valve opens rotating clockwise the handwheel. To avoid any leakage the stem has a high finish degree and a strict diametrical clearance.

Gasket

The standard gasket is in pure graphite stainless steel reinforced. This type of gasket is suitable for many different applications. For special applications (cryogenic gases, high corrosion acids etc.) we can supply special gaskets designed for the specific application or according to customer specifications. All valves with round body are provided by standard with chambered gasket.

Packing

The standard packing is made of four or more pure graphite rings with square section. The first and the last ring are reinforced with stainless steel to avoid the extrusion. Other materials like PTFE are available on request. The graphite is always treated with special corrosion inhibitors to prevent the corrosion of ferritic stainless steel stem due to galvanic cell action. For special applications (cryogenic gases, high corrosion acids, etc.) we can supply special packings designed for the specific application or according to customer requirements. To meet the TA-Luft requirements, on request, we can supply valves with special design of stem and packing. The stuffing box housing is produced with a high finish degree and a strict clearance to guarantee a perfect tight of the packing

INSTALLATION

The standard installation position for the gate valves is with the vertical stem and upright (operator on the top). This position avoids many adverse influences and stresses during the functioning. Only the gate valves with split wedge can be installed also in horizontal position. The gate valves close turning the handwheel clockwise. For a correct and easy installation please refer to the installation manual and follow the instructions there contained.

WARNINGS

§ The gate valves are not designed for throttling and regulating service. A prolonged use in partially open position would generally cause some damages.
§ The gate valves are not suitable for media which tend to produce high sedimentation or encrustation, as well as fluids containing foreign solids which, due to their hardness, present the risk to damage the seat faces.
§ In case of horizontal installation the valves shall be always provided with split wedge and, for sizes over 200 mm, the yoke shall be sustained by appropriate devices.
§ The solid wedge is always required for medium that can be subjected to solidification.
§ In case of medium that, due to temperature gradient, can change his physical phase from liquid to gas or vapour, the gate valves shall be provided by appropriate safety device.
Types of Gate Valves
There are three ways to classify the gate valve.
1. Types of Disk
1. Solid taper wedge
2. Flexible wedge
3. Split wedge or Parallel disks Valve
2. Types of Body Bonnet Joint
1. Screwed Bonnet
2. Bolted-Bonnet
3. Welded-Bonnet
4. Pressure-Seal Bonnet
3. Types of Stem movement
1. Rising Stem or OS & Y Type (Outside Stem and Screw Type)
2. Non-rising Stem type

Solid Wedge Gate Valve

Solid wedge is the most common & widely used disk type because of its simplicity and strength. A valve with a solid wedge may be installed in any position, and it is suitable for almost all fluids. It can be used in turbulent flow also.
However, it does not compensate for changes in seat alignment due to pipe loads or thermal expansion. So, this type of disk design is most susceptible to leakage. Solid wedge is subjected to thermal locking if used in high-temperature service.
Thermal locking is a phenomenon in which wedge is stuck between the seats due to the expansion of the metal. Solid-wedge gate valves are generally used in moderate to lower pressure-temperature applications.

Flexible Wedge Gate Valve

The flexible wedge is a one-piece solid disk with a cut around the perimeter. These cuts vary in size, shape, and depth. A shallow, narrow cut on wedge perimeter gives less flexibility but retains strength. A cast-in recess or deeper and wider cut on wedge perimeter gives more flexibility but compromises the strength.
This design improves seat alignment and offers better leak tightness. It also improved performance in situations where thermal binding possible. Flexible wedges Gate valves are used in steam systems.
Thermal expansion of the steam line sometime causes distortion of valve bodies which may lead to thermal blinding. The flexible gate allows the gate to flex as the valve seat compresses due to thermal expansion of the steam pipeline and prevents thermal blinding.
The disadvantage of flexible gates is that line fluid tends to collect in the disk. These may result in corrosion and ultimately weaken the disk.

Split wedge or Parallel disks Gate Valve

Split wedge Disk consists of two solid pieces and holds together with the help of a special mechanism. You can see the same in images. In case, one-half of the disk is out of alignment; the disk is free to adjust itself to the seating surface. The split disk can be in a wedge shape or a parallel disk type.
Parallel disks are spring-loaded, so they are always in contact with seats and give bi-directional sealing. The split wedge is suitable for handling noncondensing gasses and liquids at normal and high temperatures.
Freedom of movement of the disk prevents thermal binding even though the valve may have been closed when a line is cold. This means when a line is get heated by fluid and expand it does not create thermal blinding.
Types of gate valve based on body, bonnet connection
1st is screwed bonnet: This is the simplest design available and it is used for inexpensive valves.
2nd is bolted-bonnet: This is the most popular design and used in a large number of gate valves. This requires a gasket to seal the joint between the body and bonnet.
3rd is Welded-Bonnet: This is a popular design where disassembly is not required. They are lighter in weight than their bolted-bonnet counterparts.
4th one is Pressure-Seal Bonnet: This type is used extensively for high-pressure high-temperature applications. The higher the body cavity pressure, the greater the force on the gasket in a pressure -seal valve.
OS & Y Gate Valve or Rising Stem (Outside Stem and Screw Type)
For a rising stem valve, the stem will go up while opening the valve and move down when you close the valve. You can see in the image. In inside screw design, the threaded portion of the stem is in contact with the flow medium, and when you open the valve, handwheel rise with the stem.
Whereas in the case of outside screw design, the only smooth portion is exposed to the flow medium and the stem will rise above the handwheel. This type of valve is also known as OS & Y valve. OS & Y means outside steam and York.

Non-rising Stem Gate Valve or Insider Screw Valve

There is no upward movement of the stem in a non-rising stem type. The valve disk is threaded internally. The disc travels along the stem like a nut when the stem is rotated. You can see the image. In this type of valve, stem threads are exposed to the flow medium.
Therefore, this design is used where space is limited to allow linear stem movement, and the flow medium does not cause erosion, corrosion, or wear and tear to stem material. This type of valve also known as an insider screw valve.
Gate Valve Applications
• Gate valves are used in almost all fluid services such as air, fuel gas, feedwater, steam, lube oil, hydrocarbon, and all most any services.
• Some special gate valve is used in slurry and powder product also such as knife gate valve

Advantages of Gate Valve

• Gate valve provides good
• Pressure drop during operation is very less.
• Most of the gate valve can be used as bi-directional
• They are suitable for high pressure and temperature application and required less maintenance

Disadvantages of Gate Valve

• It cannot be used to control the flow.
• A gate valve is slow in operation. Opening and closing take time which is good also as it reduces the chance of hammering.
• When partially open it creates vibration and noise.
• Repairs, such as lapping and grinding of seats are more difficult due to limited access.