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Parallel Slide Gate Valve

April 29, 2022

What is a parallel slide gate valve? 

A parallel slide gate valve is a type of gate valve which uses two gates or discs to close or open fluid flow. Parallel slide gate valve manufacturers design these valves with a disc holder used to connect the two gates. These valves are meant for use in on and off applications. Parallel slide gate valve manufacturers employ design considerations that result in a valve with a compact structure, superb sealing, and reliable closing and opening. Parallel slide gate valves are used in applications such as chemical, natural gas, and petroleum industries among others. Parallel slide gate valves are suitable for use in both high-pressure and high-temperature applications. These valves can be driven using a handwheel, pneumatic actuator, electric actuator, or hydraulic actuator. 

Parallel slide gate valve

Figure: Parallel slide gate valve.

 

Components of a parallel slide gate valve 

Actuator 

The actuator is the component of a parallel slide gate valve that provides the torque needed to rotate the stem to open or close the valve. Parallel slide gate valves need to rotate for more than 360o degrees to open or close. Actuators such as hydraulic, electric, and pneumatic are used for automatic valves. When the parallel slide gate valve is manual, a handwheel is used. The handwheel is where a valve operator applies force or torque to open or close the parallel slide gate valve. 

Valve body 

The valve body is the component of a parallel slide gate valve which houses internal parts of the valve. The body is made of strong metallic material to give the valve high strength to withstand high-pressure fluids. The body contains ports of the parallel slide gate valve. The ports are used to connect the valve to the pipeline. The body can be connected to the pipeline using bolts, screws, threads, or welding.

Bonnet 

Bonnet is the component of a parallel slide gate valve which is placed on top of the valve body to prevent fluid leakage at the valve stem. The bonnet is connected to the valve body using bolts and nuts, screws, welding, or threading. 

Stem 

The stem is the component of a parallel slide gate valve responsible for raising and lowering the discs to open or close the fluid flow. The stem has threads that are used to provide rotary motion as the stem moves upwards or downwards to raise or lower the gate. The stem connects the gate and the valve actuator/handwheel. 

Gate 

This is the part of a parallel slide gate valve used to allow or block fluid flow. The gate is placed between two seats. 

Seats 

The parallel slide gate valve has two seats that provide fluid sealing with the gate. The seats can be soft seats or metallic seats. Soft seats are made of soft materials such as plastics and rubber. Metallic materials include stainless steel, carbon steel, brass, and bronze. 

Gland packing 

This is a gasket used to provide a seal between the bonnet and the stem. It is also known as packing.

Spring 

The spring is mounted between the discs. It provides initial sealing force although it does not maintain the sealing force.  

Components of a parallel slide gate valve

Figure: Components of a parallel slide gate valve.

 

How does a parallel slide gate valve work? 

A parallel slide gate valve uses a gate to control fluid flow. The gate is connected to the actuator through the stem. When the handwheel is rotated, the stem rotates as it moves linearly upwards or downwards. This vertical and linear movement raises or lowers the gate to open the valve as needed. To open a parallel slide gate valve, the handwheel is rotated in the anticlockwise direction. This makes the stem rotate in the same direction. The rotary motion of the stem is converted into vertical linear motion by the stem threads. This linear motion makes the gate move upwards since the gate and stem are connected. The movement of the gate upwards leaves space for the valve port to open allowing fluid flow. To close the parallel slide gate valve, the handwheel is rotated in the clockwise direction which makes the stem rotate while moving downwards. This downwards linear motion brings the gate down until it closes the valve. 

Working of parallel slide gate valve

Working of parallel slide gate valve.

Parallel slide gate valve manufacturers design these valves with replaceable two-disc seals with double bleeding and blocking. This is created by combining spring force and internal pressure. There is a floating seat in a parallel slide gate valve which is used to automatically relieve pressure when the middle section is under pressure. When the cavity pressure is more than the one in the channel, the cavity pressure is freed to the channel. If the channel upstream pressure is more than the downstream pressure (valve in the closed state) the middle chamber pressure is released to the upstream side. The valve seat resets automatically after pressure relief.

 

Types of parallel slide gate valves

Flanged end parallel slide gate valve 

This is a parallel slide gate valve that is connected to the pipeline using bolts and nuts. Flanged end parallel slide gate valve manufacturers design this type of valve with flanges. The flanges have holes. The bolts are placed through these holes to provide a connection between the pipeline and the valve. The pipeline also has flanges with holes. Flanged end slide gate valves uses a gasket between the flanges of the pipeline and the parallel slide gate valve. Three types of flanged ends are raised face, flat face, and ring face. The ring face is suitable for use in high-pressure applications. Flanged end parallel slide gate valves with ring face use steel ring as gasket. Flanged end connection is the most commonly used in parallel slide gate valves. This type of parallel slide gate valve is most preferred because it is easy to install and disassemble from the pipeline for operations like cleaning, repair, and maintenance. 

Flanged end parallel slide gate valve

Figure: Flanged end parallel slide gate valve.

Threaded end parallel slide gate valve 

This is a parallel slide gate valve that is connected to the pipeline using threads. The threaded end parallel slide gate valve has threads at the valve ports. Similarly, the pipeline has threads at the ends. If the pipeline has male threads, the parallel slide gate valve has female threads and vice versa. Parallel slide gate valve manufacturers produce the valve with tapered or straight threads. Straight threads need extra components to guarantee a tight seal, unlike tapered threads which do not need other components like a flat washer to enhance a tight seal. Threaded end parallel slide gate valves are meant for use in low and medium pressure applications, unlike flanged end counterparts that can be used for high-pressure applications.  

Forged parallel slide gate valve 

This is a parallel slide gate valve that is made through forging method. The forging method involves shaping the manufacturing alloys while they are in their solid-state. Forged parallel slide gate valves are suitable for use in high temperature and high-pressure applications. Forged parallel slide gate valves have low-pressure drop which makes the pumping process energy efficient. These valves are meant for on and off applications in applications such as oil and gas refining, paper and pulp, and steam power generation among others. 

Cast parallel slide gate valve 

Cast parallel slide gate valve is made through casting method. In this case, parallel slide gate valve manufacturers use metal in molten form to make the valve. The metallic material of the valve is first heated to high temperatures to change its state from solid to molten state. The molten material is then placed in a mold. In the mold, the molten material is given time to cool and solidify. Next, the material is ejected from the mold. One of the advantages of making parallel slide gate valves through the casting method is that it is cost-effective. Using this method, various types of cast parallel slide gate valves can be manufactured. Also, cast parallel slide gate valves do not need a lot of labor like forged ones which makes their cost lower as it has low machining cost. Once the valve is made through casting it is tested through non-destructive tests to ensure it has no defects which can cause fluid leakage when the valve is in use. 

 

Applications of parallel slide gate valve 

  • Parallel slide gate valves are used in oil and gas refinery systems. 
  • They are used in chemical.
  • Parallel slide gate valves are used in pharmaceutical applications. 
  • They are used in water supply systems. 
  • These valves are suitable for use in steam power generation systems. 
  • Parallel slide gate valves are used in the paper and pulp industries. 

 

Advantages of parallel slide gate valve 

  • Parallel slide gate valves are versatile as they are applicable in different industrial applications. 
  • Parallel slide gate valves are durable since their broken parts can be replaced for reuse.
  • These valves are of high strength suitable for high pressure and high-temperature applications such as steam systems. 
  • These valves have less wear relative to other gate valves. 
  • They are free from fluid leakage. 
  • Parallel slide gate valves need less torque to open/close relative to other gate valves. 
  • Parallel slide gate valves have a low-pressure drop. 
  • These valves are easy to install, assemble, operate and repair. 

 

Disadvantages of a parallel slide gate valve

  • Parallel slide gate valves are slow to operate. 
  • These valves are not suitable for throttling applications. 
  • Parallel slide gate valves need a large area for installation, repair, and maintenance. 

 

Troubleshooting a parallel slide gate valve 

High operating torque 

  • Misaligned gland flange. Check stem for damage and re-align gland flange. Replace the stem if it is damaged. 
  • The gland bolt is over-tightened. Un-tighten gland bolt to reduce packing friction.
  • Foreign materials between the gland flange and the valve stem. Unscrew gland bolt and clean foreign materials. If the item is damaged replace it. 

Parallel slide gate valve not closing completely 

  • Foreign materials in the valve do not allow full valve closure. Open the valve immediately to prevent erosion and remove those foreign materials. 
  • Valve stem or seating surfaces are damaged due to foreign materials. Disassemble the valve and remove foreign materials and replace seating surfaces and stem as necessary. 

Gate valve not opening 

  • The parallel slide gate valve has been closed using excess force. Disassemble the valve and open it carefully to prevent damage. Then assemble the valve again following parallel slide gate valve manufacturer’s installation instructions.
  • The torque has been incorrectly adjusted. Adjust torque according to parallel slide gate valve manufacturer’s instructions. 

Leakage through stem packing 

  • Damaged stuffing box or valve stem. Replace stuffing box or stem as necessary. 
  • Insufficiently tightened gland bolt or misaligned gland. Tighten gland bolt carefully until fluid leakage ceases. 

Leakage through gasket 

  • Damaged or worn-out gasket. Replace the gasket. 
  • Misaligned bonnet for bonnets connected using bolts. Re-align the bonnet. Also, replace the gasket if necessary. 

 

Summary 

A parallel slide gate valve is a type of gate valve which uses two discs to control fluid flow. These valves are meant for on and off fluid flow applications but are not suitable for use in throttling. Parallel slide gate valves can be operated manually using a handwheel or using actuators such as pneumatic, hydraulic, and electric actuators. Parallel slide gate valve manufacturers design the valve with a floating seat meant for automatically relieving pressure. The seat resets automatically after relieving pressure. Parallel slide gate valves are meant for use in different applications such as chemicals, oil and gas refineries, pharmaceuticals, paper and pulp, power generation, and water supply among others. There are various types of parallel slide gate valves which include flanged end parallel slide gate valve, threaded end parallel slide gate valve, forged parallel slide gate valve, and cast parallel slide gate valve among others. Advantages of using parallel slide gate valves are durability, versatility, high strength, low-pressure drop, zero fluid leakage, less wear, and ease to install, assemble, operate and repair.

 

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