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Double Disc Gate Valve

April 29, 2022

What is a double-disc gate valve?

A double-disc gate valve is also known as a parallel double-disc gate valve. A double-disc gate valve is a type of gate valve which has two discs and two seats. The disc in a double-disc gate valve are used to open or close fluid flow. The two-disc expand from each other when the valve is working. Double disc gate valve manufacturers design these valves for use in on and off fluid flow applications. These valves are not meant for use in throttling applications. Double disc gate valves have tight seals which ensure zero fluid leakage unless there is mechanical damage to parts of the valve. Double disc gate valves are operated manually using the valve handwheel or through actuators such as pneumatic, electric and hydraulic actuators. Double disc gate valves have two disc seals. The sealing is enhanced by combining internal fluid pressure and spring force. A floating seat is used to relieve pressure automatically when middle section of the valve is under pressure.

Double disc gate valve

Figure: Double disc gate valve.

 

Components of a double-disc gate valve 

Actuator

This is the component of a double-disc gate valve that provides torque needed to open or close the valve. For a manual double disc gate valve, a handwheel is used where the valve operator applies torque to open/close the valve. Automatic double disc gate valves use sensors that determine when to close or open fluid flow.

Valve body 

This is the part of the double-disc gate valve which is used as the main pressure barrier. The valve body houses all the internal parts of the double-disc gate valve. The valve body contains the fluid flow ports which are used to connect the valve to the piping system. The valve body must be strong enough to withstand high temperatures and high pressure. As such, double disc gate valve manufacturers design the body with strong metallic materials such as stainless steel, carbon steel, cast iron, and ductile iron materials. The body is connected to the pipeline using bolts or threads. The bolts connection is used in high fluid flow pressure.

Bonnet 

The bonnet is the component of a double-disc gate valve which is placed on top of the body to protect the stem and its sealing parts. The bonnet and the body are made of the same strong metallic material. The bonnet is connected to the body through screws, bolts, or welding. 

Stem 

This is the part of a double-disc gate valve that is used to connect the actuator/handwheel and the gate/discs. The stem has threads that make it rotate when the handwheel is rotated. The rotation of the stem makes it move linearly upwards or downwards to make the discs open or close the valve. 

Discs 

Discs are the components that are used to block or allow fluid flow in a double-disc gate valve. The discs are two in number thus the name “double-disc” gate valve. The discs are connected to the actuator via the stem so that as the stem moves the discs can move thus opening or closing fluid flow. 

Seats 

The seats are the components used to provide a seal with the discs or gate as well as support the gate/discs. The seats are either soft seats or metallic seats. Soft seats are made of plastic or rubber materials and are meant for use in temperatures below 60 oC while metallic seats can be used for high-temperature applications above 60 oC.

Gland packing

This is the component of a double-disc gate valve that is used to provide a seal between the bonnet and the stem to prevent fluid leakage through the stem.

Stuffing box 

This is a chamber where the packing seal is compressed.

Components of a double-disc gate valve

Figure: Components of a double-disc gate valve.

 

How does the double-disc gate valve work? 

A double-disc gate valve operates by raising or lowering the discs to open or close fluid flow respectively. Double disc gate valve manufacturers produce these valves to be used as manual or automatic. The working principle is the same for both manual and automatic version. The only difference between the manual and the automatic is that the latter is powered by actuators. The work of the actuator is to provide the torque necessary to rotate the stem to raise or lower the gate to allow or block fluid flow respectively. To open a double-disc gate valve, the stem is rotated by applying force on the handwheel or from the actuator. The rotation of the stem in the anticlockwise direction makes it move linearly upwards which makes the gate/double disc also move upwards. Movement of the disc upwards leaves the valve open and thus fluid flows. To close the double-disc gate valve, the handwheel is rotated in the clockwise direction. This makes the stem rotate in the same direction as it moves downwards which makes the gate move downwards until it completely closes fluid flow. 

Working of a double disc gate valve

Figure: Working of a double disc gate valve.

 

Types of double disc gate valve

Rising stem double disc gate valve 

This is a type of double disc which is designed such that the stem can rise above the handwheel when the valve is opened. This type of valve has threads machined on the outside of the stem. When the handwheel is rotated to open the valve, the stem starts rotating as it moves upwards. Rotating the handwheel of a double-disc gate valve to close fluid flow will make the stem start moving downwards. This makes it very clear for valve operators to easily tell when opening or closing a rising stem double disc gate valve. Double disc gate valve manufacturers recommend these valves for installation in places where space is not limited since they occupy a lot of space. 

Rising stem double disc gate valve

Figure: Rising stem double disc gate valve.

Non-rising stem double disc gate valve 

This is a double-disc gate valve in which the stem does not rise above the handwheel. This type of valve has the stem threaded into the gate so that it rotates to lower and raise the double-disc in the valve but not above the handwheel. Non-rising stem double disc gate valves are advantageous in that they take up less space relative to their rising stem counterparts. To make it easy to know when the valve is opening/closing, double disc gate valve manufacturers design the valve with a local visual indicator to show the position of the valve. Non-rising stem double disc gate valves are suitable for both underground and above-ground installations. 

Non-rising stem double disc gate valve   

Figure: Non-rising stem double disc gate valve.

Ceramic double disc gate valve 

A ceramic double disc gate valve is a type of valve in which internal parts are made of ceramic materials such as zirconia. The internal parts made of ceramic material include lining, seats, O-rings, and gate. Ceramic double disc gate valve manufacturers construct the body using strong metallic materials such as ductile iron, stainless steel, carbon steel, and cast iron. The strong metallic materials help to provide high strength to withstand high fluid pressure. A ceramic double disc gate valve can withstand high temperatures of around 400 oC which makes it important in high-temperature applications. Also, by using ceramic materials, these valves are highly resistant to corrosion and abrasion. As such, ceramic double disc gate valves can be used in various harsh applications such as mining, petrochemical, and steam power plants.

Ceramic double disc gate valve

Figure: Ceramic double disc gate valve.

 

Applications of double disc gate valve 

  • Double disc gate valves are used in mining industries. 
  • They are used in chemical manufacturing industries. 
  • These valves are used in oil and gas refineries. 
  • Double disc gate valves are used in water supply systems. 
  • They are used in the paper and pulp industries. 
  • They are used in petrochemical applications. 
  • Double disc gate valves are used in steam power generation plants. 

 

Advantages of double disc gate valves 

  • Double disc gate valves have zero fluid leakage unless there is mechanical damage. 
  • These valves have low-pressure drop which improves pumping efficiency. 
  • Double disc gate valves are versatile because they can be used in different industries for different applications. 
  • These valves are made of strong materials which makes them suitable for different pressure and temperature applications. 
  • Double disc gate valves are durable since their parts are available for repairs. 
  • These valves are bidirectional. 
  • These valves are easy to install, repair, and clean. 
  • Double disc gate valves operate slowly which helps to reduce hammering problems. 

 

Advantages of double disc gate valves 

  • Double disc gate valves are not suitable for use in throttling applications. 
  • These valves are slow in operation unlike other valves like ball valves. 
  • Double disc gate valves make noise when used while partially open. 
  • It is prone to wear which makes most double disc gate valve manufacturers use abrasive resistant materials to make lasting valves but which end up being expensive. 

 

Troubleshooting double disc gate valve 

The double-disc gate valve does not close completely 

  • Seating surfaces are damaged. Disassemble the valve and replace the seats. 
  • Foreign materials are stuck in the valve. Disassemble the valve as per the double-disc gate valve manufacturer’s instructions and remove foreign materials. 

The double-disc gate valve does not open completely 

  • There are foreign materials stuck in the valve. Disassemble the valve according to the double-disc gate valve manufacturer’s instructions and clean the materials. 

The double-disc gate valve does not open 

  • Torque switches have been incorrectly adjusted. Reset the switches according to the double-disc gate valve manufacturer’s instructions. 
  • The valve was closed using excess torque. Disassemble the valve and open it with caution so as not to cause damage. 

The double-disc gate valve needs a lot of torque than usual to operate

  • Excess tightened gland bolt. Un-tighten gland bolt. However, be keen to ensure the gland bolt is not loose as it will start leaking fluid. 
  • Misaligned gland flange. Realign the gland flange. 
  • Damaged stem. Replace the stem. 
  • Foreign materials between gland flange and the stem. Remove gland bolt to help clean foreign materials. Replace the stem if it is severely damaged. 
  • Wrong stem packing has been used. Replace the stem packing with one recommended by the double-disc gate valve manufacturer. 

Leakage through the seats

  • The seats are damaged. Replace the seats. 
  • Foreign materials in the valve. Clean the foreign materials. 

Leakage through gasket 

  • The gasket is damaged or worn out. Replace the gasket. 
  • Loose bolts. Tighten the bolts and nuts. 

Leakage through stem packing 

  • Loose stem packing. Tighten the stem packing. 
  • Damaged stuffing box or stem. Replace the stem or stuffing box as necessary. 

 

Summary 

A double-disc gate valve is a type of gate valve which has its gate made of two discs. The two discs are used to open and close the fluid flow. Because of the two discs, double disc gate valve manufacturers design these valves with two seats. The seats are used to provide sealing with the discs as well as support the discs. Double disc gate valve manufacturers design these valves for on and off fluid flow applications but not for throttling applications. Double disc gate valves can be manual or automatic depending on the user’s preferences. These valves are used in different industries such as petrochemicals, chemicals, mining industries, oil and gas refineries, water supply, steam power plants, and paper and pulp industries among others. There are various types of double disc gate valves which include rising stem double disc gate valve, non-rising stem double disc gate valve, and ceramic double disc gate valve among others. Advantages of using double-disc gate valves are zero leakage, long service life, versatility, high strength, low-pressure loss, bi-directional fluid flow, and ease of installation and repair. 

 

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