Showing posts with label plug valve. Show all posts
Showing posts with label plug valve. Show all posts

Sunday, July 21, 2019

Applying Gas Pipeline Block Station Valves


A block valve is used on gas transmission systems to isolate a segment of the main gas pipeline for inspection and maintenance, or for shutdown in the case of a natural disaster or pipeline damage.

The block valve is typically a full-bore, soft seated ball valve to allow for scraping. However, this type of valve cannot be opened against full differential pressure without damage to the valve seats. Therefore, a bypass system is installed around the block valve, and used to balance the pipeline pressure prior to opening. Plug valves should be used in the bypass as they are capable of opening and throttling against full differential pressure without damage.

The Requirement of a Bypass

Let's see what would happen if the block valve, which is a soft seated ball valve, was operated against full differential pressure. As it is initially opened the huge pressure drop across the valve generates high velocity flow carrying fine dust, rust, or scale particles in close proximity of the valve seats. This results in seat damage and a block valve that cannot seal bubble tight once closed.

The Bypass Valve Sequence

To avoid this, a bypass system is utilized to balance the pressure either side of the block valve prior to opening it. With the vent valve closed, bypass valve 1 is opened allowing pressure into the bypass. In this case, a plug valve should be used, as it can be opened against full differential pressure without seat damage. Now bypass valve 2 is slowly opened, gradually building pressure in the downstream section until the pressure either side of the block valve is equalized. A plug valve is also used here capable of throttling the flow without seat damaged. With the pressure now equalized the block valve can be opened safely without the risk of seat damage. The two bypass valves have now done their job and can be closed providing bubble tight shutoff against the main pipeline.

Venting a Pipeline Section

Block stations are also used to vent sections of the pipeline into the atmosphere. This operation would start with all valves in the closed position. Bypass valve 1 is then opened allowing pressure into the bypass station. The vent valve is now slowly opened to release the pipeline pressure. Once again, this is a demanding application opening against full differential pressure, hence a plug valve is used to ensure bubble tight isolation to the atmosphere once closed.

For more information, contact Swanson Flo by visiting https://swansonflow.com or by calling 800-288-7926.

Wednesday, June 27, 2018

A Process Industry Stalwart - The Plug Valve

Flowserve Durco lubricated plug valve
Lubricated plug valve.
(Flowserve Durco)
This article will focus on one of the oldest and most reliable (when properly applied) industrial valve types - the plug valve.

Fluid process control operations commonly employ pumps, piping, tanks and valves as the means of transporting, containing and controlling the fluid movement through a system.

Valves, of which there are many types, provide control over the flow rate, direction and routing of fluids in a processing operation. Flow can be started, stopped or modulated between zero and full rate using a properly sized and configured valve. Some valves enable media flow to be diverted to a selection of outlets, in lieu of a single inlet and outlet pair. Specialized valves regulate inlet or outlet pressure, or prevent fluid flow from going in an undesirable direction. All of these capabilities are packaged into differing valve product offerings that present a very large selection array to a process designer or engineer.

Floserve Durco non-lubricated plug valve
Non-lubricated plug valve.
(Flowserve Durco)
Industrial flow control valve types are generally classified according to the structure or arrangement contained within the valve body that provides obstruction to fluid flow. Some of the common types are ball, butterfly, gate, globe, and plug. Surely, there are more valve types, and this article is not intended to list them all. Some of our previous blogs have discussed selection considerations for gate, ball and butterfly valves.

Plug valves, like ball and butterfly valves, span from fully open to fully closed positions with a shaft rotation of 90 degrees. The “plug” in a plug valve is installed in the flow path within the valve body and rotated by means of a stem or shaft extending to the exterior of the body. Plugs are tapered toward the bottom and are fitted to a seating surface in the valve body cavity that prevents fluid from bypassing the plug. An opening through the plug, the port, can be shaped to provide particular flow characteristics. There are two main types of basic plug, lubricated and non-lubricated. Lubricated plug valves have a cavity into which a sealant is injected. The sealant provides a renewable seal between the plug and the body, prevents internal leakage, and protects the seating surfaces against corrosion. Non-lubricated plug valves utilize an elastomeric body liner or a sleeve in the body cavity that is pressed in to the body of the valve by the plug's wedge-like shape,  with the result of reducing the friction between the plug and the body.

Plug valve considerations:
  • 90 degree rotation from open to closed provides fast operation.
  • With proper configuration, can be well suited for frequent operation.
  • Availability of corrosion resistant liner may provide comparative cost savings because valve body can be constructed of less expensive material.
  • Design is simple and employs a low parts count.
  • Valve can be serviced in place.
  • Generally, low resistance to flow when fully open.
  • Reliable leak-tight service due to tapered plug wedging action, replaceable sleeve, and injection of lubricant in some variants.
  • Potential issues of concern for plug valve application include a short list of items.
  • Higher friction in the plug closure mechanism may require comparatively higher operating torque than other valve types.
  • Without a specially designed plug, generally not well suited for throttling applications.
  • Rapid shutoff delivered by plug design may not be suitable for some applications where hammering may occur.
Share your fluid control application challenges with a valve and automation specialist. Leverage your own knowledge and experience with their product application expertise to develop an effective solution.