Showing posts with label gate. Show all posts
Showing posts with label gate. Show all posts

Wednesday, July 13, 2016

Eight Process Control Valve Selection and Application Criteria

Sliding Gate Control valve with actuator
Sliding Gate Control Valve
Schubert & Salzer
Fluid processes will employ control valves to regulate flow or pressure in between the extremes of fully open and fully closed. Their function and design is specifically different from shutoff valves, which are designed and intended for isolation of segments of a fluid system. Improperly applying or sizing a control valve can have consequences in operation, productivity, and safety ranging from nuisance level to critical. Here are some items that should always be part of your selection  and application consideration.

  • A control valve is not intended to be a an isolation valve and should not be used for isolating a process segment. Make sure you select the appropriate valve for the function to be performed.
  • Select materials of construction that will accommodate the media and the process conditions. Take into consideration the parts of the valve that come in contact with process media, such as the valve body, the seat and any other wetted parts. Operating pressure and temperature impact the materials selection for the control valve, too. Conditions surrounding the valve, the ambient atmosphere and specific local conditions that may expose the valve to corrosives should be included in your thinking.
  • Install flow sensors upstream of the control valve. Locating the flow sensor downstream of the control valve exposes it to an unstable flow stream which is caused by turbulent flow in the valve cavity.
  • Establish the degree of control you need for the process and make sure your valve is mechanically capable to perform at that level. Too much dead-band leads to hunting and poor control. Dead-band is roughly defined as the amount of control signal required to affect a change in valve position. It is caused by worn, or loosely fitted mechanical linkages, or as a function of the controller setting. It can also be effected by the tolerances from mechanical sensors, friction inherent in the the valve stems and seats, or from an undersized actuator.
  • Consider stiction. Wikipedia defines it as "the static friction that needs to be overcome to enable relative motion of stationary objects in contact". This can be particularly evident in valves that see limited or no position change. It typically is caused by the valves packing glands, seats or the pressure exerted against the disk or other trim parts. To overcome stiction, additional force needs to be applied by the actuator, which can lead to overshoot and poor control.
  • Tune your loop controller properly. A poorly tuned controller causes overshoot, undershoot and hunting. Make sure your proportional, integral, and derivative values are set.  This is quite easy today using controllers with advanced, precise auto-tuning features.
  • Avoid oversizing control valves. They are frequently sized larger than needed for the flow loop they control. If the control valve is too large, a small percentage of travel or position change could produce an unduly large change in flow, which in turn can make stable control difficult. Unstable control can result in excessive movement and wear on the valve. Try to size a control valve at about 70%-90% of travel.
  • Think about the type of control valve you are using and its inherent flow characteristic. Different types of valves, and their disks, have very different flow characteristics. The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position. Globe control valves have linear characteristics which are preferred, while butterfly and gate valves tend to have non-linear flow characteristics, which can cause control problems.  In order to create a linear flow characteristic through a non-linear control valve, manufacturers add specially designed disks or flow orifices which create a desired flow profile.
These are just a few of the more significant criteria to consider when selecting and applying a process control valve. Consider it good practice to discuss your selection and application with a product application expert to confirm your final selection. Combing your process knowledge with their application expertise will provide the best outcome.

Monday, June 22, 2015

Selecting the Right Valve Type - Gate Valves

There are many types of valves available for industrial fluid handling and process control applications. Specifying the proper valve type for an application can be made easier with some basic knowledge about the application strengths and weaknesses of the various valve types.

gate valve cutaway view
Gate Valve Cutaway View
Courtesy DHV Industries
Gate valves open and close by changing the position of a rectangular or round wedge (the gate) in the fluid flow path. The sealing surfaces are arranged in a planar fashion and the gate, which is commonly either flat or wedge shaped, slides along the sealing surface from the open to closed position. Because of the cross-sectional shape of pipes, which is often mimicked in the valve body, the size of the opening created as the gate valve opens and closes does not change at the same rate as the percentage of total available movement of the gate. This non-linear aspect of valve operation can make a gate valve less suitable for an application where flow rate must be accurately controlled across the range from fully open to fully closed. Complicating throttling operations further is the possibility of the gate vibrating when partially open, due to the fluid flow around the gate assembly. Unless specifically designed for throttling, gate valves are generally best suited for applications requiring either full flow or no flow. Because of its operating nature and construction, a gate valve may prove to be the appropriate selection, based upon the type of media or fluid which is being controlled.

In addition to specifying the manner in which the valve will be connected within the piping system, consideration should be given to construction of the valve body. If it may be necessary to inspect, service, or clean the valve interior, look for a bonnet connection that will permit suitable access to the valve interior.

Once you have decided that a gate valve will be the most suitable type for your application, there are many other considerations in valve selection. Draw on the experience and knowledge of coworkers, maintenance technicians, and valve sales engineers to help specify a valve assembly that meets the needs of all project stakeholders.

Monday, October 20, 2014

Selecting a Stainless Steel Globe or Gate Valve

stainless steel industrial valve
Stainless steel valves
(courtesy of Aloyco)
Stainless steel globe and gate valves are available in materials including CF8M, CF3M, and CN7M (Alloy 20) and in broad pressure classes including Class 150, Class 300, Class 600, and 200 CWP.

Stainless steel valves are required in many applications found in pulp and paper, mining, chemical processing,  food processing, waste water, and fertilizer processing.

Care should be taken to select the most suitable valves for your service(s). Exact specification of each valve should be made to avoid possible ambiguity. When requesting quotations and/or ordering the product a fully adequate description should be made.

Selecting the Valve Size
Nominal size of the pipeline into which the valve will be placed must be determined.

Valve Material
The following facts should be considered in determining the correct valve material:
  • the medium or media which will be controlled
  • the temperature range of the line medium (media)
  • the pressure range to which the valve will be subjected
  • possible atmospheric conditions which may affect the valve
  • possible extraordinary stresses to which the valve will be subjected
  • safety standards and/or piping codes which must be met