Tuesday, September 29, 2015

What Cavitation Looks Like, and How It Effects Your Valve

Cavitation is the formation of "bubbles" caused by rapid changes in pressure. It is a significant cause of wear metallic on surfaces in process piping, valves, pumps and instruments. As the rapid change in pressure takes place, the bubbles (voids in the liquid) collapse (implode), and expose the surrounding metal surfaces are repeatedly stressed by these implosions and their subsequent shock wave.

The consequences for a control valve as well as for the entire control process vary and are often destructive and may include:
  • Loud noise
  • Strong vibrations in the affected sections of the plant
  • Choked flow caused by vapor formation
  • Change of fluid properties
  • Erosion of valve components
  • Destruction of the control valve 
  •  Plant shutdown
This video provides a visual demonstration, through clear piping and valve housing, what actually happens inside the piping system.

Here is an excellent paper by Samson Controls on cavitation and its effects on control valves.

Wednesday, July 22, 2015

Valve Selection for Industrial Flow Control - Ball Valves

Industrial process control often involves fluid handling and the control of fluid flow. The array of different applications in this arena is very large, which has led to the development of a similarly wide variety of valve types, sizes, materials of construction, and other features. When your valve selection options become unwieldy, inserting a filter into your selection criteria can significantly reduce the candidates to a manageable number. One such filter is valve type.

Valves can be generally characterized, and are often referred to, by their closure means. Several common types are gate, ball, butterfly, and plug. Each has particular attributes which might make it more suitable or desirable for your particular application or project. Let’s look at ball valves.

ball valve cutaway view
Ball Valve Cutaway View
Courtesy Habonim USA
Ball valves have a spherical shaped part that is held in place directly in the fluid flow path within the valve body. Most ball valves are provided with elastomeric seals between the ball and the valve body. The ball has an opening through its midsection, called a port, through which process fluid media can flow when the opening is aligned with the inlet and outlet of the valve body. With the ball connected to a stem that extends to the exterior of the valve body, control of the position of the port is provided as the stem is rotated through a 90 degree arc. Aligning the port parallel to the direction of flow provides a fully open condition, while rotating the ball and positioning the port perpendicular to the flow path will completely obstruct the flow. This is the simplest form of ball valve operation. Variants on this arrangement are numerous, providing multiple ports and flow diversion, but the selection criteria for the valve type likely still apply.

What are some of the potential application advantages of ball valves?

  • This valve type provides leak proof service.
  • With only 90 degrees of rotational movement from the open to close position, rapid operation times are possible.
  • Ball valves are comparatively compact, without long stem extensions needed for some other valve types.
  • Moderate operating torque requirements allow for lower powered actuators.
  • Valves are available in a wide variety of body and seal construction materials, enabling their possible application with a wide array of media and temperatures.
  • The port opening can be “full port”, providing an unrestricted fluid flow path in the fully open position.
  • Low maintenance requirements, no lubrication needed.

Here are some characteristics that may have an impact on your selection.
trunnion mount industrial ball valve
Large Industrial Trunnion Mount Ball Valve
Courtesy Habonim USA

  • There is a possibility of residual fluid being trapped in the valve port when it closes. Residue will be released to the flow path when the valve is reopened. If the trapped media may contaminate the restarted flow, this may be cause for concern.
  • Applications requiring throttling of the media flow may not be handled well by this valve type. A partially open position also exposes the seals to potentially deteriorating effects of the media and its flow velocity.
  • Verify that the range of available seal materials for this valve type will accommodate whatever media and operating temperature is contemplated.

A proper valve selection will provide the performance that is desired for any project, but this requires thorough knowledge of the media, operating conditions, and valve performance and application limitations. You know your process. Bring in a valve application specialist to help you match the best valve type and arrangement with your application requirements.