Showing posts with label control valves. Show all posts
Showing posts with label control valves. Show all posts

Preparation of Control Valves For Oxygen or High Purity Service

Specialty industrial control valves
Many valves can be specially prepared for high purity or oxygen service
Oxygen is used extensively throughout a wide range of industrial processes. Medical, deep-sea, metal cutting, welding, and metal hardening are a few examples. The steel industry uses oxygen to increase capacity and efficiency in furnaces. As a synthesis gas, oxygen is also used in the production of gasoline, methanol and ammonia.

Odorless and colorless, oxygen is concentrated in atmospheric air at approximately 21%. While O2, by itself, is non-flammable, it vigorously supports combustion of other materials. Allowing oils or greases to contact high concentrations of oxygen can result in ignition and possibly explosion. Oxygen service preparation of an industrial valve calls for special cleaning processes or steps that remove all traces of oils and other contaminants from the valve to prepare for safe use with oxygen (O2). Aside from the reactive concerns surrounding oxygen, O2 preparation is also used for applications where high purity must be maintained and valves must be free of contaminants.

Gaseous oxygen is noncorrosive and may be used with a variety of metals. Stainless steel, bronze and brass are common. Liquid oxygen presents unique challenges due to cryogenic temperatures. In this case, valve bodies, stems, seals and packing must be carefully chosen.

Various types of valves are available for oxygen service, along with a wide array of connections, including screwed, socket weld, ANSI Class 150 and ANSI Class 300, DIN PN16 and DIN PN40 flanged ends. Body materials include 316 stainless steel, monel, bronze and brass. Ball and stem material is often 316 stainless steel or brass. PTFE or glass filled PTFE are inert in oxygen, serving as a common seat and seal material employed for O2 service.

Common procedures for O2 service are to carefully deburr metal parts, then meticulously clean to remove all traces of oil, grease and hydrocarbons before assembly. Valve assembly is performed in a clean area using special gloves to assure no grease or dust contaminates the valve. Lubricants compatible with oxygen must be used. Seating and leakage pressure tests are conducted in the clean area, using grease free nitrogen. Specially cleaned tools are used throughout the process. Once assembled, the valves are tested and left in the open position. A silicone desiccant pack is usually inserted in the open valve port, then the valve ports are capped. A warning label about the desiccant pack's location is included, with a second tag indicating the valve has been specially prepared for oxygen service. Finally, valves are individually sealed in polyethylene bags for shipment and storage. Different manufacturers may follow slightly differing protocols, but the basics are the same. The valve must be delivered scrupulously contaminant free.

The O2 preparation of valves is one of many special production variants available to accommodate your special application requirements. Share your valve requirements and challenges with a valve specialist to get the best solution recommendations.

Cavitation - Scourge of Industrial Process Control Valves Everywhere

Cavitation produces vapor bubbles in liquids
Cavitation produces bubbles in flowing process liquids
Consider a generic industrial fluid process control operation. There are pumps, valves, and other components installed in the process lines that, due to their interior shape or their function, cause changes in the fluid motion. Let's look specifically at control valves and how their throttling operation can create conditions able to greatly impact the valve itself, as well as the overall process.

Fluid traversing a control valve can undergo an increase in velocity when passing the constriction presented by the valve trim. Exiting the trim, fluid then enters the widening area of the valve body immediately downstream with a decrease in velocity. This change in velocity corresponds to a change in the kinetic energy of the fluid molecules. In order that energy be conserved in a moving fluid stream, any increase in kinetic energy due to increased velocity will be accompanied by a complementary decrease in potential energy, usually in the form of fluid pressure. This means the fluid pressure will fall at the point of maximum constriction in the valve (the vena contracta, at the point where the trim throttles the flow) and rise again (or recover) downstream of the trim.

This is where cavitation begins.

If the fluid being throttled is a liquid, and the pressure at the vena contracta is less than the vapor pressure of the liquid at the flowing temperature, portions of the liquid will spontaneously vaporize. This is the phenomenon of flashing. If, subsequently, the pressure of the fluid recovers to a level greater than the vapor pressure of the liquid, any flashed vapor will rapidly condense, returning to liquid. This collapse of entrained vapor is called cavitation.

Flashing, the generation of vapor bubbles within the liquid, will precede and set the stage for cavitation. When the flashed vapor bubbles condense to liquid they often do so asymmetrically, with one side of the bubble collapsing before the rest of the bubble. This has the effect of translating the kinetic energy of the bubble’s collapse into a high-speed “jet” of liquid in the direction of the asymmetrical collapse. These liquid “microjets” have been experimentally measured at speeds up to 100 meters per second (over 320 feet per second). What is more, the pressure applied to the surface of control valve components in the path of these microjets can be intense. An individual microjet can impact the valve interior surfaces in a very focused manner, delivering a theoretical pressure pulse of up to 1500 newtons per square millimeter (1.5 giga-pascals, or about 220000 PSI) in water. In an operating fluid system, this process can be continuous, and is known to be a significant cause of erosive wear on metallic 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 the surrounding metal surfaces are repeatedly stressed by these implosions and their subsequent shock waves.

Consequences for control valves, as well as for the entire control process, vary and are often destructive. They may include:
  • Loud noise
  • Strong vibrations in the affected sections of the fluid system
  • Choked flow caused by vapor formation
  • Change of fluid properties
  • Erosion of valve components
  • Premature destruction or failure of the control valve 
  • Plant shutdown
The video provides a visual demonstration, through clear piping, of what happens inside the piping system when a valve is operated in a manner that causes substantial cavitation.

The solution lies in minimizing the potential for cavitation to occur through proper valve selection and sizing, along with coordinating operating characteristics of pressure drop inducing components with the total system performance. One valve manufacturer's recommendations are summed up in four basic approaches.
  • Avoidance of cavitation through proper valve selection. Use a valve with a rated liquid pressure recovery factor greater than that required for the application. Some applications may be suitable for the use of an orifice plate downstream of the valve.
  • Cavitation Tolerant Components capable of withstanding limited amounts of cavitation without excessive wear. Increased flow noise is likely to accompany this route.
  • Prevention of cavitation through the use of valve trim design that reduces pressure in several steps, avoiding excessive flashing. These valves can be expensive, but their effectiveness makes them an alternative worth considering.
  • Containment of the harmful effects of limited to moderate cavitation through trim designs that eliminate contact of the fluid with metal surfaces which are more susceptible to damage.
Share your requirements and application challenges with a valve specialist and gain insight through their recommendations. Combining your process knowledge with their product application expertise will yield a great solution.


Know Your Control Valve Basics?

Industrial Control Valve Cutaway View
Courtesy Cashco
Understanding basic operation and function of control valves, an integral part of many industrial process control loops, is essential for the process engineer, operator, or other stakeholder. This presentation outlines control valve operation, major components, and terminology used to describe valve parts, functions, and principles of operation. A useful reference for stakeholders in need of a refresher course in order to understand what the engineers are saying, it also provides detailed illustrations, charts, and description that will prove valuable to the more technical minded.


What you will find:


  • Terminology: A glossary of terms commonly used in the control valve world.
  • Control Valve Basic Designs: Control valve classifications, cutaway illustrations showing the operating structure of different valve types, comparisons of varying valve designs.
  • Characterization and Trim Design: Flow characteristic curves and comparisons for different valve types, showing how flow responds to valve position change.
  • Control Valve Technical Considerations: FTC vs FTO, illustrations showing valve operation.
  • Force-Balance Principle: Illustration and formula explanations of this basic operating principle.
  • Actuator Basic Designs: Illustrations showing the differing arrangements for actuator operation.
  • Control Valve Unit Action: Illustrations, diagrams, and explanations of a range of valve operating conditions, including loss of electrical power and loss of instrument air supply.
  • Actuator Benchset Range: Shows practical relationship between instrument air pressure and valve ability to properly operate at various pressure conditions.
  • Valve Positioner Basics: Definition of valve positioning, reasons to use a positioner, schematic illustrations of control loops.
  • Control Loop Action: Charts and provides examples of 16 combinations of Process, Controller, Positioner, and Control Valve combinations.
  • Control Valve Packing Designs: Describes and defines packing, common problems, current state of the art. Cutaway illustrations of various packing arrangements.
  • Seat Leakage: Classifications, comparisons of different materials.


There is something of value in the document for everyone, and you will undoubtedly pick up something useful. Thanks go out to the engineers at Cashco for putting this together. You can discuss any aspect of your control valve applications with a product specialist. Your contact is always welcome.




Valve Selection - When to Choose a Butterfly Valve

Industrial process control valves are available in a staggering array of materials, types, and configurations. An initial step of the selection procedure for a valve application should be choosing the valve type, thus narrowing the selection field to a more manageable level. Valve "types" are generally defined by the closing mechanism of the valve.

butterfly valve
Butterfly Valve
Courtesy Crane CPE
A butterfly valve has a disc that is positioned in the fluid flow path. It rotates around a central axis, the stem, through a 90 degree arc from a position parallel to the flow direction (open) to perpendicular (closed). A variety of materials are used in the valve body construction, and it is common to line the valve with another material to provide special properties related to the process media.

What might make a butterfly valve a beneficial selection over another valve type?

  • The closure arrangement allows for a comparatively small size and weight. This can reduce the cost, space, and support requirements for the valve assembly.
  • Generally low torque requirements for valve operation allow for manual operation, or automation with an array of electric, pneumatic, or hydraulic actuators.
  • Low pressure drop associated with the closure mechanism. The disc in the flow path is generally thin. In the fully open position, the disc presents its narrow edge to the direction of flow.
  • Quarter turn operation allows for fast valve operation from fully closed to fully open.
  • Some throttling capability is provided at partially open positions.
  • Small parts count, low maintenance requirements.
What may be some reasons to consider other valve types?
Butterfly Valve
Courtesy Crane CPE
  • Butterfly valve throttling capability is generally limited to low pressure drop applications
  • Cavitation can be a concern.
  • Some sources mention the possibility of choked flow as a concern under certain conditions.
Butterfly valves, like other valve types, have applications where they outperform. Careful consideration and consultation with a valve expert is a first step toward making a good selection.


Gate Valve: A Good Choice for Your Application?

Automatically operated gate valve
Gate Valve With Actuator
Courtesy Orbinox
Fluid flow control is an essential component of many industrial applications. Because of its prevalence, and the variety of applications, there are many types, sizes, and arrangements of flow control valves available to meet practically any need. The challenge for the specifying engineer is to select the valve type and arrangement that will provide the needed performance, while also fulfilling the need for safe performance and a desire for low maintenance burden. Sorting through the wide array of valves and targeting the correct valve technology or type can quickly narrow your focus to a much smaller circle of products to research and consider.

Industrial gate valves, like all other valves, regulate fluid flow by reducing or expanding the area through which the process fluid must pass in a closed system. It is the manner in which that restricting area is changed that serves the major discriminating factor among the different valve types. In the case of a gate valve, a sliding round or rectangular piece, known as the gate or disc, is moved by a mechanism and transects the fluid flow path. Closing the gate, completely transecting the flow path, will restrict the flow to its fullest. As the gate is retracted and the opening size increases, flow is increasingly enabled. The movement of the gate, along with valve body and mechanism construction, give this valve type an array of positive and negative attributes.

High Marks for Gate Valves:

  • Low resistance to fluid flow when the valve is completely open. Generally, the cross sectional characteristics of a gate valve will mimic those of the connected piping system. Additionally, gate valves do not impose any change in the flow direction of the fluid as it passes through the valve body.
  • Low force and energy requirements are needed to change the valve opening position (the position of the gate). Since gate movement is perpendicular to the direction of flow, it is not necessary for the mechanism to counteract the full pressure drop of the fluid in the system.
  • Gate valves can be bi-directional, controlling flow in systems that may incorporate a reversal of the flow direction.
  • The installed gate valve is shorter in length than most other designs.
  • Gate valves employ a slow closure rate. The accompanying slow reduction in fluid shutoff can inhibit physical shock (hammering) in the connected piping system.

manually operated gate valve
Manually Operated Gate Valve
Courtesy Orbinox
Things that appear as positives in favor of gate valve selection on one application may not be as desirable on another.

Gate Valve Potential Negatives:

  • Valve seals are exposed to the fluid flow when the valve is open. This might make the seals vulnerable to the wearing effects of entrained foreign matter or other components of the process fluid. The end result could potentially be prematurely worn sealing surfaces and a failure of the valve to seal properly.
  • Gate valves are generally slow to open and close. This attribute might make them a poor selection for an application requiring rapid or immediate action.
  • The gate valve will require an extended overhead service area, compared to other valve types. This may have an undesirable impact on locating the valve where desired.
  • Fluid flow control applications that require throttling of the flow are generally not good candidates for a gate valve. Fluid flow through a partially open gate valve may cause the closure mechanism to vibrate. Additionally, there are concerns associated with potential erosion of the gate and seals due to increasing fluid velocities when the valve is partially open

These, and other, very basic considerations may help point your product search in the right direction. One additional recommendation is that you contact an experienced valve specialist and take advantage of their knowledge and experience fulfilling other applications similar to yours.

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