Wednesday, January 27, 2016

Cavitation Induced by a Control Valve - Demonstrated



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.

Here is where cavitation in control valves 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.

Tuesday, January 19, 2016

Knowledge Base and Selection Guide For Magnetic Level Indicators

Orion Instruments Magnetic Level Indicators
Magnetic Level Indicators
Courtesy Orion Instruments
Industrial process control frequently involves the storage of liquid in vessels or tanks. Continuous and accurate indication of liquid level within the tank is an essential data point for safety and process management. While there are a number of methods and instrument types utilized to provide tank level measurement, the instrument of choice is often a magnetic level indicator, also referred to as a magnetic level gauge. Its use for providing level indication has a number of positive attributes:
  • Construction that is resistant to breakage.
  • Measuring indicators, switches, and transmitters mounted externally, without contacting the medium being measured.
  • Maintenance free operation. No regular cleaning needed.
  • Readable level indication from greater distance than glass sight gauges.
  • Magnetic level indicators can accommodate greater fluid level ranges without the need for multiple instruments.
Orion Instruments, a Magnetrol company and industry leader, has produced a comprehensive guide to magnetic level gauges, switches, transmitters, and related products. It delivers experts and newcomers an understandable and clear description of the technology and principals of operation behind magnetic level gauges and instruments. The guide also assists the reader in properly specifying and selecting the best instrument configuration for an application. A table of contents at the front of the document helps readers to quickly find the information they need.

Take a couple minutes to roll through the document and you are likely to find new and useful application tips and product information. Any questions about magnetic level indicators or your process measurement and control applications can be clearly addressed by a product specialist.



Thursday, January 14, 2016

Filtration Yields Returns By Protecting Fluid Process Lines and Equipment

Dual basket strainer with changeover valve
Duplex Basket Strainer With Diverter Valve
Courtesy Eaton Filtration
Most people think of "industrial" equipment as super heavy duty, virtually indestructible. Those of us responsible for operating and maintaining industrial process equipment recognize that is not the case. Even the most formidable appearing equipment can be crippled if not protected from the insidious effects of

Particulates


There are numerous strategies for mediating the impact of particulates on industrial fluid process equipment and systems. The best solutions will be customized for each process, with consideration given to:
  • Maximum particle diameter threshold: At some level, particulates may be small enough to preclude damage to the system. Above the threshold level, removal of the particles brings some benefit to process operation.
  • Pressure drop associated with any mitigation techniques: Assuming that mitigation will involve the addition of components to the fluid system, minimizing the added pressure drop is advantageous.
  • Overall volume of particulate matter to be removed: Most often, mitigation equipment traps and retains particulate matter. The retaining capacity of the unit must match the particulate production rate of the process. Be mindful that certain events, such as routine maintenance or cleaning of process equipment, may produce surges of particulates in some types of systems.
  • Location of the filtration equipment: Filtration units must be placed in the process flow upstream of the equipment or system portion to be protected. An additional consideration is a provision for maintenance through placement in a convenient, easily accessible location.
  • Filtration equipment materials of construction: The filtration gear must be fabricated of materials compatible with the process media.
I have provided a data sheet below with cutaway illustrations and detailed performance data for one type of filtration unit. This particular equipment is manufactured by the filtration division of Eaton and features a duplex strainer basket arrangement with a diverter valve. The process fluid flows through one strainer, with the other clean and ready to be brought on line when the active basket becomes clogged. When the active basket becomes clogged and pressure drop excessive, the operator moves a lever to divert the flow to the second basket, sealing off the now clogged basket area so that it can be opened and cleaned. This design provides for uninterrupted process operation.

Browse the provided data sheet. You will likely pick up something you did not already know, or get a quick refresh of your technical knowledge. The duplex basket strainer is one type in a wide variety of filtration products available for every conceivable process application. Share your challenges with a product specialist. Combining your process knowledge and experience with their thorough product application expertise will generate great solutions.



Wednesday, January 6, 2016

Fluid Flow Control - Slurries, Entrained and Suspended Solids

Slurry, suspended solids in fluid, toxic fluid
Industrial process control can be confronted with
hazardous, corrosive, or other fluids containing
suspended solids.
Industrial process control can involve the manufacture, storage, or transport of almost any imaginable fluid. Media can range from water to concrete, hydrogen gas to steam, and anything in between or outside of those boundaries. Valves are the favored control device for regulating fluid flow and they are available in uncountable varieties, each with particular aspects making them more of less suitable for a particular media or application.

Most industrial valves consist of a body, a stem, and some form of flow obstruction which is located within the media flow path. Operation of the stem repositions the obstruction to allow or block the flow. All of these valve types have a defined sealing surface where the obstruction contacts the body. They also have additional seals where the stem penetrates the body. These design features, while providing certain functions and application advantages, also add to the operational complexity and parts count for the valve.

There is a valve type with a simple operating principle that provides superior performance when the application involves certain media characteristics. It is called a pinch valve, and here is where it excels.

  • Resistance to abrasion and corrosion from slurries or fluids containing suspended solids and the ability to provide tight shutoff around particulates
  • Media and environmental temperature range -40 deg F to +300 deg F
  • Low to moderate operating pressure
  • Flow regulating capability and tight shutoff
  • Non clogging
  • Straight through full bore design with minimal flow resistance
  • Isolation of the valve body and workings from the media
  • Low parts count, low maintenance, easy repair/replacement
cutaway view of manually operated industrial pinch valve
Cutaway view of manually
operated industrial pinch valve
A pinch valve consists of a sleeve, through which fluid flows, and a means to compress or "pinch" the sleeve to reduce the open area inside the sleeve. The sleeves are most often fabricated from elastomers with various types of fiber reinforcement. Closure is commonly achieved through movement of one or two bars to squeeze the sleeve, providing throttling or positive closure. The flexibility of the sleeve material allows for tight shutoff, even with fluids containing suspended solids. The valves can be coupled with electric or pneumatic actuators and are available with industry standard connections. One valve variant has a body that can be pressurized to close the sleeve, without the need for a separate actuator. Pinch valves are available with and without an enclosing body.

You should be familiar with the capabilities and forms of this unique valve type. When confronted with certain application challenges, a pinch valve can be a superior solution. I included a product line data sheet from one manufacturer, General Rubber Corporation, so you can see all the different variants that are available. You can get even more information, or start a conversation about any of your process control challenges, by contacting a product specialist.