Wednesday, February 24, 2016

Pressure Relief Valves

Brass safety valve
Brass Safety Valve - One of many types of safety valves
Danger and hazards are an integral part of industrial processes. The mitigation of these dangers and hazards, as well as reducing the probability of their occurrence, is the primary charge of industrial process engineering. Every product intended for use in a process control setting has safety and protection included in its design criteria. Pressure relief valves fall in that category of products designed and intended solely for safety purposes.

Manufacturers of what most generally refer to as pressure relief valves break the genre down into two distinct groups, relief valves and safety valves. One manufacturer, Anderson Greenwood (a Pentair brand), distinguishes the two valve types in their "Pentair Pressure Relief Valve Engineering Handbook"...
Relief Valve: A pressure relief valve characterized by gradual opening or closing generally proportional to the increase or decrease in pressure. It is normally used for incompressible fluids.
Safety Valve: A pressure relief valve characterized by rapid opening or closing and normally used to relieve compressible fluids.
The difference between the two valve types is found in their response to an excessive pressure condition. The relief valve, according to the definition, responds proportionally to the pressure increase, whereas the safety valve provides a non-proportional rapid response. Note also that the relief valve is generally intended for use with liquids (incompressible) and safety valves are commonly applied to compressible fluids, which would include steam and air.

Pressure relief valves are found anywhere pressure is contained, be it a piping system, vessel, even a household pressure cooker. The purpose of the relief or safety valve is to protect a pressurized system or vessel, should the system pressure exceed the maximum allowable working pressure. Simply put, keep it from breaking apart.

Because of the potentially catastrophic nature of a pressurized system failure, there is a high level of scrutiny, regulation, and testing focused on pressure relief and safety valves. The proper sizing and selection of the valves is also critical to providing proper function.

More detailed information, product data, and application assistance is available from the specialists at CTI Controltech.

Tuesday, February 16, 2016

Control Valve Cavitation - A Demonstration and Some Considerations

Cavitation in process fluid exhibits as bubbles
Cavitation can be damaging to process components.
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.

Tuesday, February 9, 2016

ISA Offers Free White Paper on Cybersecurity for Industrial Control Systems

cybersecurity for industrial control systems
Industrial control system pose special
cybersecurity challenges.
The International Society of Automation is offering a free white paper entitled “What Executives Need to Know About Industrial Control Systems Cybersecurity”. The article provides useful commentary and information that establishes the scope of cybersecurity in the industrial process control space and provides a basic framework for understanding how every process may be impacted by lax cybersecurity efforts. The author, Joseph Weiss, differentiates Industrial Control System (ICS) cybersecurity from that of organizational IT through a review of various attributes common to both types, including message confidentiality, integrity, time criticality, and more. Any reader’s awareness and understanding of the cybersecurity risks to their operation will be enhanced through this article. I finished reading the article wanting more on the subject, and ISA is certainly a resource for additional content.

A quote from the article...
“Cyber incidents have been defined by the US National Institute of Standards and Technology (NIST) as occurrences that jeopardize the confidentiality, integrity, or availability (CIA) of an information system.”
ICS cybersecurity extends beyond preventing malicious outside intruders from gaining access. It is an important part of maintaining the overall operating integrity of industrial processes. A holistic approach is advocated to identify physical risk factors to the process and its componentry (previous post on device protection), as well as vulnerabilities that may prevent exploitation by unauthorized parties. Weiss goes on to describe the role and qualifications of the ICS Cybersecurity Expert, essentially an individual that can function effectively as an IT cybersecurity tech with the added skills of an industrial control systems expert.

A synopsis of attack events is provided in the article, with the author’s conclusion that not enough is being done to secure industrial control systems and the risk exposure is substantial in terms of potential threats to personnel, environment, and economy. By providing your name and email address, you can obtain the white paper from the ISA website. Your time spent obtaining and reading the article will be well spent.

For any specific information or recommendations regarding our products and cybersecurity, do not hesitate to contact us directly. We welcome any opportunity to help our customers meet their process control challenges.

Understand These Areas of Process Instrument Protection

process measurement instruments
Properly protect process measurement instrumentation
to prevent mishap 
The performance of every process is critical to something or someone. Keeping a process operating within specification requires measurement, and it requires some element of control. The devices we use to measure process variables, while necessary and critical in their own right, are also a possible source of failure for the process itself. Lose the output of your process instrumentation and you can incur substantial consequences ranging from minor to near catastrophic.

Just as your PLC or other master control system emulates decision patterns regarding the process, the measurement instrumentation functions as the sensory input array to that decision making device. Careful consideration when designing the instrumentation layout, as well as reviewing these five common sense recommendations will help you avoid instrument and process downtime.

Process generated extremes can make your device fail.

Search and plan for potential vibration, shock, temperature, pressure, or other excursions from the normal operating range that might result from normal or unexpected operation of the process equipment. Develop knowledge about what the possible process conditions might be, given the capabilities of the installed process machinery. Consult with instrument vendors about protective devices that can be installed to provide additional layers of protection for valuable instruments. Often, the protective devices are simple and relatively inexpensive.

Don't forget about the weather.

Certainly, if you have any part of the process installed outdoors, you need to be familiar with the range of possible weather conditions. Weather data is available for almost anywhere in the world, certainly in the developed world. Find out what the most extreme conditions have been at the installation site....ever. Planning and designing for improbable conditions, even adding a little headroom, can keep your process up when others may be down.

Keep in mind, also, that outdoor conditions can impact indoor conditions in buildings without climate control systems that maintain a steady state. This can be especially important when considering moisture content of the indoor air and potential for condensate to accumulate on instrument housings and electrical components. Extreme conditions of condensing atmospheric moisture can produce dripping water.

Know the security exposure of your devices.

With the prevalence of networked devices, consideration of who might commit acts of malice against the process or its stakeholders, and how they might go about it, should be an element of all project designs. A real or virtual intruder's ability to impact process operation through its measuring devices should be well understood. With that understanding, barriers can be put in place to detect or prevent any occurrences.

Physical contact hazards

Strike a balance between convenience and safety for measurement instrumentation. Access for calibration, maintenance, or observation are needed, but avoiding placement of devices in areas of human traffic can deliver good returns by reducing the probability of damage to the instruments. Everybody is trained, everybody is careful, but uncontrolled carts, dropped tools and boxes, and a host of other unexpected mishaps do happen from time to time, with the power to inject disorder into your world. Consider guards and physical barriers as additional layers of insurance.

Know moisture.

Electronics must be protected from harmful effects of moisture. Where there is air, there is usually moisture. Certain conditions related to weather or process operation may result in moisture laden air that can enter device enclosures. Guarding against the formation of condensate on electronics, and providing for the automatic discharge of any accumulated liquid is essential to avoiding failure. Many instrument enclosures are provided with a means to discharge moisture. Make sure installation instructions are followed and alterations are not made that inadvertently disable these functions.

Developing a thoughtful installation plan, along with reasonable maintenance, will result in an industrial process that is hardened against a long list of potential malfunctions. Discuss your application concerns with your instrument sales engineer. Their exposure to many different installations and applications, combined with your knowledge of the process and local conditions, will produce a positive outcome.

Monday, February 1, 2016

Valves Used For Oxygen and High Purity Service Get Special Handling And Treatment

Industrial valve for oxygen or high purity service
Industrial valves can be specially cleaned, handled,
and packaged for oxygen or high purity operations
Courtesy Mogas Industries
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.