If you perform a Google search on the words “critical service valves,” you quickly discover that the number of hits is endless. The term is one of the most over-used phrases in the valve industry today, with “severe service” running a close second. Everyone seems to use the terminology in describing their valves. No doubt there is someone out there selling a “critical service” 1/4-inch 200 water, oil, gas bronze gate valve or a “critical service” 3/4-inch hose bib.
If you work in a plant surrounded by valves and piping, you probably consider the service that equipment lends to be critical. That may be true to you and your plant, but there are a number of valve applications and service environments deemed critical by end users, governmental agencies and standards development organizations.
Still, defining critical service applications is a difficult task, especially for isolation-type valves. However, in the field of control valves, a bit more clarity and agreement exists. For example, the following are conditions considered critical service applications by some control valve manufacturers:
- The flowing media may harm either persons or the environment.
- The potential for cavitation exists (water service).
- There is a high vibration or noise (steam service).
- Tight shutoff is required (ANSI/FCI 70.2, class V or VI).
- The valves are for flashing service.
- High pressures exist (ANSI class 300 and above).
- There are pressure drops of over 650 psi.
Meanwhile, with non-control valves, the American Society of Mechanical Engineers (ASME) provides a definition of critical service in its B31.3 Process Piping Code. The term used is “Category M,” defined as “a fluid service in which the potential for personal exposure is judged to be significant and in which a single exposure to a very small quantity of toxic fluid, caused by leakage, could produce irreversible harm to persons on breathing or bodily contact, even when prompt restorative measures are taken.”
Many individual petrochemical and process plants have their own company-wide definitions for critical service application criteria. One global refining company, for example, classifies as critical service, applications in which the following will be included:
- Toxic materials such as phenol, hydrogen sulfide and chlorine
- Highly corrosive materials such as acids and caustics
- Flammable materials, including light hydrocarbons
- Boiler feedwater and steam requiring class 300 ratings and higher
- Oxygen in concentrations greater than 35%
IT’S IN THE FLUID
Many fairly common process applications are considered critical because of the fluids involved. At the very top of the list are valves in nuclear applications. Some well-known services are:
|Hydrogen Sulfide||Emergency shutdown|
Nuclear applications, particularly those involved in primary cooling or isolation service, are especially critical because if these valves fail, the result can be catastrophic. It is why the extent of quality control and assurance procedures and paperwork is exhaustive for this field. It’s also why the standards and codes that govern the construction of this equipment are extensive and without equal. It’s why manufacturing valves for nuclear service requires that the manufacturer earn and maintain an “N” stamp in accordance with ASME requirements.
It’s also why requirements for castings in nuclear service are precise, requiring the highest quality levels, which in turn is why many times it is cheaper to manufacture valves from forged material rather than going through initial casting creation and casting repair.
Hydrogen and Hydrogen Sulfide
Hydrogen is a popular raw material for key refining processes, including several focused on producing low-sulphur fuels. Popular refining processes such as hydrocracking, hydroprocessing and hydrotreating all require a steady supply of hydrogen. Other refinery processes produce hydrogen as a by-product.
Meanwhile, two parameters in refinery processes are high temperature and pressure, which call for the utmost scrutiny. When hydrogen is the process fluid, the concern for safety and integrity is great. This is because a failure mode known as hydrogen attack can occur when carbon and alloy steels are exposed to hydrogen under high pressure or temperature. Hydrogen enters the steel and reacts with the carbon to form methane (CH4), which can result in the formation of cracks and fissures when the operating temperature is above 392°F (200°C).
Another failure mode associated with hydrogen service occurs predominately in low-strength alloys when atomic hydrogen diffuses into internal defects such as voids or laminations. This hydrogen then precipitates as molecular hydrogen. The internal pressure created by this phenomenon is so strong that if it occurs near the metal’s surface, blisters are created in the metal.
Probably the most insidious form of refinery hydrogen damage, however, is called hydrogen embrittlement. This generally occurs at near-ambient temperatures when the hardness of the metal is 22 HRC or greater. It can cause a brittle fracture of normally ductile steels under a sustained load in the presence of hydrogen at levels of less than 100 parts per million (ppm).
Because of the hydrogen permeability of steel, it is important that every step be taken to ensure the base material of a valve is of the highest homogeneity. This is why high-quality, near-defect-free castings are desired. It is also why forged materials are highly valued, since their structure is much more compact, dense and free of internal defects.
Hydrogen’s evil cousin, hydrogen sulfide (H2S), is one of the more lethal fluids handled in the oil and gas industry. Also called sour gas, the highly toxic material is often found in crude oil and gas production. H2S is inherent in some raw petroleum products until it is removed in the early stages of the refining process. In levels above 100 ppm, sour gas is hazardous to humans. What’s more, the unmistakable low-concentration, “rotten egg” odor of sour gas disappears at high concentration, which makes it undetectable and always lethal.
The other, very bad attribute of H2S is its extremely high corrosivity. H2S material recommendations to prevent corrosion are covered in two NACE International material standards: MR0175, Sulfide Stress Corrosion Resistant Metallic Materials for Oilfield Equipment, and MR0103, Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments.
Chlorine (Chlor-Alkali) and Oxygen
If you have a swimming pool, you know what it’s like to open a container of chlorine powder or pellets: You can sense this is something bad to breathe. Your sense of smell is accurate: Chlorine in concentrations of as little as 35 ppm can be lethal. In addition to the health hazards of breathing in this chemical, it can also be very corrosive. Valves and piping systems in chlorine service need to be cleaned and dried to ensure they are free of all grease, oil or other materials that can react adversely with the chlorine. Because of the unique requirements of many chlorine process systems, valves are often specified to be designed especially for chlorine service.
Meanwhile, while oxygen is non-flammable by itself, it vigorously supports the combustion process, a process that can be intense enough to melt metals. All it takes is a miniscule drop of oil in an oxygen system combined with a spark to cause a catastrophic ignition. Like valves and piping for chlorine service, valves and piping for oxygen service must be thoroughly cleaned and free of all oils, grease and other contaminants.
Guidance for chlorine valves and piping can be found in Chlorine Institute Pamphlet 6, Piping Systems for Dry Chlorine, while guidance for cleaning valves for oxygen service is found in Compressed Gas Association and National Fire Protection Association literature.
Highly Corrosive Media
Highly corrosive media are primarily acids and some caustics. The choice of materials to handle such media is broad—usually more than one metal or non-metallic material will work. When choosing materials to handle such service, it is important that the concentration of the corrosive media, the temperature and velocity of the flow and the pressure all be taken into account. Quite often, the valve trim material is a key consideration because it will face the highest velocity or the most challenging component stress for the valve, along with the corrosivity of the fluid.
Emergency Shutdown or Isolation
Just the description “emergency” reflects the fact these valves are in critical service applications. They usually are the first and sometimes last line of defense, and they have to work when required. Emergency shutdown (ESV) or emergency isolation (EIV) valves are found in many demanding applications. EIVs are also required at intervals in most pipelines containing flammable or hazardous fluids, and are required on each river bank where a pipeline crosses a river.
Flammable fluids include hydrocarbons such as oil, gasoline and natural gas. Usually the major concern with valves in this service is preventing unwanted leakage past the valve closure member, although pressure boundary integrity and packing leakage are important as well. With flammable fluids, additives and unwanted trace materials sometimes create corrosion issues. These are addressed in choosing the materials of construction. The need for better control of this broad range of materials has traditionally led the way in creating new valve designs, such as the triple-offset butterfly valve.
CRITICAL SERVICE VALVE DESIGN
Soft-seated ball valves made their entrance on the valve scene in the early 1960s. They are ideal choices for zero leakage. However, the soft elastomers that make them seal so well also limit their maximum service temperature. Additionally, soft-seated valve closure members are easily damaged. To alleviate the drawbacks of the soft-seated ball valve, the metal-seated ball valve was developed in the late 1960s.
The key to metal-seated ball valve success is twofold: 1) precision lapping techniques for precisely mating the ball with its seats, and 2) the development of ultra-hard and abrasion-resistant ball and seat coatings. These two areas of design advancement enabled the ball valve to perform feats of closure not successfully attained before. This success allowed the metal-seated ball valve to become the severe service valve of choice for many applications.
Another valve design that found great favor in many critical service applications is the triple-offset butterfly valve. Unlike non-offset butterfly valves, the triple offset design relies on torque to seal tightly, instead of disc position. Many triple-offset valves can attain a very high degree of closure tightness.
The triple-offset is used more and more in critical applications such as the light-end, non-lubricated services in oil refineries. What’s more, their nearly friction-free metal or composite seating components enable the valves to be used for high-temperature applications. Both the metal-seated ball valve and the triple-offset butterfly valve have replaced gate valves in many process plant applications.
The bellows-seal valve is a design that has been around since the 1950s when it was used in early nuclear applications. The flexible bellows is welded to the valve stem on one end and the valve body on the other, providing a hermetically sealed valve interior. When combined with a welded bonnet and butt-weld or socket-weld ends, the valve is virtually guaranteed to be leak-free and ideal for controlling dangerous fluids.
Another critical service valve design is the internally or externally refractory-lined slide gate valve used in flue gas or catalyst service. These valves usually operate at low pressures but temperatures up to 1400°F (760°C). Extremely fine closure adjustment and regulation is usually required, and these valves often employ exotic actuation systems to precisely adjust the flow rate.
The coking industry has relied on specific valve designs for critical and severe service requirements for many years. Unique coker de-heading valves are used on the bottom of the coke drum, for example, while highly engineered ball or plug designs are used to direct product into the coke drums from above.
In addition to specially designed valve solutions, flow control of critical fluids has incorporated the use of a range of special valve materials from stainless steels such as 347H to high-nickel super-alloys.
IT’S NOT SQUARE TO BE HIP
High-integrity protection systems (HIPS) are unified control systems designed to prevent over-pressurization in a process plant. Traditionally, over-pressure situations were handled in a reactive manner by those silent sentinels—pressure relief valves (PRVs). The goal of a HIPS is to be proactive and eliminate the overpressure and probable atmospheric release of fluids by controlling the source of the overpressure at its root. HIPS programs are becoming very popular in refineries and chemical plants. Driven by environmental issues, regulatory directives (i.e., the need to reduce flare emissions) and inline testing of relief valves, the HIPS programs are becoming necessities instead of luxuries in critical service applications.
One of the fundamental issues related to valves in critical service is what type and extent of testing is necessary to qualify the valves for that service. Leakage rates and test durations in test standards such as American Petroleum Institute (API) 598 and 6D as well as Manufacturers Standardization Society (MSS) SP61 often are not stringent enough for critical service applications.
Aside from valves with resilient soft seats, all metal-seated valves have acceptable leakage rates during required production testing duration. API 598, Valve Inspection & Test, does not allow any leakage for many small-diameter valves during the 15 or 30 seconds of the actual test. A drop or bubble after the required duration does not fail the valve.
As metal-seated valves increase in size, the allowable leakage rates during production testing become proportionally larger. But how much leakage is accepted for critical or severe service applications? Looking at a typical valve that might be used in flammable service and tested to the required specification reveals much.
The valve used in Table 1 is a 4-inch class 300 metal-seated gate valve.
Table 1: Testing of a 4-inch class 300 metal-seated gate valve
Allowable leakage during 60-second test
8 drops/minute = .5 ml
Allowable leakage per hour
Allowable leakage per day
Allowable leakage per month
Allowable leakage per year
Total acceptable leakage per year
68.5 gallons—from a valve that passed its production testing requirements for seat leakage
Many valves will exhibit zero leakage during these test durations; but what is needed are standardized testing procedures and protocols that match the levels of severity requirements the valve may actually see in service.
This specialized, tiered approach has been suggested before at standards development organization meetings when revisions of production test standards were underway. The addition of tiers and severity levels was rejected for a general-purpose production test standard because of the concern that purchasers would over-specify requirements for general-purpose valves. The solution now being proposed within MSS is to write a severe/critical service test document that will create meaningful testing procedures and acceptance criteria to better match elevated performance requirements for critical and severe service applications.
The new proposed standard practice will also tackle some of the thorny issues, such as establishing definitions for vague phrases (e.g., bubble-tight shut-off).
Some of the areas being considered for the new standard are:
- Longer test durations
- Higher test pressures
- Cycling requirements
- Inert gas testing as a requirement
- Gas underwater testing for casting integrity
The primary goal of the new standard will be to create common test methods to better confirm the performance of a valve for critical service applications. A secondary goal will be to give valve buyers added confidence the critical service valve they are buying (and having specially tested) will maintain a high level of performance for an acceptable period of time.
Since the valve research and development boom of World War II, the valve industry continues to develop new products and processes to meet the ever-growing needs of industry. From Teflon to triple offsets, the improvements have been exceptional. Today, with a much tighter focus on safety and the environment, the industry is now tackling the toughest of sealing challenges. These newer product designs and even newer ones on the drawing board are making quantum leaps in sealing efficiency.
When combined with proposed new critical service testing standards, the net result will be even higher confidence that these valves can meet the test of coping with tough sealing challenges. VM