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Update on valves used in hydrogen service

06.01.2012  |  Sequeira, T.,  Tyco Flow Control, Houston, Texas

Recommended practices can better scrutinize performance issues such as design, packing and metallurgy to mitigate failures.

Keywords: [valves] [hydrogen] [leak detection] [corrosion] [environment] [caskets] [packing] [valve seat]

Processing of heavier crude oil, stricter environmental regulations and detrimental effects from impurities on very expensive refinery assets are increasing the use of and demand for hydrogen. The main hydrogen-consuming refinery processes are hydrotreating to remove sulfur and hydrocracking to convert heavy hydrocarbons and create higher-value fuels.

For valve makers, this has increased demand for specialty materials, treatment procedures and zero-leakage performance. Adding further impetus to this demand are rigorous new US and German regulations regarding fugitive emissions and resulting industry and corporate standards.

From the metallurgy side, manufacturers must consider the adverse effects that hydrogen at elevated temperatures and pressures can have on carbon and low-alloy steels. They include hydrogen embrittlement, high-temperature hydrogen attack (HTHA) and hydrogen blistering.

Hydrogen is colorless, odorless and highly explosive, low-viscosity, low-molecular-weight gas. It is an asphyxiant. These characteristics make zero leakage a necessity. Leakage must be addressed not only at the seat but also at all other valve components. Metal gaskets, requiring more accurate and precise machining, can be useful where temperature limits the use of soft materials. Packing design becomes critical to avoid stem leakage. Additional inspection practices, such as radiography, may be necessary.

Adding to the detrimental effects that hydrogen can have on the valve, operating conditions such as temperature, pressure and concentration of hydrogen should be considered when selecting the right materials. Additional steps, such as post-weld heat treatment (PWHT), are required. This article will present recommended practices used by manufacturers to assure safe and optimal valve performance in hydrogen service.

Refining and hydrogen

Crude oil refineries are the world’s largest hydrogen consumers. Refining operations account for almost 90% of the global hydrogen consumption in 2008.1 The gas is central to many refinery unit operations. Hydrogen use is expected to increase 3.4%/yr from 2008 to 2013 and reach 475 billion m3. Of this anticipated 73 billion m3 of new global demand, refineries will consume almost 84%.

Several factors are driving this demand: greater production of heavier crude oil with higher sulfur and nitrogen content, lower demand for heavy fuel oil requiring more upgrading requirements and more rigorous regulations for cleaner transportation fuels. Hydrogen is used to upgrade crude oil into light transportation fuels and to also remove sulfur and nitrogen compounds.

Operational and safety challenges

Refineries are substantial hydrogen users. The outlook for hydrogen demand growth requires understanding the scope of issues regarding this gas use. In short, hydrogen poses unique material and operational challenges.

Hydrogen is a significant safety risk; it is an explosive and an asphyxiant. It is colorless and odorless, and, thus, it can’t be detected by human senses. Because hydrogen is lighter than air, accumulations can be difficult to detect. Its small atomic size compounds the risk by making traditional materials used in gaskets and packing permeable and therefore unsuitable for use.

Hydrogen corrosion. Operationally, the hydrogen is corrosive, and it can degrade material performance in many ways. Hydrogen corrosion weakens metals internally when the relatively small atom penetrates metals to adversely affect strength and ductility. Metals can absorb hydrogen when exposed during production, processing and service. For refinery valves, this corrosion is shown in several ways, including hydrogen attack, embrittlement and blistering.2

High-temperature hydrogen attack. HTHA occurs when high concentrations of hydrogen are used under extreme temperatures and pressures. The result is a difficult-to-detect reaction within the steel; the reaction causes the steel to lose strength and ductility. Material failure occurs significantly below the yield stress, with little to no prior sign of weakness.

Embrittlement. Hydrogen embrittlement, unlike HTHA, occurs at hydrogen levels as low as a few parts per million. It reduces steel ductility, making the metal brittle and resulting in static-load failures based on stress and time. Due to the difficulty in detecting cracks in welds and hardened steels, embrittlement is a particularly challenging problem. In addition, hydrogen does not affect all metallic materials equally. Most vulnerable are high-strength steels, titanium alloys and aluminum alloys.

Hydrogen blistering. Hydrogen blistering occurs mostly in low-strength alloys and in metals that have been exposed to hydrogen-charging conditions. It occurs when hydrogen is absorbed into the metal and diffuses inward. This can precipitate molecular hydrogen at laminations or inclusion and matrix interfaces that can build up enough pressure to cause internal cracks. When these cracks are just below the surface, the hydrogen gas pressure causes the exterior layer of the metal to lift up and form a blister.

Risk to valves

The risk analysis for valves used in hydrogen service is based on three primary factors: pressure, temperature and concentration of hydrogen. In each instance, end users and manufacturers consider the most extreme conditions under which the valve could be subjected.

Much of this risk analysis associates the presence of hydrogen in the stream to the possibility of metallic material corrosion such as hydrogen attack, embrittlement and blistering. Metal specifications are usually provided by end users based on their full knowledge of the operating environment. These specifications are typically done in accordance with American Petroleum Institute (API) 941 Recommended Practices, although end users also specify metallurgy developed from internal best practices.3

In addition to metallurgical considerations, risk reduction for valves in hydrogen service should also consider several other factors. The relative merits of valve design features such as fabricated, forged or cast processes, and rounded vs. angular features must be understood for the best performance. Recent advances provide new hydrogen service leak prevention capabilities with gaskets and packing. Heat-treating and inspection methodologies further reduce risk in hydrogen environments.

Metallurgy for hydrogen service

API 941 RP guides metallurgical selection for hydrogen service in refineries. It summarizes the results of experimental tests and actual data acquired from operating plants to establish practical operating limits for carbon and low-alloy steels in hydrogen service at elevated temperatures. The steel discussed in the RP resists HTHA but not necessarily other corrosives present within a process stream or other metallurgical damage mechanisms.

Central to the RP is a set of plots called “operating limits for steel in hydrogen service to avoid decarburization and fissuring,” as shown in Fig. 1. These so-called Nelson curves illustrate the resistance of steel to hydrogen attack at high temperatures and pressure. This plotted data is based on experience gathered since the 1940s. It was originally plotted by G. A. Nelson, using two parameters—operating temperature and partial pressure of hydrogen. It has been updated multiples times with further experience and new steels.

  Fig. 1. Operating limits for steel in hydrogen service to avoid decarburization
  and fissuring.


In selecting a valve, the end user provides the manufacturer with the correct material specification because the risk is strongly related to the specific process and its characteristics, such as hydrogen concentration, other corrosive stream components and exposure time. Operating pressure and temperature, while generally known to valve manufacturers, are subordinate to these total process considerations. All of these parameters are important to selecting the most suitable material based on the Nelson curves.

Design

The potential for hydrogen attack, embrittlement and blistering can be greatly reduced by various design considerations. A key objective is the reduction or elimination of sharp edges and abrupt angles. Such edges concentrate stress that can accentuate hydrogen embrittlement and cracking. The tapering or thinning of metal at a sharp edge also creates stress areas that are more easily invaded and degraded by hydrogen. As a result, large-radius designs that produce a uniform stress typify hydrogen service valves, as shown in Fig. 2. These curves are based on stress calculations for the crotch areas of the valve, using finite-element analysis to avoid peak stress.

  Fig. 2. Hydrogen-service valves typically
  feature large-radius designs to avoid stress
  concentrations.


The forming process is also very important to valve performance. Both casting and forged steel have advantages and disadvantages that should be considered. Welding should be minimized or eliminated; it is one of the most critical points where embrittlement is likely to occur. Casting has no welds and, therefore, offers an advantage. In addition, casting frequently eliminates sharp edges and resulting stress concentrations. Conversely, casting is more prone to defects—such as voids and porosity—and impurities than forged steel. Therefore, if casting is used, the valve usually will undergo nondestructive testing to identify possible defects. Foundries are increasingly applying casting simulations to reduce potential flaws and faults.

Zero leakage. Valve leakage occurs through two key paths. Leakage at the seat allows hydrogen to pass when the valve is closed. In this leakage, the gas is contained within the process. The more critical leakage is through the stem, which can result in gas escaping to the atmosphere.

Leakage at the seat. To prevent leakage at the seat, metal-to-metal technology is preferred. The technology used in hydrogen service applies a flexible, resilient metal on the disk that seals against a stellite hard-faced seat. The metal-to-metal design provides a durable, high-temperature seal, thus ensuring a leak-proof seal. As mentioned, from an environmental point of view, the most critical leaks occur when the media escapes to the atmosphere, and it is then subject to legal scrutiny. Before addressing that type of leakage, we will briefly mention some regulatory issues.

Regulatory issues. Two applicable laws—the US Clean Air Act (CAA) and the German TA-Luft, address fugitive emissions and specify the acceptable emission limits, particularly volatile organic compounds (VOCs). In addition to these laws, standards and specifications include ISO 15848 1 and 2, Shell MESC 77 – 300/312, and API 622. Most recently, regulations under the CAA affecting US fugitive emissions have been significantly revised. These changes are implemented through an enhanced leak detection and reporting (LDAR) program administered by the US Environmental Protection Agency (EPA). Key provisions of the enhanced LDAR program apply to certification of leaking valves and valve-packing technology. This certification reduces previous emission limits from 10,000 ppm to no more than 100 ppm.

API 622 is recognized by the EPA as an industry testing standard applicable to packing materials. The standard specifies the requirements for competitive testing of block-valve stem packing for process applications where fugitive emissions are a consideration. It is, at present, being revised to comply with the enhanced LDAR with changes.

Leakage at the stem. Preventing stem leakage entails a number of basic design considerations, including:

  • Live-loaded packing for temperature variations
  • Packing to be prevented from rotation
  • Providing a very efficient shaft seal
  • Very smooth shaft surface
  • Ensuring the packing segments are in touch with the stuffing box and the shaft simultaneously
  • Applying independent PTFE and graphite packing.

Two valve components are key to these considerations: the packing and gaskets. Softer graphite, which is typically used for packing and gaskets, is highly effective for leak prevention in many applications. But graphite presents a unique problem in hydrogen service. Because the graphite is permeable to the small hydrogen atom, the soft material cannot prevent leakage. Reducing leakage by impregnating graphite with PTFE, a synthetic fluoropolymer, is unacceptable. PTFE can evaporate in a fire with disastrous results.

Gasket technology

Hard-metal gaskets between the body and bonnet are necessary in hydrogen processes. Softer metals are too porous for the application. The harder, nonporous materials have significantly higher operating temperatures exceeding 250°C. However, metal gaskets require more accurate machining. If the body or the gasket is not perfectly round and precisely within tolerances, it will not seal effectively. For this reason, valve bodies must be precisely machined.

Packing technology

As with gaskets, graphite packing material also presents permeability problems. An alternative in low-temperature applications (below 200°C) are packing designs that use O-rings in various rubber compounds or Chevron-type packing to provide leak prevention with multiple seals.

However, these materials are not suitable for high-temperature (HT), high-pressure (HP) applications. Under these extreme environments, engineered graphite packing technology provides an effective alternative. The technology uses special graphite packing interposed with metal sheets to minimize gas losses. This design has been factory tested with different valves to check for leakage at pressures corresponding to ANSI classes 600, 1500 and 2500. The tests were carried out under the requirements of Shell’s MESC 77-300/312. Test results showed that recognized losses were lower than those imposed by the specification for Class “A” tightness in a range between seven to ten times. The packing design, as shown in Fig. 3, consists of:

  • Top and bottom rings of polyacrylonitrile (PAN) fiber to provide mechanical resistance to pressure
  • Outer rings of laminated, graphite/steel to provide a barrier to hydrogen molecules
  • Central graphite rings especially designed for low emissions.

  Fig. 3. Operating limits for steel in hydrogen service to avoid decarburization and
  fissuring. Source: API.



Inspection

Testing and Inspections are critical steps in certifying valves for hydrogen services. Radiographic x-rays and dye-penetration testing are used to check for internal defects, including cracks, hot tears, holes and gas inclusions. The internal volume of the valve is checked to address hydrogen blistering.

Manufacturers are increasingly testing fully assembled valves with HP helium to search for leakage. This testing is done on the finished product. Because the helium atom is very close in size to the hydrogen, this testing best simulates operating conditions. Low-pressure testing indicates losses due to leaks and porosity, while the HP testing ensures there is no deformation of sufficient magnitude to cause losses.

Post-weld-heat treatment

For valves in hydrogen service, manufacturers typically prefer PWHT. Industry experience and research indicate that PWHT of 0.5 molybdenum (Mo) and chromium-Mo steels improves resistance to HTHA. The PWHT stabilizes the alloy carbides, which reduces the amount of carbon available to combine with hydrogen. The treatment also reduces residual stresses, making the material more ductile. Manufacturers typically use PWHT for all low-alloy steels while plain carbon steels are only treated at the end-users’ request.

Final thoughts

As refiners use more hydrogen in various processing, more scrutiny will be applied on valve specifications. Hydrogen service places special demands on valves. The potential for corrosion, HTHA, embrittlement and blistering presents operational, safety and environmental considerations. In addition, government regulations limiting fugitive emissions set strict standards regarding valve performance. Due to these issues, multiple factors must be considered when specifying valves for hydrogen service and include:

  • Metallurgy selected through API and/or company specifications
  • Forming process, i.e., casting vs. welding
  • Design features to avoid susceptibility to embrittlement, etc.
  • Gasket/packing design and material selection
  • A robust inspection process. HP

ACKNOWLEDGMENT

An upgraded and revised presentation from the AFPM Annual Meeting, San Diego, California, March 11-13, 2012.

LITERATURE CITED

1 Freedonia, 2010, World Hydrogen Industry Study with Forecasts for 2013 and 2018: http://www.freedoniagroup.com/brochure/26xx/2605smwe.pdf.
2 Avery, M., B. Chui, Y. Kariya and K. Larson, “Hydrogen-induced corrosion,” Materials Science 112 Group Research Paper, March 12, 2001. http://www.mavery.com/academic/Hydrogen_Corrosion_Report.pdf.
3 API Recommended Practice 941, Seventh Ed., “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants,” August 2008.

The author

Tito Sequeira is the global marketing manager for refining at Tyco Flow Control. He is responsible for the strategy and marketing mix required to serve the global refining market. He has experience in product management and industry marketing manager for power, refining and petrochemical. He has provided strategic leadership to pursue business opportunities into refining for different valve and control leading manufacturers. Mr. Sequeira holds a BS degree in industrial engineering from the Monterrey Institute of Technology in Mexico and an MBA from Yale University. He has five years of experience in the valve industry. 




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