Hydrogen is widely employed for hydrogenation and other purposes in chemicals manufacture, petroleum refining and other chemical process industries (Hydrogen: The Real Action Is Today, CE, February, pp. 28ff), and it holds promise as a fuel. However, this gas is highly explosive, prone to leakage and permeation, and difficult to detect, so special care must be taken during process engineering design of equipment and systems that handle or contain hydrogen. This knowledge and attention to detail is all the more important inasmuch as the reactions involving hydrogen tend to be exothermic and in many cases are conducted under high pressures and temperatures.
Hydrogenation and other hydrogen-handling processes involve a considerable amount of process equipment, instrumentation and piping components, such as reactors, catalyst feed vessels, spent catalyst filters, pumps, valves, pressure relief devices, pressure regulators and check valves. Many such systems, particularly those for hydrogenation of organic chemicals, are located inside a building. Such facilities must be designed with four levels of safeguards, namely:
•A high degree of automation, with remote operations, interlocks and alarms to monitor process and environment conditions
•Certified relief devices for process equipment and piping. The relief devices vent discharges must be directed to safe locations
•Adequate, dependable space ventilation to prevent accumulation of hydrogen gas pockets for systems located inside buildings
•Damage-limiting building construction to protect personnel and property.
When gas is released or escapes from containers, it presents detonation, deflagration and fire hazards. The wide flammability range, high burning rate, low ignition energy and non-luminous flame of hydrogen accentuate the combustion hazards.
The flammability range for hydrogen in dry air at atmospheric pressure and ambient temperature is about 4 to 75%. With so wide a range, virtually any release of hydrogen has a great potential of igniting. The minimum energy required for ignition of hydrogen in air at atmospheric pressure is about 1.6 X 10–8 Btu, which is considerably less than the value for other fuels, such as methane (2.7 X 10–7 Btu at 14.7 psia). As a consequence of low ignition energy, even a small heat-producing source, such as friction and static charge, may result in ignition when hydrogen gas is released at high pressure. In fact, hydrogen is frequently thought of as self-igniting. All ignition sources in the hydrogenation system must be eliminated or safely isolated.
The outcome of hydrogen combustion due to a release in air is cannot be predicted. Some of the many possible hazardous outcomes of hydrogen combustion are as follows:
Fire: In this case, a release of hydrogen gas in air ignites and burns like fuel at a burner. The size and type of flame will depend on the hydrogen release rate. In any case, the flame radiates very little heat, and is visually imperceptible under artificial light or daylight. Therefore, reliable methods of fire detection must be provided in facilities that handle hydrogen.
Deflagration consists of a flame that propagates through a combustion zone at a velocity less than the speed of sound in the unreacted medium. The flammability range is the same as that for fire. The presence of confining surfaces such as piping, ducting or vessels can elevate the pressure and accelerate the flame speed. If the flame speed exceeds the speed of sound, the deflagration process can transition into a detonation.
Detonations, propagating at a rate greater than the speed of sound within the unreacted media, generate high pressures. Detonation requires a richer hydrogen-oxidizer mixture and a more-energetic source of ignition to occur than does a deflagration. Very high pressures can be generated in a detonation when a pressure wave is reflected from wall to wall inside a building. Detonation is associated with shock waves and an accompanying blast wave that can severely injure personnel and damage property.
BLEVEs (boiling-liquid expanding-vapor explosions): Theoretically, cryogenic containers with liquid hydrogen present are subject to BLEVE. Under rapid heating (for example, due to engulfment by fire), a vessel containing pressurized liquid hydrogen may fail suddenly, producing this explosive effect. On the other hand, a cylinder containing compressed hydrogen gas is not subject to BLEVE if it fails.
Deflagrations and detonations alike are perceived as explosions. The resulting shock waves and hot product gases impinging upon the surroundings outside of the combustible region can also be referred to as blast waves. There is no combustion in a blast wave, but it physically displaces the surrounding gases and it propels shrapnel.
Most hydrogenation and hydrotreating reactions are performed at high pressures (pressures as high as 100 atm or more are not unheard of) and over wide temperature ranges; for instance, for some types of reactions, –20 to 350°F. These conditions generate high stress in equipment and piping systems, so that achieving leak-tight design is not a trivial matter — especially in light of hydrogen having the lowest molecular weight of an industrial gas. Aside from being prone to leak easily through seals, the gas readily permeates through various materials that are impervious to other gases.
Key considerations that must be taken into account when selecting materials of construction for hydrogenation and other hydrogen-handling system include the following:
•Design temperature and pressure
•Permeability and porosity
•Compatibility between dissimilar metals
In the design of low- and high-pressure hydrogen systems alike, stainless steel (Types 304, 316) is the most commonly used material for equipment, tubing, piping, fittings, and components. Other construction materials that are satisfactory in hydrogen service include Monel, Hastelloys, aluminum alloys, suitable grades of carbon steel, glass-lined carbon steel and copper alloys. However, the final selection should be based on full awareness of all the raw materials, catalysts and other substances that are used in the process. Some process compounds can significantly affect materials’ suitability even when present only in trace amounts.
Ordinary carbon steel, iron, low-alloy steels, chromium, molybdenum, niobium, zinc, nickel, etc are not acceptable for use at cryogenic temperatures. Cast iron is not acceptable for hydrogen service due to its porosity. Nickel should not be used because it is subject to severe hydrogen embrittlement.
Elastomers and plastics such as polytetrafluoroethylene are in many installations used for gasketing, O-rings, packing, seats and other sealing elements. Ideally, their usage in hydrogen service should be limited, because they can fail in the event of fire.
In a hydrogen environment, most welds are susceptible to hydrogen embrittlement. Therefore, post-weld annealing is recommended to restore the microstructure.
Outside is best
Some important factors in determining the location for a hydrogen facility include the following:
•Climate condition (warm or cold)
•Process condition of the hydrogen reaction system (pressure, temperature, other properties)
•Quantity of hydrogen involved
•Type of adjacent property
•Nature and presence of other fuels or oxidizers in the facility or vicinity
•Protection afforded by shielding, barricading, or other means.
From a safety standpoint, the ideal location of a hydrogenation or other hydrogen-handling facility is outdoors. This allows any hydrogen leak to quickly disperse into the atmosphere, thus minimizing the potential for a deflagration or detonation. A separate, dedicated building, away from the rest of other buildings is the next best option for location of a hydrogenation system.
In cases where a reactor must be located indoors, the building should be designed to prevent leakage and migration of hydrogen vapors into other parts of the building, as discussed below. The reactor should be installed on an outer building wall on the top floor; or at a building corner, where there are at least two walls for venting. A missile containment courtyard with limited access should be provided in front of the hydrogenation building, facing pressure relief panels.
The minimum distance requirements between properties constitutes a critical parameter that must be evaluated early during the design phase. There are several useful sources of information that relate hydrogen quantity to separation distance. Two such sources are the U.S. National Fire Protection Assn.’s NFPA 59 (Table 188.8.131.52 Non-refrigerated Container Installation Minimum Distances) and Data Sheet FM 1-44 of FM Global (Johnston, R.I.).
As a matter of good engineering practice, the separation distance should be checked by energy release calculations involving the rupture of an overpressurized vessel. The energy release is converted to equivalent pounds of TNT, a quantity which is related to shock and gas pressure waves. From the calculation, one can determine the resultant blast wave pressure at any distance from the source of explosion.
All buildings used in hydrogen services must be designed to limit personnel injury and facility damage in the event of fire or explosion. The building should be constructed in accordance with the International Building Code, the NFPA (e.g. 55, 68), the Code of Federal Regulations 29CFR1910.103 (in the U.S.), as well as with any other codes and insurance regulations that have jurisdiction in the location. Damage-limiting construction for the building requires pressure resistant walls to contain explosion and pressure-relieving panels to vent an explosion.
Both NFPA 68 and FM Global’s datasheet FM 1-44 provide useful methods for sizing deflagration vent panels. Under the FM 1-44 guidelines, for example, the facility must meet the following design criteria:
•The ratio of the enclosure surface area (As) to vent area (Av) should equal less than 7.25.
•A minimum of 1 ft2 of vent area is required for every 15 ft3 of room volume. (Note, however, that this requirement results in large facilities, which are costly in today’s economy. It is therefore recommended to consult with all stakeholders early in the design, to establish the appropriate codes, guides and standards for use in the design. This point is particularly important because other codes, such as NFPA 68, do not require this ratio criterion.)
•The pressure-resistant walls must be designed to a minimum pressure of 100 and a maximum of 216 lb/ft2.
As good engineering practice, the pressure-resistant-wall rating should also be confirmed by calculations of the shock and gas pressure waves generated by the energy release of an overpressurized reactor (equivalent pounds of TNT).
Some features of a safely designed building in which hydrogen is stored or used are as follows:
•Hinged doors swing outward in an explosion
•There are no pockets or space where hydrogen gas could accumulate
•Window panes (if installed) are shatterproof or plastic in frame
•Floors, walls and ceilings are designed and installed to limit the generation and accumulation of static electricity
•Floors, walls and ceilings are designed for at least 2 h of fire resistance
•Walls or partitions are continuous from floor to ceiling, and securely anchored
•The building is constructed of noncombustible materials, on a substantial frame
•Restrained deflagration vent panels are present
•There is adequate ventilation, and any heating in rooms is limited to steam, hot water, or other indirect means
•Deflagration venting is provided in exterior walls or the roof
There are no national codes, standards or guides that cover design-limiting construction for detonation occurrence. Since detonation takes microseconds to occur, per NFPA 68, it cannot be successfully vented. Also, there are currently no known pressure relief devices that can react to a detonation speed.
Piping and supply lines
The attributes of a piping system suitable for handling hydrogen are summarized in Table 1. Although some of those attributes, such as the ones relating to structural matters, are shared by other piping systems, the adherence to then is especially important with hydrogen.
The piping to convey hydrogen gas from cylinders entails special considerations. If the cylinders cannot be stored outside, then a well ventilated storage shed is required. The temperature in the shed must be kept below 50°C. Hydrogen cylinders must be segregated from cylinders, tanks, silos or other containers that store oxidant gases or other oxidants.
Hydrogen cylinders supplying the process must be connected through approved gas manifold. The following components and attributes are typical in such a hydrogen supply line:
•The gas manifold usually consists of pressure regulators, pressure gauges, relief valve(s), vent connections, and provision for automatic switchover between online and standby cylinders. An alarm for alerting operator to a switchover must be provided. A supply of inert gas, usually nitrogen, must be provided for purging the hydrogen lines before and after use. Hydrogen detection sensors with alarms are also required in the area
•An excess flow valve is required to shut down the hydrogen supply in the event of downstream line rupture or similar failures
•Automated block and bleed valves should be provided to isolate the cylinders from the reactor or other process equipment. A bleed line having a flame arrestor must be vented outside at a safe location
•Cylinders must be adequately earthed, and piping line bonded.
•Check valves must be installed in appropriate locations to prevent backflow of process contents into the line
Control and monitoring
Minimum instrumentation and control practices for hydrogen plants are summarized in Table 2. Note the importance of adopting a fail-safe design philosophy. Critical process-safety interlocks, soft- and/or hard-wired, should be actuated by redundant instrument sensors. Examples of critical safety instruments installed with redundancy are those for hydrogenation-reactor process temperature and pressure, and hydrogen detection in confined spaces. Some of the common interlocks associated with hydrogenation reactors are found in Table 3.
Hydrogen detection: Because hydrogen gas is colorless and odorless, means for its detection must be provided in all areas where leakage or hazardous accumulations may occur. For hydrogen-detection sensors installed in confined spaces, current standard practice at hydrogenation plants calls for alarm setpoint at 1% concentration by volume in the ambient air (which is 25% of the lower flammability limit, as given earlier). Furthermore, operators should not enter any confined space in which the ambient hydrogen concentration is greater than 0.25%.
Indoor process units that employ hydrogen should be supplied with fixed and portable hydrogen sensors. Portable sensors are used by personnel when entering an area where a leak may have occurred. It is suggested that the fixed detectors be located in the following areas:
•Reactor room, where hydrogen leakage, accumulation or spill is possible
•Storage area, where hydrogen connections are routinely made and separated
•Building-air intake ducts, if hydrogen could be carried into the building
•Building-air exhaust ducts if hydrogen could be released inside the building
The appropriate response to a detection of the presence of hydrogen in ambient air varies with the likely degree of risk. Examples of common responses include isolation of the hydrogen supply source, shutdown of the hydrogen-handling and process system, provision for issuing a visual and audible warning, and/or increased ventilation of the enclosed space. In addition, remote television monitoring should be considered for systems not visible from the control room.
Fire detection: Because a hydrogen flame in air is usually almost invisible and because the emissivity of a hydrogen flame is low, the flame is hard to see or feel. So, aside from the detection of the gas itself, the design of a hydrogen-using facility must provide detection of a hydrogen flame in all areas where leaks or hazardous accumulations omay occur. Infrared (IR) and ultraviolet (UV) are two technologies commonly used.
In any hydrogen-using facility, provision must be made for safe disposal of unused hydrogen. The most-common methods employ burn-off flares or roof-venting.
For large quantities of unused hydrogen (the definition of “large” varying by industry), a flare system is generally the choice. The hydrogen-containing exhaust process gas is piped to a remote area, where it is burned with air in a multiple burner arrangement. Flare systems are equipped with pilot ignition and a warning systems in case of flameout The exhaust-gas header to the flares is usually kept under slight negative pressure, so extra care must be taken to ensure that the vent systems is leaktight to avoid air intrusion and possible detonation.
Although flare-system design technology is mature, flares continue to pose hazards of flame stability, flame blowoff, and flame blowout. To minimize the malfunction of flare systems, it is important to keep the stack discharge velocity between 10 to 20% of the sonic velocity in hydrogen at the temperature prevailing in the exhaust line. Velocities above the recommended range may cause the flame to blow off or blow out.
For roof venting, the main variables to consider are such site-specific conditions as the prevailing-wind direction and speed, proximity to adjacent buildings, vent stack height, and local discharge limitations or other environmental restrictions.
In a roof-vent system for a process plant in which hydrogen is used, it is better to vent each major piece of equipment (such as each of several hydrogenation reactors) separately instead of using an interconnected collection header. The separate-vents approach avoids the possibility of any high-pressure, high-throughput discharges overpressurizing the low-pressure parts of the system. If, however, collection headers cannot be avoided, be sure to size the header to handle the flows from all discharges with only minimal back pressure developing at the lowest-pressure equipment.
The roof-vent lines should be designed with inert-gas (usually nitrogen) purging and steam snuffing at the end of each line. The purging takes air out of the system before introduction of hydrogen, and removes hydrogen at the end of the process. The design activity must include a review of all piping and equipment system to ensure that they can be adequately purged and leak tested prior to admission of hydrogen. The vent piping must be designed with care to prevent steam condensate from flowing back into the process equipment. During plant operation, both nitrogen and steam flow should be commenced upon the introduction of hydrogen gas in the process equipment.
Avoidance of flammable atmospheres is a key aspect of preventing combustion hazards. If the exhaust vent stream happens to be solvent-laden, the stream should be passed through the condenser, scrubber or absorber prior to discharge to the atmosphere. For additional safety, a certified flame arrester should be installed in the vent line. Be aware, however, that flame arresters certified for use with hydrogen are still not common; the equipment suppliers are still in the process of development and certification.
The venting system must be designed to handle both normal and emergency venting requirements. Normal venting usually involves, for example, exhaust gas streams from process reactions, distillation, and equipment and piping-systems purges, which are handled in the venting systems as just discussed. For emergency venting, the plant must install safety valve or rupture disks, as appropriate, on vessels, lines, and component systems for emergency venting to prevent damage by overpressure. Each safety device must, of course, be sized for the complete system that it protects.
Although there are a few exceptions, most hydrogenation reactions are not usually subject to runaway. In cases where a potential for runaway does exist, the emergency vent system should be sized by a rigorous methodology such as that developed by AIChE’s Design Institute for Emergency Relief Systems (DIERS). As a rule, rupture disks are used for runaway conditions because their response time is faster than that of safety valves. In many cases, a rupture disk is installed in series with a safety relief valve, to protect that valve from corrosive chemicals or sticky solids such as catalyst particles. As a minimum, if there is no indication of a runaway-reaction potential, the relief devices should be sized for fire exposure conditions and set to relieve at the maximum allowable working pressure (MAWP) of the vessel.
The vent line from emergency devices should be routed to a catch tank. This tank should be sized for at least one and half times the volume of the largest process vessel at the facility, to provide liquid containment and phase separation. Both horizontal and vertical catch tanks are common in practice. The tanks and vent headers are usually purged with nitrogen to reduce oxygen concentration to below 5% by volume. All nozzles, attachments, supports, and internals for the catch tank should be designed for shock loadings resulting from thermal effects and slugs of liquid during emergency relief.
In light of hydrogen’s wide flammability range and low ignition energy, hydrogen leaks or spills in a non-ventilated, confined space can readily form an ignitable gas mixture. Accordingly, all such spaces should be provided with room ventilation, in addition to the aforementioned hydrogen-concentration monitoring. Confined spaces for hydrogenation systems are usually designed for 15 to 20 air changes per hour during normal conditions, and 30 to 40 when high hydrogen concentration has been detected. To accommodate the necessary ramped-up volumetric flowrate for emergencies, the ventilation exhaust fan should be a variable-speed or two-speed unit. Exhaust fans must be fabricated of non-sparking materials, and their motors rated for the same electrical classification as that of the other motors in the room.
Rooms in which hydrogenations or other hydrogen-using operations take place must be kept at a negative pressure with respect to outside areas to prevent outward hydrogen flow. The room pressure should be monitored, with provision for an alarm to sound.
Most fires in hydrogenation facilities are caused by electrical faults, so careful consideration needs to be given to the design of electrical equipment or wiring in such a facility. Electrical systems should be designed to comply with the Electrical Area Hazardous Classification. Per the NFPA 70 Code, an area where flammable hydrogen mixture is normally present is classified as Class I, Division 1, Group B, whereas an area where hydrogen is contained and only present under abnormal conditions is classified as Class I, Division 2, Group B. All potential sources of ignition should be prohibited in such areas.
Electrical installations in Class I, Division 1, should meet the following requirements:
•Certified for use in hydrogen environment
•Intrinsically safe per NFPA 70 and Underwriters Laboratories Specification UL 913
•Non-certified electrical equipment to be located in purged enclosures. In this case, the enclosure should be maintained under positive pressure and purged with an inert gas such as nitrogen. This is a meaningful requirement at present, because there are no commercially available motors that are suitable for use in Class 1 Division 1 Group B environment; therefore, purged enclosures are employed routinely in this application
•All piping joints electrically bonded
•All portable equipment electrically grounded prior to use
For Class I, Division 2, Group B environments, explosionproof motors are not available. Standard totally enclosed, fan cooled (TEFC) motors can be used, provided that there are no arcing devices in the motor. Motors suitable for this area classification should include an approved thermal switch, which limits the external surface temperature of the motor housing for the Group B rating. Installations for explosionproof equipment should meet NFPA 70 and 496 guidelines as a minimum.
A fire protection system is required for all hydrogenation facilities. At a minimum, the design of the system should include all the following features:
•Automatic process shutdown system on fire detection
•Water sprinkler system
•Water deluge system
•Dry-chemical extinguishing system
Dry-chemical extinguishers, carbon-dioxide extinguishers, nitrogen and steam are all acceptable for use to extinguish small fires. As precaution to prevent major explosion hazards, it is recommended that a hydrogen fire should not be extinguished until the hydrogen source has been isolated, to prevent ignition of a large combustible cloud of the gas.
Hydrogenation equipment should be protected by water-deluge sprinkler systems designed for coverage of at least 0.35 gallons per minute per square foot. The fire-water supply system must be designed with enough capacity that when a hydrogen fire is detected, water can be applied on equipment in the nearby surroundings as well. The facility should be provided with a spill protection and drainage sized for the largest possible single spill to prevent the spread of fire to other areas.
A hydrogenation or other hydrogen-using facility is designed safely if it minimizes the severity of the consequences of a mishap. The people and facilities are separated from the potential effects of fire, deflagration, or detonation originating from failure of hydrogen-handling equipment. All the confined spaces are adequately ventilated to prevent accumulation of flammable mixtures and all unused hydrogen gas from the process is safely disposed of by flare system or vented above other facilities. The instrumentation and control system is a “fail-safe design” with features such as redundant sensors for critical safety instruments, remote monitoring of critical information, remote operation with process safety interlocks.
Edited by Nicholas P. Chopey
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