Commonly used at power generation facilities, gaseous hydrogen (H2) poses a serious risk of explosion if ignited. This Quick Tip is intended to highlight risks associated with hydrogen systems for power generation facilities, and to help members understand their options for mitigating the potential for loss from an uncontrolled gaseous hydrogen bulk supply release into a building, an enclosure or the atmosphere.
In air, H2 has a flammability range of 4% to 74%, and an explosive range of 18.3% to 59%. This implies that any mixture of H2 in air containing between 4% and 74% would support combustion and that a slightly narrower range can result in detonation. The potential for an explosion exists if an ignition source is present while hydrogen is enclosed. This wide flammability range makes it a particularly hazardous gas to use in buildings or enclosures by comparison to natural gas, which has a flammability range of 5% to 15%, or propane, which has a range of 2% to 10%. Outside the flammability range, the air and natural gas or propane mixture is either too lean or too rich to support combustion. The ignition energy required to ignite H2 is significantly less.
Hydrogen is commonly used at power generation facilities as a coolant for generators and, more recently, as a potential blended fuel for combustion turbine generators (CTG). A release of hydrogen in an enclosed space such as a turbine enclosure or turbine generator building could result in a devastating explosion if ignited. Such explosions have occurred in the power generation industry during incorrect H2 venting, purging and filling operations, following machine abnormal conditions, or as a result of H2 system valve misalignments.
To prevent a hydrogen-related fire or explosion, the NFPA 2 Hydrogen Technologies Code, 2023 Edition, Section 7.1.25, “Emergency Isolation” provides four options for emergency isolation requirements of an H2 bulk source:
(1) Automatic shutoff valves, located as close to the bulk source as practical, tied to leak detection systems
(2) Attended control stations where trained personnel can monitor alarms or supervisory signals and can trigger emergency responses
(3) A constantly monitored control station with an alarm and remote shutoff of the gas supply system.
(4) Excess flow valves at the bulk source
In instances where H2 is being used in a fuel mixture with other fuel gases for a CTG, Options 1 through 3 above should be evaluated to determine the most appropriate alternative for the hydrogen portion of the fuel supply system. These options are reasonable, given the relatively small size of the CTG and fuel supply enclosures, the potential for the effective use of hydrogen leak detection systems, appropriately designated electrical devices, and limited personnel access. Leak detection systems can be gas detection, low header pressure, or both. Additional considerations may apply when the CTG and fuel supply equipment or enclosures are located within a larger building. Where hydrogen is used as part of a fuel mixture in a CTG, Option 4, the use of excess flow valves, would be impractical since variable and high flow rates are required.
Option 4 – excess flow valves at the bulk source – is considered acceptable in instances where continuous H2 makeup is necessary for a generator located in a building or enclosure. With the continuous makeup of hydrogen, Option 1 alone – automatic shutoff valves with leak detection (gas detection or low header pressure) – while possible, may not result in detection before ignition, given ever-present ignition sources and the volume of hydrogen involved. Options 2 and 3 are impractical given the requirement for manual actions by operators who may already be overwhelmed in responding to a system abnormal condition, e.g., turbine generator failure. In Options 2 and 3, a means for detecting a gas release would also need to be provided for signaling the operators to respond, which again ignition may occur before detection given ever-present ignition sources.
With Option 4, an in-line excess flow valve on the hydrogen header, immediately downstream of the low-pressure regulators at the bulk supply, provides acceptable protection against an uncontrolled release without the need for operator intervention. The excess flow valve should be sized for normal consumption for generator scavenging. If an H2 system is capable of filling the generator with an excess flow valve installed, the excess flow valve is not properly sized. A manual bypass valve is needed around the excess flow valve to allow for generator filling, and typically, pressure must be vented upstream and downstream of the excess flow valve to reset following its operation. Option 4 supplemented with low header pressure and an automatic shutoff valve, may be considered the best arrangement.
An alternative to Option 4, where only periodic manual topping up of the generator is performed, is to keep the hydrogen bulk supply source isolated and open only during manual topping up of generators through the associated unit gassing stations. The risk associated with this alternative includes the operator’s limited ability to ascertain if a significant leak exists when charging the header(s). For this reason, the manual operation should be procedurally controlled. After the low-pressure header is charged to the generator gassing stations, the operator should isolate the supply and check for leak-off before proceeding. Excess hydrogen consumption should trigger a leak detection investigation with handheld monitors.
The location of discharge from hydrogen relief valves and vent systems should also be considered. Vertical discharge of relief valve vent pipes and process vent systems is recommended. Angled discharge, no greater than 45 degrees, can be used in locations where vertical discharge may not be practical due to environmental considerations. The use of gooseneck or straight-down discharge is discouraged as it can create significant exposure to storage vessels, adjacent process equipment and buildings, specifically roof coverings.
Members that modify or add these safety systems to their hydrogen supply systems should contact AEGIS for a review of the design prior to installation. Have questions or want more information on hydrogen system safety? Please contact Peter Anderson by e-mail.
Note: This AEGIS Quick Tip is not intended to address all hydrogen risks, e.g., off-gassing of UPS battery systems.
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