[ safety ] that the downstream pressure to the burner is maintained at an appropriate level and can be controlled by the user .
As the optimization of burner operation is heavily dependent on the supply flowrate of fuel , a gas flow regulator must be included in the gas train . The flow regulator should be able to change positions quickly based on the operation and requirements of the downstream burner . Both electronically and pneumatically actuated systems work for this application . Additionally , a flow element and flow transmitter should be connected upstream of the flow regulator to provide information and control to the regulator .
When considering the gas velocity through the pipe , typical factors include ensuring sub-sonic flow to avoid choking and limiting noise generated by the gas flow through the pipe . 7 For hydrogen , both conditions are easily met since its sonic velocity is four times that of most flammable gasses . 8 There are no special velocity restrictions for piping in hydrogen service ; however , a velocity of 20 m / s is suggested as a target ( piping should be sized accordingly ). 9 , 10
A gas train using gaseous fuel is required to have two safety shutoff valves in series , each with a proof-of-closure switch . 3 Each of these valves should include a position sensor and be interlocked with low- and high-pressure switches located before and after the series of valves , respectively . This ensures that if pressures are too high or too low for safe burner operation , the system will shut down . The shutoff valves should also be interlocked with the downstream burner sensor to close if , for whatever reason , there is a loss of flame . Between the two safety shutoff valves , an automatic vent valve should be provided with the vent termination in an appropriate location . The reader should consult NFPA 2 for details on where to locate the termination of a hydrogen vent line .
There are varying opinions about the efficacy of using an inert gas dilution to prevent autoignition or using a flame arrestor to inhibit backwards propagation of a flame in the event of venting . The use of either or both is acceptable , though not required . Diluting pure hydrogen to below 25 % of its lower flammability limit ( 1 vol % hydrogen in air ) would require an impractical volume of inert gas , which itself has safety considerations for storage . However , purging the vent stack volume itself before venting could help ensure the safe release of gas and would require a much smaller volume of inert gas to be stored at the facility .
To prevent over pressurization that could lead to equipment and instrumentation damage , or even rupture and explosion , a pressure relief valve is included in the gas train . The opening pressure of the pressure relief valve should be set below the smallest maximum design pressure of any components in the gas train itself , as well as the upstream and downstream components , to ensure that pressure can be safely dissipated before any failures occur .
Figure 2 illustrates the main piping and instrumentation that should be included in the section leading to the burner of a hydrogen gas train .
Leak detection
Detecting leaks is important not only to minimize hydrogen losses , but due to its wide flammability limit range and low ignition energy , an accumulation of hydrogen from a leak could cause a fire or an explosion , especially indoors . Various types of hydrogen sensors and leak detection technologies are available , and while none of them provide perfect detection , a combination of different technologies , coupled with routine inspections , is the best approach to ensuring safe operation . GCT ’ s article , Hydrogen gas safety standards and best practices – Part 2 : detection
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