Chemical Engineering August 2021 - 36

Feature Report
Hydrogen Piping Systems:
Mitigating Pitfalls by Design
This overview presents guidance for the safe design of piping systems that are used for the
distribution of hydrogen gas
William M. Huitt
W. M. Huitt Co.
IN BRIEF
PROPERTIES OF H2
PIPING MATERIAL
MITIGATING LEAKS
EXAMPLE SPECIFICATION
HONORABLE MENTION
LEAK TESTING
FINAL REMARKS
H
ydrogen gas is a very volatile
fluid that has a high
propensity to leak. This is a
very dangerous and lethal
combination of tendencies, a volatile
fluid that is difficult to contain. These
are tendencies that need to be taken
into consideration when selecting
material, gasketing and sealing, as
well as the design characteristics of
such a system. These topics regarding
H2 gas distribution are the focus
of this discussion, not the manufacture
of H2, H2 in its liquid state, nor
slush H2 (see sidebar on p. 35).
FIGURE 1. Hydrogen pipelines are a common sight at petroleum refineries
Properties
of H2
The following are a few key points of hydrogen
gas and H2-air mixtures that are helpful to
know. There are two modes in which H2 gas
will burn: by deflagration and by detonation.
Deflagration. Deflagration is the ordinary
mode of burning in which the flame travels
through the mixture at subsonic speeds. As
an example, this happens when a free cloud
of H2-air mixture is ignited by a small ignition
source. In such a case the flame will
travel at a rate anywhere from ten to several
hundred feet per second. The rapid expansion
of hot gases produces a pressure wave
whose force is in direct proportion to the
size of the cloud. The force of the pressure
wave can be strong enough in some cases
to damage building structures and
other objects in its path and cause
injury to personnel.
Detonation. With a detonation, the
flame and the shock wave travel
through the mixture at supersonic
speeds. The pressure ratio across
a detonation wave is considerably
greater than that found in a deflagration.
The hazards to personnel,
structures and nearby facilities are
greater in a detonation due to the
added force. A detonation can build
up from an ordinary deflagration when
ignited in a confined area. In such a
34
confined area, ignition can be caused by a
minimal energy source. But detonation of a
H2-air mixture in an unconfined area requires
a more powerful ignition source.
The pressure ratio across a detonation
wave in a H2-air mixture is about 20. At atmospheric
pressure, a ratio of 20 equals 300
psi. When this wave of pressure strikes a
fixed object the pressure ratio increases to
between 40 and 60. This is due to the pressure
wave bouncing back on itself from a
fixed obstacle.
Propensity to leak. Due to its low viscosity
and low molecular weight, gaseous H2 has a
high propensity to leak, and even permeate
through or into various materials.
H2 gas is eight times lighter than natural
gas, 14 times lighter than air, 22 times lighter
than propane and 57 times lighter than gasAmerican
Petroleum Institute
Upper left - Carbon-steel elbow segments
(view of inside surface)
Above - Flange segments
Lower left - Close-up of fissure on middle
elbow segment
FIGURE 2. Shown here is the damage caused by high-temperature
hydrogen attack
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Chemical Engineering August 2021

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