Hydrogen Compressors

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-1536

Hydrogen Compressors

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Engineering Design Guide:

Hydrogen Compressors

CONTENTS

SECTION

1

SCOPE

2

2

PHYSICAL PROPERTIES

2

2.1
2.2

3

Data for Pure Hydrogen
Influence of Impurities

2

3.1

Hydrogen from Electrolytic Cells

3

3.2

4

MATERIALS OF CONSTRUCTION

Pure Hydrogen

3

3

4.1

Pulsation

3

4.2

5

DESIGN

Bypass

3

TESTING OR COMMISSIONING RECIPROCATING
COMPRESSORS

4

6

LUBRICATION

4

7

LAYOUT

4

8

REFERENCES

4


FIGURES
1

MOLLIER CHART - HYDROGEN

2

COMPRESSIBILITY CHART

3

NELSON DIAGRAM

4

WATER CONTENT IN HYDROGEN FOR OIL-LUBRICATED
COMPRESSORS AS GRAMM/M2 SWEPT CYLINDER AREA


1

SCOPE

This Engineering Design Guide describes those requirements for a compressor
centered system which arise from the duty with hydrogen.

2

PHYSICAL PROPERTIES

2.1

Data for Pure Hydrogen
Molecular Weight

2

Critical values:

TK = 33.25°K
PK = 12.95 bar

(Reference I)

VK = 0.0323 m3/kg
Cp = 14.23 kJ/kg/o C at 10°C

Speed of sound:

1286 m/sec @ O°C

Cp/C v = 1.41

(Reference 2)

A Mollier chart is given in Figure I, a compressibility chart on Figure 2.
(Ref 3)

2.2

Influence of Impurities

The density of the hydrogen is so low that small quantities of impurities cause
large increases in density, e.g. the density is doubled by 7.5% N2 in hydrogen.
The consequent effects are:
(a)

Reciprocating Compressors
The increase in density leads to increases in pulsation and also increased
loads on the valves. Both pulsation levels and valve velocities should be
checked when operating on the heaviest gas to be handled.


(b)

Centrifugal and Axial Compressors
Increase in the density of the gas handled results in a nearly equal
increase in pressure ratio. For this reason the use of centrifugal
compressors on hydrogen rich gases subject to impurities is not
recommended.

3

MATERIALS OF CONSTRUCTION

At temperatures normally encountered with compressors all ferrous materials are
suitable. At temperatures above 200°C and elevated pressures, however, special
steels should be used. A Nelson diagram is given in Figure 3 (Reference 4).
Non-metallic materials are not affected at moderate pressures.
Non-ferrous materials are not attacked by pure hydrogen below 200°C.

3.1

Hydrogen from Electrolytic Cells

Hydrogen from electrolytic cells is:
(a)

Saturated with water

(b)

Contains other impurities (Hg, etc) particularly mercury.

If mercury is carried over, then aluminium and aluminium alloys should not be
used. All non-metallic materials are suitable at moderate pressures.

3.2

Pure Hydrogen

There is limited evidence that extremely pure hydrogen - purer than 99.9% causes the speed of fracture propagation to be increased by several magnitudes.
The presence of a small amount of impurity (oil, water or other gases) eliminates
this effect. The first indication of this phenomenon with reciprocating
compressors is expected to be frequent valve failures as valves are the highest
stressed parts within the compressor cylinder (Reference 5 & 6).


4

DESIGN

4.1

Pulsation

The basic pulse (d p = δ CV) is low in systems handling hydrogen because of the
low density - in spite of the higher speed of sound. However, the volume required
for a given attenuation is large; pulsation dampers for hydrogen are therefore
expensive. By keeping velocities low in the pipework pulsation dampers can be
avoided. A relationship between line pressure cylinder arrangement and
permissible velocities is given by:

where
V
P
C
K

4.2

=
=
=
=

velocity in m/sec in the pipe
pressure in bar
speed of sound in m/sec
0.4 for single cylinder, single acting machines, and
0.2 for single cylinder, double acting machines

Bypass

When hydrogen is expanded from high pressures (above 110 bar) it warms up in
the process. However, the effect is small (see Figure. 3).

5

TESTING OR COMMISSIONING RECIPROCATING COMPRESSORS

Reciprocating hydrogen compressors are usually tested on air. Under these
conditions the velocity in the valves will be excessive and leading to excessive
pressure drop. Either valves of a different kind are to be used or other
precautions taken.


6

LUBRICATION

On lubricated machines the oil film will tend to be starved of oxygen. To replenish
the oxide film on the cylinder wall it is necessary to either operate the machine
periodically on air or to add small quantities of water to the hydrogen or to the oil.
(see Figure 4.) This can be done by adding a small percentage of wet hydrogen
either from another source or by passing some hydrogen through a saturator.

7

LAYOUT

The minimum area classification for machines operating on hydrogen is Division
2 Class B. Machines should preferably be installed in buildings with sufficient
ventilation to prevent the build-up of an explosive mixture under normal
operation. They also ensure that any accidental release of hydrogen is quickly
removed from the building.


REFERENCES
1

Comprehensive Inorganic Chemistry:
Bailar, J C, Emeleus, H J, Sir Ronald Nyholm, Trotman-Dickenson, A F
Tables of Physical & Chemical Constants

2

Kaye and Laby

3

F Frohlich Kolben Verdichter

4

API 941: Steel for Hydrogen Service at Elevated Temperatures and
Pressures in Petrochemical Refineries and Plants

5

Walter, R J and Chandler, W T:
'Cyclic Crack Growth in ASHE SA 105 Grade 2 Steel in High Pressure
Hydrogen at Ambient Temperature' printed in 'Effect of Hydrogen on
Behavior of Metals', published by AIME 1976, p 273

6

Nelson, H G:
'Hydrogen Induced Slow Crack Growth under Cyclic Loading'
Ibid p 602


FIGURE 1

- MOLLIER CHART – HYDROGEN


FIGURE 2

- COMPRESSIBILITY CHART


FIGURE 3

- NELSON DIAGRAM


FIGURE 4 - WATER CONTENT IN HYDROGEN FOR OIL-LUBRICATED
COMPRESSORS AS GRAM/M2 SWEPT CYLINDER AREA


Hydrogen Compressors

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