M. Mohitpour
Novacorp International Consulting Inc.
C.L. Pierce
Novalta Resources Inc.
Calgary
Peter Graham
Novacor Chemicals Ltd.
Joffre, Alta.
In lieu of uniform codes or standards for hydrogen-gas pipeline design and construction, Novacorp International Consulting Inc., Calgary, developed a design basis and used it in the design, engineering, and construction of a hydrogen-gas cross-country pipeline.
The pipeline joined the Alberta Gas Ethylene Co. Ltd. hydrogen purification plant to the Cominco Fertilizers/Alberta Energy Co. Ltd. anhydrous ammonia plant.
STORAGE, TRANSPORTATION DIFFICULTIES
Despite more than 750 km of hydrogen pipelines in place worldwide and approximately 40 years of operating experience, no codes and standards exist for the design, construction, and safe operation of hydrogen pipeline systems.
Design of hydrogen facilities still relies on each owner's or operator's experience and data published by researchers in the field.
Hydrogen is a gaseous substance except at cryogenic temperatures necessary to maintain it in a liquid or solid state. It is very light (density: 0.09 kg/cu m at 0 C., 101.325 kPa; 32 F., 14.7 psi) and as a result is one of the most difficult gases to store and to transport in large quantities.
Standard welding bottles of hydrogen weighing in the order of 57 kg (126 lb) can only hold about 0.45 kg (1 lb) of hydrogen gas. This low payload factor limits the extent of hydrogen transportation in pressure vessels.1
Truck-delivery facilities are only used for the transportation of liquid hydrogen. And liquefying hydrogen is very expensive. Hydrogen gas is used for many purposes.
It is a good clean fuel. It burns in air to produce heat without the simultaneous production of contamination, the consequence of burning being merely the formation of water.
It is used commercially in chemical industries for various reduction processes, for ammonia and methanol production and heavy-oil upgrading. Its largest usage is in ammonia and urea production. In heavy-oil upgrader plants, hydrogen is used for hydrotreating for production of lighter products: gasoline, diesel fuel, and petroleum substances.
While hydrogen has commercial uses and is used for some power generation, a much greater demand is seen in the future when larger quantities of synthetic fuel will need to be produced from lower grade feedstock than oil. Large quantities of hydrogen gas are most economically transported by pipelines.2
PROPERTIES, APPLICATIONS
Normal hydrogen is a gas at atmospheric conditions (15 C., 101.325 kPa absolute; 60 F., 14.7 psi). This hydrogen is a mixture of 75% ortho-hydrogen (o-H2) and 25% of para-hydrogen (p H2).3 At temperatures of less than 200 K., it is mainly para-hydrogen. Table 1 provides typical hydrogen properties.
Important properties of pure hydrogen gas for typical pipeline operating pressure and temperature ranges, and useful to pipeline hydraulics and design, are provided elsewhere.4 These include specific volume, density, conductivity, viscosity, enthalpy, entropy, specific heat, compressibility, Joule-Thomson coefficient, and velocity of sound. Some of these parameters for a pipeline operating range of 3,000-6,000 kPa (435-870 psi) are provided in Table 2.5 Other properties and parameters are given elsewhere.3 5
The history of transportation of gaseous hydrogen by pipelines dates from 1938 when the first world hydrogen pipeline network was put in operation in Germany by Chemische Werke Huels AG.6 Table 3 is an extension of the world hydrogen gas-pipeline experience previously provided.2
The last known pipeline design and construction was performed by Novacorp, Calgary.
Table 4 provides the typical composition of raw hydrogen gas mixtures transported by pipelines. In Canada, production of hydrogas (pure or commercial compositions) is generally 75% by chemical methods (ammonia and methanol) and 25% by refining and oil sands (steam-methane reformers).4
LOSS ESTIMATES
Accurate pipeline and facilities sizing, as well as pipeline hydraulic design and operational simulation, are only possible with computer techniques which solve flow, compression, and heat balance (continuity, momentum, and energy) equations and which consider actual flow properties at pipeline segments.7
A nomogram, however, has been developed which estimates pipeline losses for raw hydrogen gas.8 The nomogram has limited applications in that it gives first-order estimates of system requirements.
Material selection concerns are that hydrogen gas has a detrimental effect on toughness, ductility, burst strength, and fatigue life.
The main problem associated with the exposure of pipelines to high-pressure hydrogen gas is the possibility of hydrogen embrittlement because it occurs at temperatures at which pipelines normally operate (
In hydrogen-gas pipeline systems, the process involves the diffusion of hydrogen into the steel and coalescence at voids or inclusions. This results in a hydrogen pressure in the steel that causes an increase in internal stress and thus the lowering of the apparent fracture stress.
Hydrogen gas pressure, composition, the strength level and chemical compositions of steel, and other factors including strain-rate effect (pressure reversal cycles) also have an impact.
One other aspect of the hydrogen pipeline design is the consideration for hydrogen delayed failure. This type of failure is common in hydrogen gas pipeline subjected to a sustained load.
Specific design criteria for hydrogen-gas pipeline materials operating in normal temperature ranges (
Table 5 provides standard notch toughness requirements for hydrogen service line pipe. Other requirements for valves, fittings, and operating limits of materials (temperature and pressure) are given elsewhere.4
Fig. 2 summarizes experiences with various pipeline materials in place and their reported operating pressure.
Most transmission lines operate at the 3,000-6,000 kPa range and use low grades of steel typically equal to or lower than API 5LX X42 (CSA Grade 290). At these pressures very few or no failures have been reported.9
A typical range of pipeline steel and appropriate carbon and manganese compositions well suited to hydrogen service is summarized in Fig. 3. Steel with a low percentage of carbon and manganese is found to be the least susceptible to hydrogen embrittlement and attack.6
SAFETY, INSTALLATION
The major hazard associated with pipeline transmission of hydrogen is leakage followed by fire and explosion in the presence of a small amount of ignition energy.
Hazardous properties of hydrogen are provided in an accompanying box. Because of the small molecular size of hydrogen, it is more likely to leak than any other gas.
As shown, flammability limits are very large for hydrogen. The ignition temperature (570 C.; 1,058 F.) for hydrogen is higher than most hydrocarbons (200-370 C.; 392-698 F.), but ignition energy (the amount of heat required to ignite the hydrogen) is an order of magnitude lower (Fig. 4).
Once ignited, hydrogen flame speed is of the order of ten times faster than that for natural gas-10 As a consequence of low ignition energy requirement and high flame speed, it is important to design piping in hydrogen-bound facilities with the minimum amount of flange connections. Adequate ventilation is essential in such facilities.
Generally, hydrogen pipeline facilities construction and installation techniques follow that of natural gas. But because of the explosive nature of the hydrogen-air mixture in the presence of a small source of ignition, special precautions in piping configurations and pipeline fabrication are necessary.
Some of these precautions (see box), including welding, testing, cleaning, and commissioning, were previously covered.2
Since 1985-86 design, construction, and operation of the Alberta Gas Ethylene Co. (AGEC) hydrogen pipeline and associated plant facilities, lessons have been learned from the design and construction technique used.
Briefly, the AGEC pipeline designed and constructed by Novacorp International Consulting Inc. comprises 3.7 km (2.3 miles) of cross-country 10-in., 4.8-mm (0.190-in.) W.T. Grade CSA 290, cat 11, 5 C. pipe.
The line currently carries 4,825 kg-mole/hr of 99.99% pure hydrogen at a maximum operating pressure of 5,790 kPa from the AGEC hydrogen purification plant to the Cominco Fertilizers/Alberta Energy Co. Ltd. plant.
Following are some of the experiences gained from the construction of the AGEC pipeline and associated plant and facilities piping.
- Welding. Welding procedures specified in the initial project had laboratory-proven micro-hardness levels of less than HRC 22.
For proof to the client and to ourselves that the line welders were achieving the hardness requirements, macro hardness tests were randomly specified. All tests indicated hardness levels less than HRC 22.
Now the thinking has changed in that field verification of shop-proven welding procedures providing acceptable hardness levels are no longer required. It is felt that macro-hardness field testing of micro-hardness welding phenomena is not practical.
The initial design was based upon the thinking that the fewer flanged or screwed couplings in the system the better. This was only carried out to the extent that it was easily practical.
For example, all valves were flanged, threadolets/weldolets were used as usual for pressure gauges, temperature gauges, and blowdowns.
Today, the line would be built with welded valve fittings and only with pressure/temperature/pressure relief indicators deemed absolutely necessary.
The adage "the more ... the better" no longer applies for hydrogen pipelines.
- Valve tests. To simulate in-service conditions, all valves selected were subjected to hydrogen-gas seat tests.
Although the valves selected and the in-service history of these valves have shown no erroneous assumptions from these tests, the thinking now would be to use helium gas.
Helium gas would be safer and would provide a small molecule with which to test the valve seats. Hydrogen molecules are quaternary molecules as compared to helium which is a diatomic.
Even though the hydrogen atom is the smallest known, the hydrogen molecule is larger than the helium molecule.
- Piping spools. Although less applicable to pipeline work than to plant work, predesigning the spool pieces and tie-ins is extremely important. Field changes cause an increase in the number of field joints which increases the possibility of joint failure or hydrogen leakage.
The involvement of maintenance personnel in the initial design would eliminate most of the field-implemented changes.
- Flange finish. All flanges used in hydrogen-rich service (pipe flanges, process valves, pressure safety valves, and control valves) were specified as 125-200 AARH (Average Arithmetic Roughness Height). In the specification, concentric-ring finish was specified over a spiral serration design.
Some manufacturers were unable to supply the concentric ring finish, and spiral serration fittings were accepted. Both of these have provided excellent in-service performance.
- Control, pressure safety valves. The design called for the minimum body pressure specification on all control and pressure safety valves to be the equivalent of ANSI 300 flange rating.
This was done to ensure that the body would not be distorted if "pulling up" was required to seal small leaks during the operation of the facilities.
- Piping, flange spacers, blind points. The piping design philosophy used in the plant was to reduce the number of possible leaking points (flange connections, vent valves). Although these cannot be entirely eliminated, they were significantly reduced during each design drawing review.
An additional concern was the stressing of piping components caused by blinding of process equipment during routine facilities maintenance. Typically, the flanges are spread to install slip blinds during which time the piping is displaced on its supports.
To eliminate this stress, blind points were identified and removable spacer rings were designed and installed between flanges to allow stress-free blinding to take place. Since the operation of the system, no leaking of undisturbed piping joints has been experienced.
- Gasket materials, specification. All gasketing on associated plant process piping was originally completed with spiral wound "Flexittalic" asbestos-filled gaskets API 601 series. These gaskets have performed very well.
Replacements are required purely for maintenance purposes. Graphoil-filled gaskets of the same style will be used in the future due to concern about asbestos.
- Process gate-valve specifications (stuffing box valve spindle). All the process valves for associated facilities were specified to have a packing box and valve spindle finish of 32 RMS (Roughness Measurement System).
START-UP PROCEDURES
With the recognition of the difficulty of containing high-pressure concentrated hydrogen in process piping systems, a final plant integrity test was completed just prior to the charging of feedstocks to the plant.
This was done by feeding a mixture of 2% by volume helium in nitrogen into all process piping vessels and then pressuring to working pressure.
At this time all joints in the plant were visually inspected and soap tested.
The nitrogen/helium compression units were supplied by Linde Union Carbide Gas Products Division. After original leaks were located and repaired, the plant and pipeline were charged with feedstocks.
No leaks have occurred since start-up.
ACKNOWLEDGMENTS
The authors are grateful for permission by the management of Novacorp International Consulting Inc., Alberta Gas Ethylene Co. Ltd., Novacor Chemicals Ltd., and Novalta Resources Inc., for publication of this article.
REFERENCES
- Angus, Hamish C., "Storage, Distribution and Compression of Hydrogen," Chem. & Ind., January 1984, pp. 68-72.
- Mohitpour, M., Pierce, C. L., and Hooper, R., "The Design & Engineering of Cross Country Hydrogen Pipeline," JERT, December 1988, Vol. 110, pp. 203-207.
- L'Air Liquide; "Gas Encyclopedia," 1976, pp. 885-931.
- Mohitpour, M., "A Guideline Manual for Design of Hydrogen Pipeline Systems," NOVA Corp. of Alberta, January 1987.
- McCarty, Robert D., "Hydrogen: Its Technology & Implications," Hydrogen Properties, Vol. 3 (1979), CRC Press Inc.
- Thompson, Anthony W., "Hydrogen: Its Technology & Implications," Vol. 2, CRC Press, Cleveland, 1977.
- Mohitpour M., "Practical Pipeline Design & Construction," lecture, October 1989, Continuing Education Department, Univ. of Calgary, Calgary.
- RTM Engineering Ltd., "Alberta Hydrogen Pipeline Study," Draft Report, 1985; prepared in conjunction with the Canadian Hydrogen Industry Council, Calgary, Alberta.
- Wyle Laboratories, "Safety Criteria for the Operation of Gaseous Hydrogen Pipeline," 1984 Report #DO7/RSPA/DMT-10-81.
- Drell, I.L., and Belles, F. E., "Survey of Hydrogen Combustion Properties," NACA Report 1383, Washington, 1958 Natural Advisory Committee for Aeronautics; 1976 Gas Encyclopedia, pp. 885-931.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.