Perspectives: Field Notes
Carefully Commission Hydrogen Pipe
Safety depends upon scrupulously performing a variety of inspections and tests
A low-pressure alarm sounded on a refinery hydrotreater-compressor suction line during startup. Two operators were dispatched. Minutes later, hydrogen collecting under the roof of the poorly ventilated compressor building exploded, killing them. Investigators later determined the cause to be a leaky, poorly sized gasket that passed inspection during construction.
This accident highlights the need for greater care in commissioning hydrogen systems. Unfortunately, advice is sparse. Background yourself on such systems by reviewing: ASME STP-PT-006, “Design Guidelines for Hydrogen Piping and Pipelines;” NACE Standard TM 0284-2003, “Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking;” and API-RP 941, “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants.”
Begin the commissioning process by focusing on the spools. Examine every component before assembly. Require vendors to provide complete records on materials and testing data for gaskets and pipe. Welds and parent metal both should have a maximum Rockwell hardness of 22 and a Brinell hardness of 250. Don’t treat instruments as whole devices: inspect the parts. After reviewing the data files and correcting component issues, weld the piping; follow with commercial-grade cleaning. Assemble the spools completely: do bench assembly tests. Pay attention to weight — badly balanced, heavy spools are difficult to assemble, so add lifting lugs as needed. Redesign poorly placed vents and drains to ensure proper slope. Polish smooth all rough welds and perform surface treatment to reduce thermal expansion and cold working. If pigs will be used in cleaning, confirm a clear path and the means of pigging. Take special precautions with relief vents: sharp angles and a sudden discharge promote mixing that can lead to spontaneous H2 fires. Inspection failures take you back to square one.
Effective cleaning of the spools eliminates future field-assembly problems. Some components will require disassembly and separate cleaning and drying. Pigs usually are used for pipe: first, soft shell pigs to look for obstructions; then rubber shell pigs for moving cleaning agents and dewatering; and finally wire brush pigs to remove pipe scale from carbon steel (CS), followed by a descaler chaser. Wire brush pigs generally aren’t used with stainless steel (SS) pipe. If you intend to send a wire brush pig through SS pipe, confirm the pig previously hasn’t been used for CS. Do a ferroxyl test of SS to detect rust contamination. Cleaning generally involves a slug of detergent or mild caustic, followed by water and a citric acid solution to passivate the SS, and finally buffered water and dewatering. Muriatic acid is a descaler often used for CS after detergent. Verify that gasket materials are compatible with cleaning solvents.
Drying is next, usually with nitrogen or air, although argon has been used because it sweeps slightly better than nitrogen. Dry by intermittent purging until the difference between input and output dew points is < 5°C; vacuum drying works best — if you can avoid leaks. Venting is important because H2 can form when acids react with metals. If air is used, the requirements are a dew point ≤ -40°C and a hydrocarbon content < 140 mg/1,000 std ft3. Perform a helium leak test after drying to detect flange surface irregularities; helium is mixed as a trace gas in nitrogen and detected by mass spectrometer. Once purged to < 1 mole % O2, seal and maintain the spool with 10 psig pressure. By the way, going the other way, to remove H2, aim for <1 mole % H2. (Keep in mind the lower explosive limits: 5.7% for O2 and 5% for H2.) These values are at 1 atm and 25°C: values decrease with increasing pressure or temperature. You may need to mix vent gas with a secondary fuel for the flare once the H2 is depleted.
Field assembly is next. Record the flange torques after inspecting the final gaskets; tighten again during initial operation. Fires have resulted from leaks caused by poor monitoring of flange torques during hot-bolting, especially where SS bolts are used with CS flanges (because SS undergoes greater thermal expansion). Take care to ensure that instruments are included. A standard pressure test is next on the checklist. This is followed by cleaning, drying and, perhaps, another helium leak test, and then finally by a hydrogen sweep to eliminate trace impurities. Follow a broad sampling plan during purging and leak-testing.
For further information, see:
Hydrogen Transportation Pipelines
Gaseous Hydrogen Piping Specification
Chemical Process Safety
For tips on material selection and piping design for hydrogen systems, see: “Head off Hydrogen Hazards."
DIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at firstname.lastname@example.org
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