Pipework & Distribution
The connecting tissue of the plant, and the discipline that decides whether it is a good one - because a small gas finds every joint that was made carelessly, and the whole trade is organised around making fewer joints and making them properly.
Last updated 2026-08-25
Typical duration
Typically 5-10 months, overlapping equipment installation and running right up to commissioning. The welding is rarely the constraint. Examination, repair and the assembly of a complete record usually govern the tail of the phase.
What is Pipework & Distribution?
Pipework on a hydrogen plant is not technically exotic. It is welded steel, supports, valves and flanges, and the people who build it are the same people who build process pipework everywhere else. What is different is the control wrapped around it. Hydrogen molecules are small and they will pass through connections that would hold other gases perfectly well, so the design responds in three ways: it minimises the number of joints, it prefers welded connections to mechanical ones wherever it can, and it routes pipework where it can be seen and reached rather than buried in a duct or hidden behind equipment. That last point is why a hydrogen plant looks so exposed compared with other process plants, and it is a deliberate choice rather than a lack of finish.
Materials carry the second half of the story. Metals behave differently in this service over long periods, and the designer selects them accordingly and specifies the certification that must come with them. That turns an ordinary procurement job into a traceability job. Every length of pipe, every fitting, flange, gasket, fastener and welding consumable installed has to be the one specified, with a documented chain back to where it came from, and that chain has to survive a fabrication shop, a lorry, a laydown yard and a site store. Substituting an item because it is on the shelf and the specified one is three weeks away is the single most expensive shortcut available on the whole project, and it is the one most likely to be taken by somebody who does not understand why the material was chosen.
Prefabrication does most of the heavy lifting. Spools are built in a controlled shop where the welding is done in position, on a bench, by people who are not standing on a scaffold in the rain, and where inspection is straightforward. Site work is then reduced to erection and a limited number of closing joints, which is faster, safer and far easier to prove. Cleanliness runs through all of it. Debris left inside a pipe does not stay where it was left - it ends up in a valve seat, an instrument tapping or a machine, and it is discovered during commissioning at the worst possible time. Ends are capped from the moment a spool is finished until the moment it is connected.
Compare the methods at a glance
When and why is Pipework & Distribution used?
Pipework follows the equipment because you cannot fabricate accurately to a machine that is not yet standing, and it precedes commissioning because nothing can be introduced into a system that is not complete and proven. It is also the last large labour peak on the project, which makes it the phase where programme pressure is felt most sharply and where the temptation to cut corners is strongest. That is worth naming, because the corners available here are exactly the ones that matter most: substituting a material, skipping a record, moving a joint to somewhere easier to weld, or signing off inspection that has not actually been done. The commercial logic pushes the other way. The weld and joint record is not paperwork for its own sake - it is the asset's history, and the operator uses it for the whole life of the plant to decide what to inspect, when, and how urgently. A plant handed over with a complete, traceable record can be maintained intelligently. A plant handed over with gaps in that record has to be treated as suspect everywhere, which costs the operator far more over twenty years than the record would have cost to keep properly. There is a safety argument too, and it is the plain one. The inspection regime, the material selection and the testing procedure are all set by the designer for reasons specific to this service, they are not generic process-plant defaults, and the site's job is to execute them exactly as written and prove that it did.
Types of Pipework & Distribution
Explore each method in depth - benefits, limitations, plant and quality control on its own page.
Above-ground process pipework within the plant
The main body of the work - lines connecting production, compression, storage and the utilities, run on racks, sleepers and supports where they can be seen, inspected and reached. Mostly prefabricated into spools and erected, with a limited number of closing joints made on site. Exposed routing is deliberate, and any proposal to box it in, bury it or hide it behind equipment is a design question rather than a site one.
Buried site distribution and export mains
Sections that have to cross roads or leave the plot go underground, with the material, coating, protection and depth all specified by the designer. Buried work is inherently harder to inspect later, so the control moves to the front: coating integrity, joint records and as-built survey are all completed before backfill, because nothing about a buried line can be checked casually once the trench is closed.
Loading and offtake facility pipework
The connection to the customer, whether that is a vehicle loading facility, a connection to a network operator's system or a direct line to an industrial user. It is a small quantity of pipework with a large amount of interface, because the arrangement, the metering, the isolation philosophy and the operating procedures all have to satisfy two organisations rather than one, and the boundary between them has to be unambiguous.
Vent, relief and utility headers
Relief and vent collection routes, nitrogen and air services, cooling, drains and the small-bore lines that connect instruments to the plant. Individually minor and collectively enormous in labour terms. Relief and vent routing is engineered by the designer and built exactly as drawn; utility lines are simpler but they are also where connections get made to whatever is closest instead of to what is on the drawing.
Best suited for
- Plants where whole-life integrity and availability matter more than the cost of the initial installation
- Sites exporting to a network operator or an industrial user, where the connection has to satisfy two parties
- Layouts that deliberately keep pipework visible and reachable rather than concealed for appearance
- Schemes where a complete, traceable joint record forms part of what is handed to the operator
Pipework & Distribution: step by step
- 1
Step 1: Procure the materials and set up traceability that will survive the job
The specification for this service is written by the designer and it covers more than the pipe: fittings, flanges, gaskets, fasteners, valves and welding consumables are all named, and all of them come with certification that has to be checked at receipt and kept. The system for holding that certification is set up before the first delivery arrives, not improvised halfway through, because a traceability system started late never catches up. Material is marked so that identity survives cutting, and offcuts are marked as they are made. Storage is organised so that specified material cannot be confused with anything else on the site, which usually means physical separation and controlled issue rather than a fenced area and good intentions. The rule that everyone on the phase has to understand is that substitution is not a site decision under any circumstance, however tight the programme and however similar the alternative looks.
- 2
Step 2: Survey the routes and confirm the supports before fabricating
Spools are fabricated to dimensions taken from the plant as it actually stands, not from the model as it was drawn eighteen months earlier, so the routes are surveyed once the equipment is landed and the racks are up. Support positions, guides and anchors are set from the design, and it is worth being clear about the difference: a guide allows a line to move in one direction and a support that was meant to guide but was installed to clamp will lock a line that has to move, and it will keep loading it every time the plant changes temperature. Connection points on equipment are measured rather than assumed, because a package sitting a little off position changes the geometry of everything joining it. Where the survey and the drawing disagree, the resolution is a documented change and not a site adjustment, because the difference may matter to the flexibility analysis the designer carried out.
- 3
Step 3: Prefabricate spools in a controlled shop
As much as possible is built off site or in a covered on-site shop, where pipe can be rotated, welding is done in the easiest position available, and lighting, power, environmental conditions and inspection access are all under control. Cutting and preparation are done by machine to the specified profile. Each spool carries its own identity from the moment it is created and that identity links to the material it was made from, the welder who made each joint, the procedure used and the consumable batch. Finished spools are cleaned internally, inspected, capped at every open end, marked and stored so that they can be found without being dragged over each other. Shop fabrication is not just quicker - it produces welds that are easier to make well and far easier to inspect, and every joint moved from site to shop is a joint that is more likely to be right first time.
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Step 4: Weld to qualified procedures with qualified people
Welding procedures for this service are approved before any production welding starts, and every welder working on the system holds a current qualification for the procedure and position they are working in. That is checked at the start and monitored throughout, not assumed from a certificate produced on the first morning. Consumables are stored, issued and controlled as specified, because consumables that have been left out in a damp store will produce defects no amount of skill will prevent. Conditions at the joint are controlled - shelter, cleanliness, preparation and fit-up all get attention before the arc is struck, since most weld defects are decided before welding begins. Every joint is numbered and recorded against its welder, procedure, consumables and parent material as it is made. Site closing joints get the most attention of all, because they are made in the worst positions, they are the hardest to inspect, and they are the ones made under the most programme pressure.
- 5
Step 5: Inspect, examine and build the weld record
Inspection follows the regime the designer specified for this service, which is normally more demanding than a general process plant would attract, and the coverage, the methods and the acceptance criteria are the designer's to set. Visual inspection happens first and it catches more than people expect. Non-destructive examination follows, carried out by qualified technicians whose reports name the joint they refer to. Any joint that fails is cut out, repaired to the approved procedure and re-examined, with the repair recorded as part of the joint's history rather than quietly replacing it. The record that comes out of this step is the deliverable: a complete, traceable set showing every joint in the system, what it is made of, who made it, how it was examined and what was found. Where radiography is used, the exclusion arrangements are controlled by the specialist under their own permit and they take priority over everything else happening in the area.
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Step 6: Erect, support and complete the systems
Spools are lifted and erected in a planned order, supported as they go rather than left hanging on temporary arrangements, and connected. Flanged joints are brought together square and parallel and then bolted, never pulled into alignment by tightening the fasteners, because a joint forced closed carries a permanent load and will find a way to release it. Fasteners are tightened in the specified pattern with calibrated tools and recorded. Supports, guides, anchors and any spring or sliding arrangement are installed exactly as detailed and set to the cold position specified, and any transit restraints on spring supports are removed at the right time - forgetting them is one of the most common faults in the trade. Systems are completed and marked up against the drawings as they are finished, so that mechanical completion can be declared system by system, which is how commissioning will want to receive them.
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Step 7: Install the buried sections, protect them and prove them before cover
Buried pipework is a different discipline because everything about it becomes invisible on the day it is backfilled. Trench preparation, bedding, surround and cover are all to the designer's detail. Coating is applied and made good at every joint, then checked for integrity across the whole run, since a small coating defect is a large problem over twenty years. Any cathodic protection provision, marker tape, warning boards and route markers are installed as specified. Joints in buried sections are examined to the same regime as everything else and their records are kept with the rest. The line is surveyed as-built - position and depth - before backfill, and that survey goes into the operator's records rather than into a site file, because the next person to dig near it will be relying on it years from now. Backfill and reinstatement then follow the detail, with compaction controlled rather than eyeballed.
- 8
Step 8: Clean, prepare and hand the systems over for testing
Before anything is introduced, the systems are cleaned and prepared to the designer's requirement. The methods, the media, the acceptance criteria and the sequence are all specified by the designer and carried out under the operator's or commissioning team's control, and they are not something for the pipework contractor to improvise. Temporary items used to keep systems clean - blanks, strainers, spades and caps - are logged on a register as they are fitted, because an unrecorded blank left in a completed system is a fault that surfaces in the most disruptive way possible. Pressure testing is carried out to the designer's written procedure, with the area controlled and the results recorded. The systems are then handed over to commissioning with their line marked up, their punch list honest, their test and inspection records complete, and their temporary items either removed and signed off or clearly listed as still in place.
Plant & equipment
- Pipe preparation machines, cutting and bevelling equipment for controlled weld preparation
- Welding plant, consumable ovens and controlled issue for procedure-specified consumables
- Non-destructive examination equipment, with radiography run by specialists under their own controls
- Pipe handling equipment, stands, rollers and cradles for shop fabrication and site erection
- Mobile cranes, telehandlers and powered access for erecting spools and working on racks at height
- Calibrated torque and tensioning tools for flanged joints, with records kept per joint
- Coating application and integrity testing equipment for buried sections
- Survey equipment for route survey, support setting and as-built recording of buried lines
Quality control & testing
- Welding procedures approved and welder qualifications current, checked at the start and monitored throughout
- Every joint numbered and traceable to welder, procedure, consumable batch and parent material
- Material certification checked at receipt and again at installation, with identity preserved through cutting
- Examination coverage, method and acceptance to the regime the designer specified for this service
- Repairs carried out to the approved procedure, re-examined, and recorded as part of the joint history
- Supports, guides and anchors installed as detailed, with transit restraints removed and recorded
- Cleaning, preparation and pressure testing carried out to the designer's written procedure, results recorded
Safety watchpoints
- Hot work throughout a partly built plant, controlled under a permit system with fire watch arrangements
- Radiography exclusion arrangements, run by the specialist and taking priority over other work in the area
- Work at height on racks, structures and equipment, much of it with awkward loads in restricted positions
- Manual handling and crush risk from heavy spools, valves and in-line components during erection
- Excavation, collapse and buried services during the installation of buried and export sections
- Confined and restricted space entry for cleaning, inspection and closing joints inside equipment
- Testing operations run as controlled activities under the designer's procedure, with the area cleared
- Simultaneous operations, with pipework crews sharing space with electrical, instrument and commissioning teams
Common defects to hunt
- Material substituted to keep a programme moving, defeating the reason the material was specified
- Incomplete weld records, so the operator has to treat the whole system as unproven rather than the one joint
- Joints positioned for ease of welding rather than for inspection, ending up permanently unreachable
- Flanged joints pulled into alignment by their fasteners, carrying a locked-in load from day one
- Supports installed rigid where the design called for a guide, so the line is restrained instead of guided
- Spring support transit restraints left in place, so the support never does the job it was installed for
- Debris left inside the pipework, discovered later in a valve seat, an instrument tapping or a machine
- Coating damage on buried sections, or an as-built survey never recorded, leaving a line nobody can locate
How long does Pipework & Distribution take?
Typical duration: Typically 5-10 months, overlapping equipment installation and running right up to commissioning. The welding is rarely the constraint. Examination, repair and the assembly of a complete record usually govern the tail of the phase..