Pipe Fabrication: Shop and Field Fabrication of Process Piping

Pipe fabrication is the work of turning straight pipe, fittings and flanges into finished, welded, tested sections that are ready to install, and Paul Industries does it in its own shop and in the field, in carbon steel and stainless steel, for process plants and plant utilities nationwide. We cut, bevel, fit and weld to isometric drawings, examine and pressure test the work, clean and passivate stainless, and then install what we build. This page explains how fabricated piping is produced, which codes govern it, what the fabricator needs from the engineer, and how to compare the firms that bid it.

What is pipe fabrication?

Pipe fabrication is the controlled, documented conversion of raw pipe material into assemblies with exact dimensions, prepared ends and qualified welds, so that installation in the plant becomes a matter of setting, aligning and closing a small number of joints. Installation is the other half of the job: rigging the assemblies into place, supporting them, making the field welds and bolted joints, and tying into equipment. A fabricator controls geometry, weld quality and documentation; an installer controls fit, support, slope and connection to the rest of the system. When the same crew handles both, the drawings, the weld numbers and the test records follow one chain from the bench to the plant floor.

Industrial pipe fabrication differs from plumbing and from building HVAC work in three ways. It is built to a piping code with written welding procedures and qualified welders. Every weld carries an identity that ties it to a welder, a heat of material and an examination record. And the assemblies are designed around a process: pressures, temperatures, fluids and cleaning requirements that set the material, the wall thickness and the joint type before anyone strikes an arc.

How is pipe fabricated? The process step by step

Pipe is fabricated in a fixed sequence: the drawings are converted into cut lists, material is received and traced, pipe is cut and beveled, components are fitted and tacked, joints are welded under a qualified procedure, welds are examined, the assembly is tested and cleaned, and finished pieces are marked, protected and shipped. Skipping or reordering a step is where most rework comes from. The table below sets out the pipe fabrication process as it runs in a code shop, with the record each step leaves behind.

The sequence matters because each record depends on the one before it. An examination report is only useful if the weld number on it matches the weld map, and the weld map is only useful if the heat numbers were transferred before the pipe was cut. Shops that pass audits are the ones that treat the paperwork as part of the welding, written at the bench while the joint is in front of the welder.

Spools and spool pieces: the unit of fabricated piping

A spool is a prefabricated length of piping, made of pipe, fittings and usually flanges, whose ends are prepared to connect to the next piece or to equipment. Most fabricated piping leaves the shop as spools, because a spool is small enough to handle and ship and large enough to carry most of the welds. A buyer asking “what is pipe spool work, and what is field work?” is really asking where the field welds sit on the drawing: everything between two field welds is shop scope.

The short answer to what is a pipe spool used for: to move welding off the plant floor. A header with branch outlets, a pump suction and discharge set, or the interconnect between a heat exchanger and a skid can be built and tested before the plant shuts down, so the shutdown only has to set and close the pieces. A pipe spool piece can be as simple as a flanged straight run that replaces a corroded section, or as complex as a multi-plane assembly with valves, instrument bosses and a reducer. A stainless steel pipe spool for purified water or product service also arrives already purged, examined, cleaned and capped, which is far easier to control in a shop than in an operating area. Detailed spool fabrication sequencing, size limits and handling are covered on our process piping page.

What a fabricator needs from the engineer before cutting starts

A fabricator needs complete isometric drawings, a piping specification for each line class, the governing code and fluid service category, the examination and testing requirements, and any cleaning or documentation requirements, all issued and frozen before the first cut. Incomplete inputs are the single biggest cause of fabrication delay, because a spool built to an unapproved revision becomes scrap or rework.

Each pipe fabrication drawing is normally an isometric: a single-line, not-to-scale view showing every fitting, dimension, elevation, slope, weld and flange orientation in a line, with a bill of materials in the corner. The fabricator then produces its own pipe spool drawing for each piece, breaking the line into transportable sections and numbering every shop and field weld. Where the plant has no current drawings, Paul surveys the existing piping in the field and produces the isometrics as part of our engineering and design work.

For stainless and hygienic systems, our stainless piping RFQ guide lists the extra items to put in the request: interior finish, orbital welding percentage, borescope rate and passivation method.

Pipe fabrication standards: ASME B31.3, B31.1, Section IX and BPE

The rules for fabricated piping in North American plants are set mainly by the ASME B31 piping codes: B31.3 Process Piping for chemical, pharmaceutical, food and general process plants, and B31.1 Power Piping for steam, boiler external piping and power systems. Welding procedures and welder qualifications under both are governed by ASME Boiler and Pressure Vessel Code Section IX, and hygienic bioprocessing systems add ASME BPE on top of B31.3.

Under B31.3, the fluid service category drives most of the inspection cost. Normal Fluid Service calls for random radiographic or ultrasonic examination of at least 5 percent of circumferential butt welds, Category D service (non-flammable, non-toxic, low pressure and moderate temperature) generally needs visual examination, and Category M service for toxic fluids raises the sampling rate considerably. The owner or engineer assigns the category; the fabricator builds and examines to it. Section IX also requires that each welder keep using a process to stay qualified: a welder who has not welded with a given process for six months loses qualification for it until requalified, which is one reason to ask a shop for its continuity records.

How tight are pipe fabrication tolerances?

Pipe fabrication tolerances are usually specified by reference to PFI ES-3, which holds linear dimensions on smaller pipe to about an eighth of an inch, limits flange bolt-hole rotation to 1/16 inch, and limits flange facing alignment to 3/64 inch per foot across the diameter. Larger pipe sizes get proportionally larger linear allowances, and project specifications can tighten any of these.

Tolerances are only half of what makes fabricated piping fit. The other half is weld joint preparation and fit-up. Butt-weld ends are commonly beveled to 37.5 degrees with a root face of about 1/16 inch, per ASME B16.25, and the root opening is set by the welding procedure rather than by the fitting standard. Socket welds need a gap of about 1/16 inch between the pipe end and the bottom of the socket before welding, so that weld shrinkage does not crack the joint. Internal misalignment, or hi-lo, has to stay inside the procedure and code limits, which is why counterboring or careful matching of wall thickness is part of fit-up on heavier pipe.

Because small errors add up over a long run, a good fabricator plans where the tolerance goes. One weld in each run is usually designated a field-fit weld, and the adjoining piece is shipped a few inches long so the installer can cut it to the measured gap. Tie-ins to existing piping are measured in the field before the closing pieces are cut, never scaled from old drawings.

Carbon steel pipe fabrication vs stainless

Carbon steel suits steam, condensate, cooling and utility water, compressed air, glycol and many hydrocarbon and chemical services where the fluid does not corrode steel or where a corrosion allowance covers it; stainless is used where corrosion, iron contamination of the product or repeated cleaning would destroy carbon steel. The two materials are fabricated with different tools, different welding practice and different cleaning after welding.

Carbon steel has a lower material cost per foot and welds faster on heavy wall, but it needs coating, insulation or a corrosion allowance and has a finite life in wet or oxygenated service. Stainless costs more per foot and per weld, yet it often allows a thinner schedule and skips internal corrosion allowance, which narrows the gap on installed cost in corrosive or clean service. For grade selection between 304L and 316L and the joining options for stainless, see stainless steel piping.

Made-to-order piping for process lines and plant utilities

Custom pipe fabrication means building piping to a project’s own drawings and service conditions rather than buying standard catalog assemblies, and it covers both the process lines that carry product and the utility systems that keep a plant running. Process pipe fabrication follows the product: transfer lines, CIP supply and return, reactor and tank connections, and equipment interconnects, often in stainless with tight cleanliness requirements. Industrial piping fabrication for utilities follows the plant: steam and condensate, chilled and cooling water, glycol, compressed air and treated water, mostly in carbon steel with stainless where water quality demands it.

Skid piping is the same work at a tighter scale. When we build a pump skid, the suction, discharge, bypass and instrument piping is fabricated on the frame in the shop and tested with the skid; the planning side of skid packages is covered in our process skid fabrication guide.

Should piping be built in the shop or in the field?

Piping should be built in the shop wherever the pieces can be shipped and set without losing fit, and in the field where existing conditions are uncertain, access is tight, or the run is too long or awkward to move. On most projects the answer is a split: the majority of welds made at the bench, with the field crew making closures, tie-ins and connections to equipment.

A pipe fabrication shop gives welders a rotating positioner, good light, controlled purge gas and no weather, so welds are faster, more consistent and easier to examine. Work in the plant brings fire watches, hot work permits, scaffolding, restricted hours and the cost of a running process nearby. The field has its own advantages: pieces are cut to real dimensions, nothing has to fit through a door or onto a truck, and changes found during installation are handled on the spot.

Orbital welding blurs the line, because the same machines and procedures run in the shop and in the plant. Our orbital welding services cover both, and the hygienic side of welding practice, including purge and borescope standards, is explained on our sanitary welding page.

Pipe fabrication and installation as one scope

Fabricating and installing piping works best under one contractor because fit problems, weld records and test results then belong to a single party, with no argument at the interface between a shop that built the pieces and a crew that set them. The same drawings carry the shop welds and the field welds on one weld map, and the system is tested, flushed and turned over with one package of records.

After installation, the system is pressure tested as a whole. B31.3 piping is normally hydrostatically tested at 1.5 times design pressure, corrected for temperature, and held long enough to inspect every joint, at least ten minutes; pneumatic testing at 1.1 times design pressure is reserved for systems where water cannot be tolerated, with added safety controls. Our hydrostatic testing crews run those tests and document them. Stainless systems are then flushed and, where specified, cleaned and passivated by our passivation services team so the internal surface goes into service with its protective oxide layer restored.

The turnover package that comes with one-contractor work typically includes the as-built isometrics, the weld map, welder qualifications and procedure records, material certificates traced to heat numbers, examination reports, pressure test records, cleaning and passivation records and any heat treatment charts.

How to compare pipe fabrication companies

Compare fabricators on the evidence behind the work: qualified welding procedures for your materials, welder qualifications with continuity records, a working traceability system, examination capability, the ability to test and clean in-house, and a crew that can install what the shop builds. Price per weld or per foot means little until those are equal.

Ask pipe fabrication shops for a sample weld map and turnover package from a comparable job, with the client’s name removed. It tells you more than a brochure: whether every weld is numbered, whether heat numbers follow each piece, and whether examination reports match the map. Ask which procedures they hold for your base metals and thicknesses, how they keep stainless away from carbon steel contamination, and how they handle a weld that fails radiography. Pipe fabrication contractors who also install will tell you where they plan the field-fit welds and how they survey tie-ins; that answer shows whether they have carried their own spools into a plant before. Our guide to choosing a process piping contractor has a fuller set of questions.

Check capacity against the schedule, too. A shop that has to fabricate pipe for a shutdown needs enough welders, positioners and examination resources to finish before the outage starts, and the bid should show when drawings must be approved for that to happen.

Pipe fabrication services from Paul Industries

Paul Industries covers the whole job from drawing to turnover: field survey and isometrics when there are no current drawings, material procurement with full traceability, cutting and beveling, fit-up, orbital and manual welding in stainless and carbon steel, examination, heat treatment where required, shop and field pressure testing, flushing, cleaning and passivation of stainless, and installation of the piping in the plant. We build spools and fabricated pipe assemblies for process lines, plant utilities and skids, and offer pipe spool services on their own for maintenance replacements and shutdown work. Paul works nationwide for pharmaceutical, biotech, food and beverage, dairy, data center, oil and gas and general industrial facilities, and the same team handles process piping contracting for complete systems. Request a quote with your isometrics, line list and schedule.

Standards referenced: ASME B31.3 · ASME B31.1 · ASME BPVC Section IX · ASME BPE · ASME B16.25 · ASME B36.10M · ASME B36.19M · PFI ES-3 · ASTM A106 · ASTM A312 · ASTM A380 · ASTM A967

Frequently asked questions

What is the pipe fabrication standard for ASME work?

There is no single document. Process plants build to ASME B31.3 and power and steam systems to ASME B31.1, both of which reference ASME Section IX for welding qualification. Hygienic systems add ASME BPE, and most project specifications point to PFI ES-3 for dimensional tolerances. The engineer of record names the governing code for each line class.

What should be in a pipe fabrication RFQ package?

Approved isometrics, the line list with design pressure and temperature, the piping specification for each class, the code and fluid service category, examination and test requirements, cleaning and passivation requirements, the documentation list and the required delivery or shutdown dates. A complete package lets bidders price the same scope.

How long does a fabrication job take from drawings to delivery?

It depends mostly on two things outside the shop: how quickly drawings are approved and how long material takes to arrive. Alloy fittings, large flanges and specialty valves often set the schedule. Once drawings and material are in hand, weld count and examination rate drive the shop time.

What is a field-fit weld?

A field-fit weld is a joint deliberately left unmade in the shop, with one adjoining piece shipped long, so the installer can measure the actual gap in the plant and cut the piece to suit. It absorbs the build-up of dimensional tolerance along a run and any difference between the drawing and the existing piping.

How do fabricators make new piping fit existing lines?

By measuring the existing flanges and nozzles in the field, or laser scanning congested areas, before closing pieces are cut, and by planning field-fit welds at the tie-ins. Measurements taken from old drawings are treated as approximate until confirmed on site.

Can carbon steel and stainless be fabricated in the same shop?

Yes, with segregation. Stainless is cut, ground and stored in its own area with dedicated saws, brushes and grinding discs, and it is handled with clean slings rather than chain, so that particles of carbon steel do not embed in the surface and rust later.

What does a weld map record?

A weld map is a marked-up isometric or spool drawing on which every weld has a unique number. Each number is linked to the welder who made it, the welding procedure used, the date, the heat numbers of the joined components and the examination and test results. It is the backbone of traceability at turnover.

Is radiography required on fabricated process piping?

Under B31.3 Normal Fluid Service, at least 5 percent of circumferential butt welds are randomly examined by radiography or ultrasonic testing, and a rejected weld triggers additional examination of that welder's work. Owners often specify higher rates for critical lines, and ultrasonic testing can replace radiography where the code and specification allow.

What test pressure is used on B31.3 piping?

A hydrostatic leak test at not less than 1.5 times design pressure, raised when the test temperature is lower than the design temperature in proportion to the material's allowable stress. A pneumatic test, used only when water cannot be introduced, is at 1.1 times design pressure with additional safety precautions.

When does carbon steel piping need postweld heat treatment?

B31.3 generally requires postweld heat treatment of P-No. 1 carbon steel welds when the nominal wall is thicker than 3/4 inch, unless an exemption applies. Some services, such as caustic or sour duty, require it at any thickness by specification, and the welding procedure may require it regardless of thickness.

Why are shop welds cheaper than field welds?

In the shop the pipe rolls on a positioner so the welder works in the flat position, purge and shielding gas are controlled, and there are no permits, fire watches or scaffolding. The same weld made overhead in a pipe rack during a shutdown takes longer and costs more in support labor, so moving welds to the shop lowers installed cost.

What end preparation is used for butt-welded pipe?

Most butt welds use a single V-bevel of 37.5 degrees with a root face of about 1/16 inch, giving a 75-degree included angle when two ends meet. Heavy-wall pipe may use a compound bevel to reduce weld volume, and thin-wall stainless tube for orbital welding is cut square and faced with no bevel at all.

What is the difference between socket weld and butt weld fabrication?

A socket weld slips the pipe into a recessed fitting and closes it with a fillet weld, which is quick for small bore, typically 2 inch and under. A butt weld joins beveled ends with a full-penetration groove weld that can be radiographed and leaves no crevice inside, so it is used on larger pipe, severe service and anything that must be cleanable.

What limits the size of a shop-built piping section?

The truck, the route and the building. Loads wider than 102 inches need oversize permits on most highways, and the piece must also pass through plant doors, under racks and into position with the rigging available. Fabricators break long or multi-plane lines at field welds placed where the installer can reach and close them.

What is positive material identification in pipe fabrication?

Positive material identification uses a handheld analyzer, usually X-ray fluorescence, to confirm the alloy of pipe, fittings and weld metal before or after fabrication. It catches mixed-up stainless grades or a carbon steel fitting in an alloy line, and many specifications require it on alloy and stainless systems.

How are fabricated pieces protected for shipping?

Open ends are capped or plugged, flange faces are covered with wood or plastic protectors bolted in place, stainless is wrapped to keep road grime and carbon steel dust off it, and pieces are braced on the trailer and tagged with their drawing and piece numbers so they come off in installation order.

Does Paul Industries install the piping it fabricates?

Yes. Paul crews install, align, support and close the pieces built in our shop, make the tie-ins to equipment and existing lines, and run the system pressure test, flushing and cleaning, so fabrication and installation records are delivered as one package.

Can piping be fabricated without current drawings?

Yes. Paul surveys the existing installation, measures tie-in points and equipment nozzles, and produces isometrics as part of our engineering and design service. The drawings are reviewed with the owner before fabrication, then updated to as-built condition at turnover.

Related: Process Piping · Stainless Steel Piping · Process Piping Contractor · Process Skid Fabrication: The Seven Stages · Tank Fabrication · Orbital Welding Services for Sanitary Pipe · Request a quote