Process Skid Fabrication: How to Scope a Skid Package for Your Plant
- Shop Limits Set the Design: Crane capacity, hook height, and road transport permits determine what a skid can actually be, and they should be established before the process design is fixed.
- Alloy Segregation Protects Reliability: Stainless and high-nickel fabrication needs to be physically separated from carbon steel work, because particulate transfer causes surface contamination that outlives installation.
- Code Scope Belongs in the Specification: A pressure vessel mounted on a skid falls under ASME Boiler and Pressure Vessel Code Section VIII and requires a shop holding the applicable Certificate of Authorization.
A process skid is a structural base carrying a functional section of a process, assembled and tested as one unit before it ships. Plant expansions and equipment replacements arrive at a facility less and less as loose equipment and pipe. They arrive as skids.
A well-scoped package shortens the field schedule and moves work out of an operating unit. A poorly scoped one produces a module that cannot be lifted or cannot be trucked. Most of that difference is settled before anyone cuts steel.
What a Skid Package Covers
Scope usually includes the structural frame, mounted equipment, interconnecting piping, valves and instrumentation, and the access needed to operate and maintain what sits on it. It tends to get thin at the boundaries: termination points, utility connections, instrument wiring, and surface preparation. Naming those early keeps field scope from expanding after delivery. PALA fabricates process skids and structural skid units alongside coded vessels, tanks, and piping, and when a vessel forms part of the skid it carries its own code requirements. For the fuller definition, see what a modular process skid is.
Scoping the Package
Shop-Fabricated or Field-Built?
Shop fabrication moves labor hours off the site and into controlled conditions. The Construction Industry Institute found it helps project teams meet the challenges of demanding schedules, adverse site conditions, and limited availability of skilled labor.
| Factor | Shop-fabricated skid | Field-built system |
|---|---|---|
| Schedule | Parallel with civil and foundations | Sequential, waits on site readiness |
| Conditions | Controlled shop, fixed cranes | Weather and congestion dependent |
| Permits in a live unit | Most work performed outside it | Hot work and entry permits per task |
| Size ceiling | Shop crane and road transport | Site access and rigging only |
| Tie-in flexibility | Terminations fixed at design | Adapts to verified conditions |
| Best fit | Congested sites, tight shutdowns | Oversize systems, uncertain geometry |
Field construction still fits when a system is too large to transport in one piece, or when tie-in geometry cannot be fixed until existing conditions are verified.
What Sets the Size Limit?
Three limits set the envelope, and they are rarely the ones a process designer starts with. Overhead crane capacity and hook height determine what can be lifted, turned, and stacked. Testing is the second, because a skid hydrotested as an assembly has to be supported somewhere capable of holding it. PALA’s Hammond, Louisiana facility has 70,000 square feet under roof, a 75-ton overhead crane, a 23-foot hook height, and a hydrotest pad rated for approximately one million pounds.
The third is the road. Width, height, and weight all trigger permit thresholds, and transport is often the binding constraint rather than fabrication. Establishing the shipping envelope early costs less than redesigning a module that cannot legally leave the yard.
How Should Alloys and Welding Be Specified?
Material selection follows the process. Carbon steel serves many utility and hydrocarbon applications, while corrosive, high-purity, or elevated temperature service calls for stainless, duplex, or a high-nickel alloy. PALA works in stainless steel, duplex stainless, chrome, nickel, Monel, Hastelloy, Inconel, Alloy 20, and Haynes alloys, supported by more than 350 welding procedures.
Segregation matters more than the alloy list. Carbon steel particulate transferred onto stainless causes surface contamination and rust bloom that can compromise corrosion resistance long after installation, which is why PALA dedicates 10,000 square feet of its Hammond facility to a segregated alloy shop. Specifications should also state weld examination requirements and acceptance criteria rather than leaving them to shop standard.
Which Codes Should the Specification Name?
Different parts of a skid answer to different standards.
| Scope on the skid | Governing standard | Authorization required |
|---|---|---|
| New pressure-retaining vessel | ASME BPVC Section VIII, Div 1 or 2 | ASME U or U2 certificate |
| Repair or alteration of a coded vessel | National Board Inspection Code | National Board R certificate |
| Power boilers and external piping | ASME BPVC Section I | ASME S certificate |
| Aboveground storage tanks | API 650, API 620 | Per-standard qualification |
| Bioprocessing and biopharmaceutical | ASME BPE | ASME BPE certificate |
| Dairy and food contact | 3-A Sanitary Standards | 3-A Symbol, third-party verified |
PALA holds R, U, U2, and S stamps and fabricates Division 1 and Division 2 vessels, Section VIII pressure parts, and vessel walls up to 3.5 inches thick. An unstamped shop cannot deliver a coded vessel, so confirming authorization early avoids a mid-project substitution.
From Shop to Startup
What Ships With the Skid?
Pressure testing, functional checks, and a factory acceptance test give an owner the chance to reject or correct work while the skid is still reachable from all sides. Documentation should travel with the module: an inspection and test plan agreed before fabrication, weld records, material test reports, and a turnover package at delivery. PALA runs quality this way, from the inspection and test plan through turnover.
What Happens at the Site?
Field scope generally includes foundations sized for the module, offloading and rigging, setting and alignment, piping tie-ins, electrical and instrumentation connections, and commissioning support. Tie-ins are where schedule risk concentrates, and hot taps or in-service welding can sometimes allow connections without a full shutdown. PALA self-performs mechanical piping, structural steel, equipment setting, and civil work, which allows shop scope and field scope to be planned together rather than handed between contractors.
Process Skid FAQs
What is a process skid?
A process skid is a structural base carrying a functional section of a process, with the equipment, piping, valves, and instrumentation mounted, assembled, and tested as one transportable unit before it ships.
How large can a process skid be?
Three limits set the ceiling: the fabricator’s crane capacity and hook height, the shop’s ability to test the skid as an assembly, and road transport permit thresholds for width, height, and weight. Transport is often the binding constraint.
What certifications should a process skid fabricator hold?
It depends on what sits on the skid. A new coded pressure vessel requires an ASME U or U2 Certificate of Authorization, and repair or alteration of an existing coded vessel requires a National Board R certificate. Bioprocessing service adds ASME BPE, and dairy or food contact adds the 3-A Symbol.
What drives the cost of a process skid?
Alloy selection, code scope, physical size, testing requirements, and transport distance. High-nickel and duplex alloys carry higher material and welding cost than carbon steel, coded vessels add inspection and documentation, and any dimension crossing a permit threshold adds transport cost.
Partner With PALA for Process Skid Fabrication
A few questions separate fabricators quickly. Match the shop’s crane capacity, hook height, and testing capability against the module you need. Confirm which code stamps your equipment requires, ask how alloy work is kept away from carbon steel and what arrives in the turnover package, then ask who performs the field setting and tie-in work. A skid that transfers cleanly from shop to field is usually one where the same team planned both.
PALA has fabricated industrial equipment in Louisiana since 1973, starting with mechanical piping and equipment installation for the state’s sugar industry and expanding into pulp and paper, refining, petrochemical, and food and beverage work. The company is employee-owned and provides services in over 30 states from offices in Louisiana and Texas. To discuss a process skid package or shop capacity, explore PALA’s fabrication shop capabilities or contact our team.
