A surprising number of skid packages get redesigned after fabrication — not because the process equipment was wrong, but because nobody checked the finished skid against the truck that was supposed to carry it. The reactor sizing was fine. The pump curve was fine. The 2.7-metre width that needed a wide-load permit and a three-week route survey was not fine, and nobody caught it until the skid was already welded together.
Modular skid design lives at the intersection of process engineering, structural design, rigging, and logistics — and most of the failures we see come from treating it purely as the first one. Here’s a full worked example: equipment weight rollup, structural frame check, lifting lug sizing, electrical load, and the transport envelope check that should happen on day one, not at the loading dock.
What a Skid Design Actually Has to Survive
A piece of process equipment only has to work where it’s installed. A skid has to work — and survive intact — through several distinct load cases:
- Fabrication and shop testing — equipment mounted, piped, and pressure-tested in a controlled environment
- Lifting — onto a trailer, off a trailer, and onto its foundation, each time loaded dynamically through a finite number of lift points
- Transport — over road, rail, or sea, against real dimensional and weight limits that don’t care how efficient your equipment layout is
- Operation — fully loaded with process fluid, exposed to whatever wind, seismic, or vibration loads the actual site demands
Most of the rework we see traces back to designing for the fourth case and assuming the first three would just work out.
Step 1 — Build the Equipment List and Weight Rollup
Start with the equipment going onto this skid: a jacketed reactor, a shell-and-tube heat exchanger, a centrifugal pump and motor, and a feed/surge tank — a fairly typical small process package. The process duty that sizes the reactor and exchanger comes straight from your heat and mass balance. See our material takeoff and BOQ guide for weight rollup and costing workflow.
| Equipment | Dry Weight | Wet (Operating) Weight |
|---|---|---|
| Jacketed reactor | 2.5 MT | 4.2 MT |
| Shell & tube HX | 0.8 MT | 1.1 MT |
| Pump + motor | 0.35 MT | 0.4 MT |
| Feed/surge tank | 1.2 MT | 3.8 MT |
| Structure, piping, cable tray | 2.0 MT | 2.0 MT |
| Total | 6.85 MT | 11.5 MT |
Two numbers, not one — and they get used for different checks. Dry weight governs the lift (the skid ships empty and gets commissioned with process fluid after it lands on its foundation). Wet weight governs the in-service structural and foundation check. Use the wrong one for the wrong check and you’ll either over-design the lift rigging or under-design the foundation.
Step 2 — Structural Frame Check
For a quick conservative check, take the heaviest single equipment item — the feed tank at 3.8 MT wet — sitting at the worst-case location: midspan of a simply supported 6.0 m frame beam.
P = 3.8 MT × 9.81 kN/MT = 37.28 kN
M = P × L / 4 = 37.28 × 6 / 4 = 55.92 kN·mRequired section modulus, using an allowable bending stress of 150 N/mm² for IS 2062 structural steel:
Z_required = M / σ_allow = 55.92 × 10⁶ N·mm / 150 N/mm² = 372,800 mm³ ≈ 373 cm³An ISMB 250 section, with a published elastic section modulus around 410 cm³, clears this with roughly 10% margin. That’s a workable starting point — though a real design would superpose every point load along the span plus the frame’s own distributed self-weight rather than checking one item in isolation. The single-load check tells you quickly whether you’re in the right neighborhood before running the full analysis.
Step 3 — Lifting Lug Design
The lift happens at dry weight — 6.85 MT — through, say, four lifting lugs. Two things make the lift load higher than a simple “divide by four”:
- A dynamic/impact factor (commonly 1.5 for routine lifts) accounts for the jerk of picking the load off the ground
- Sling geometry rarely distributes load evenly across all points — a conservative design allows each lug to take up to 40% of total weight rather than an idealized 25%
Design load per lug = 6.85 MT × 1.5 × 0.40 = 4.11 MT
= 4.11 × 9.81 = 40.32 kN per lugThat 40.32 kN is what each lug, its weld, and the local frame reinforcement underneath it actually need to be checked against — not 6.85 MT ÷ 4. Skip the dynamic and uneven-load factors and you’ll size lugs for roughly half the load they’ll actually see in the field.
Step 4 — Electrical and Utility Load
Tally connected load across the skid: pump motor (15 kW), control panel and instrumentation (2 kW), and an allowance for trace heating if the process fluid needs it (3 kW) — 20 kW connected.
Apply a diversity factor (not everything runs at full load simultaneously) of 0.8:
Design load = 20 kW × 0.8 = 16 kWThis is the number that sizes the incoming supply cable and the local MCC — not the connected load, which would oversize both for no benefit.
Step 5 — The Transport Envelope Check (Do This Before Fabrication, Not After)
This skid measures 6.0 m × 2.5 m × 3.0 m. In most Indian states, 2.5 m is right at the threshold before a load is classified as “wide” and starts requiring special permits, route surveys, and a police escort — so this skid sits right on the line. Push the equipment layout 200 mm wider for “convenience” during detailing, and the project picks up weeks of permit lead time it never budgeted for.
Weight is the easier check here: at 6.85 MT dry, this skid is well within the capacity of a standard flatbed trailer — width and height are almost always the real constraint, not weight, for a package this size.
The rule worth holding onto: check the transport envelope against actual local road transport regulations before the layout is finalized, not after the skid is built and someone’s trying to book a trailer.
Step 6 — Costing, BOQ, and Commissioning Rollup
Roll up equipment cost, structural steel, piping and instrumentation, electrical, shop labor for assembly, and shop testing into a single skid cost. Leverage our engineering design templates for BOQ and costing automation — then compare it against what the same scope would cost field-erected. The per-item equipment cost is identical either way; the difference shows up almost entirely in labor and schedule. Multiple trades working in parallel in a shop on one skid is a fundamentally different schedule profile than sequencing the same trades on a congested, weather-exposed site.
Before shipping, the commissioning checklist needs to confirm: mechanical completion (bolting, alignment, torque records), instrument loop checks, hydrotest results and pass/fail documentation. See our reliability and maintenance procedures guide for best practices, electrical continuity and motor rotation direction, and a leak test on all piped connections — all signed off in the shop, where rework is still cheap, rather than discovered after the skid is already on site.
Where This Goes Wrong on Real Skid Projects
Designing the footprint for layout efficiency, not transport. Equipment gets arranged for the tightest, most efficient footprint with zero reference to road width limits — until logistics flags it weeks before shipment.
Using the wrong weight for the wrong check. Dry weight for the in-service foundation check, or wet weight for the lift, both under- or over-design the thing they’re checking.
Even-split lug loading. Dividing total weight by the number of lugs without a dynamic factor or an allowance for uneven sling distribution — fine until an actual lift puts more load on one corner than the math assumed.
Underestimating piping and cable tray congestion. A 3D model that looks clean at the equipment-arrangement stage can turn into a routing nightmare once every pipe spool and cable tray actually needs space — usually discovered at fabrication, when changes are expensive.
No area classification check for hazardous service. If the skid handles a flammable process fluid, the electrical equipment and instrumentation need to match the zone classification for that area — not get specified generically and corrected later.
Doing This Without Starting From a Blank Sheet
This exact structure — equipment weight rollup, structural frame check, lifting lug sizing, electrical load, and BOQ/commissioning checklist — is the kind of workflow our engineering template library is built around, with skid layout, structural frame, and foundation sizing sitting alongside the utility and costing sheets.
Explore the engineering design & calculation template library →
If your skid involves hazardous area classification, multiple interconnected skids, or non-standard lifting arrangements, the templated sheets get the structure right — but the project-specific certification needs an engineer’s eyes on it.
Request a custom skid design review from our engineering team →
Prefer to talk it through first? Book a free engineering consultation, or message us directly on WhatsApp. Once the skid scope is fixed, the BOQ from Step 6 feeds straight into your project economics and CAPEX estimate.
Frequently Asked Questions
Why does skid design need both dry and wet weight? Dry weight governs the lift, since skids typically ship empty and get filled with process fluid only after landing on their foundation. Wet (operating) weight governs the in-service structural and foundation check. Using one for both checks under- or over-designs whichever case you skipped.
How many lifting lugs does a process skid need? It depends on size and weight, but four is common for small-to-medium skids. The key design point isn’t the count — it’s applying a dynamic factor and an uneven-load-distribution allowance rather than simply dividing total weight by the number of lugs.
What width should I design a skid to, to avoid transport permits? This varies by country and even by state within India, so it has to be checked against local road transport regulations directly — but staying at or under 2.5 m width is a common threshold worth designing toward from the start, rather than discovering the limit after fabrication.
Is a skid-mounted package always cheaper than field erection? Not necessarily on pure equipment and material cost — the savings show up mainly in schedule and labor, since shop fabrication allows multiple trades to work in parallel under controlled conditions instead of sequencing the same work on a congested site.
Standards & Reference Frameworks
ASME B30.20 / ASME BTH-1 (below-the-hook lifting device design), IS 800 (general construction in steel, India) and IS 808 (rolled steel section properties), IEC 60079 (electrical area classification for hazardous locations), NFPA codes where fire/gas detection applies to the skid scope, and the relevant ASME/API codes for the individual pressure equipment items mounted on the skid.
