Equipment Skid Design: Beam Sizing, Mounting Pads and Lifting Lugs
An equipment skid is a structural base that carries a package, pump and motor, compressor, dosing unit, filtration set, so that the whole assembly can be built and tested in a shop and installed as one lift. Done well, it turns weeks of site work into a day of rigging. Done badly, it arrives twisted, the pump will not align, and the lifting lugs are in the wrong place.
This is what goes into one.
The frame
Perimeter beams
Most skids use wide-flange (W-section) beams around the perimeter, occasionally channel where the flange would obstruct. Rough guidance from common practice:
| Package weight | Typical perimeter beam |
|---|---|
| Under 2 t | W6 / W8 (roughly IPE 150–200) |
| 2–6 t | W8 / W10 (IPE 200–250) |
| 6–15 t | W12 (IPE 300) |
| Over 15 t | W14 and above, often with built-up sections |
These are starting points, not design values. The real driver is deflection under the worst load case, which is often not operation, it is the lift, or transport over a bad road.
Cross members
Cross members run between perimeter beams under the equipment feet. Two principles:
- Put a member directly under every equipment foot. A foot bearing on unsupported deck plate is the classic cause of alignment drift.
- Spacing of roughly 600–900 mm is typical on medium skids; closer where there is plate decking that has to carry foot traffic.
Cross members are usually the same depth as the perimeter beams so the top flanges are flush, giving a single plane to machine.
Deck
Three options, in ascending cost:
- Open frame: no deck. Lightest and cheapest. Fine where the equipment bolts straight to the beams and nobody walks on it.
- Chequer plate deck: walkable, standard where operators need access.
- Plate deck with drip containment: a continuous plate with an upstand kerb and a drain point. Required for anything handling oil or chemicals.
Mounting pads: the part that decides alignment
Equipment with rotating elements, pumps, compressors, gearboxes, needs its feet on a common plane. How you get there matters for cost.
The cheap way that works: weld the frame, stress relieve, then machine all mounting pads in one setup so they are coplanar. Specify coplanarity, not individual flatness on each pad, see flatness tolerance on large machined weldments for why that distinction saves money.
The expensive way that also works: machined pads set into pockets, grouted or epoxy-levelled on site.
The way that does not work: unmachined beam flange as a mounting surface for a coupled pump set. Rolled section flanges have mill tolerance and are not flat.
Add oversized bolt holes or slots in one direction for adjustment, and design in a shim pack, typically 3 mm of shims under each foot, so the millwright has somewhere to go.
Lifting lugs
The lugs are a structural element and a safety item, and they are frequently an afterthought.
- Rate them for the lift, not the weight. A four-lug lift is rarely equally shared. Common practice is to design each lug for the full load divided by two, not by four, to cover sling angle and uneven rigging.
- Sling angle changes the load. At 60° from horizontal each leg sees about 115% of its share; at 30° it is about 200%. If you want a spreader beam used, say so on the drawing and on a plate welded to the frame.
- Put them over structure. A lug welded to unstiffened plate tears the plate out. It belongs over a web, with a doubler if not.
- Mark the centre of gravity. Paint it on the frame. The rigger on the far end of the journey has never seen your assembly.
- Specify the hole size and the intended shackle. A 40 mm hole and a 35 mm pin is a bad afternoon.
Get the lugs checked as part of the design, not drawn by eye. Everything else on the skid can be recovered with a grinder; a failed lift cannot.
Transport and the tie-down case
The road journey to the port frequently applies higher loads than the operating case. Longitudinal deceleration during braking, and vertical shock over bad surfaces, can both exceed 1 g.
Two consequences:
- Design tie-down points and mark them. Lashing over a pipe run or an instrument is what happens if you do not.
- Consider a transport frame or bracing for tall, top-heavy packages. Removable bracing between the skid and a tall vessel is cheap insurance.
Whether the finished package fits a container or becomes out-of-gauge cargo is often decided here, see out-of-gauge and breakbulk shipping for oversize frames.
Piping, cable and the interfaces
The skid is a structural product, but it lives inside a package. Three interface details that cause the most site trouble:
- Flange terminations at the skid edge: clearly marked, with orientation shown. Terminating pipework in the middle of the frame guarantees a site weld.
- Cable tray or conduit routed clear of lifting and tie-down points.
- Access. Leave room for a spanner. A bolt you cannot reach is a bolt that will not be torqued.
Finish
Skids live outdoors or in wet plant more often than machine bases do, which pushes the finish decision harder. Hot-dip galvanising is common and durable, but it has design consequences, vent and drain holes in every closed section, and distortion risk on thin decking. The comparison is in surface finish systems for structural steel.
Why packages are built on skids at all
The argument is schedule and quality: the assembly is built and functionally tested in a controlled shop, then lifted into place. It moves labour off a congested site and into a workshop. The economics are covered in modular skid packages.
A short specification checklist
- Overall footprint, height limit, total package weight and centre of gravity
- Equipment layout with foot locations and loads
- Beam grade, usually A36 or S355, see steel grades for heavy weldments
- Deck type and drip containment requirement
- Mounting pad coplanarity and shim allowance
- Lifting lug rating, sling arrangement, spreader beam requirement
- Weld standard and NDT scope
- Finish system
- Tie-down points and transport limits
- Destination port and Incoterm
For the whole path from scope to acceptance, read our steel skid fabrication guide, and for the choice between beams and tubes see skid frame sections.
Have a package that needs a base under it? Send the general arrangement, even a rough one, and we will come back within 24 business hours with the shops that clear the envelope. They quote you directly.
See also: equipment skids, the reference page with skid types, load cases and the RFQ list.