Process Skids and Equipment Baseplates
A process skid is a frame that carries pumps, drives, vessels and pipework so a plant can be assembled in a shop and installed in a day. The steelwork itself is rarely difficult.
What makes a skid a specialist job is everything wrapped around the steelwork: coplanar mounting pads, lifting points that are actually certified, and a documentation package that satisfies whoever signs the installation off.
What this application demands
- Coplanar mounting padsPump and driver feet have to sit in one plane so the shafts can be aligned without shimming the whole machine. The pads are machined, usually in one setup, after the frame has been stress relieved.
- Alignment that survives transportA skid aligned in the shop and dropped onto an uneven foundation loses the alignment. Stiffness in the frame, defined support points and a grouting or shimming plan are what preserve it.
- Lifting and handlingLifting lugs on a loaded skid are a safety item, not a convenience. Their design, weld quality level and inspection belong on the drawing and are usually the joints that get volumetric inspection.
- Drainage and containmentDrip trays, bunding and slopes to a drain point are frequently required and are usually the source of the fiddly, expensive fabrication on a skid.
- DocumentationMaterial certificates, welding procedure references, welder qualification records, inspection reports and a dimensional report against the drawing revision. On regulated work this is a substantial part of the price and it belongs in the quote.
What goes wrong
In roughly the order we see it on drawings.
Coplanarity assumed rather than specified
If the drawing does not say the pads are coplanar within a number, they will be as flat as the fabrication happened to come out.
Lifting lugs added at the end
Lugs welded on after the structure is designed often land where the frame cannot carry the point load. Design them in.
Documentation discovered after the order
Traceability and records are real hours. Asking for them after the price is agreed is the most common commercial argument on skid work.
No agreed support condition
A large frame sags under its own weight. A flatness number measured with the skid supported differently from how it is used gives a different answer.
What to put on the drawing
Every line here removes an assumption a shop would otherwise price as risk. Risk is always priced against you.
The full package is on the RFQ checklist, which is printable and remembers your ticks in your own browser.
- Coplanarity of equipment mounting pads, with the number and the support condition.
- Which pads are machined and in how many setups, if the machining envelope forces more than one.
- Lifting point locations, design load and the inspection required on those welds.
- Grouting or shimming plan and the defined support points.
- Drainage, bunding and containment requirements.
- The full documentation package: certificates, procedure references, inspection and dimensional reports.
- Coating system, with machined pads masked.
Questions on this part type
Why do skid frames need machined mounting pads?
So rotating equipment can be aligned without shimming every foot. Machining the pads in one setup after stress relief puts them in a single plane, which is what the alignment procedure assumes.
Does a process skid fall under EN 1090?
It depends on what the skid is and which market it goes to. EN 1090 covers structural components placed on the European market. A skid that is part of a machine is often governed by machinery rules instead. Get it confirmed for your specific product rather than assumed, because the execution class changes the whole quality regime.
Who is responsible for the lifting lug design?
Whoever designs the skid, unless it is explicitly handed to the fabricator in writing. It is a safety critical item and it should never be left ambiguous in the RFQ.
Go deeper
- AWS D1.1, ISO 3834 and EN 1090 Explained for People Who Buy WeldmentsWhat AWS D1.1, ISO 3834 and EN 1090 each cover, which one applies to your part and market, and what to ask a supplier to show you for each.
- Which Weld Inspection Method to Specify, and on Which WeldsVisual, dye penetrant, magnetic particle, ultrasonic and radiographic testing on weldments: what each method finds, what it misses and what it adds to the quote.
- What a WPS Is, and Why You Should Ask to See OneThe difference between a WPS, a PQR and a welder qualification record, what a qualification range covers and the three questions to ask a weldment supplier.
- How Much Stock to Leave on a Weldment Before You Machine ItHow much machining allowance to leave on a welded frame, how to split weldment and machining tolerances on the drawing and where to put the datums.
Have a drawing for one of these?
We read it and match it with up to 3 shops that weld, stress relieve and machine parts like it in house. Free for buyers, reply within 24 business hours.