A process skid is a structural frame with the piping, valves, instrument supports, and equipment mounts already fitted and welded. It arrives at the site as one piece, gets set on its foundation, and connects at a handful of flanged or welded tie-in points. Everything that would have been field work is already done.
Field welding is expensive for reasons that have nothing to do with the weld: travel, permits, scaffolding, weather, waiting on other trades, and the fact that a positioner cannot follow you onto a plant floor. In the shop, pipe turns on rollers, joints are welded flat, and the welder is standing on concrete under lights.
Quality goes up for the same reasons. Fit-up is checked on a table, purge is easy to control, and inspection happens before the assembly ships rather than after a crane has set it forty feet up. When something needs correcting, it is corrected in the shop.
The design work is deciding the shipping split. Modules have to fit on a truck and through the site's access, so a large system becomes two or three skids with defined interconnects. That decision is made early, with the general arrangement drawings, and it drives everything downstream.
For clients in San Diego, Los Angeles, and Orange County, distance from our Colton shop is an argument for skids rather than against them: more of the work travels finished, and fewer people spend fewer hours on your site.