Consider Modular Pilot-Plant Construction

This under-utilized approach can provide high-quality cost-effective units.

By Richard Palluzi, ExxonMobil Research & Engineering Co.

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Our experience indicates that shop fabrication is 20%–50% more efficient than similar field construction. This is particularly true when onsite work would involve cramped spaces, such as laboratory hoods, or exterior locations exposed to weather.

Safety also benefits from the more organized and efficient approach to construction. It helps eliminate potentially unsafe field expedients and shortcuts. The more-open access typical of shop construction reduces ergonomic-related injuries because bending, stretching and contorted positions can be avoided through proper unit placement in the shop. The lack of hazardous materials decreases potential for worker exposure, fires, explosions or spills.

But Resistance Remains
Given these advantages why do companies still resist modular construction?

First there's a perception that units constructed offsite won't fit in their intended location. Horror stories abound, although upon investigation many appear to be more urban legends -- or at least classic cases of lack of adequate planning -- than truly relevant examples.

Modular construction does require more upfront design. You must determine how the unit can be moved into place, not just how to build it in place. This often creates problems because low beams, intruding piping or wiring, undersized doors and a host of similar problems arise. Yet, as anyone who's ever moved into a house knows, these issues almost always have viable solutions.

Modules can be made in sections and easily reassembled in place. You can temporarily remove or permanently enlarge doors. Walls often can be opened by detaching panels or even removing and replacing masonry. You can make modules to rig through roofs or upper walls. Existing site piping and wiring frequently -- with planning and advance notice -- can be economically rerouted or temporarily taken out and reinstalled. Such efforts can add to cost but usually are offset by savings modular construction can deliver.

Second, there's a sense a unit can't be economically and safely shipped long distance. Anyone who's ever seen major process units on highways will understand how even incredibly large items can be moved. Of course, as units get larger, routes available get more limited, preparations grow more involved, permits and escorts required more extensive and costs higher. However, most modules can be shipped with minimal problems at reasonable cost if they're properly designed to meet a few easy guidelines.

Most U.S. interstate highways are limited to a 15-ft. clearance. Low-boy air-ride trucks as low as 18 in. are available, thus affording a tight but realistic 13-ft. module height. Widths and lengths are more flexible but, in general, holding lengths to 20 ft. and widths to 10 ft. isn't a problem. Expansion to 40-ft. long and 15-ft. wide, while more costly, usually is an option. Recognize also that a module sometimes can be laid on its side for taller units (Figure 1) or stood up for wider units to meet these dimensions.

Modules must be structurally strong enough to endure lifting and transport. This doesn't mean, as some fear, they always must be enclosed with massive and expensive steel frames. In many cases, proper bracing or temporary supports add more-than-enough stiffness and strength. Other alternatives include crating for smaller modules or shipping skeletons that are applied over the module but removed upon installation (Figure 2).

While an added expense, these often can be reused or at least fabricated more crudely to reduce their cost. In many cases, standard strut pallets and frames can move across the continent with only minimal extra bracing if properly designed and constructed. Simple tricks like double nutting threaded rods, tack welding joints and clips can greatly increase vibration resistance.

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