Tackle Tray Trade-Offs

Find the right compromise between process and mechanical requirements.

By Andrew Sloley, Contributing Editor

Successful equipment purchase involves balancing process, mechanical, cost, schedule and many other demands. For distillation trays, specifications include a number of highly interrelated process and mechanical requirements that can conflict with each other. To determine the most-appropriate set of trade-offs, you must understand the mechanical issues. So, here, we'll look at some common ones and provide general guidelines.

Ensure your tray sections will fit through the manhole.

Strength. Three factors are critical: point load, distributed load, and uplift resistance. Point load refers to the weight of a person standing or working on the tray surface. It's normally defined at expected ambient conditions. If you don't specify a value, the tray usually will be designed for a 240–300-lb point load. Distributed load is the weight of liquid expected to be on the tray deck at a severe "normal" operation. It may be defined at ambient conditions, operating conditions, or both. The industry default value usually is 60 lb/ft2 for a tray on a 2-ft spacing. For larger tray spacings, the required load must be higher. No allowance generally is made for uplift resistance — but severe services may require a specification as high as 1 or even 2 psi.

Thickness. This has both mechanical and process aspects. However, mechanical considerations control for most applications. Thicker trays are stronger but also more expensive and slower to install because the sections are heavier.

Tray spacing. Unless other factors over-ride, cost and ease of inspection favor trays on a 2-ft spacing. A gap in the range of 18 in. to 30 in. usually is considered normal. With spacing less than 18 in., inspection becomes difficult. Also, at very tight spacing (<12 in.), designers must pay more attention to weir heights and beam depths. At spacing above 36 in., capacity benefits from the extra height decrease. Very tight or very wide spacing may make moving through the tower during inspection more difficult.

Manhole dimensions. Little is worse than finding that your newly arrived tray sections don't fit through the manhole into the tower. In severe situations, trays have been cut apart and then welded back together inside the tower; unless precisely done, this can impact tray hydraulic performance. Remember to specify the internal manhole dimensions. Forgetting about cladding and lining thicknesses has caused problems. In a related vein, pay attention to special dimensional requirements if tray parts must be moved past internal obstructions (piping) or restrictions inside the tower.

Manway dimensions and removal. Include the dimensions for the minimum-size internal manways in trays. Also, don't forget to define if the manways must be removable from the top, the bottom, or either. Industry practice has moved toward equipment with manways removable from either side — but not every tray is built that way.

Assembly. Normally clamps attach the tray to the tray ring. Panel-to-panel connections generally involve either overlapping sections held together with bolts and washers or a tongue-and-grove design. If you have a preference, or a need, for one type or the other, specify it. Standard practice assumes trays are installed with available access to both the top and bottom of the tray. Special assembly requirements may include through-bolted trays for high-strength applications.

Spare hardware. Vendors usually provide from 5% to 10% extra bolting and hardware parts. If you want more (or none), you must include this information in the specification.

Tower attachments. Standard industry practice is to not include weld-in attachments for new trays in new vessels unless you ask for them.

Leak rates. Trays assembled from sections will leak. Gaskets, if used, can reduce but not eliminate leaks. Process, not mechanical, requirements determine if preventing leakage is important. If it is, the only truly leak-free active tray is a bubble-cap tray that's fully welded — including the bubble cap risers to the tray deck, the tray deck sections to each other, and the tray to the tower or tray ring.

Storage or shipping requirements. Include special storage or shipping requirements, such as surface treatment (oils or anti-corrosion additives), crating, and protection from environmental conditions, in the mechanical specification.

Additionally, for a revamp, the specification always should include the best available tower elevation and orientation drawings as attachments. They will allow a quality vendor to verify dimensions for proper equipment layout in the tower.

ANDREW SLOLEY is a Chemical Processing contributing editor. You can e-mail him at Asloley@putman.net.

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