TD2K's method is the intelligent and smart manner of going about the preparation of preparing formal specification's when dealing with such a proprietary and specialty item such as distillation or tower internals. The vendor is clearly the expert - by common sense and definition of the subsequent business bargain. It it the vendor who holds the key expertise and design decisions on how to meet specifications and expectations and with how much assurance.
The best we, the purchasers, can do is come up to the vendor's level of general knowledge in the design - but we should never assume the position of "expert" - unless we are willing to take on the related liabilities. That being the case, do the smart thing just as TD2K does: work with the vendors, giving them ALL your scope of work and letting them tell you what they can/can't build and what their limitations are and why. You basically want to know if they can meet your specifications; if there are any tradeoffs in meeting those specifications, you certainly want to know about them up front - with no post surprises.
This is the technique I've always used in specifying and purchasing major equipment as a project engineer and project manager. To do otherwise is not to take advantage of the maximum know-how a vendor can lend to a project in the area of special, proprietary design. Distillation trays and tower internals is that type of design.
