In refining applications, system limit tends to govern at pressures above 150 psig or below atmospheric, and in atmospheric-pressure services involving high liquid rates. Internals vendors almost never report or even check this value. Published models, their derivations, comparisons with test data and recommendations are summarized in references 5 and 6. The original versions developed by FRI have been released and are available through the Oklahoma State University Library. For packing and dualflow trays, the correlation given in its Topical Report No. 34  applies. For trays with downcomers, a version of this correlation is described in the FRI Annual Report for 1961 . Some proprietary devices on the market assist gravity and claim to get past this barrier. They tend to be very expensive, have relatively high pressure drop and normally are only justified when the column loading approaches the system limit.
Several other important constraints must be determined before you actually go out for quotations. Vendors need to know how much downtime is acceptable. Another consideration that affects both time and cost is whether welding to the column shell, such as for moving tray rings, will require the vessel to be recertified. The best source of advice on this point is your insurance company or state inspector. They also can advise if the vessel was fabricated to a superceded edition of the code and whether this presents any complications. If the vessel will have to be recertified and it was shop fabricated and tested, you need to verify that the foundation can sustain the hydrostatic test load. Up-to-date shell drawings showing manway sizes and locations must be assembled for inclusion in the inquiry document package. Other information that the suppliers might require before a firm quote can be given are discussed below.
Preparing the RFQ documents
If you want to be able to compare quotations on a reasonable basis, it is essential that you do not hand the vendors a duty specification followed by a blank sheet of paper. Specify as many things as you are certain of, including your company standards. To take advantage of the vendors’ experience and know-how, tell them you will accept alternatives to the base bid, not in the base bid. Also, you need the vendors to supply their definition of percent of usable capacity or percent of flood for all devices offered. These commonly used terms do not mean the same thing to all vendors. Some use a capacity definition based upon loss of separation efficiency, whereas others use hydraulic operability as an upper limit. There can be a considerable difference between these two loadings depending upon the device and system. In addition, particularly for proprietary devices, ask the vendor to state the number of installations of this device, and how many are in this service and the size; also, request operating or test data. If a vendor is going to report on throughput of an existing column before and after installation of this device, make sure the vendor tells you all that was altered (the existing unit might have been limited by a reboiler and/or condenser that also were changed).
For the technical specification part of the RFQ, the first thing obviously is the design column loadings. Modern simulation programs give the loading for each stage. For practical purposes, columns are divided into sections in which one design loading is used to set a constant tray design or packing size (although loads from top and bottom of a section might be checked if large changes are occurring). Feed and product withdrawal points, etc., usually define such sections. For each section, you need to specify the number of trays or height of packing. (Note that the prediction of stage efficiency falls to the purchaser.) Within each section, the stage with the maximum load is chosen. Common practice is to use vapor to and liquid from a stage. For distillation columns with a feed near its boiling point, the bottom section normally governs. There are some common cases, such as strippers or packed fractionating beds, where the maximum load occurs at the top and the limiting section loads consist of liquid to and vapor from the section. You must specify the vapor and liquid mass rate and density. Liquid viscosity and surface tension are required by most rating methods. Vapor viscosity and molecular weights are also useful. Diffusivities and slope of the equilibrium curve are needed if efficiency estimation is desired.
You might wish (or need) to specify the maximum-allowable-pressure-drop per tray or foot of packing. Based on the considerations discussed above, you might wish to set the maximum allowable approach to flood (or entrainment effect point for low-pressure columns). Also, if you have determined a derating factor for this system, you must specify it and explain its basis — e.g., foaming.
Under trayed column requirements, the first item is the diameter and number of trays in each section followed by tray spacing and tray type (moving valve, fixed valve, dualflow, sieve, bubble cap, etc.). You might wish to fix the number of passes or hole size for a sieve or dualflow tray. If you are asking for a valve tray, it might make sense to specify fixed or moving valves (and whether you want one or two weights of valve if moving) and set valve size (standard or mini). This is where you need to get all offerings on a common basis, with vendor alternatives as options. Also, you should specify the material, particularly if there is a process requirement. Based on your experience or company standards, you might wish to set the maximum downcomer velocity or calculated backup.
For a packed column, the first item is the diameter and height of each bed. A single section might have more than one bed if redistributors are being used. The packing type can be structured or random, the choice of which is outside the scope of this article. For structured packing, the type and size is required. For random packing, the type and size must be specified, as well as the volume to be shipped. (The packing is shipped in boxes or bags and then dumped into the column where it settles differently – usually 10% to 15% extra is required.) Also, for random packing, specify if you desire bed limiters or hold-downs.