Avoid Costly Design Mistakes

“Just build it to the Code.” That’s the most common response you hear during a design review that involves purchasing a new pressure vessel. Yet, there’re as many choices within the ASME code as there are when searching for your next vehicle. Smart choices can save you money during fabrication as well as over the lifecycle of the vessel. So, here, we’ll attempt to condense the 5,000+ pages (50+ lbs) of the ASME Boiler and Pressure Vessel Code, or “the Code,” as it’s affectionately known, into a simple guide when specifying vessels, heat exchangers and tanks. We’ll focus on 10 key factors.

1. Inside diameter versus outside diameter. Process engineers often specify a vessel’s diameter based on inside diameter (ID) to ease volumetric calculations. This also will simplify fabrication/installation of internal hardware (e.g., support rings, trays, distributors, etc.). However, sometimes specifying a vessel based on outside diameter (OD) is better. For instance, after a purchase order is issued, heads are the first things ordered — obtaining off-the-shelf heads is more likely if specified by nominal pipe sizes (NPS), which is OD from diameters 14 in. to 36 in. (For thin-wall heads, i.e., 2-in. thick or less, choosing ID or OD makes little difference, while most heads more than 36 in. are custom made.) As heads get thicker, hot forming is necessary and dies are based on ID. Thick, hot formed heads can be OD ordered but require an extra manufacturing step.

In addition, specifying a vessel based on OD has advantages when the project is modeled using 3D plant design software.  If vessel wall thicknesses change during project development, the adjustment is done to the ID keeping nozzle projections, bolt circles, steel design unaffected, etc.

Figure 1 -- Curved heads predominate and avoid the pressure limitations of flat heads. Click on illustration for a larger image.

2. Design pressure and temperature. Required wall thickness is more sensitive to pressure than temperature. Therefore, specifying a design pressure 100 psig over the maximum operating pressure is more costly than specifying a design temperature 100°F higher than what’s needed. A design pressure of 25 psig–50 psig above that at the maximum operating/upset condition and not less than 90% of maximum allowable working pressure is industry practice. Keep design temperature to no more than 50°F–100°F above that at maximum operating/upset conditions. Also, watch your design pressure and temperature so as not to cross into the next higher flange class. Check ASME B16.5 for design temperature and limitations for flanges.

3. Vacuum rating. Although current project needs may not require a vessel to be vacuum rated, over its lifetime, changes in feedstock, products and technology will occur. A large number of re-rates currently performed are on older vessels originally not documented for vacuum that now require it due to process changes. Many new vessels will rate for full vacuum and all for partial vacuum. So have the fabricator evaluate your proposed design for vacuum and apply it to the code stamp. With today’s software, this calculation can be easily performed and at no cost. You may get full vacuum rating without any modifications — if not, consider spending a little extra now by welding on a stiffening ring and a couple of re-pads to avoid having to go through the cumbersome re-rate process and field hydro-test down the road. (For more on the value of vacuum rating, see www.ChemicalProcessing.com/voices/plant_insites.html.)

4. Head choices. Functionality, not cost, should determine head choice; so understanding the functional differences is crucial. Dished heads for ASME vessels typically are available in three styles; elliptical (2:1), flanged and dished (F&D), and hemispherical (hemi-heads). Under 600 psig, elliptical heads are the most common and least expensive in terms of wall thickness and forming costs. Above 600 psig, hemi-heads are economically attractive due to their inherent low-stress shape; below 600 psig, they are the most expensive choice because they are constructed of welded, segmental parts not a single piece. F&D (torispherical) heads have the lowest profile (height/diameter ratio) and compete well with elliptical heads under 100 psig, although they have half the volume. The low profile of the F&D head only is advantageous when top head accessibility is required for maintaining instruments, agitator, etc., or when space is limited below or, for horizontal vessels, to the sides. For vessels 24 in. or less, off-the-shelf pipe caps (elliptical) provide the most economical design.