Gone are the days when most flange companies retain their own materials engineers/metallurgists or fabrication savvy personnel on staff. With the swings in the process industries over the past two decades, a number of flange companies have elected to dispense with their materials/metallurgy group and instead rely on a process engineer or a consulting metallurgist to specify materials of construction.
Process engineers are specifying welded equipment more and more, and often with a lack of fabrication/materials know-how. Their approach is to rely on a fabricator to guide them through the materials decisions and to point out any oversights.
Furthermore, with the widespread use of sophisticated vessel design software, many small- to mid-sized fabricators no longer employ engineers. Instead they depend on technicians to design vessels, many of whom lack the technical insight or materials background often required. In today's market, fabricators often do not have the time to challenge material/fabrication datasheet abnormalities and merely add these additional costs to their bid or choose not to bid, putting the engineer in a less competitive position. As a result, if the engineer receives a quote from the fabricator, it's weeks later, higher than expected, and with exceptions, deviations and surprises, all of which must be reconciled before proceeding. In the end, the project will have incurred unnecessary schedule delays, higher equipment costs, and then finds itself over budget and behind schedule before it gets off the ground.
You can pre-empt such problems with a bit of guidance. So, in this first article in our three-part series, we'll look at a dozen important factors to consider in materials selection. We won't get deep into the technical weeds but will provide pointers gleaned from our first-hand experiences that can help you avoid costly mistakes and delays.
Select the right material. For non-corrosive service, use design temperature to choose a readily available, cost-effective material. Table 1 offers a general guide [1, 2]. For corrosive or hydrogen service, consult a materials engineer.
Avoid specifying materials by trade name. Many projects involve replacement-in-kind of existing or similar equipment. The original design may have specified a particular brand or trade name alloy such as Hastelloy C276, Carpenter 20-Cb3, Monel or Inconel 600, and so these words are used throughout project development. Citing brand or trade name materials was necessary in the 1970s because many were unique and protected by patents. Today however, most major metals manufacturers produce their own and competitors' alloys. So, unless sticking with an exact proprietary alloy is mandated, using generic names, such as Alloy C276, Alloy 20, Alloy 400, or specifying the trade name “or equal” on the data sheet is more appropriate.
Take your bid expiration date seriously. Prices for commodity metals change daily on world metal exchanges. There was a time when mills/suppliers only adjusted their prices once per month and you could hold onto a firm quote for two to four weeks while it was evaluated. However, in recent years, metal pricing has become more sensitive to world events and more frequent and dramatic pricing swings occur. A fabricator recently reported that its quote for several large heat exchangers had to be adjusted upward $300,000 when the order was placed two months later — solely because of increases in stainless-steel tube cost due to a surge in nickel and molybdenum prices.
In today's market, fabricators must contend with material pricing from suppliers that can expire at the end of day. So, take your bid expiration date seriously. On the other hand, carbon-steel costs, while rising, tend to be less volatile than those of alloy steels; so it's safe to assume normal escalation during the project development/estimating phase.
Specify dual-grade stainless steel. There's much confusion about “L” grade, straight grade, and dual-grade 300-series austenitic stainless steels — in particular, Types 304 and 316. Engineers often specify lone “L” grade materials such as Type 304L or 316L on data sheets. The reason: during welding, such low-carbon stainless steels resist chromium carbide sensitization that can lead to preferential heat-affected zone corrosion in some corrosive processes. However, L grade stainless steels have lower strength than straight (non-L) grade stainless steels and the ASME code penalizes the design 15% to 20% with additional shell thickness and lower flange rating.
What's important to understand here is that a lot of the weldable forms of stainless steels (Types 304/316) produced today in the U.S. come dual certified as Type 304/304L or Type 316/316L. These steels have the higher strength of straight-grade stainless steels and have the superior resistance to sensitization during welding of the L grade stainless. This is because they're now made in a melt furnace process that substitutes nitrogen for carbon. Nitrogen strengthens the steel (like carbon) but won't promote sensitization during welding. Fabricators often will purchase dual certified materials but will use the lower strength values of the L grade material in their calculations if you specify L grade material on your data sheet.
Properly use corrosion allowance. This allowance adds extra thickness to account for uniform metal loss over the equipment's expected service life. The key word here is uniform. Mild carbon steel uniformly corrodes due to the galvanic cell potential of the interlaced ferrite-cementite grain structure, called pearlite.
Specifically, there are millions of anodic (ferrite) and cathodic (cementite) sites that in the presence of moisture provide the four necessary elements for corrosion (anode, cathode, metallic bridge, and electrolyte). Alloyed materials in aggressive service will also uniformly corrode because their strong protective oxide layer is breached.
This results in unnecessarily adding extra wall thickness and possibly crossing into a higher flange rating.
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