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VMA Technical Event: Engineering Valves in the Extreme

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Industry Headlines

GE Oil & Gas Expands with New Sub Saharan Africa Facility

Thursday, 23 March 2017  |  Chris Guy

GE Oil & Gas opened a new facility in Takoradi Port, Ghana, expanding its global footprint and supporting local investment. The facility, which wi...

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Industry Headlines

GE Oil & Gas Expands with New Sub Saharan Africa Facility

5 HOURS AGO

GE Oil & Gas opened a new facility in Takoradi Port, Ghana, expanding its global footprint and supporting local investment. The facility, which will be the primary service center for deep-water offshore projects in Ghana, has a 1,600 square meter indoor test area with capability for testing thre...

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Metso Launches Digital Program to Accelerate Growth

1 DAY AGO

Metso has set up a Digital Program to accelerate the company’s digital capabilities, which are required to succeed in the future of minerals processing and flow control. Metso's ambition is to become one of the digital leaders in the industries it serves. This means for example turning custome...

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Shell Looking to Lower Cost of Offshore Projects

10 HOURS AGO

“With oil prices hovering around $50 a barrel or less, and a rising amount of renewable energy chipping away at market share, Shell is going lean on its deepwater projects to make sure it can eke out a profit from all of its operations,” writes ChemInfo .

For example, Shell is attempting to ...

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Report Says PA Can Support Four More Ethane Crackers

11 HOURS AGO

A new report from IHS Markit, commissioned by the state of Pennsylvania, forecasts $2.7 to 3.7 billion in investments in natural gas liquid (NGL) assets as well as the opportunity to attract additional cracker plants, and petrochemical and plastics manufacturing. IHS Markit believes the Marcellus an...

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Consumer Sentiment Reaches Highest Level Since 2000

3 DAYS AGO

The overall level of consumer sentiment remained quite favorable in early March due to renewed strength in current economic conditions as well as the extraordinary influence of partisanship on economic prospects. The current economic conditions component reached its highest level since 2000, largely...

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U.S. Manufacturing Output Rose 0.5% in February

3 DAYS AGO

Industrial production was unchanged in February following a 0.1% decrease in January. In February, manufacturing output moved up 0.5% for its sixth consecutive monthly increase . Led by advances of more than 1% for nonmetallic mineral products, fabricated metal products, and machinery, the production ...

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Which Valve for Nitric Acid Service?

materials_q_and_a_graphicQ: I requested CF8 valves but the supplier is offering CF8M valves at less cost. However, I am not sure this alternative is a good choice for my nitric acid service. Are my concerns warranted?

 

A: Suppliers of cast valves may offer certain higher alloys at the same or less cost for a couple of reasons. Before getting into your concern about nitric acid, let’s address the issue about the cost of certain cast alloys. First, we’ll look at the cast versions of 304 and 316 stainless steel, known as CF8 and CF8M respectively. The compositional makeup of these two alloys are similar: chromium and nickel contents. However, CF8M contains 2% to 3% molybdenum that CF8 does not have, so we might expect CF8M to cost more. That is not usually the case. This is because alloy content is only one variable in the cost of an alloy; an equally important one is volume. While wrought producers make a significant quantity of 304 SS at less cost than 316 SS, foundries traditionally have standardized cast 316 (CF8M). The reason is that CF8M has much broader applications than CF8, so foundries produce a much greater volume of CF8M valves and benefit from the economies of scale. Thus, CF8M valves are usually less expensive than CF8 valves.


OTHER PRICING ISSUES

There are a few other reasons some cast alloys are priced at levels that seem contradictory, including whether an alloy is “foundry friendly.” In other words, can the alloy be produced without any consistent problems as a casting? For example, CA15 (410 SS) should be expected to cost less than its newer version, CA6NM. However, when you take into account castability and weldability issues, most foundries would prefer to make CA6NM rather than CA15. Because of this, these foundries will either price CA6NM less or the same as CA15, and the customer benefits by getting an alloy that has better ductility, impact toughness and corrosion resistance than CA15.

A foundry unfriendly alloy is grade 12 titanium. Grade 12 contains a small amount of nickel and molybdenum and was developed as a less expensive alternative to the palladium-stabilized titanium grades while having almost the same corrosion resistance. While this may not be a difficult alloy for wrought producers to make, the alloy is more difficult to cast than the palladium grades. In addition, weld repairs on grade 12 must be stress relieved, which adds cost to the foundry. As a result, some reactive alloy foundries will price grade 12 titanium the same as the more corrosion-resistant palladium grades. In this case, there may be no cost benefit in specifying grade 12 cast titanium valves.

NITRIC ACID APPLICATION

Let’s return to the concern about using CF8M in nitric acid services. 304 SS is certainly a good choice for most nitric acid applications, and it is used extensively as wrought piping, vessels, tanks, etc., that are handling nitric acid. As a result, when it comes to valves, it is only natural to select CF8 thinking it will be less expensive than CF8M. For reasons discussed previously, this may not be true. Therefore, the remaining question is whether CF8M is suitable for nitric acid service. In my experience, it is. I have reviewed considerable corrosion data and conducted numerous corrosion tests for CF8 and CF8M and found little difference in corrosion rates between the two alloys in nitric acid applications. The difference usually is only a couple of mils per year, sometimes in favor of CF8, but other times in favor of CF8M (Figure 1).

vmsum11_materials_fig1Figure 1.
THE GRADE LEVEL

Another issue people struggle with concerning nitric acid is whether they should use the low-carbon grades: 304L, 316L or the cast equivalents CF3 and CF3M. The concern is that nitric acid is a strong oxidizing acid that can cause intergranular corrosion—one way to minimize the susceptibility to intergranular corrosion is to use the low-carbon grades. The use of these grades makes sense for wrought components such as piping, vessels and tanks that are welded in the field and cannot easily be post-weld heat treated. However, it is practical and sometimes required by ASTM specifications such as A744 to post-weld heat treated castings. Therefore, if the higher carbon CF8 and CF8M alloys are post-weld, heat treated, they can be used and will not experience intergranular corrosion in a nitric acid service.

Because of all of this, the choice between these CF8 and CF8M for nitric acid service should be based on price and availability.


THOMAS SPENCE is director of materials engineering for Flowserve Corp. (www.flowserve.com), Dayton, OH. Reach him at This email address is being protected from spambots. You need JavaScript enabled to view it..

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