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Basics of Elastomeric Seal Design

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Controlling Our Water Systems, Part II

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Industry Headlines

Sharpest Rise in U.S. Manufacturing Production Since November

Monday, 25 July 2016  |  Chris Guy

July data signaled a further rebound in business conditions across the U.S. manufacturing sector, led by a robust expansion of incoming new work and t...

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Industry Headlines

Emerson Exploring Acquisition of Pentair Valves & Controls

9 MINS AGO

Reuters UK has spoken to sources that confirm Emerson has made an offer to acquire Pentair Valves & Controls. Pentair Plc added the Valves & Controls division after its merger with Tyco Flow Control in 2012.

“Pentair has received offers for the valves and controls business from companies o...

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Pentair Valves & Controls Rolls Out Customer Education Program

3 DAYS AGO

Pentair Valves & Controls has introduced the Pentair University customer education program for past, present and prospective clients and industry leaders in various locations throughout the world. Pentair University’s invitation-only seminars are free to attend. In some regions of the worl...

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Canadian Oil & Gas Earnings Signal Industry Recovery

2 DAYS AGO

The Canadian oil and gas earnings season began yesterday with “signs of an industry recovery as Encana Corp and Precision Drilling Corp outlined plans to boost activity,” Reuters reports .

Analysts say “the uptick in optimism might be mirrored by some U.S. shale companies like Pioneer ...

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U.S. Lower 48 Sustains $150B in Cuts by Upstream Developers

3 DAYS AGO

Out of the more than $370 billion in global capital expenditure cut by upstream developers across 2016 and 2017, $150 billion was slashed in the U.S. Lower 48 alone — more than three times any other single country. Largely due to responsiveness and flexibility in the unconventional space, spen...

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Sharpest Rise in U.S. Manufacturing Production Since November

-1 DAYS AGO

July data signaled a further rebound in business conditions across the U.S. manufacturing sector, led by a robust expansion of incoming new work and the fastest upturn in production volumes for eight months. Job creation also strengthened in July, with the latest increase in payroll numbers the fast...

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Texas Manufacturing Activity Stabilizes

23 MINS AGO

Texas factory activity held steady in July, according to business executives responding to the Texas Manufacturing Outlook Survey . The production index, a key measure of state manufacturing conditions, came in near zero after two months of negative readings, suggesting output stopped falling this mon...

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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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