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Last updateThu, 30 Jun 2016 6pm

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Fugitive Emissions—Issues and Opportunities

Fugitive Emissions—Issues and Opportunities

Fugitive emissions, ppm, consent decrees...

Problem-Driven Innovation

Problem-Driven Innovation

Developing Alternative Technology to Imp...

Valve Repair Takes Center Stage in Houston

Valve Repair Takes Center Stage in Houston

Attendees gathered June 2-3 in Houston t...

What Internal Best Practices Can Do for Valve Selection

What Internal Best Practices Can Do for Valve Selection

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Industry Headlines

Watson Valve Upgrades Oxygen Clean Room

Thursday, 30 June 2016  |  Chris Guy

In an effort to ensure proper cleaning procedures are performed on valves intended for oxygen service, Watson Valve Services has upgraded its existing...

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Industry Headlines

Watson Valve Upgrades Oxygen Clean Room

13 HOURS AGO

In an effort to ensure proper cleaning procedures are performed on valves intended for oxygen service, Watson Valve Services has upgraded its existing clean room within its manufacturing plant in Houston. Watson Valve Services’ clean room is operated by trained technicians, and is constructed wi...

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Emerson Selected to Upgrade SE Virginia Wastewater Utility

1 DAY AGO

Emerson Process Management is replacing an outdated supervisory control and data acquisition (SCADA) system as well as remote terminal units (RTUs) used throughout Hampton Roads Sanitation District’s (HRSD) wastewater collection system. Currently, HRSD serves about 1.7 million people over an are...

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U.S. Natural Gas Making Up for Coal Plant Closures

16 HOURS AGO

"Five years ago, opponents of newly proposed clean-air rules sounded dire warnings of blackouts and surging electricity prices if coal-burning plants were shuttered,"  Bloomberg  reports .

"Instead of rising, the price of electricity in the nation’s largest grid is now 40% lower than it ...

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U.S. Crude Storage Capacity Utilization Rises Even as Capacity Grows

1 DAY AGO

Weekly U.S. commercial crude oil inventories have increased by more than 71 million barrels (15%) since the end of September, pushing crude oil storage capacity utilization to a near record high of 73% for the week ending June 10.

The U.S. Energy Information Administration (EIA) measures crude oil stor...

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Trump Comes Out Against Trade, Draws Pushback

1 DAY AGO

GOP presidential candidate Donald Trump, speaking Tuesday near Pittsburgh, expressed fierce opposition to free-trade agreements and criticized Hillary Clinton for supporting them. Trump blamed Clinton and her husband for the loss of millions of manufacturing jobs and threatened to pull the U.S. out ...

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Consumer Confidence Climbs to 8-Month High

2 DAYS AGO

The Conference Board Consumer Confidence Index, which had decreased in May, improved in June. The Index now stands at 98.0 , up from 92.4 in May. Those stating business conditions are “good” increased slightly from 26.1% to 26.9%, while those saying business conditions are “bad&rdquo...

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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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