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The Dos and Don’ts of Isolating Pressure Relief Valves

The Dos and Don’ts of Isolating Pressure Relief Valves

Typically, isolation valves are used to ...

Valve Basics Training Helps Fill Industry Skills Gap

Valve Basics Training Helps Fill Industry Skills Gap

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An End-User’s Perspective on Valve Selection and Risk

An End-User’s Perspective on Valve Selection and Risk

I am not a valve expert, although I ofte...

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Why Gas Won’t Be Traded Like Oil in the Future

Wednesday, 31 August 2016  |  Chris Guy

Natural gas can be transported anywhere on LNG carriers, but will “probably remain regionally priced for the time being, with some contracts con...

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Industry Headlines

Chemours Selects Wilmington, DE for its Global Headquarters

1 DAY AGO

The Chemours Company will locate its global headquarters in Wilmington, DE. Chemours began evaluating headquarters location options more than 6 months ago with Delaware, New Jersey and Pennsylvania all under active consideration. Recent changes to Delaware’s corporate tax structure were key comp...

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ValvTechnologies Successfully Completes NUPIC Audit

7 DAYS AGO

ValvTechnologies, Inc. recently achieved NUPIC-approved suppliers list status, upon successful completion of the Nuclear Procurement Issues Committee (NUPIC) audit conducted at the Houston facility. NUPIC members include all domestic U.S. nuclear utilities as well as several international members.

Form...

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Why Gas Won’t Be Traded Like Oil in the Future

9 HOURS AGO

Natural gas can be transported anywhere on LNG carriers, but will “probably remain regionally priced for the time being, with some contracts continuing to track oil, said Roger Bounds, senior V.P. for global gas at Shell. Prices will depend on location, regulation and infrastructure, as some c...

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Feds Warn of Bolt Failures in Offshore Drilling

1 DAY AGO

On Monday, Bureau of Safety and Environmental Enforcement (BSEE) director Brian Salerno held a press conference aimed at oil & gas regulators. The issue he wanted to draw attention to is the failure of bolts in offshore drilling equipment such as blowout preventers. This problem, Salerno says, has...

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U.S. Consumer Confidence at 11-Month High

11 HOURS AGO

The Conference Board Consumer Confidence Index, which had decreased slightly in July, increased in August. The Index now stands at 101.1 , compared to 96.7 in July. Consumers’ appraisal of current conditions improved in August. Those stating business conditions are “good” increased f...

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Advanced Manufacturing Outpaces Rest of Production Sector

1 DAY AGO

A new report from the Brookings Institution finds that high-tech advanced manufacturing bucked both global economic challenges and the longstanding shrinking of the traditional manufacturing sector over the last two years. While job losses were felt in other pockets of the production sector, advance...

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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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