Industrial PVF

Special Requirements Needed For HF Service

It's not just ordinary carbon steel anymore for refineries and many other PVF users in process industries that generate hydrogen fluoride (HF), a poisonous gas that when dissolved in water produces hydrofluoric acid. In order to stave off a potential disaster of Bhopal magnitude or worse, customers are requesting that PVF suppliers provide product with material chemistry that meets HF service requirements.

John Lafferty, marketing manager and sales engineer for Penn Machine, addressed the background and specifics of HF service in a presentation to Houston's PVF Roundtable last Feb. 18. Some highlights of his presentation follow:

Hydrofluoric acid is used in an alkylation process that is critical to petroleum refining and certain other chemical processing. A tremendous amount of alkylate production takes place in the United States - more than 300,000 barrels per day, using upwards of 47,000 pounds per day of HF. The U.S. has 47 HF units in production, with 120 units worldwide.
Under unlikely but not impossible conditions, an HF accident could release a dense, toxic, aerosol cloud that can travel intact 6-10 miles from the accident site. Such an HF cloud could potentially kill thousands of people in that vicinity. According to the National Oil Refinery Action Network, more than 2.3 million people, mainly in the west, southwest and midwestern regions of the U.S., live within a 1- to 5-mile radius of a refinery. Even the smallest of these refineries with an HF accident would kill an estimated 5,000 people.
Carbon steel PVF products may be used safely for HF service, as long as HF acid concentration stays at least 65% and moisture level below 4% - in essence, virtually NO WATER. A Refinery Task Group concluded that residual elements of nickel and copper should be no more than .15%, and established a carbon equivalent (CE) maximum of .43%. Other restricted carbon steel elements include vanadium (V), maximum = .02%; niobium (Nb) = .02%; V + Nb maximum = .03%; minimum carbon (C) = .18%.
In high temperature areas such as the acid boiler or the bottom of the acid regeneration of the alkylation unit, only Alloy 400 (Monel UNS#NO4400) PVF materials should be used. Alloy 400 also is recommended when the material has shown past performance of having hydrogen blistering, cracking or severe corrosion.

Some other HF service requirements include:

Gas tungsten arc (TIG) welding is preferred for a slag free process.

Screwed ends are preferred over socket weld or butt weld in small diameter piping products.

Flanged faces are made with an Alloy 400 overlay, and carbon steel valves with Alloy 400 trim and PTFE packing.

B7 or B7M bolting for flanges and valves are used to hold up if there exists any potential leakage on these products.

The Refinery Task Group concluded that a "normalized" heat treatment must be used. This guarantees that the material will have a homogeneous grain structure so the material is sound enough to withstand fluid corrosion rates to the material and maintain a longer service life in this type of corrosive service.

At the time of Lafferty's presentation, ASTM was balloting committee members to incorporate the preceding requirements into ASTM A960 and ASTM A961, covering steel pipe and fittings.

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