Reynolds Wrap Up: Substantial Changes Coming in API 751 on HF Alkylation Unit Mechanical Integrity Requirements and Expectations

By John Reynolds, Principal Consultant at Intertek. April 30, 2020


API RP 751 on Safe Operation of Hydrofluoric Acid Alkylation Units is in the final stages of revision before balloting and publication of the fifth edition of the standard. The largest changes pending are in Section 3 (soon to become Section 6) and Annex D (soon to become Annex G). This ‘Reynolds Wrap Up’ will highlight some of the most significant changes that are pending in the section on Mechanical Integrity, Damage Mechanisms and Maintenance Work Practices (hereafter referred to as the MI section). There are just over 130 refineries left in the USA and a bit over one third of them have HF Alkylation (HFA) units to produce a high octane gasoline blending component.

A task group (TG) of HFA MI subject matter experts (SMEs) with both API and NACE contributors has been busy over the last 2-3 years working on the MI section to update it in order to continue progress to enhance the process safety aspects of operating HFA units. The SMEs contributing to the new revision include those from operating companies and consultants with long experience in HFA unit design, processing, inspection and auditing. The majority of HFA operators are involved in the revision, bringing to bear thousands of hours of operating experience. Over the years since the publication of the fourth edition, owner-operators have made a lot of changes to continue to improve the mechanical integrity and process safety of operating their HFA units. The effort of this API 751 MI TG is to capture most of those improvements and standardize them for all to follow.

Some of the Big Changes in the Fifth Edition

From my perspective, the ten most significant changes being considered for the MI section (out of upwards to a hundred total) of API RP 751 in the fifth edition include:

  1. Changing process system classifications of acid handling equipment from two types of acid handling services (Main and Trace Acid) to five different corrosion zones a (CZ):
    • Fresh HF CZ
    • Rich HF CZ
    • Phase Change HF CZ
    • Dilute HF in Water CZ
    • Fractionator HF CZ
  2. Increased guidance on the low residual element (RE) issue for carbon steel causing preferential, localized corrosion in most of the corrosion zones mentioned above.
  3. Requirements and expectations for inspection programs to detect localized preferential corrosion in HFA units.
  4. New requirements and expectations for HFA Corrosion Control Documents (CCDs) and Integrity Operating Windows (IOWs).
  5. Substantially enhanced informative Annex on the Damage Mechanisms (DM) associated with HFA processing and what can happen to equipment/piping when and where these DMs occur.
  6. New section on Bolted Joint Assembly guidance to reduce the likelihood of flange leaks.
  7. Increased emphasis on deadleg identification and inspection for localized corrosion and fouling using as much Profile RT and/or UT scanning; plus the need for periodic review of all deadlegs to identify and eliminate non-essential ones.
  8. New guidance on investigating all HFA leaks and near-leaks (including surprise findings during turnarounds) to identify and eliminate their causes.
  9. More information and experience on alloy upgrades to solve corrosion problems and the reasons behind why some materials of construction need to be excluded from the HFA unit.
  10. Significantly more guidance on the need for and recommended elements of a materials verification program and Positive Material Identification (PMI) for HFA units, including the newer field PMI instruments that can detect low levels of residual elements.

New Decisions Based on the Five Corrosion Zones

In this ‘Reynolds Wrap Up’ I will cover some of the major decisions that sites will need to make based on the five new Corrosion Zones (No. 1 change on the list above) after transitioning away from the older Main and Trace Acid categories of the HFA process unit. Here is a very brief overview of each corrosion zone:

  1. Fresh HF Corrosion Zone contains HF as delivered by the supplier.
  2. Rich HF Corrosion Zone contains HF Acid with hydrocarbon and water.
  3. Phase Change HF Corrosion Zone occurs in areas where HF is either being condensed or vaporized, either by design or inadvertently: such as condensing in overhead lines.
  4. Dilute HF in Water Corrosion Zone occurs where the water phase condenses which contains a low concentration of HF.
  5. Fractionator HF Corrosion Zone is the section of fractionator towers between the bottom where HF is being produced by thermal defluorination and where the HF and water in the tower feed has been stripped out.

Clearly there is much more understanding of the five zones presented in the new Annex G. Once each site has successfully converted their HFA P&IDs and/or PFDs and C&MDs from the color coding of Main/Trace to the five Corrosion Zones outlined in Annex G, the site has several decisions to make in order to facilitate improvements in their inspection and MI programs relative to each Corrosion Zone. Among those decisions that are affected by which Corrosion Zone is involved are:

  • Which operating parameters may require mandatory and/or recommended IOW limits
  • Where to be more cognizant of and to inspect more closely for localized/preferential corrosion
  • When PWHT of repair welds will be necessary and when they may be optional
  • When on-stream inspection can or cannot be substituted for a scheduled internal inspection
  • How to prioritize the special emphasis inspection program of 100% component inspection
  • Guidance on localized and/or preferential corrosion of carbon steel due to low/high RE connections
  • Guidance on the need for thermal stress relief of high alloy components in some zones
  • Guidance on CML selection and placement for localized/preferential corrosion in some zones
  • Where RBI can or cannot be used for inspection scheduling
  • How to schedule inspection intervals for flange surface crevice corrosion
  • When inspection intervals in the propane/butane rundown piping may need to be adjusted
  • Where acid-detecting paint is required in HF services

So clearly one of the early adjustments that site MI/Inspection groups will need to make when the fifth edition is published is to complete their transition from designating Main/Trace processing areas to the five new Corrosion Zones.

Stay up to date with all things mechanical integrity and inspection.

Every Monday, we send out a newsletter containing the latest Inspectioneering articles, blog posts, industry news and events, and more.

Sign up below to start getting the newsletter in your inbox.

Comments and Discussion

Posted by Chandra Vykanti on January 9, 2021
Is it possible to get a copy of API 751 5th... Log in or register to read the rest of this comment.

Posted by John Reynolds on January 9, 2021
The fifth edition of API RP 751 is still in final... Log in or register to read the rest of this comment.

Add a Comment

Please log in or register to participate in comments and discussions.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Training Solutions

Improve your skills in key mechanical integrity subjects.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.


Read short articles and insights authored by industry experts.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

Videos & Webinars

Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.