Figure 2: Where API RP 751 Low-RE limits apply on an HF Alkylation Valve. π www. valve-world. net Valve World November 2026 9
COVER STORY
MRC Global’ s approach to low-RE requirements: The chemistry and standards behind HF alkylation valves
Introduction to HF alkylation service
In HF alkylation service, small variations matter. This process uses hydrofluoric( HF) acid to produce high-value gasoline blending components. In its concentrated form, HF is highly corrosive and classified as a systemic toxin with exposure effects that may not be immediately apparent. These hazards are major reason HF alkylation units are held to a higher standard of material control and safety requirements than a general refinery service. Valves carry a large share of the material control requirements in HF alkylation units, being present at each pressure boundary and sealing point in the process. However, material consistency does not always receive the same level of attention as mechanical design; the margin for error at the material level is narrow, as even the slightest inconsistencies in residual elements can be the difference between a component that is acceptable for HF service and one that fails. That complexity shifts part of the burden ahead of installation. Design defines how a component is intended to perform, but material chemistry determines how it will perform, making confirmation of whether materials meet these requirements much more important. MRC Global approaches this challenge by working to identify missing supplemental requirements or miscoded purchase orders before they result in a component that does not meet HF service requirements. To better understand how these gaps can be addressed, it is important to first understand where these material limits come from and why they are so narrowly defined. This work starts with American Society for Testing and Materials( ASTM) whose specifications define material grades from the start. For carbon steel, each grade is defined by a specified carbon content range. A given heat( meaning a single furnace batch with its own chemical makeup) is then analyzed to determine whether its chemistry falls within the limits of that grade. In HF alkylation service, meeting the ASTM grade alone is not enough. API RP 751 imposes additional limits on residual copper, nickel, and chromium content to reduce corrosion risk. The criteria below determine whether a carbon steel component qualifies as low residual element or Low-RE.
Low-RE criteria for carbon steel
A carbon steel component qualifies as a low residual element( Low-RE) material under API RP 751, the industry recommended practice governing HF alkylation units, if it meets any of the following:
• Combined copper and nickel content stays at or below 0.15 % by weight, provided carbon content is 0.18 % or higher.
• Below that carbon threshold, or when carbon content is unknown, copper, nickel, and chromium combined must stay under that same 0.15 % limit.
• Weld metal is held to that same 0.15 % limit regardless of carbon level.
Two variables sit behind the 0.18 % carbon threshold: 1) carbon content itself; and 2) acid concentration. Carbon content determines how well the protective iron fluoride scale( a thin, corrosion-resistant film that forms on the steel’ s surface) holds up against residual copper, nickel, and chromium, which is why the residual-element limit tightens at the 0.18 % threshold. Acid concentration works alongside it: that same scale holds up in relatively concentrated HF with low water content, but in the dilute HF corrosion zone, where water content is higher, the scale breaks down and a nickel-based alloy becomes the more practical choice. Getting both right is what keeps a component matched to the conditions it truly operates in during service, and it’ s why those limits are applied zone-by-zone across a valve. API RP 751 sets that threshold for the unit itself, while MSS SP-160 carries it into valve design, inspection, and testing. Together, these two practices define what a compliant HF valve component looks like on paper( API RP 751, § 3.1.36, § 3.1.48; MSS SP-160, § 2.5, § 2.7). Confirming that
Figure 2: Where API RP 751 Low-RE limits apply on an HF Alkylation Valve. π www. valve-world. net Valve World November 2026 9