Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Monel 400 in HF Alkylation - API RP 751 Material Default
Monel 400 (UNS N04400, ASTM B164 bar, ASTM F468 bolts) is the industry-default alloy for HF alkylation unit wetted internals, HF-circuit piping, valve trim, and HF-wetted flange bolting. The governing recommended practice is API RP 751 Safe Operation of Hydrofluoric Acid Alkylation Units, which names Monel 400 for every surface that contacts liquid HF or HF vapour and for the bolting on every HF-wetted flange joint. The nickel-copper oxide film holds across the acid concentration band run inside a commercial HF alky unit (70 to 95 percent acid in the settler). Austenitic stainless and copper-alloy hardware corrode aggressively in the same service. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals. Numbers cited are from API RP 751, ASTM B164, ASTM B165, ASTM B564, ASTM F468, ASTM F467 and NACE MR0175 / ISO 15156-3.
The HF Alkylation Process
HF alkylation combines light olefin (butylene, propylene, amylene) with isobutane over an anhydrous hydrofluoric acid catalyst to produce a high-octane alkylate. The reactor runs at roughly 30 to 40 degrees Celsius and 8 to 12 bar; the acid settler separates spent acid from hydrocarbon at the same conditions. Acid concentration sits at 88 to 95 percent HF and is regenerated continuously to remove acid-soluble oil and water. Water content (target below 2 percent) controls the corrosion rate of every wetted surface, which is why API RP 751 binds the alloy selection to the inspection plan.
Where Monel 400 is Used Inside an HF Alky Unit
Monel 400 is specified at every surface that contacts HF in the liquid or vapour state. The inventory below is non-negotiable in current API RP 751 practice.
| Equipment | Monel 400 part | Specification |
|---|---|---|
| Acid settler | Internals, baffles, weir, demister pad frame | ASTM B127 plate and sheet |
| Reactor riser, depropanizer feed | Internal lining, distributor, tray attachments | ASTM B127 plate, ASTM B164 bar |
| HF acid stripper | Trays, tray support, downcomer | ASTM B127 sheet |
| HF-wetted piping | Seamless pipe and seamless tube | ASTM B165 |
| Flanges and forgings | WNRF flanges, blinds, fittings | ASTM B564 |
| HF valve trim | Plug, seat, stem, gland follower | ASTM B164 bar |
| Flange bolting (every HF-wetted joint) | Stud bolts + heavy hex nuts | ASTM F468 + ASTM F467 |
| Instrument tubing and connectors | Seamless tube, compression fittings | ASTM B165 + ASTM B164 |
API RP 751 Material Requirements
API RP 751 binds the alloy selection to inspection regime, bolt-up procedure and turnaround interval. Monel 400 is named as the default alloy for HF-wetted surfaces; surfaces are classified by service (continuous acid, intermittent, vapour, hydrocarbon-only) and assigned inspection method and frequency. Hardness on Monel 400 bolting is the F468 25 HRC cap; welding consumables are AWS A5.11 ENiCu-7 and A5.14 ERNiCu-7. Hydrogen-blistering monitoring on the carbon-steel pressure boundary is the parallel mechanism in the unit.
Companion Materials in the HF Alky Circuit
Monel 400 hardware pairs with specific non-metallic companions. Gaskets on HF-wetted flanges are virgin PTFE or PTFE envelope with aramid filler; graphite-filled spiral-wound gaskets are excluded. Valve packing is PTFE V-ring sets or HF-grade braided PTFE. Instrument diaphragms use Monel 400 wetted parts with PTFE secondary seals.
Fastener Selection for HF Alky Flanges
Every flange in the HF-wetted circuit uses Monel 400 stud bolts to ASTM F468 with Monel 400 heavy hex nuts to ASTM F467. The F468 25 HRC cap sits below the NACE MR0175 35 HRC ceiling, so the bolt is sour-service-compliant by design. Threading is ASME B1.1 8UN above 1 inch and UNC / UNF below. See Monel 400 stud bolts, heavy hex nuts and Monel 400 bolts. Lubrication is a PTFE-filled anti-seize; nickel-graphite anti-seize is excluded because graphite is incompatible with HF.
| Flange (Class 300) | Stud size + length | Studs / joint |
|---|---|---|
| 2 in WNRF | 5/8 in x 4-1/4 in | 8 |
| 4 in WNRF | 3/4 in x 5 in | 8 |
| 6 in WNRF | 3/4 in x 5-1/2 in | 12 |
| 8 in WNRF | 7/8 in x 6 in | 12 |
| 12 in WNRF | 1-1/8 in x 7-1/4 in (8UN) | 16 |
| 16 in WNRF | 1-1/4 in x 8 in (8UN) | 20 |
Inspection Requirements on Monel 400 HF Hardware
Every Monel 400 fastener heat is verified by positive material identification (PMI) on the bolt body before bolt-up; the PMI alloy ID is recorded against the joint number in the turnaround dossier. The mill test certificate is EN 10204 type 3.2, witnessed by a third-party inspector. Settler and stripper internals receive hydrostatic pressure test with low-chloride water and full dry-out before HF reintroduction. Welded joints on Monel 400 piping receive 100 percent radiography per ASME B31.3. Monel 400 surfaces are exempt from hydrogen-blistering monitoring; the nickel-copper film does not generate atomic hydrogen at the cathodic site.
Why Stainless Steel Fails in HF Service
Austenitic stainless (SS 304, SS 316, SS 316L) relies on a chromium-rich passive film. HF attacks the chromium oxide directly: chromium fluoride is soluble, the film cannot self-repair, and the substrate corrodes at hundreds of mils per year. Ferritic and duplex stainless fail by the same mechanism. Copper alloys corrode by cuprous-fluoride solubility above 5 percent HF. Carbon steel develops a protective iron-fluoride film in anhydrous HF above 70 percent acid and below the water dew point, which is why the pressure boundary uses carbon steel with strict water-control and blistering monitoring. Monel 400 carries the nickel-copper oxide film into both acid and vapour phases and is the only ASME-listed structural alloy that holds across the full HF alky band. Detail on HF acid resistance and Monel 400 vs stainless steel.
Frequently Asked Questions
What alloy is used for HF alkylation units? Monel 400 (UNS N04400). API RP 751 names it for HF-wetted internals, valve trim and every flange bolt in the HF circuit. Stainless and copper alloys are excluded from HF-wetted service.
Why Monel 400 and not stainless steel? HF dissolves the chromium-oxide film on stainless steel. SS 304, SS 316 and SS 316L corrode rapidly at any HF concentration. Monel 400 forms a stable nickel-copper oxide film that holds across the 70 to 95 percent acid band.
What is API RP 751? API Recommended Practice 751 is the industry standard for safe operation of hydrofluoric acid alkylation units. It binds the Monel 400 alloy selection to the inspection regime, the flange bolt-up procedure and the turnaround interval.
Which Monel 400 fasteners go on HF flanges? Stud bolts to ASTM F468 (UNS N04400, 25 HRC cap) with heavy hex nuts to ASTM F467. Threading is ASME B1.1 8UN above 1 inch. EN 10204 type 3.2 plus PMI per heat is the refinery standard.
Related Pages
Application: HF acid resistance, vs stainless steel, NACE MR0175, corrosion resistance. Fasteners: bolts, stud bolts, heavy hex bolts, heavy hex nuts, nuts, washers. Product forms: plate, seamless pipe, tubing, flanges, fittings, forgings. Standards: ASTM B127, ASTM B164, ASTM B165, ASTM B564, ASTM F467, ASTM F468. Pricing: Monel 400 pricing.
Request a Quote
For Monel 400 hardware on an HF alkylation turnaround or capital project, send the line list, flange schedule and stud / nut bill of materials with project-specific PMI / certification requirements to info@torqbolt.com or WhatsApp +91-22-66157017. TorqBolt QA-QC returns the alloy-conformance memo, EN 10204 type 3.2 statement, mill source, NACE MR0175 statement and Ex-Mumbai price within one working day. Domestic India 3 to 7 working days; export through JNPT.
References: API Recommended Practice 751. ASTM B127, B164, B165, B564 (Monel 400 product forms). ASTM F468 and F467 (UNS N04400 bolts and nuts). NACE MR0175 / ISO 15156-3 (Part 3 CRAs). AWS A5.11 and A5.14 (nickel-copper welding consumables).
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