Reference profile
- Designation
- API 660
- Title
- Shell-and-Tube Heat Exchangers
- Issuing body
- American Petroleum Institute (API)
- Current edition
- 10th Edition (May 2020)
- Status
- Active; purchasing and technical specification standard adopted by refineries worldwide.
- Category
- Refinery Equipment Standard & Specification
Overview
API Standard 660 (ISO 16812) specifies the mechanical design, material selection, fabrication, inspection, and pressure testing requirements for shell-and-tube heat exchangers used in petroleum, petrochemical, and natural gas processing services.
It serves as a stringent purchasing and engineering supplement overlaid on top of TEMA Class R and ASME BPVC Section VIII Division 1, ensuring maximum operational safety, leak tightness for hazardous hydrocarbons, and ease of maintenance in heavy process plants.
Scope & Applicability
Covers
- Special mechanical design rules for stationary heads, floating heads (TEMA T and S), tubesheets, and shell covers.
- Corrosion allowance rules (minimum 3.2 mm / 1/8 inch for carbon steel and low-alloy pressure components).
- Allowable nozzle load limits (forces and moments acting on nozzles from connected piping).
- Tube-to-tubesheet joint requirements (strength welding, seal welding, and hydraulic expansion limits).
- Gasket type restrictions: Soft non-metallic gaskets prohibited for hydrocarbon processes; metallic or spiral wound required.
- Inspection and pressure testing procedures, including mandatory shop hydrostatic and bundle testing provisions.
Does NOT cover (exclusions)
- Air-Cooled Heat Exchangers (Fin-Fan Coolers)—covered by API 661.
- Plate Heat Exchangers (gasketed or welded)—covered by API 662.
- Thermal & hydraulic performance calculations (LMTD, overall heat transfer coefficient, pressure drop Sizing)—refer to TEMA / HTRI.
- In-service inspection, fitness-for-service, and repair procedures—refer to API 510.
Key Requirements
Design & Construction Overlay (API 660 over TEMA R)
- Floating Head Design: Floating heads for TEMA S and TEMA T must feature a split backing ring or bolted design for reliable sealing and easy maintenance. Threaded internal fasteners are strictly regulated.
- Bundle Pulling Provisions: All exchangers with removable bundles must be fitted with lifting lugs, bundle pulling eyes, and skid bars rated for bundle pulling loads.
- Vents & Drains: High-point vents and low-point drains must be provided on both shell and tube sides for complete flushing and deaeration.
Materials & Corrosion Protection
- Corrosion Allowance: Minimum 3.2 mm (1/8 inch) for carbon steel and low-alloy pressure-retaining components (shells, channels, covers, tubesheets).
- Tube Materials: Seamless tubes are mandatory for severe/hazardous services. Electric resistance welded (ERW) tubes are prohibited unless explicitly approved for mild service.
- Sour Service Compliance: Materials exposed to wet H2S must comply with NACE MR0175 / ISO 15156 (hardness testing, PWHT, and HIC/SSC resistance).
Nozzle Loads & Piping Interfaces
- Nozzles must be designed to withstand at least 2x the standard TEMA allowable moments and forces transmitted from external piping thermal expansion.
- All shell and channel nozzles 2 inches and larger must be flanged (welded neck or studding outlet) with full-penetration attachment welds.
Examination & Hydrostatic Testing
- Hydrostatic Test: Shell and tube sides must be tested at a minimum of 1.3x MAWP (or ASME VIII Div 1 rules) with clean water; hold time minimum 1 hour.
- Floating Head Test: Floating head bundles must undergo a shop bundle hydrotest using a test ring before final assembly into the shell.
Technical Details
API 660 Nozzle Loading Standards: Standardizes calculation of maximum allowable nozzle moments (Mx, My, Mz) and shear forces (Fx, Fy, Fz) using WRC 107/WRC 537 or FEA analysis to prevent shell distortion.
Tubesheet Ligament & Joint Strength: Joint strength calculation for expanded and welded joints complies with ASME VIII-1 Appendix A and API 660 pull-out resistance testing.
Minimum Shell & Tube Dimensions: Tube OD minimum is typically 3/4 inch (19.05 mm); tube wall thickness minimum is 16 BWG (1.65 mm) for carbon steel in refinery service.
Engineering Notes
- API 660 is a procurement specification meant to be referenced in project datasheets and purchase orders alongside ASME VIII-1 and TEMA Class R.
- When API 660 requirements conflict with TEMA Class R or ASME VIII-1, the more stringent requirement shall govern.
- Ensure adequate layout space in front of the exchanger equal to the tube bundle length plus 1.5 meters for bundle pulling crane operations.
Related Standards
- TEMA
Base mechanical standard defining exchanger types (AEM, AES, BEM) and Class R rules.
- ASME BPVC VIII-1
Mandatory construction code for pressure-retaining components (shells, channels, nozzles).
- API 661
Companion or referenced standard; verify its exact relationship in the adopted edition.
- API 662
Companion or referenced standard; verify its exact relationship in the adopted edition.
Source, Edition & Status
- Publisher
- API
- Edition Year
- N/A
- Industries
- Petrochemical, Refinery, Oil & Gas
- Content Status
- published
- Last Reviewed
- 2026-07-01
- Last Updated
- 2026-07-01
- Verified By
- Engineering Reviewer
- Official Link
- www.api.org
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