What Is API 660? Requirements for Shell-and-Tube Heat Exchangers

API 660 is one of the key standards for shell-and-tube heat exchangers used in petroleum, petrochemical, and natural gas service. It helps establish consistent requirements for equipment that often operates under demanding pressure, temperature, corrosion, fouling, and thermal-cycling conditions.

Published by the American Petroleum Institute, API 660 covers key areas including design, materials, fabrication, inspection, and testing.

This guide explains what API 660 requirements cover and how the standard works with ASME, TEMA, and ISO 16812 in real heat exchanger projects.

CONTENT:

• What Is API 660?
• What Does API 660 Require for Shell-and-Tube Heat Exchangers?
  • Mechanical Design Requirements
  • Material Requirements
  • Fabrication Requirements
  • Inspection and Testing Requirements
  • Documentation Requirements
• How Do API 660, ASME, and TEMA Work Together?
• FAQ About API 660
• Conclusion

What Is API 660?

API 660 is an industry standard published by the American Petroleum Institute for shell-and-tube heat exchangers used in petroleum, petrochemical, and natural gas processing. It establishes a common technical basis for how these exchangers should be designed, manufactured, inspected, tested, and prepared for delivery.

The standard applies to common exchanger services such as heaters, coolers, condensers, and reboilers. It does not apply to vacuum-operated steam surface condensers or feed-water heaters.

API 660 at a Glance
Scope	Shell-and-tube heat exchangers for petroleum, petrochemical, and natural gas service
Equipment	Shell-and-tube heat exchangers
Main industries	Petroleum, petrochemical, natural gas
Typical services	Heaters, coolers, condensers, reboilers
Main coverage	Mechanical design, materials, fabrication, inspection, testing, shipment preparation

Why is this important? In refinery and petrochemical service, a heat exchanger has to manage more than pressure alone. Temperature differences, thermal expansion, corrosion, fouling, vibration, erosion, and cyclic operation can all affect long-term reliability.

Specifying only the required heat duty is rarely enough. Exchanger configuration, materials, mechanical details, fabrication controls, inspection, and project requirements need to be considered together. API Standard 660 provides a consistent framework for doing that.

What Does API 660 Require for Shell-and-Tube Heat Exchangers?

API 660 requirements are not limited to a single design calculation. They follow the exchanger from mechanical design and material selection through fabrication, inspection, testing, and preparation for shipment.

For a typical project, the requirements can be understood in five connected areas:

Requirement Area	What It Covers
Mechanical Design	Pressure and temperature conditions, tubes, tubesheets, baffles, thermal expansion, nozzle loads, vibration and other mechanical details
Materials	Material selection, corrosion resistance and corrosion allowance
Fabrication	Welding, tube-to-tubesheet joints, dimensional tolerances and PWHT where required
Inspection & Testing	NDE, dimensional inspection, pressure testing and leak testing
Documentation	Design calculations, drawings, material records, welding/NDT records and final project documents

These requirements are closely connected. A change in operating temperature or material, for example, can affect mechanical design, welding procedures, heat treatment and inspection requirements at the same time.

The following sections look at each requirement area in more detail.

Mechanical Design Requirements

Mechanical design is the first major requirement area in API 660. Some values come from the project conditions, while others are defined by the standard as minimum limits or allowable values.

Item	Specific API 660 Requirement*
Minimum tube outside diameter	19.05 mm (3/4 in.), unless otherwise specified or approved
Minimum tube wall thickness – carbon / low-alloy steel	2.11 mm (0.083 in.)
Minimum tube wall thickness – copper alloys	1.47 mm (0.058 in.)
Minimum tube wall thickness – high-alloy steel / other nonferrous materials	1.47 mm (0.058 in.)
Minimum tube wall thickness – titanium	1.07 mm (0.042 in.)
U-bend radius	Normally at least 1.5 × tube OD; certain alloys require 2.0 × tube OD
Slip-on flange restriction	Not permitted above 2100 kPa(g) / 300 psig, above 400°C / 750°F, or under certain corrosive / cyclic services
Chemical-cleaning connection	Minimum DN 50 (NPS 2) when specified
Nozzle loads	Allowable forces and moments are specified by nozzle size and flange rating

*Values are based on API 660 Ninth Edition requirements and should be checked against the project-specified edition.

The API 660 nozzle loads table is a good example of how specific the standard can be: it provides defined allowable forces and moments instead of leaving piping loads entirely to engineering judgment.

Other items, such as design pressure, design temperature, and thermal expansion, depend on the actual service conditions. These design choices also affect material selection, which is covered next.

Material Requirements

Material selection follows the actual process conditions. Pressure, temperature, fluid chemistry, corrosion mechanisms, and expected service life all influence the final choice.

API 660 also sets several specific material requirements:

Material Area	Specific API 660 Requirement*
Alloy lining	Must be weld overlay, integrally clad, or explosion-bonded. Loose liners or sleeves require purchaser approval.
Weld overlay	Specified chemical composition must be maintained to a depth of at least 1.5 mm (1/16 in.) from the finished surface.
Tubesheet cladding – tube side	Minimum 10 mm (3/8 in.) for expanded-only tube joints and 5 mm (3/16 in.) for welded tube joints.
Tubesheet cladding – shell side	Minimum 10 mm (3/8 in.).
Carbon steel in sour / wet H₂S service	Normally supplied in the normalized condition, unless otherwise approved by the purchaser.
Sour-service material records	Pressure-retaining components require a Certified Material Test Report (CMTR) with chemistry needed to determine carbon equivalent.
Gaskets	Asbestos is prohibited. Metallic gasket materials must have corrosion resistance at least equal to the gasket-contact surface.
Compressed-sheet gaskets	Not permitted for hydrocarbon, steam, hydrogen, sour, or wet-H₂S service.

*Values are based on API 660 Ninth Edition requirements and should be verified against the edition specified for the project.

For sour or high-temperature hydrogen service, additional material requirements may also come from NACE MR0103 / ISO 17945 or API RP 941.

Corrosion allowance is project-specific. A typical value may be around 1.5–3 mm, but the final requirement should follow the actual corrosion assessment and purchaser specification.

Once the material requirements are confirmed, the next step is controlling how those materials are welded, machined, and assembled during fabrication.

Fabrication Requirements

Fabrication turns the approved design into the finished exchanger. Key controls include welding procedures, tube-to-tubesheet joints, machining tolerances, and PWHT where required. WPS, PQR, qualified welders, and controlled tube joining are therefore part of the fabrication basis.

API 660 also gives specific fabrication limits:

Fabrication Item	API 660 Requirement*
Tube holes for selected alloy tubes	Austenitic stainless steel, duplex, titanium, cupronickel, and nickel-alloy tubes require TEMA Special Close Fit tube holes
Roller-expanded joints	Maximum tube-wall reduction: 8% for carbon/low-alloy steel, 6% for stainless/high-alloy steel, and 5% for titanium/work-hardening nonferrous materials
Welded + expanded joints	Tube expansion should begin at least 6 mm (1/4 in.) from the weld
Expansion near shell-side face	Expansion should stop at least 3 mm (1/8 in.) from the shell-side tubesheet face
Weld hardness	Maximum 225 HBW for carbon steel and Cr steel up to 3% Cr; 241 HBW for 5–17% Cr steel
Stacked exchanger flange alignment	Mating nozzle flanges: out-of-parallel ≤ 0.8 mm (1/32 in.); separation after gasket installation ≤ 3 mm (1/8 in.)

*Values are based on API 660 Ninth Edition requirements and should be checked against the edition specified for the project.

Tube-to-tubesheet joints may use expansion, strength welding, seal welding, or a combination of welding and expansion. The method must achieve a leak-tight joint without excessive tube-wall thinning or damage.

PWHT is not defined by one universal API 660 temperature or thickness limit. Its need depends on the material, thickness, service, welding configuration, and applicable pressure design code. Where PWHT is required, subsequent hardness testing and inspection must account for the heat-treated condition.

These fabrication controls are then verified through inspection and testing.

Inspection and Testing Requirements

After fabrication, inspection and testing are used to verify weld quality, joint integrity, dimensional accuracy, and pressure containment.

Inspection / Test	Specific API 660 Requirement
Welded-and-expanded tube-to-tubesheet joints	Pneumatic leak test at 50–100 kPa (7.5–15 psi) before final tube expansion
Shell and tube sides	Independent hydrostatic tests are required, except for differential-pressure designs
Hydrostatic test duration	Test pressure maintained for at least 1 hour
Test water	Potable water
Austenitic stainless steel	Chloride content in test water ≤ 50 mg/kg (50 ppm)
Helium leak testing	Performed after final expansion when specified by the purchaser
Nonmagnetic welds	Liquid penetrant testing is used where magnetic-particle examination is not applicable
After PWHT	Required weld examinations and final visual inspection are carried out after heat treatment where applicable

*Values are based on API 660 Ninth Edition requirements and should be checked against the edition specified for the project.

API 660 does not use one fixed hydrostatic test pressure for every exchanger. The test pressure depends on the applicable pressure design code, material allowable stresses, and project conditions.

Radiographic testing (RT), liquid penetrant testing (PT), dimensional inspection, hydrostatic testing, and leak testing may therefore be combined according to the exchanger design and specified inspection scope.

These records are then included in the final documentation package.

Documentation Requirements

API 660 also requires the exchanger to be supported by a complete technical record. These documents allow the purchaser to verify design, materials, fabrication, inspection, and final as-built condition.

Document Type	Typical Contents
Design documents	Mechanical calculations, stress calculations, thermal rating data
Drawings	General arrangement, fabrication drawings, nozzle schedules, component details
Material records	Material Test Certificates, mill certificates, heat-number traceability
Welding records	WPS, PQR, welder qualifications
Inspection records	RT, PT and other NDT reports, dimensional inspection records
Test records	Hydrostatic and leak-test reports
Final documentation	As-built drawings and approved vendor data

These records remain useful well after delivery. They support installation, commissioning, future inspection, maintenance, revamp, and replacement work.

The heat exchanger specification and API 660 datasheet bring together the process conditions, design basis, materials, exchanger configuration, and purchaser requirements used by the manufacturer.

How Do API 660, ASME, and TEMA Work Together?

A shell-and-tube heat exchanger project may reference all three, but they serve different purposes:

• ASME Section VIII: Focuses on pressure-vessel safety, including pressure-boundary design, allowable stress, welding, examination, and pressure testing.
• TEMA: Defines how shell-and-tube heat exchangers are configured and mechanically constructed, including exchanger types, tubesheets, baffles, bundles, clearances, and fabrication practices.
• API 660: Adds more specific requirements for shell-and-tube exchangers used in refinery, petrochemical, and natural-gas service. In many areas, it places additional limits or requirements beyond the basic TEMA provisions.

For an ASME heat exchanger design, Section VIII establishes the pressure-boundary basis, while TEMA and API 660 address exchanger-specific construction and service requirements.

A typical project may therefore specify:

ASME Section VIII + TEMA Class R + API 660 + purchaser specification

Because ASME mainly governs the pressure boundary, the most direct requirement-by-requirement comparison is between API 660 and TEMA.

API 660 vs TEMA: Key Requirement Differences

Subject	API 660 Requirement	TEMA Requirement
Tubesheet cladding thickness	Minimum 10 mm for expanded tube-to-tubesheet joints; 5 mm for welded joints	Class R/B: 7.8 mm expanded, 3.2 mm welded; Class C: 4.8 mm expanded, 3.2 mm welded
Sliding saddle anchor-bolt slot	Slot width = anchor-bolt diameter + 8 mm (5/16 in.); slot length also includes longitudinal movement allowance	No specific numerical slot dimensions
Floating-head exchanger types	TEMA Type P and W exchangers are not allowed	Type P is allowed; Type W is allowed for limited services
Backing-device corrosion allowance	Shell-side corrosion allowance is included on the back side of the floating-head backing device	No corrosion allowance required
Minimum tube OD	19.05 mm (3/4 in.)	Smaller tubes, down to about 6.4 mm, may be permitted
Minimum tube thickness	Minimum thickness defined according to tube material	No equivalent material-specific minimum thickness requirement
Tubesheet extension	Full-diameter stationary tubesheet required for removable bundles with bonnet-type front heads	No specific tubesheet-extension requirement
Baffle thickness	Carbon/low-alloy steel baffles and support plates must include the specified shell-side corrosion allowance	Thickness based mainly on shell diameter and plate spacing; corrosion allowance not generally required
Impingement plate thickness	Minimum 6 mm (1/4 in.)	No specific minimum thickness
Perforated impingement plate	Perforated impingement-plate baffles are not permitted	Perforated distribution devices may be used
Bypass sealing devices	Specific provisions for seal strips, tie rods, dummy tubes, and other bypass-control devices	No equivalent numerical requirement
Seal-strip thickness	At least the transverse-baffle thickness or 6 mm (1/4 in.), whichever is less	No specific thickness requirement
Stud washers	Hardened washers required for studs 38 mm (1½ in.) and larger; minimum washer thickness 6 mm (1/4 in.)	No equivalent requirement
Flange allowable stress	Short-time tensile-strength-based allowable stress is not permitted for girth flanges, gasketed tubesheets, and gasketed flat covers	No equivalent restriction
Thin-wall expansion joints	Detailed requirements covering service, cycle life, liner design, and related conditions	Not specifically covered
Gasket construction	Requirements specified for welded/non-welded, serrated, jacketed, corrugated, and spiral-wound gaskets	No equivalent detailed requirement
Integrally finned copper-alloy tubes	Required in the annealed-temper condition in accordance with ASTM B359/B359M	No specific requirement
Shell ovality	For removable bundles, ovality checked using a metal template made from at least two disks and spaced at least 300 mm apart	Fabricated shell ID limited by circumferential measurements; no metal-template requirement
Pass-partition plate weld	First 50 mm from gasket face must be full-penetration welded where partition plate is welded on both sides	No equivalent full-penetration requirement
PWHT – channel	Additional PWHT requirements apply to certain CS/LAS channels, including multi-pass or high nozzle-to-ID ratio designs	No equivalent specific requirement
PWHT – floating head	CS/LAS floating heads fabricated from dish-and-ring construction require PWHT	No equivalent specific requirement
Stacked exchanger tolerances	Mating nozzle-flange tolerances are more restrictive	Uses standard TEMA Figure F-1/F-2 tolerances
Tube holes	“Special Close Fit” holes required for austenitic stainless steel, duplex, titanium, cupronickel, and nickel-alloy tubes	No equivalent material-based requirement for standard vs special close fit
Maximum tube-wall reduction	Specific maximum roller-expansion reduction defined for different tube materials	No equivalent material-specific limit
Expansion length inside tubesheet	For welded-and-expanded joints, expansion begins 6 mm from the weld and stops 3 mm from the shell-side face	Expansion generally extends through the tubesheet with a smaller end clearance
Pneumatic test for welded tube joints	Additional pneumatic test required before final expansion of welded-and-expanded tube-to-tubesheet joints	No equivalent specific requirement

*The detailed values above are based on the API 660/TEMA comparison data provided for this article. Exact requirements should always be checked against the editions and purchaser specifications specified for the project.

This comparison shows why API 660 vs TEMA is not simply a question of choosing one standard. TEMA provides the basic exchanger construction framework, while API 660 adds more prescriptive requirements in areas where refinery service demands tighter control of corrosion allowance, tube construction, flow distribution, fabrication, and inspection.

FAQ About API 660

Conclusion

API 660 sets key requirements for shell-and-tube heat exchangers used in demanding refinery, petrochemical, and natural-gas service. It covers mechanical design, materials, fabrication, inspection, testing, and documentation.

In practice, API 660 is often used together with ASME, TEMA, and project specifications. For API 660 exchanger projects, Gelan can support design clarification, fabrication, testing, and documentation.