A recent project involved a shell-and-tube exchanger in a refinery hydrocracker unit. The process engineer specified the channel based on TEMA selection logic: moderate pressure, clean service on the tube side, so a TEMA B-type bonnet with a standard gasketed cover plate seemed reasonable.
Then the procurement team flagged it. The tube-side design pressure was 10.5 MPa—just above the 10.3 MPa (1500 psig) threshold in API 660. That single number changed the required channel type from B to C. The cover plate design was rejected, and the exchanger had to be reconfigured before fabrication could begin.
This is not an isolated case. It happens because engineers apply TEMA correctly but stop there—without checking whether API 660 adds a mandatory overlay.
TEMA does not prescribe channel types based on pressure thresholds. It defines the available configurations—A, B, C, N, and D—and describes their construction features:
TEMA's selection guidance is qualitative. It tells you that C-type is preferred when the gasket should not be disturbed during tube bundle maintenance, and that N-type is appropriate for high-pressure service. It does not say “above 1500 psig, use C-type.” That is an engineering judgment left to the designer.
API 660 converts TEMA's qualitative guidance into mandatory, numeric rules. For channel selection, four thresholds trigger specific requirements:
| Condition | Required Action |
|---|---|
| Tube-side design pressure > 10.3 MPa (1500 psig) | TEMA C-type channel for removable bundles; TEMA N-type for fixed tubesheets |
| Tube-side to shell-side design pressure differential > 5.2 MPa (750 psig) | TEMA C-type channel |
| Tubesheet tube-side temperature differential > 110°C (200°F) | TEMA C-type channel with internal gasket-type pass partition box |
| Tubesheet thickness-direction temperature differential > 140°C (250°F) | TEMA C-type, or a design that eliminates the double gasket seal |
The first threshold is the most commonly triggered in hydrocracker and hydrotreater service. A tube-side design pressure of 10.5 MPa is not unusual in these units—and it is exactly the kind of number that slips past a TEMA-only review.
The reason is not arbitrary. It comes down to gasket exposure and leak paths in high-pressure service.
In a B-type channel, the cover plate gasket is compressed by the same bolts that hold the cover in place. When the cover is removed for inspection, the gasket is disturbed. In high-pressure service, re-making that joint reliably in the field becomes a critical path activity—and a potential leak point.
In a C-type channel, the gasket is not in the bolting path. The channel is bolted to the shell or tubesheet, and the cover plate is a separate closure. This means the tube bundle can be accessed without breaking the high-pressure seal between channel and shell. For a refinery unit where downtime costs are measured in tens of thousands of dollars per day, this distinction matters.
API 660 also references the ASME Code and specific material requirements that go beyond TEMA. For example, for austenitic stainless steel, duplex steel, and titanium tubes, API 660 requires the “special tight fit” tolerance from TEMA—turning what TEMA lists as an option into a mandatory requirement for vibration-prone petrochemical service.
A simple three-step check before finalizing channel selection:
If none of these thresholds are triggered, TEMA selection logic applies—and a B-type channel may be perfectly adequate.
TEMA tells you what channel types exist and when each is generally preferred. API 660 tells you when a specific type is required. For high-pressure hydrocracker and hydrotreater service, the 10.3 MPa threshold is the one that most often turns a routine TEMA selection into a mandatory API 660 requirement.
The cost of missing it is not just a design change—it is a procurement delay, a fabrication hold, and potentially a re-rating of the entire exchanger.
Yuhong Group manufactures TEMA and API 660 shell and tube heat exchangers, backed by ASME-certified fabrication, TEMA class R, C and B designs, third-party inspection experience and full material traceability — contact us for high-pressure channel selection support on your next refinery project.
