Why Must Shell-Side and Tube-Side Pressure Cases Be Specified Together for Heat Exchangers?

An exchanger enquiry may list a design pressure for each side and still leave an important question unanswered: what pressure and temperature will exist on the other side during the condition being assessed?

Specify shell-side and tube-side cases together so the designer can evaluate the relevant simultaneous pressures, temperatures and boundary conditions. Two independent maximum pressures do not establish every required combination, and subtracting them is not a complete differential-pressure design basis. Provide the applicable operating, shutdown, upset and test scenarios, then have the responsible designers confirm the required load cases and any restrictions.

For EPC teams and industrial equipment buyers, a heat exchanger differential design pressure review should produce a traceable case schedule. It should also distinguish the information supplied by the process team from the mechanical assessment and any formal approval to use differential-pressure design.

Why Two Maximum Pressures Do Not Define Every Case

Two design-pressure entries do not say what happens on the opposite side during a particular event. They also do not identify the corresponding temperatures, pressure direction or restrictions on the equipment.

Keep three kinds of information distinct:

• Specified design values: the requirements assigned to each side.
• Process scenarios: the pressures and temperatures that can occur together during a defined operating state or event.
• Mechanical load cases: the combinations the designer evaluates under the applicable rules and project specification.

TEMA’s public specification sheet reflects this distinction. In addition to separate design-pressure fields, it provides a case schedule with shell-side and tube-side conditions. The separate fields are useful, but they do not replace the schedule. TEMA specification sheet, pages 1–2

Hypothetical comparison—not project design data: suppose shell pressure is above tube pressure. If shell pressure remains unchanged while tube pressure falls, the shell-to-tube pressure difference increases. Subtracting the two independent maximum pressures would not describe that later state. Whether that state is possible, and how it must be assessed, still requires confirmation.

For a stated case, a signed difference such as shell pressure minus tube pressure can help communicate direction. Use compatible units, the same pressure reference and defined locations. That subtraction describes a pressure relationship; it is not a complete strength calculation for the exchanger.

For the underlying terminology, see design pressure vs MAWP. Here, the procurement task is to connect the two sides within each case, rather than replace their individual design requirements with one difference.

Identify Applicable Scenarios and Required Design Load Cases

Start with the process team’s documented operating envelope, then have the mechanical designer identify the required analytical combinations. These are related reviews, with different purposes.

Use the following questions to screen the project. Include a scenario when it applies, or record the basis for excluding it:

• Normal operation and turndown: what paired pressures and temperatures occur across the required operating range?
• Startup, shutdown and standby: can the two sides change state at different times, or remain at different temperatures?
• Upset or utility interruption: which credible events change one side while the other retains pressure?
• Single-side shutdown or cleaning: if this is part of the intended arrangement, what condition exists on the other side?
• Vacuum or external pressure: is either relevant to the specified service or transient, and what pressure boundary does the requirement concern?
• Shop or site testing: what are the simultaneous conditions on both sides for each proposed test configuration?

Codeware’s public design overview explicitly includes conditions beyond the basic design point, such as upset, shutdown and steam-out conditions. That supports a multi-case review; it does not establish that every listed scenario occurs in every exchanger. Codeware design conditions

Keep the mechanical review separate from the process credibility check. Codeware’s published example report distinguishes design loading cases from operating loading cases and lists different shell/tube pressure combinations. Do not remove a load case required by the applicable design rules solely because it is not a normal operating state. The designer must establish which rules and combinations apply to the selected construction. Codeware example report, PDF pages 89–90

For a changing event, request the relevant pressure and temperature history or an agreed set of representative points. Record duration and expected repetition instead of assuming that “temporary” means negligible.

Build a Traceable Paired Pressure-Case Schedule

A useful schedule lets the reviewer trace each pressure pair to one identified condition and its supporting documents. Attach it to the exchanger datasheet and use the same case identifiers in technical clarifications.

The following is a suggested information structure, not a completed engineering datasheet:

TEMA’s public case table includes both fluid and mean metal temperatures, alongside the two pressure columns and design cycles. Additional traceability fields above are procurement recommendations, not a reproduction of mandatory TEMA fields. TEMA specification sheet, page 2

Illustrative schedule—not project data or approved operating conditions:

Here, P and T stand for project-supplied pressure and corresponding temperature data, including inlet/outlet temperatures where needed. Each row also needs the reference, timing, boundary and traceability fields above. Its status remains awaiting clarification until the project confirms applicability and supplies the data. A case description alone is not design approval.

Be particularly careful with pressure references. Gauge pressure is referenced to ambient atmospheric pressure; absolute pressure uses an absolute-vacuum reference. A zero gauge value therefore does not mean zero absolute pressure. Identify the basis before comparing sides or describing vacuum. WIKA pressure types

Agree on Who Confirms the Cases and Design Assumptions

Assign confirmation duties in the contract or project responsibility matrix. A fabricator should not have to infer plant operating relationships from two pressure figures, and a process schedule should not be mistaken for mechanical design approval.

A practical allocation to agree is:

• Owner or process/licensor team: establishes the intended operating envelope, credible events and restrictions on plant operation.
• EPC or package coordinator: checks interfaces and makes sure equipment, connected systems and controls use consistent assumptions.
• Responsible mechanical designer: translates the accepted inputs into the required load cases, evaluates the specified construction and identifies limitations.
• Party responsible for testing: defines the proposed test conditions and boundaries for review against the equipment design and applicable requirements.
• Named purchaser reviewer: closes contractual deviations and records acceptance of the agreed basis.

This is a suggested division of work. The actual parties and approval authority depend on the contract.

Paired data is not permission to use differential-pressure design. A proposal that relies on pressure on one side to offset pressure on the other needs an explicit, reviewed basis. It cannot be justified merely by subtracting two datasheet values.

For example, IOGP S-614, December 2018, section 7.1.3 does not permit differential-pressure design unless the purchaser specifies otherwise. Section 10.3.1 also addresses identifying applicable design and testing restrictions. Those are requirements of that particular supplementary specification where adopted; they are not presented here as a universal rule for every exchanger. IOGP S-614

If an offer credits a minimum opposing pressure or another operating restriction, ask who establishes it, verifies its validity and carries it into the final documentation. Any associated protective arrangements need their own appropriate review. For the separate question of fluid-mixing consequences and containment, see heat exchanger cross-contamination procurement.

Keep the Approved Case List Attached to the Quotation and Order

Compare offers against the same issued case schedule. Otherwise, apparently similar exchanger quotations may rest on different assumptions about the pressure on the opposite side.

Use this suggested document-review sequence:

1. Issue one basis. Send the equipment datasheet, paired case schedule, applicable code and edition, and purchaser specifications together. Identify which inputs remain preliminary.
2. Obtain a case-by-case response. Ask each bidder to state included cases, assumptions, deviations and exclusions. A general statement such as “designed to the specified pressures” should not hide an assumed minimum counterpressure.
3. Reconcile the design documents. Request a mapping from the accepted schedule to the mechanical calculation cases and proposed test conditions. Ask the designer to identify the governing checks rather than treating the largest arithmetic pressure difference as the only acceptance criterion.
4. Control later changes. When a pressure, temperature, sequence, cleaning arrangement or test boundary changes, revise the affected case and obtain a documented impact assessment before relying on the earlier acceptance.

Retain the case ID and revision in the calculation package and technical clarifications. For each open input, identify who will resolve it and what decision cannot proceed until it is closed. A quotation can identify a provisional assumption; that assumption should not silently become the final operating restriction.

At order finalization, ask for a coordinated record of the accepted cases, unresolved deviations, design limitations and required test documentation. This gives the complete exchanger purchase an auditable basis without assuming that every plant-system responsibility belongs to the equipment supplier.

Frequently Asked Questions

Is differential design pressure the same as pressure drop?

No. In this context, heat exchanger differential design pressure concerns the pressure relationship across the boundary separating the shell and tube sides. Flow pressure drop concerns the change between specified points along a flow path, such as an inlet and outlet on one side. TEMA lists flow pressure drop separately from mechanical design conditions. Label the boundaries whenever using “differential pressure” or “ΔP.” TEMA specification sheet

Can an unknown pressure on the other side be entered as zero?

Not as a confirmed process value. Mark it as missing and identify the party responsible for supplying it. If the designer uses zero in a required load case or an explicit provisional assumption, keep that analytical choice separate from the unresolved process input. Codeware’s example report illustrates zero-pressure entries within defined loading cases; it does not make zero a default for unknown plant data. Codeware example report

Does a shop pressure test replace the paired operating-case review?

No. A test record addresses its stated test conditions. It does not, by itself, evaluate the different pressure and temperature combinations in service. Keep test conditions and operating/design assessments separately identified and linked to the agreed equipment basis. Codeware’s design documentation treats design and hydrotest conditions as identifiable inputs. Codeware design conditions

Submit One Coordinated Exchanger Enquiry

Provide the two sides as a coordinated set of cases, with temperatures, references, boundaries and confirmation responsibilities. Keep the individual design requirements and any proposed differential-pressure design basis explicit.

For a complete shell-and-tube heat exchanger enquiry, prepare the datasheet, both fluid descriptions, proposed case schedule and project specifications, then submit your project requirements. Ask the proposed supplier to identify the design work, assumptions and documentation included in its quotation.

References

• TEMA. TEMA Heat Exchanger Specification Sheet, SI units, public file identified as 2026, pages 1–2. Used for datasheet fields and case organization; not a review of the complete TEMA Standards.
• Codeware. Multiple Heat Exchanger Design Conditions, page dated May 21, 2026. Used for the scope of multiple design and test conditions.
• Codeware. Heat Exchanger Example Report, SI, publication year not confirmed, PDF pages 89–90 (printed pages 86–87). An illustrative calculation report, not project data or the ASME standard text.
• IOGP JIP33. S-614: Supplementary Specification to API Standard 660, Shell-and-Tube Heat Exchangers, Version 1.0, December 2018, sections 7.1.3 and 10.3.1. Its requirements apply where adopted by the project.
• WIKA. Types of Pressure: Absolute Pressure, Gauge Pressure, Differential Pressure, publication year not stated. Used for pressure-reference terminology.