10/13 Rule: Containment Alternative, Not PSV Sizing

The low-pressure side of a heat exchanger has a lower design pressure than the high-pressure side, creating a tube-rupture overpressure case. Two design paths are available: rate the complete low-pressure boundary to contain the communicated pressure, or provide pressure relief sized for tube rupture. The 10/13 rule is a screening ratio for the containment path; it is not a PSV sizing formula.

Containment and PSV approaches

Before anything else, confirm which design path the relief-system basis uses. Do not apply the ratio and simultaneously assume that it sizes or validates a PSV.

Use the containment approach when the entire low-pressure system already has a suitable pressure rating or can be economically upgraded. Use the relief approach when raising the rating of every affected component costs more than providing and routing adequate relief capacity. Compare lifecycle cost, discharge-system impact, inspection requirements, and modification scope before selecting the path.

Pressure-ratio mechanism

The ratio comes from comparing low-side hydrostatic test pressure with high-side design pressure. With a test multiplier of 1.5, the limiting ratio is:

With a test multiplier of 1.3, the corresponding ratio is:

Here, is the low-side pressure rating used as the basis for its hydrostatic test, and is the high-side pressure being compared. The shorthand asks whether the low side's test pressure reaches the high-side rating:

Low-side test pressure ≥ applicable high-side pressure

The cited fourth edition of API RP-521, Section 3.18.2, dated March 1997, used the 150% test-pressure premise and the two-thirds criterion. The 10/13 shorthand follows the later 1.3 multiplier described for ASME Section VIII, Div 1. Also review UG-133(d) and Interpretation VIII-1-04-38 under the governing code edition.

Use the actual required test pressure rather than relying only on either shorthand. Code editions and the pressure basis recorded on equipment documents decide which multiplier applies. A passed hydrostatic test is not, by itself, an operating pressure rating or a blanket exclusion of pressure-relief requirements.

Tube-failure scenario boundary

A tube failure connects the high-pressure fluid source to the low-pressure system. The resulting low-side pressure depends on the source pressure, rupture opening, fluid state, isolation arrangement, normal outlets, check valves, control valves, and relief paths. A blocked or isolated low side presents a different case from a low side with a verified open discharge path.

1. Mark the complete low-pressure pressure boundary exposed after a tube failure. Include both upstream and downstream systems where pressure can propagate.
2. Identify every credible valve position, including blocked isolation. Do not credit an outlet unless its availability is part of the protected design basis.
3. Read the high-side and low-side pressure ratings from equipment and piping records. Keep design pressure, maximum allowable working pressure, operating pressure, and hydrostatic test pressure as separate entries.
4. Identify the governing construction-code edition and project relief criteria. Confirm the test-pressure requirement from that edition rather than selecting 1.3 or 1.5 from memory.
5. Check whether any component within the exposed boundary has a lower rating than the exchanger low side. The lowest applicable rating controls the containment decision.

For example, a shell rated at 100 psig and tested at 100 × 1.5 = 150 psig formed the basis of the older comparison with a tube side rated at 150 psig or less. Conversely, rating an exchanger shell at 300 psig provides no containment benefit to connected shell-side piping or equipment rated at only 150 psig when tube-side pressure can reach that lower-rated boundary.

Containment-path procedure

1. Set the comparison pressure to the applicable high-side pressure defined by the design basis. Record where that value comes from.
2. Determine the code-required low-side test pressure from the governing edition and equipment documentation. Do not move on until the pressure basis and multiplier are traceable.
3. Compare the test pressure of each exposed low-side component with the selected high-side pressure. A simple 10/13 nameplate ratio is insufficient when connected components use different ratings or code bases.
4. Remove the containment option if any exposed component fails the comparison. Either upgrade that component, restrict pressure propagation by an accepted engineered design, or use tube-rupture relief.
5. Review the proposed exclusion against API RP-521, ASME Section VIII, Div 1, UG-133(d), Interpretation VIII-1-04-38, and the editions adopted for the installation.
6. Document the exchanger sides, exposed piping limits, valve assumptions, component ratings, calculated ratios, required test pressures, and the decision to exclude or retain tube rupture in the relief basis.

Tube-rupture relief procedure

1. Retain tube rupture as a PSV sizing scenario when the complete low-side boundary does not satisfy the accepted containment criterion.
2. Define the relieving case using the credible high-side source, tube-failure opening, fluid phase, thermodynamic state, and low-side pressure. Obtain missing values from the exchanger datasheet and process design records.
3. Account for normal and blocked-isolation configurations. Do not credit operator action or an unverified open path as immediate protection.
4. Calculate the tube-failure inflow and size the PSV using the governing relief methodology. The ratio 10/13 supplies no relieving rate, orifice area, backpressure correction, or discharge-system capacity.
5. Check the inlet line, outlet line, flare or discharge destination, and backpressure for the calculated load. Confirm that adding the case does not overload a shared disposal system.
6. Record tube rupture as a named sizing case on the relief-device calculation and link it to the exchanger and piping configuration used in the analysis.

Verification and recurring pitfalls

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