in hot process vapour condensers, or heating media such as
methanol or propylene in ethylene heaters). The 10/13th rule (or 2/
3rd rule, as may be applied in some jurisdictions), is based on ASME
Boiler and Pressure Vessel Code Section VIII, Div. 1 (2014). This code
requirement eliminates the need to evaluate tube-rupture scenarios
if the LP side design pressure is equal to or higher than 10/13
times the HP side's design pressure. However, API 521 6th Ed.
(2014) also notes that “Pressure relief for tube rupture is not required
where the LP exchanger side (including upstream and downstream
systems) does not exceed the criteria noted above. The tube-rupture
scenario can be mitigated by increasing the design pressure of the LP
exchanger side (including upstream and downstream systems), and/or
assuring that an open flow path can pass the tube-rupture flow
without exceeding the stipulated pressure and/or providing pressure
relief”.
The key phrase in the API 521 clause above is “including upstream
and downstream systems”. Further explanation were pro- vided in API 521 stating that “upstream and downstream piping