Engineering guide
Before being placed in service, every pressure vessel per ASME BPVC Section VIII Division 1 must undergo a pressure test — hydrostatically per UG-99 as the standard, pneumatically per UG-100 in justified cases. This module calculates the required test pressure and compares it against the load capacities of the components per UG-27 (cylindrical and spherical shells), UG-32 (formed heads) and UG-34 (flat heads and covers).
Per UG-99(b), the hydrostatic test pressure is at least 1.3 times the maximum allowable working pressure (MAWP), multiplied by the lowest ratio of the allowable stress at test temperature to the allowable stress at design temperature (LSR, Lowest Stress Ratio). For the pneumatic test per UG-100, the factor 1.1 applies with the same stress ratio. The stress ratio ensures that a vessel designed for high temperature is actually loaded correspondingly higher during the cold test.
The manufacturer must calculate the test pressure per UG-99 for every new acceptance test, but also for periodic inspections, after repairs and in re-rating procedures. The critical part is always the counter-check of whether all components — including flanges and flat heads — can withstand the test condition without inadmissible stress.
Typical workflow
Define the test type
The standard is the hydrostatic test per UG-99 with water. The pneumatic test per UG-100 is permitted only if the vessel cannot be filled with liquid (structural load capacity, process reasons) or residual moisture is unacceptable — because of the high stored energy, stricter constraints apply.
Determine the governing pressure
The basis of the test pressure is the MAWP of the vessel (or, as a substitute, the design pressure). It results as the smallest allowable pressure of all components per UG-27, UG-32 and UG-34 in the corroded condition, referred to the highest point of the vessel.
Form the stress ratio LSR
For each pressure-retaining component, the ratio of the allowable stress at test temperature to the allowable stress at design temperature is formed; the smallest value governs (Lowest Stress Ratio). For a room-temperature design the ratio is 1.0; for a high-temperature design it is significantly greater.
Calculate the test pressure
Hydrostatically, Pt = 1.3 · MAWP · LSR applies; pneumatically, Pt = 1.1 · MAWP · LSR, each measured at the highest point; for the hydrostatic test, the static head of the water column must additionally be considered for lower-lying components.
Counter-check the components in the test condition
The module checks whether shells, heads and covers per UG-27/UG-32/UG-34 can withstand the test pressure: in the test condition, the membrane stress must not exceed the customary limits (typically 90 % of the yield strength as the visual limit for permanent deformation); otherwise the test pressure must be limited or the test planned in sections.
Frequently asked questions
Why is the test pressure multiplied by the stress ratio?
A vessel designed for 350 °C uses a reduced allowable stress at design temperature. During the cold pressure test, the material is stronger — if you tested with only 1.3·MAWP, the test loading relative to the cold strength would be lower than the operating loading relative to the hot strength. The ratio S_test temperature/S_design temperature compensates for this, so that the test constitutes a true overload proof.
What is the difference between MAWP and design pressure as the test pressure basis?
The MAWP is the highest allowable pressure at the topmost point in the corroded condition — the actual capacity limit of the weakest component. UG-99 permits, as a simplification, taking the design pressure as the basis if the MAWP is not calculated. If the MAWP is calculated and lies above the design pressure, it yields a higher test pressure — relevant for later re-ratings, since the test pressure once applied can limit the later rating.
Is there an upper limit for the hydrostatic test pressure?
1.3·MAWP·LSR is a minimum value. On the upper side, UG-99(d) limits the test by requiring that no visible permanent deformation occur; as the customary calculation limit, the membrane stress in the test condition should not exceed 90 % of the yield strength. Flanges, flat heads and gasketed joints are frequently the limiting components — which is why the module counter-checks the test condition against UG-27, UG-32 and UG-34.
When is a pneumatic test per UG-100 justified?
Only when filling with water is not practicable: the foundation or supporting structure cannot carry the fill weight, the process does not tolerate residual moisture (e.g. oxygen, cryogenic or catalyst plants), or the vessel cannot be drained. Because the stored energy of the gas is orders of magnitude higher, the lower factor 1.1, a staged pressurization program and extended safety distances apply; many owner specifications additionally require a brittle fracture assessment at test temperature.
Worked example
For a vessel per ASME VIII-1 with a design temperature of 350 °C, the hydrostatic test pressure per UG-99(b) is to be determined; for comparison, the pneumatic test pressure per UG-100 is given.
Given
| MAWP (at the highest point, corroded) | 16.0 bar |
|---|---|
| Allowable stress at test temperature (room temperature) | 138 MPa |
| Allowable stress at design temperature (350 °C) | 118 MPa |
| Test medium | Water (hydrostatic) |
Solution
Stress ratio (LSR)
LSR = Stest temperature / Sdesign temperature = 138 / 118 = 1.169
(The smallest ratio of all pressure-retaining components governs; here simplified to one material.)
Hydrostatic test pressure per UG-99(b)
Pt = 1.3 · MAWP · LSR = 1.3 · 16.0 · 1.169 = 24.3 bar
For lower-lying components, the static head of the water column is added.
Comparison: pneumatic test pressure per UG-100
Pt,pneu = 1.1 · MAWP · LSR = 1.1 · 16.0 · 1.169 = 20.6 bar
Result
| Stress ratio LSR | 1.169 |
|---|---|
| Hydrostatic test pressure (minimum value) | 24.3 bar |
| Pneumatic test pressure (minimum value) | 20.6 bar |
Technical content source: LV-Solved-Problems-Content 628d313247e7