ASME BPVC Sect. VIII Div. 1 – minimum, calculated and maximum test pressure

Successfully passing this test according to ASME Section VIII Div. 1 UG-99 is the ultimate proof of your vessel’s integrity and safety and is part of the final inspection.
ASME BPVC Section VIII Div. 1 in UG-99 structured the pressure test with

1. Minimum test pressure
2. Calculated test pressure
3. Maximum test pressure

1. Minimum test pressure

The calculation of the minimum test pressure PT_min is based on the smallest MAWP (hot and corroded), the lowest of the values determined for all essential parts of the vessel according to the principles specified in UG-98 (b) and Mandatory App. 3 / 3-2:

Minimum pressure test:

PT_min. = MAWP • 1.3 • LSR

PT_min. = {S_d • E • (t_n–CA) / [(R_i+ CA) + 0.6 • (t_n–CA)] – P_static} • 1.3 • LSR

where:

S_d = max. allowable stress at design temperature
S_t = max. allowable stress at test temperature
E = Joint Efficience of longitudinal weld
t_n = Nominal thickness (actual measured thickness)
CA = corrosion allowance
R_i = radius of shell (without CA)
P_static = static head (for liquids and most vertical pressure vessels)
S_t = the stress value at test temperature
S_d = the stress value at the design temperature
LSR = S_t / S_d (lowest stress ratio for each pressure-boundary material)

Note 1:
The MAWP for a vessel is the maximum pressure permissible at top of the vessel in its normal operating position (e.g. vertical or horizontal) at designated coincident temperature specified for that pressure.

Note 2:
Ref. to Endnote 35, the maximum allowable working pressure (MAWP) may be assumed to be the same as the design pressure, when calculations are not made to determine the MAWP.

Note 3:
All components of a vessel satisfy the Code rules for the Maximum Allowable Working Pressure that is marked on the vessel nameplate including any coincident static head acting at each component in its normal operating position.

2. Calculated test pressure PT_calc

Calculated test pressures of the respective components (the weakest component applies) based on App. 3 / 3-2 with the design formulas, also multiplied by 1.3 and reduced by static pressure heads (if necessary);
This option is only permitted if agreed between the USER and the MANUFACTURER. Conducting a hydrostatic test based on calculated test pressure requires an agreement between the user and the manufacture, and the inspector has the right to review the calculations used to determine this calculated test pressure.

The “New and Cold” pressure P_max is the value determined as the basis for a calculated test pressure as discussed in UG-99(c). It references the reader to Appendix 3 /3-2, which defines a calculated test pressure as “the highest permissible internal pressure (P_max) as determined by the design formulas, for each element of the vessel using the nominal thicknesses with the corrosion allowances included and using the allowable stresses given in Section II, Part D for the temperature of the test“.

Highest permissible internal pressure P_max (cold and new) :

P_max = S_t • E • tn / ( R_i + 0.6 • tn )

where:

S_t = max. allowable stress at test temperature
E = Joint efficiency of longitudinal weld
t_n = Nominal thickness (actual measured thickness)
R_i = inner radius of shell

The calculated test pressure:

PT_calc = P_max • 1.3 – P_static

If this value is applied and the inspector involved reserves the right to inspect the manufacturer’s calculations of the hydrostatic test pressure for each component.

Why is the necessary agreement between the user and the manufacturer required?
The reasons for this requirement are not entirely clear, but one assumption is that it is necessary for the purpose of performing a pressure acceptance test of the system with connected piping at the user’s premises, because the test pressure of the installed piping would often be limited by the MAWP of the vessel during pressure tests.

3. Maximum test pressure

This section explicitly states that there are no upper limits for the test pressure. However, and stated that intermediate values from par. 1. and par. 2 above can be used.

Maximum test temperature PT_max:

PT_min ≤ PT_max ≤ PT_calc

If this value is exceeded (eitherintentionally or accidentally) and the pressure vessel suffers visible permanent deformation as a result, the inspector involved reserves the right to reject the vessel.

Read More

MAWP calculation procedure for pressure vessels

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