4.1 Baseline Measurements
Baseline measurement is very important in the evaluation of vessels condition. The comparison of baseline data with future data will enable the evaluation or assessment of vessels integrity be made.
To establish a baseline condition for a system or component, initial wall thickness measurements (WTM) shall be carried out in the pre-commissioning stage. If such records do not exist, the first measurements obtained during operation will be considered as the compared with the baseline in order to find any actual loss in thickness. For vessels with external insulation, the baseline measurements shall preferably be performed before insulation is applied. The number of baseline WTM points may be larger than number of points for periodic inspections. This is to ensure a reference if the number of inspection points have to be increased at a later stage.
4.2 Calculation for Pressure Vessel
All pressure vessels are normally calculated according to a pressure vessels code. These shall give information about stresses in corroded and un-corroded conditions, as well as allowable stresses. If corrosion of an older vessel is found to be near the design corrosion allowance, new calculations can be necessary. One shall the use actual measured wall thickness as input and see how far calculated stresses are from allowable ones. By varying the thickness in areas where the most serious corrosion may be expected and do repeated calculations, one can find maximum allowable corrosion in these areas. Where pitting corrosion is detected, the minimum thickness shall be used. All calculation are usually to acceptable principles and formulae in various codes and standards.
4.3 Pressure Vessel Calculation Related to Code and Standard (For New Fabrication):-
i. ASME VIII, Div 1
- Shell and heads
- Opening and nozzle reinforcements shell and heads
- Narrow faced flanges
- Vessels of rectangular cross section
- Bolted head (floating head) and backing device (split flange ring) 2 cartridges
ii. ASME VIII, Div. 2
- Opening and nozzles reinforcements in shells and heads
- Narrow faced flanges 1 cartridge
iii. BS 5500
- Cylindrical shell/Test pressure
- Opening and nozzle reinforcements in shell and heads
- Local loads on cylindrical shells
- Narrow faced flanges
- Fixed tube-sheets
- Floating head or U-tube tube-sheets
- Boiled head and backing device
- Horizontal vessels on two saddles support 3 cartridges
iv. TBK1 (TBK2)
- Cylindrical and spherical shells
- Opening nozzle reinforcements in shells and heads
- Flanges and holts in flange joints 1 cartridge
v. TEMA “R”
- Tubesheets 1 cartridge
The pressure vessel calculation can be found in the relevant code and standard and not shown in this manual.
4.3 Pressure Testing
The strength testing requirement for pressure vessels following a period of service is subject of some complexity, but as a general rule, pressure vessels will not be subject to regular or periodic tests except in the following circumstance
(i) where statutory regulations make it a mandatory requirement
(ii) where repairs or modifications have been made and the basic design, strength and integrity of the vessel must be verified.
(iii) where the pressure vessel is to be re-rated
(iv) where lack of internal access prevents a meaningful inspection and a hydraulic test acceptable in lieu.
Safety precaution of the pressure vessel shall be established for the safety consideration. In general the safety valves or valves shall be removed prior to the pressure test. A hydro test of pressure vessel up to 1.5-design pressure is necessary following by leak detection. Air test of double-plate to manhole or other large opening is also required. The holding time of hydro test shall be 1 hour per 25 mm of shell thickness but with the minimum of 20 minutes. Visual inspection is to be carried out, no deflection or distortion is allowed.
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