1. INTRODUCTION
This blog deals with the comparative study of pressure vessel MAWP (Maximum Allowable Working Pressure at Design Condition) and MAP (Maximum allowable Pressure at New and Cold Condition) as per minimum requirement of ASME Code Section VIII Div 1 and the common oil and gas client specification such as Saudi Aramco, Shell DEP, ADNOC Etc which asks for
1. The vessel MAWP shall be governed by shell or Head
2. The vessel MAP shall be governed by shell or head
3. The vessel MAWP and MAP shall not be governed by Nozzle reinforcement and Standard Flanges
4. The minimum thickness of the vessel to be 6mm + CA
The purpose of this study is to optimize the design without compromising safety and integrity
In addition to the above requirements, clients wants vessel to be designed and marked with calculated MAWP at design temperature and don't allow MAWP to be considered as equal to design pressure specified in their design specifications / data sheets, which is otherwise permitted by ASME Code
2. BACKGROUND
While complying with all the above conditions specially for low pressure vessels, it has been noticed that there is substantial increase in calculated MAWP which in turn requires increase in nozzle thickness and requires higher ratings of flanges to be used. This over designing further affects the cost of the vessel.
3. DESIGN MODEL
A design study has been performed to explain this situation on various vessel diameters (ranging from 273mm to 2500mm) at the following design conditions
Design pressure : 5 barg @ 100 Deg. and 10 barg @ 100 Deg.
Corrosion Allowance : 3 mm
For each design condition, calculations were made for the following two cases
Case 1 : ASMESection VIII Div 1 Code requirements
Case 2 : Client Specification considering minimum specified thickness and MAWP/MAP to be governed by shell / head
For each size of the vessel, one nozzle has been selected and designed for meeting Case 1 requirements. For the same size nozzle, increase in thickness, weight and the required change in flange rating are analyzed when designed for Case 2.
For the purpose of this study, nozzles have been designed without any additional reinforcement pad
The analysis done with PV Elite 2020 software.
4. STUDY RESULTS
4.1 Design Pressure 5 Barg.
*Pipe thickness is reduced with 12.5% mill tolerance
Comments:
1. Nozzle flange ratings are required to be increased for vessel diameter ≤ 1000mm due to calculated MAWP. For smaller diameter vessels, this change is significant.
2. Substantial change in thickness and weight is observed for vessel diameter ≤ 1000 with percentage change increasing as the vessel diameter gets smaller.
4.2 Design pressure 10 barg.
*Pipe thickness is reduced with 12.5% mill tolerance
Comments:
1. Nozzle flange ratings are required to be increased for vessel diameter ≤ 1000mm due to calculated MAWP. For smaller diameter vessels, this change is significant.
2. Substantial change in thickness and weight is observed for vessel diameter ≤ 1000 with percentage change increasing as the vessel diameter gets smaller.
5 CONCLUSION
Based on the above analysis, a flow chart has been prepared as a guide to ensure compliance with Code as well as client requirements and to achieve an optimized and safe design.
