USER SPECIFICATIONS FOR PRESSURE VESSELS AND TECHNICAL INTEGRITY

Specifications translated from user requirements are prescribed in an attempt to capture and incorporate best practices with regards to the design, fabrication, testing, and operation of pressure vessels. The question as to whether these requirements affect the technical integrity of pressure vessels is often a subjective matter. This paper examines typical user requirement specifications against technical integrity of pressure vessels.

The paper draws on a survey of a convenience sample of practising engineers in a diversified petrochemical company.When compared with failures on selected pressure vessels recorded by Phillips and Warwick, the respondent feedback confirms the user specifications that have the highest impact on technical integrity.

i) design and constructability requirements ii) quality and material requirements iii) welding and fabrication requirements
The respondents were then asked to rank these requirements with respect to the effect on the technical integrity of a process vessel.
The profile of the respondents ranged from junior engineers who had just joined the company, to chief engineers who had been with the company for over 20 years.For the company this meant that four different grades of engineers had responded to the questionnaire and given their rating for the impact a requirement would have on technical integrity.Because of this the ratings were preferentially weighted towards the higher job levels.

Actual defects
Historical data for 78 process columns in a refinery was analyzed to determine the types of defects that occur in practice.Data was gathered from routine inspection reports for the process columns, dating back to the fabrication of the vessels in about 1977.Metallurgical data reports were sourced as a second basis for recording material defects for the vessels.

Results
It can be seen that with respect to design and constructability, the requirements that the respondents ranked highest address the design of weldments.The top ranking requirement is the use of integral reinforcement (as opposed to compensation pad reinforcement) for nozzle openings.This reduces the amount of welding on the vessel.The requirement that addresses the weld design of the head to skirt attachment welds for process vessels classified as being slender (i.e. a length over diameter ratio of greater than 10) was ranked second highest.Here, attention is given to the weld to ensure that fatigue cracking does not occur.
The third highest ranking requirement addresses the connection detail of nozzles where cyclic loading occurs.In this case the nozzle to shell attachment weld is changed from a corner weld to a full penetration butt weld, where the root can be inspected.This takes the weld away from the area that would see the highest stresses during nozzle loading, and so reduces the risk of crack generation in the weldment of the corner weld.

Comparisons
When the results from the respondents are compared with the Phillips and Warwick data, it can immediately be seen that ten of the twelve requirements rated by respondents as having the greatest impact on vessel integrity relate to preventing crack formation in the vessel material.When the Phillips and Warwick data is considered, we see that 89.3% of the cases causing failures result from cracks.On further investigation of the reason for these cracks, it is observed that fatigue constitutes the largest cause (35.6% of the total number of cases).Three of the twelve SASOL specification requirements address fatigue specifically.The second highest cause for cracking in the Phillips and Warwick data is corrosion (18.2%).When the results from the respondents are analysed, it is observed that four of the top twelve ranked SASOL requirements address cracking as a result of corrosion.
The specification requirements ranked highest in the questionnaire are similar to the requirements that the Phillips and Warwick data rates highest for failures in industry.
When the data is compared with the historical data reports and history of the vessels in the refinery investigated, it is apparent that the area regarding corrosion as a result of incorrect material selection was where most defects were experienced.

SUMMARY
It is reassuring that the requirements given a priority ranking by the respondents align with the causes for failure identified in the Phillips and Warwick research data.However, the area contributing to the majority of defects experienced was material selection, and these requirements would need to be specified in more detail.