Sorry for a lack of posts these last couple of weeks, they have been crazy with travel and work. But I wanted to follow-up on a lot of questions and comments I got regarding my “Internal inspections… of my pressure vessel??? We don’t need no internal inspections” post a few weeks back. If your engineers/management is telling you that the vessel does not need (or require) an internal inspection then all we can do is provide the RAGAGEP facts and ask them to explain why not. I can tell you this, OSHA/EPA will take this minimal position as well. But for those of you that seek more info, here is what I have been sending SAFTENG subscribers and clients:
This is from my API 510 Training Course from way back in the early 1990’s and guess what? It still holds true TODAY in 2019!!!
Facts:
- A vessel has been in service for 8 years (since it’s the last inspection!)
- The original shell thickness was 0.625″ and Full RT was performed during fabrication many years ago
- The required thickness is 0.575″
- Current measured thickness indicates a remaining thickness of 0.600″
What is Internal Inspection Interval? (Section 6, par 6.5.1v)
Since the “CORROSION RATE” is unknown, the 1st Step is to determine the Corrosion rate.
What is known:
t-previous = 0.625
t-last= 0.600
Step #1: Calculate the Corrosion Rate
Number of years between the last 2 inspections = 8 (e.g. the construction inspection and the 8-year old inspection)
0.625 – 0.600 ÷ 8 = 0.003
The CORROSION RATE for this vessel is 0.003
Step #2: Calculate the Remaining Life
What is known:
t-required = 0.575
t-last = 0.600
Corrosion Rate = 0.003
0.600 – 0.575 ÷ 0.003 = 8 years is the REMAINING SERVICE LIFE of this vessel
Step #3 – Calculate the Inspection Interval
Inspection Interval is the Lesser of:
- 0.5 Remaining Service Life, or
- 10 years
whichever is the LESSER!
- 0.5 X 8 years= 4 years
- 4 years is LESS THAN 10 years, therefore the INTERNAL inspection interval is 4 years from the last inspection
PLEASE NOTE: Your API or NBIC inspector SHOULD provide you with these calculations and specifically call out the DUE DATE for the next INTERNAL INSPECTION in their report.
WARNING! I am NOT an API certified inspector, but I did stay at a Holiday Inn Express last week.
ALWAYS Remember this about API 510 Internal Inspections…
- Internal Inspection Interval is LESSER of ½ remaining life or 10 years. If remaining life is LESS THAN four (4) years, the interval can be the full remaining life up to a max of 2 years. (API 510 par 6.5.1.1).
- The inspection MUST be conducted by the CERTIFIED INSPECTOR (API 510 par 5.5.2.1)
- The primary reason for the internal inspection is to find damage that cannot be found by external CML’s (API 510 par 5.5.2.1)
- Inspector should consult with Corrosion Specialist to determine if it is necessary to remove any linings and/or deposits (API 510 par 5.5.2.3)
- Vessels in non-continuous service, the interval is based on the number of years of ACTUAL SERVICE, instead of calendar years, provided the vessel when idled is separated from process stream & not exposed to corrosive streams.