ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America.
REPAIRS AND ALTERATIONS – Part 2
Pressure Vessels are containers which are designed to hold liquids, vapors, or gases at high pressures, usually above 15 psig. Examples of common pressure vessels used in the petroleum refining and chemical processing industries include, but are not limited to, storage tanks, boilers, and heat exchangers. Each individual vessel has its own operating limits built in by design that it has to work under, referred to as its design pressure and design temperature. Operating outside of these limits could damage the equipment and potentially lead to loss of containment or catastrophic failure.
Because they work under immense pressures, a ruptured pressure vessel can be incredibly dangerous, leading to poison gas leaks, fires, and even explosions. For this reason, pressure vessel safety is imperative. There are several standards and practices that cover the construction, maintenance, and inspection of pressure vessels. Chief among these standards are ASME Section VIII, NBIC and API 510.
ASME Section VIII is the section of the ASME Boiler & Pressure Vessel Code (BPVC) that covers pressure vessels. It gives detailed requirements for the design, fabrication, testing, inspection, and certification of both fired and unfired pressure vessels.
API 510, "Pressure Vessel Inspection Code: In-Service Inspection, Rating, Repair, and Alteration" is an inspection code, written and published by the American Petroleum Institute, that covers the in-service inspection, repair, alteration, and rerating activities for pressure vessels and the pressure relieving devices protecting these vessels.
National Board Inspection Code is the Code that a majority of the Jurisdictions in the United States have accepted by law. This means under the rules of the Jurisdiction, any weld repair to a boiler and/or a pressure vessel must be performed in accordance with the NBIC. When the National Board was formed years ago, it was at the request of the ASME Boiler Code committee to enforce accepted construction design rules thereby ensuring uniformity and safety across the industry. The original rules that were drafted by Jurisdictions and ultimately passed by State legislators started out with the boiler and over the years included pressure vessels.
When it comes to inspections, most pressure vessels should be examined once before being placed into service and again every 5 years after every alteration or major repair. An inspection can be internal, external, or both and should involve a thorough examination, a thickness evaluation, a stress analysis, an inspection of the vessel’s pressure release valves, and a hydrostatic pressure test. It is also important to perform a surface inspection, examine the insulation and any structural connections, and finally inspect any welds or joints.
API 510 applies to most refining and chemical process vessels that have been placed into service. It covers vessels which have been constructed under certain construction codes, such as the ASME Boiler and Pressure Vessel Code. It also covers vessels that have been constructed without a construction code or to a code that has not been recognized. Furthermore, it covers vessels which were constructed to a code but have since lost their nameplate or stamping. Finally, it can be used to cover pressure vessels in other industries at the user’s discretion and as acceptable per jurisdictional rules. Absent from the list of covered vessels are those that have been officially retired from service and abandoned and those that are specifically excluded per section 1.2.2.
Internal, external, and on-stream pressure vessel inspections are covered under API 510. Minimum requirements for implementing risk-based inspection as an alternate to conventional inspection planning criteria are also provided. API RP 572 supplements API 510 to provide logic and best practices to assist practitioners in the “how to” inspect pressure vessels.
On-Stream Inspections (tank not taken out of service) should be performed every five years. Typically, these inspections include the following:
• Visual inspection, performed by an API-510 certified inspector, of the following components:
- External shell
- External heads
- External nozzles
• UTT inspection of the following components:
- External shell
- External heads
- External nozzles
A final report should be completed to show the inspection’s results and recommendations. In addition, this report should provide the short- and long-term corrosion rates as well as a remaining life calculation. Finally, a client-approved engineer must review the report to determine if the vessel is suitable for continued service.
OFF-STREAM INSPECTIONS (vessel taken out of service) are required every 10 years. If possible, the vessel needs to be opened and cleaned for confined space entry. Normal inspections performed during this examination include the following:
• Internal & External Visual Inspections (VT) are performed by a certified API-510 inspector using the recommended visual checklist. They include the following components:
- Shell (internal and external), including out-of-roundness and bulge inspection
- Heads (internal and external)
- Nozzles (internal and external)
- Ancillary equipment (i.e. gauge levels)
- Pressure relief valves
• Ultrasonic Thickness Testing (UTT) Inspections are performed by an ASNT Level II Certified UT Inspector. The following components are usually examined for ultrasonic thickness:
- Shell (internal and external)
- Heads (internal and external)
NOTE: Additional inspections may be required depending on the results of the initial inspections (i.e. Magnetic Particle (MT) on the welds).
A final report should be completed showing all of the inspection results and recommendations. In addition, this report should show short- and long-term corrosion rates as well as a remaining life calculation. A client-approved engineer must review the report to determine if the vessel is suitable for continued service.
In a change from the last issue, vessels that have been abandoned in place and no longer an asset of record from a financial/accounting standpoint are no longer covered by API 510. This relieves the owner from continuing to include these vessels in its overall inspection program. However, should it be decided later to put the vessel back into service, then its condition must be evaluated and meet API 510 requirements.
WHAT ABOUT SPHERES?
What are the In-Service Inspection Requirements for Spherical Shell Pressure Vessels?
In-service inspection requirements are similar to those for the cylindrical pressure vessel and are based on the requirement of the API STD 510 Standard.
Some important points about in-service inspection are:
The inspection plan needs to be developed for the vessels, and generally, internal inspection is a mandatory requirement.
You need to do internal inspection in intervals not exceeding one half of the remaining life, or 10 years, whichever is less.
You can calculate the remaining life with the following simple formula:
RL = (Tc-Tmin)/CR ;
When Tc is Actual Thickness, and Tmin is Minimum Required Thickness, it is can be obtained by the deduction of nominal thickness from corrosion allowance, and CR is corrosion rate.
Example: you have a spherical shell storage vessel with an actual thickness of 1.121 in., and the minimum required thickness is 1.0 and the corrosion rate is 0.005 in./year, so the reaming life will be:
(1.121-1.0)/0.005 = 24.2 years;
So the internal inspection interval will be:
Half-life = 24.2/2 = 12.1, so 10<12.1, then the internal inspection interval will be 10 years.
But in most plants, evacuation of storage vessels is very difficult and costly, so integrity engineers try to find a new technique to replace internal inspection.
API STD 510 allows the On-Stream inspection to be replaced with the internal inspection with some conditions.
This is the definition of On-Stream Inspection per the API 510 Standard:
“An inspection performed from the outside of a pressure vessel while it is on-stream using NDE procedures to establish the suitability of the pressure boundary for continued operation.”
But only you may replace this technique if all of following conditions are met with your vessel:
The general corrosion rate is less than 0.005 inch. per year, remaining life is greater than 10 years, there is no corrosive character in the contents, no questionable condition is observed in the External inspection and the vessel is not subject to SCC cracking.
What is the Best Technique to Use as On-Stream Inspection?
The best technique, which is widely used, is acoustic emission testing. In this technique, the spherical vessel is completely filled by content fluid to maximize stresses.
Meanwhile highly-sensitive proprietary acoustic sensors are attached to the vessel wall to detect the fracture of corrosion products during the monitoring period.
The suspect areas are identified and located. If the suspect shows a high risk issue, then the vessel would be evacuated for internal inspection and necessary repairs.
What are the Repair Methods for a Spherical Shell Pressure Vessel?
There are three sources for repair of a spherical pressure vessel; API STD 510 has provided some repair requirements and instructions.
Based the API 510, only repair organizations are allowed to do repairs, and the definition and condition for being a repair organization has been provided in the API 510.
The other source is the ASME-PCC-2, which provides you detailed instructions and requirements.
The other source is the NBIC Part 3 Code book, which similarly provides requirements and instructions.
Please note if your spherical pressure vessel is a “U” or “U2” stamped pressure vessel, only repair organizations that are ”R” stamp holders from NBIC are allowed to do repair on your vessel.
At Boardman, we are certified to both fabricate ASME Code Stamped Pressure Vessels and to repair Pressure Vessels and apply the “R” SYMBOL. Our experience in fabricating and repairing pressure vessels enables us to not only meet customer needs for new pressure vessels, but to effectively execute repairs of existing pressure vessels as well. We are just a call away from assisting you in repairing or altering your equipment.
Source: National Board Inspection Code
This is presented to you as a service from BOARDMAN, LLC. located in Oklahoma City, Oklahoma.
Since 1910, Boardman has been a respected custom fabricator. We take pride in our ability to take the most stringent specifications and requirements to provide a high quality solution to our customers. With more than 75 years of ASME Section VIII, Division I engineering experience, we have the unique ability to provide custom solutions to our customers.
Fabricated Projects Include:
- Trayed Towers & Columns
- ASME Pressure Vessels
- Molecular Sieves
- Rotary Dryers & Kilns
- API Tanks
- Acid settlers
- Stacks, Scrubbers
- Thermal Oxidizers
- Accumulators, Condensers
- Large Diameter Piping
- Shell & Tube Heat Exchangers
The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.