Files: PVE-4711/6847, Last Updated: Feb 11 / 2016, By Laurence Brundrett
Quick Links (to topics on this page)
You Should Be Registering Generic Vessels!
Back in the pre-history* of the CRN system it was recognized that the cost and delay of registering vessels was an unacceptable obstacle to business in Canada. These were the 2 week / $100 days, not our current era. A solution was worked out: jurisdictions could register generic designs.
*The pre-history of the CRN system as used here is defined as the time before PVEng started up in 1999, but in this case it predates the spinning off of the safety organizations like ABSA and TSSA from their parent government departments in the mid-1990s.
What is a Generic Design?
A generic registration covers a range of possible vessel configurations in one CRN. Without changing the operating conditions, diameter, wall thickness and material, a huge variety of changes can be made to a vessel. It can be vertical or horizontal. Its length can change within an acceptable specified range. It can have a huge variety of nozzles on the heads and shell. The location and spacing of the nozzles can change. All these changes can be covered in one drawing and calculation set.
ABSA has written a guideline outlining what can and cannot change in a generic design. Reading the ABSA guide is a good start. Also please refer to our sample below.
The designer needs to choose how much to include in the generic design. Not every option can be included; some unusual designs will still have to be registered one off. The number one option for generics is a large range of nozzle designs: configurations, sizes. Note that Alberta restricts the number of variations allowed on each nozzle size. ABSA:
…only one configuration of minimum nozzle neck thickness, minimum internal projection, minimum weld size(s) and added reinforcement shall be permitted [per shell or per head].
The largest problem is normally covering the location and spacing of the nozzles. This is covered in a nozzle spacing table (see our sample job). ABSA again:
Nozzle quantities and positions shall preferably be fixed. If this is not the case, tables specifying the minimum centre-to-centre distance in inches or millimetres between any two nozzles shall be provided.
More time is usually spent on the issue of nozzles than all others combined. Even with these restrictions a huge number of options are possible.
Also typically covered are options like vertical and horizontal mounting, varying shell lengths and varying mounting locations for external equipment. The generic drawing needs to have all the desired options documented.
The calculation set covering all of the options tends to be long and the matching drawing dense. The weight of paper of a generic registration is about 2.5x more than a comparable one-off. We estimate that there is about 2.5x as much work for designer and the reviewer. The jurisdiction reviews often take twice as long. We budget 2x the registration fees.
Using the Registered Generic Design
Once registered, the work is not done. The registered generic drawing is usually too complicated to build from. Usually the fabricator makes a new drawing for each application, which refers to the registered drawing. The Authorized Inspector can verify that the new drawing falls within the scope previously registered and therefore it is acceptable to use the generic CRN number. No new registration is required!
The generic calculation set is reviewed by the shop and Authorized Inspector on a yearly basis, the same as for repeat production of unique CRN designs or regular National Board fabrication. For fabricators outside of Canada, National Board registration is also required on each production batch with vessels destined for Canada.
The photo here is from a generic registration for seven related vessels differing in diameter. The weight of paper shown is 38.5lbs, representing the mail out after the first province signed off. The total weight of submitted paper is 45 lbs (6.5 lbs per vessel). For reference, a full box of paper with 10 reams weighs 49 lbs. The registration process Canada wide took about one year. Registration fees totalled $16,800 (or $2400 per vessel), higher than would be expected for a series of related one-off designs. If these were not generics, the total weight would be around 15 pounds and the expected total review time about half a year.
Fabricators located outside of Canada still have to pay the National Board registration fees on each batch produced. This batch of 7 vessels can be National Board registered for $128 (cheaper if you file electronically). There is no registration delay with National Board.
Once the cost and delay of the first registration process is covered, the remaining production costs and times are the same as for vessels sent to countries other than Canada. Production can continue until the code changes or the manufacturer updates their design. Recent B16.5 flange weld size and Appendix 2 flange flexibility updates are examples of code changes that could lead to design changes ultimately leading to the need to update the generic design.
All Vessel Registrations Are Generic
The generic drawing is deliberately set up to allow the AI to review a derived production part without need to review code calculations. The generic drawings with limits on length and nozzle spacing tables are specifically set up to make this process easy. However all designs are generic to a limited extent, even if derived designs were not considered at the time of original registration. ABSA’s guideline is “an inspector performing his Authorized Inspector duties in the Manufacturer’s shop or looking at a vessel in the field should be able to quickly see that the vessel is covered by the registered design.” AI’s are willing to sign off on some designs as being generic even if they do not meet the guidelines presented here. Following these guidelines makes the process easier and more certain.
What is accepted as generic even if it is not registered as generic? When manufacturers build left/right pairs of vessels, we only register one hand – the drawing does not have to state that a mirror image part will be built. This has worked every time. Also, producing a vessel with a lower pressure rating without changing the design is acceptable. Shortening the length of a shell is usually accepted. Although this changes nozzle spacings, often the AI can see at a glance that nozzle interferences are not a problem. Lengthening the shell is less likely to be accepted. Removing nozzles is not a problem unless they are required for inspection. Adding nozzles that are identical to existing nozzles is usually acceptable.
Adding new nozzle designs, raising design temperatures, lowering MDMT, changing inspection and changing materials beyond the original scope are all grounds for new or revised CRNs. If an AI will allow the CRN to be used on a derived design, then the original design is generic.
The Big Idea – A Return to Sanity
Yes the process is long and expensive, but picture the upside – once complete you have a series of vessels that can be built the same as if the CRN system did not exist and Canada was only part of National Board. Future registration costs and delays – an unknowable variable – have been removed. Your designs have been locked down because different reviewers with differing requirements will not be looking at each job you are building. More companies should seriously consider generic registrations.
These concepts also apply to the registration of fittings. The difference is that fitting registrations can be much more flexible. The restrictions on the diameter, material, wall thickness, pressure and temperature do not apply to fitting registrations. All details must still be covered in the calculations, testing and drawings, but with fewer restrictions. The balance of this article only considers generic vessels.
The important point to take home is that manufacturers with lines of related products can save cost, time and pain by doing more registration work up front. Go for it!
Generic Vessel Sample
The above discussion covers the whys of generic vessel registration, here an actual sample is used to illustrate how it is done.
For this sample job we assume a manufacturer regularly produces 36 inch diameter vessels in a variety of lengths and nozzle variations. They currently register each design as it is sold. By doing a generic design, they expect to move all registration costs up front and then just build vessels later with no delay.
The manufacturer has surveyed their production of 36 inch vessels and has come up with a single pressure/temperature rating, a maximum and minimum shell length, and single materials and thicknesses for the shell and for the head that covers the majority of their production. This is the basis for their 36 inch “generic” design. Once registered, the manufacturer will make a number of derived designs; one called the “36×60 derived vessel” is also shown below. The manufacturer will still need to individually register some unusual designs that do not fall within the scope of the generic design.
The Maximum Pressure Temperature Rating and the MDMT
The generic vessel design is limited to only one temperature rating at one pressure. One shell thickness and one material is used. Some alternate materials are allowed if stated on the drawing and covered in the scope of the calculations.
Once registered, a lower maximum pressure and temperature can be assigned to individual vessels without the need to re-register. The MDMT can be increased to a higher number without re-registering, but cannot be lowered. The generic design has a rating of 200 psi at 250°F. The derived 36×60 vessel shows the pressure reduced to 180 psi at 200°F but the original rating could also have been used. Even with the reduced pressure, the wall thickness and material have not been changed.
Similarly the generic design shows an MDMT of -20°F at 200psi. The derived shows -10°F at 180psi but could also have used -20°F at 200psi.
The Shell, Heads and Supports
The generic design has a shell length of 48 to 96 inches. The 36×60 derived vessel is 60 inches long. It falls within the range registered, so it is acceptable. Only one shell material was registered so the shell material on the 36×60 must remain SA-414 Gr G. The thickness cannot be increased or decreased.
Likewise, the head dimensions and material have not changed from the generic to derived design and the testing and inspection is identical on both.
The manufacturer registered both a saddle and skirt on the generic design so either a vertical or horizontal vessel can be produced. Two calculation sets are merged to provide both skirt and saddle calculations. Seismic calculations are done for the maximum dimensions to the worst Canadian zone to allow registration Canada wide.
Nozzles are the big pain with generic designs. Every nozzle variant that will be used needs to be registered. The calculation covers the worst case static head for vertical / horizontal mounting options. Nozzles that can be used on heads or shells are calculated on both. Nozzles on heads can be either calculated to work in the knuckle region or excluded from installation in that location on the drawing. Because a huge number of nozzle variants can be created, the Alberta generic vessel guideline limits the wall thickness, projection and material to one per size. In practice you can often register more than one variant per size as long as the calculation set is kept to a reasonable size. This is what makes the calculation set for a generic vessel much longer than a regular vessel.
Every nozzle has complete area replacement calculations with a defined limit radius. Limit radii for individual sizes are combined to create a nozzle separation table. The generic design shows a separation table format that is accepted Canada wide. The flanged nozzle N1 is available in 2 1/2 to 6 inch sizes. The 3 inch option has a limit radius set to 2.375 inches in the calculations. Two 3 inch nozzles would need to be 2x this or 4.75 inches apart center to center. The table shows this as the intersection of the 3″ pipe row with the 3″ pipe column showing 4.75 inches. As a further example, a 2″ coupling has to be kept 5.063″ from a 4″ pipe center to center.
The design does not call up any radiography. These nozzles are all calculated without any allowance for shell weld efficiency. Therefore the nozzles and their replacement area must not intersect any long or circ weld. The last column on the Separation Table “Seam” gives a closest allowed nozzle center to weld seam distance.
The derived vessels will be vertical and horizontal, vertical having the highest possible static head. The nozzles are calculated on the vertical vessel calculation set, head nozzles are calculated on the bottom head, shell nozzles at the lowest possible point on the shell.
With the correct minimum spacing accounted for, any number of the generic nozzles can be included on a derived design. The “36×60” derived design uses different mark numbers to identify the nozzles, but the designs can all be found on the generic design.
The Registration Process
The registration process is handled identically to other CRN designs. Generic registrations tend to take longer and cost more due to the number of options involved. This is part of the paying up front aspect of the generic process.
The Derived Drawings
The generic design drawing is usually too complex and is missing vital dimensional information to be useful on the shop floor. A fabricator makes derivative drawings like the 36×60 sample linked below. The derived drawing for each variant built has the following features:
- – The head and shell are identical to the generic, only the length changes.
- – The inspection is identical to the registered design.
- – All nozzle details have been taken from generic drawing without changes. The quantity and location can change within the limits as specified in the separation table. Multiple copies of the same generic nozzle are acceptable.
- – All nozzles necessary for inspection are kept.
- – All design conditions are identical to or more conservative than those on the generic drawing.
- – The support system (horizontal or vertical) is the same as registered.
- – The derived drawing references the generic drawing so that the CRN validity can be traced.
For cases where it is not possible to follow these restrictions, a new CRN would be applied for. The generic registration has not covered all design possibilities, just the most common.
The Authorized Inspector (AI), Code Revisions and Design Changes
Internationally, a National Board inspector is tasked to review the drawings and calculations for a vessel, agree to the design and put construction hold points on prior to start of construction. The work continues with inspections during and after construction ending with final sign off on the completed paperwork. A Canadian AI will also do this in for vessels without CRN that will be exported.
The Canadian CRN process limits the scope of work of the AI. The review of the design has been shifted to the jurisdiction review engineers and gets repeated for each of the province the design will be installed in. The AI is still responsible for setting the hold points, in process and final inspection and signing off on the paperwork. The CRN is no guarantee, but AIs usually accept designs that have been registered without requiring changes.
The CRN for a pressure vessel does not expire unless code rules make it obsolete or the manufacturer wants to change the design (CSA B51-2009 4.1.2 and 4.1.4). For each build, the AI checks that the calculations are updated to the latest code and addenda and that no design changes are needed to meet the latest rules. This is identical to the process used for National Board registered designs that are not CRN registered.
Once the design changes either from changing code requirements, or by the manufacturers choice and the AI is no longer able to clearly link the derived designs to the registered generic design, then the design has to be sent back to all the jurisdictions to update the CRN.
For this sample vessel, it is the AI’s is responsibility to determine that the 36×60 derived design falls within the scope of the registered generic design. Once satisfied, the AI will allow the CRN number to be used and put on the official paperwork. By relying on the AI instead of a review engineer for each derived design the CRN registration delay and cost has been eliminated. Once the generic design is in use, the CRN process becomes as efficient as the National Board system until design changes are required.
- The calculation set for this registration sample. The calculation set covers all possibilities of the generic vessel
- The generic registration drawing. All the possible production options are covered.
- The “36×60” derived vessel drawing. The actual produciton drawing is simpler than the generic registration, and does not do anything not covered in the generic drawing.
- ABSA’s guide to registering generic designs. This is a good starting point when trying to make a generic design that will survive review by multiple Canadian vessel reviewers in multiple provinces.
- TSSA’s guide to registering generic designs. This is a simpler guide than the ABSA document, but shares much of the wording. See page 5.