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.

CRN registered generic vessel vs derived vessel

The generic vessel gets registered, but it is not what gets built

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

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.

Nozzle separation table

The nozzle separation table is a critical component of CRN registrations

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.

Downloads

Resources

  • 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.