Hydrostatic Test of a New Nozzle in an Existing Vessel per NBIC 1

[AWDMIKE]

We have an existing field fabricated vertical vessel that has multiple design temperatures and materials; the upper-most section having a design temperature equal to 650 F and the bottom-most section having a design temperature of 1050 F. We are adding a new nozzle in the bottom-most section.

The original hydrostatic test pressure was calculated by 1.5 * MAWP * LSR.

In the days when this vessel was built, the allowable stress of the material in the top section (carbon steel) was the same up to 650 F thereby having a LSR = 1.0. For all other parts of the column the stress ratio is higher, so a value of 1.0 would be considered the lowest stress ratio (LSR) which would validate the original Code requirement of 1.5 * MAWP * LSR which is shown in the original documentation. The current NBIC requirement is also shown as 1.5 *MAWP * LSR.

For this new nozzle we are planning on using a weld cap to perform a hydrostatic test (for which the AI has agreed) but since the new nozzle is in the section where the stress ratio is closer to 3.0 if that stress ratio was used that area would be hydrostatically tested at a pressure three times higher than what it was originally tested.

Should the requirement in the NBIC (for which we must comply) be calculated for the entire vessel (even though we are performing a localized hydrostatic test) or should it be calculated for the part of the vessel subjected to the test? I am thinking that it should be the latter, which would subject that new nozzle to a higher percentage of the in-service stress to allowable stress. If so, it must be very clear that this higher test pressure never be used for the entire vessel, which we would do so in our own new documentation.

Thanks for any other insight anyone may have here.

 

[metengr]

First, is this a repair or alteration?
If this is an alteration it would be the lower hydrostatic test pressure regarding the weakest material in a composite materials vessel. The other option is to use a combination of design calculations to show adequate strength with or without reinforcement, NDT of the nozzle attachment weld and lower pressure hydrostatic test.

 

[chicopee]

Adding a new nozzle would be an alteration, not a repair.

 

[metengr]

Adding a new nozzle can be a repair per the 2017 edition of the NBiC.

3.4.4 EXAMPLES OF ALTERATIONS
c) The addition of new nozzles or openings in a boiler or pressure vessel except those classified as
repairs;

3.3.3 EXAMPLES OF REPAIRS
i) Installation of new nozzles or openings of such a size and connection type that reinforcement and
strength calculations are not a consideration required by the original code of construction;
j) The addition of a nozzle where reinforcement is a consideration may be considered to be a repair,
provided the nozzle is identical to one in the original design, located in a similar part of the vessel, and
not closer than three times its diameter from another nozzle. The addition of such a nozzle shall be
restricted by any service requirements;

So, as I stated before to the OP who has the data report with attachments is this a repair or alteration?????

 

[AWDMIKE]

We are planning on calling this a repair per 3.3.3.j. I did not think that whether it was a repair or alteration mattered for a new nozzle and hydrostatic test but perhaps I was mistaken.

To be clear, we are classifying this work as a repair but the section of the vessel in which the repair will occur has a stress ratio of close to 3.0, which would yield a hydrostatic test pressure about three times higher than what was performed as part of the original field hydrostatic test many years ago. That is why I asked whether we would use the lowest stress ratio for the entire column, or if we should be using the stress ratio for the specific area which will be hydrostatically tested.

Thanks again.

 

[metengr]

No, you don't need to duplicate the code hydrostatic test for a repair. Only a pressure test with NDT.

 

[AWDMIKE]

Thank you for the responses thus far.

Metengr you may have answered the question I asked but I may have asked the wrong one.

We are looking to comply with the NBIC hydrostatic test requirement of 1.5 * MAWP * LSR (lowest stress ratio). If I calculate the hydrostatic test number of the whole vessel I get the same value to which the vessel was originally tested (75 psig).

What I'm unsure about is the following:
1. Should I be taking the LSR of the entire vessel of 75 psig?
2. Should I be taking the stress ratio of the section in question which yields 3 * 75 = 225 psig?

 

[metengr]

You would use the value for the entire vessel, and not locally.