Impact Testing Exemption for SA 266 Gr. 2 Tubesheet

[ashaykadam]

We are designing one S&T heat exchanger with Carbon Steel MOC.
The Design MDMT for shell side is -29 degree C.

Tubesheet MOC is SA 266 Gr. 2 & its thickness is 67mm.

We can not take Impact Testing exemption as per UG-20(f) as specified in the project documents.
When we designed the heat exchanger in COMPRESS we found that it is not considering reduction in rated MDMT for Tubesheet as per Fig. UCS-66.1.

However, the same is being considered for body flanges with same MOC SA 266 Gr. 2.

I shall be thankful if you kindly inform why COMPRESS software is not considering reduction in rated MDMT for Tubesheet as per Fig. UCS-66.1?

Does ASME Section VIII Div. 1 specifies that this exemption is not applicable for tubesheet?
I am also not able to differentiate two terms specified in UCS-66 -> (b) -> (1) -> (b)
- maximum design pressure at the MDMT &
- maximum allowable pressure (MAP) of the component at the MDMT

 

[brkmech1234]

Whether your body flange is standard flange? An MDMT up -29 deg C is allowed for standard flanges in accordance with UCS 66(C)

 

[ashaykadam]

Thanks for your reply brkmech1234.
Our Body Flanges are not Standard Flanges. They are designed as per ASME Sec. VIII Div. 1, App.-2

 

[LCeballos]

Hi,

Is the tubesheet integral or gasketed between channel and shell? It seems that tubesheet are exempted by UCS-66.3 (governing thickness). Please, see fig UCS-66.3 (c) Bolted flat head or tubesheet and flange. If tubesheet is "sandwiched" UCS-66.3 applies also.

Regards.

Luis C.

 

[ashaykadam]

Thanks LCeballos.

However, kindly have a note that our tubesheet thickness is 67 mm & MOC is SA 266 Gr. 2 (non-normalized).

As per FIG. UCS-66.3 the governing thickness for Tubesheet works out to be 67 / 4 = 16.75mm

We could have straight away taken Impact Testing Exemption as per UG-20(f) which give relaxation for Impact Testing upto governing thickness 25 mm.
But unfortunately we can not take this exemption as Project Specification says Exemption as per UG-20(f) shall not be considered.

Hence, we wish to know whether we can reduce minimum design metal temperature for tubesheet without impact testing using FIG. UCS-66.1 or not.

We found that COMPRESS Software do not consider reduction in MDMT without impact testing as per FIG. UCS-66.1 for tubesheet. So we wish to know reason behind the same.

Does that mean reduction in MDMT without impact testing as per FIG. UCS-66.1 is not applicable for Tubesheet?

I am herewith attaching our COMPRESS Software design file. See if you can provide further information regarding this issue.

 

[SnTMan]

ashaykadam, as you no doubt know, UCS-66(b) and Fig. UCS-66.1 provide methods to reduce MDMT based on reduced stress (or excess thickness) in a component. It is fairly straightforward for a number of components, however tubesheets are a more complex situation (see Part UHX) and I doubt it would be considered feasible to program, with any confidence, such a reduction calculation. This does not mean that an experienced designer cannot calculate a reduction. UCS-66(b) gives several bases to do so, and the designer is free to determine which is most applicable and can be backed up.

Regardng your question about max design pressure versus max allowable pressure, see the definitions in Appendix 3. I might add these are pretty fundamental concepts.

Regards,

Mike

 

[Tony1t]

Hi,ashaykadam,
As you know,Figure UCS-66.1 may also be used for components not stressed in general primary membrane tensile
stress, such as flat heads, covers, tubesheets and flanges(including bolts and nuts).
If your COMPRESS can not calculate reduction in MDMT, or may try PV Elite software.
The MAP is defined as the highest permissible pressure as determined by the design formulas for a component using the nominal thickness less corrosion allowance and the maximum allowable stress value from the Table 1A of Section II, Part D at the MDMT.

Regards
TonyTao
HSB CT (China division)