Is Hydrotest required??

[AravindSujay]

Hi all,

The Heat Exchanger has a design pressure of 1100 Psig and design temp of 150 F with Flange rating of Class 600. The design temp is now changed to 300 F (Design press & others remaining the same) and the calculation was done supporting this value including the name plate change. My question is "Is the hydro test necessary for this change of temp alone?"

Thanks,
Aravind Sujay

 

[dtn6770]

Subject to the Code having changed since I last looked at it, if the new temperature doesn't have an effect on the material stress values the original and "new" hydrotest pressure should be the same.

Going from 150F to 300F probably doesnt' change your stress values but I don't know that for sure based on your description. Basically, calculate what your "new" test pressure would need to be and if it's the same as the original...there you go.

Of course your AI may trump you but I doubt it.

 

[jte]

Well, you didn't state the material of construction, but taking a wild stab at it and presuming its SA516-70 you do have a loss of allowable stress in making this temperature change. This rerate is an alteration, and thus waiving a hydro is contingent on jurisdictional approval. My jurisdiction would typically not approve such a request since the allowable stress is changing. You'll have to check to see if the allowable stress is changing and contact your AI and/or jurisdiction for guidance.

Presuming you have material group 1.1 flanges you appear to be ok, but have you considered piping loads and left some margin for them?

As dtn 6670 alluded to, if the original hydro was greater than the newly calc'ed hydro then that may be sufficient to satisfy the jurisdiction...

jt

 

[deanc]

HX....what kind of tubes does it have in it? Copper perhaps?

Be careful,does your U-1 list Appendix 23?

I would need more info. See NBIC RC-3022.

 

[AravindSujay]

Hi,

The MOC is 0.75" SA-179 smls for Tubes and 10" SA-106 B for shell cylinder and SA 516 70 N for Tube sheet.

As per calculations,
Local test pressure = 1890.57 @ 300 F
= 1892.80 @ 150 F

Allowable test stress @ 300 F
shell=31,500 psi
Tube =23,400 psi

Design stress @ 150 F
shell=17,100 psi
Tube =17,100 psi

Design press @ 300F / 150F remaining 1100 psi

As per the calculation of local test press, hydrotest is not required. But when compared with stress value, there is a vast difference and Iam totally confused - Is hydrotest really required?

Waiting for all of your expertise answers on this.

Thanks in advance,
Aravind Sujay

 

[0707]

I would say yes, not because of materials and temperature itself but because of gaskets behaviour in the new temperature conditions.

What are the fluids in the shell side and in the tube side?


luis

 

[JoeTank]

0707,
How would a hydrotest tell one anything about the gasket performance? The hydro is done at ambient temps not at service temps.

AravindSujay,
I would expect that the allowable stress at 150F and 300F to be the same for SA516 and SA105. Are you seeing differnt allowaables in your Code book at these two temperatures?

Have you inspected the unit to verify the conditions of the tubes, shell and tubesheet?

Joe Tank

 

[jte]

'Doh! I was looking at Div 2 allowables... Yup, no change in stress values for 516-70 or 106-B... Never mind...

jt

 

[0707]

Joe Tank

Of course a hydro test is done at ambient temperature. But if the equipment was already in service but and it is going to work at more than double temperature (you pass from 65.5 to 149ºC) you should perform any kind of checking to see gasket condition to see the possibility of having leaks through the gaskets in new service conditions mainly if leaking fluids are dangerous. If I were Aravind Sujay I will put new gaskets and for that reason I will perform a hydro test

Another reason for testing should be if the exchanger was dismantling for maintenance inspection or so, which we don’t know.

Regards

luis

 

[JoeTank]

Luis,
Thanks for the additional comments.

Joe Tank

 

[AravindSujay]

Hi Everyone,

Thanks for all your answers. But Gaskets is not a Matter. The system is never being used and it was in our stock from the day it was built (exactly one year). All the Shell & Tube conditions remain in good condition.

To narrow down my stuff,
"Is hydro necessary for the change in stress values".
Pls look at my values which I gave previously.

Thanks,
Aravind Sujay

 

[dtn6770]

Aravind,

Are you referencing ASME Sec. VIII UG-99(b)? All I've got is the 2001 Code and it offeres the following test pressure criterial....

Pt = 1.3(MAWP)(St/Sd)

PT = test pressure
MAWP = MAWP
St = material allowable stress at test temperature
Sd = material allowable stress at design temperature

My experience is that as the design temperature increases Sd decreases, the ratio of St/Sd increases, and test pressure increases. The values you posed earlier are counterintuitive to me. That is, a higher "local test pressure" at 300F vs 150F doesn't make sence. Neither do the stress values you listed. If only metal got stronger at higher temperatures!!!

I'm familiar with all the MOC you identified and don't believe their stress values change between 150F and 300F but I don't have ease access to Sec. II to verify that. Make sure those are the only materials you need to consider.

If your MAWF is 1100 and the original test pressure, the one that was performed, was 1892.80 psi I back calcuate your stress ratio as being 1.32 wich is further from unity (1) than I would have expected for the MOC and temperatures. That MAY mean the original hydrotest was over-kill which MAY mean you don't have to re-hydro.

I don't think anyone is going to flat out tell you that you don't have to re-test; our/my hope is that we've provided enough information for YOU to answer your own question.

 

[JoeTank]

AravindSujay,
The hydrotest pressure will be affected by an increase in design temperature if the allowable stresses are diminished at the elevated temperature. In general, the allowable stress decreases if the design temperature increases. However, the change in design stress for most carbon streels will not be noticed until the temps rise above the the 400F to 500F range. I suggest that you recheck the the allowable stress values to see if there is indeed a difference at 300F. Please let us know what you find.

Joe Tank

 

[0707]

Again if the system was never being used and it is in your stock from the day it was built (exactly one year) I recommend you to perform a hydrostatic test before putting it in service independently of stress values had been changed. This is my advice.

Good Luck

luis

 

[AravindSujay]

Friends,

Finally the hydrotest was decided to be carried out in the field.

Thanks for all your suggestion

Regards,
Aravind Sujay