Flange Rating Dillemma

[ericvd]

Hi everybody,

I am currently busy with a steam line design for our client's process plant, and we ran into a bit of a situation.

The current conditions on plant is two package package boilers supplying to a common line. The one boiler has a design pressure (SV lift) of 1600kPa and the other 1415kPa. They deliver sat steam. The flanges that they have ordered are (all 200 of them and 150 valves) ASTM A182 F316L class 150. They basicly just ordered it the same class as what is currently intalled on the plant, class 150. Now according to us, the design consultants, B16.5 limits us to 1100kPa (or 160psi). According to me this means everything has to be scraped and class 300 ordered. I was hoping there might be some way to get around this but according to the code, just cant see a way. There boiler are mostly running below design pressure at 1200 - 1350kPa, which is still higher than the rated 160psi, but not much. All the 20 year old existing flanges on the plant are class 150, but is carbon steel, which has a slighltly higher rating, but still below 1600kPa of the boiler.

So i was wondering or hoping there is somebody out there that knows the code better than we do that can tell us that the it allows one to go down a percantage below the design pressure, or something like that that I have missed! Although, I think they have srewed up big time...

Thanks in advance.
Erwil

 

[BigInch]

That dosn't sound very good. I think there is little hope for an easy solution, other than rerating the system for a max of ANSI#150 operation? Surely not an attractive option. What code?

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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[C2it]

You could check out the flanges to ASME VIII Div 1 App 2, and see what the stress levels are at the B16.5 pressure limit and at your design pressure. It largely depends on gasket type, but some standard flanges have lots of reserve capacity. Beware though, others are notably weak.

 

[ione]

The class 150 flanges (ASTM A182 F316L) are definitely under rated for this application (as per ASME B 16.5). ASTM A 105 (or other materials) class 150 would be under rated too, because of the design pressure of the boilers.

IMHO two options are available:
1. Change class
2. Re-rate the system for a max of ANSI#150 operation (as stated above). This second option requires a revision of the pipeline to check whether the diameters are adequate for the reduced pressure.

 

[JLSeagull]

If in new condition consider exchanging for the higher flange class with a credit for the Class 150 toward the purchase of the Class 300 valves and fittings.

 

[rneill]

The majority of 316 SS material these days that is supplied by manufacturers is actually dual graded as 316/316L. In the case of dual graded material, you are permitted to use the mechanical properties of either grade.

Using the flange rating values for 316 material (instead of 316L) material, the flange rating curve and the saturated steam curve would intersect at a maximum of 194 C and 1383 kPa.

If you could confirm that these were dual rated flanges (which they most likely are) then they would have this maximum rating of 1383 kPa (200 psi). However, I'm not sure that this would solve your problem since that is really to tight to an operating pressure of 1350 kPa - you'd have problems with relief valves on a regular basis.

I also agree with ione that your existing CS flanges are really not adequate for this either. Carbon steel flange pressure rating intersects the steam curve at a pressure of 1400 kPa and 195 C so really are about the same as 316 SS flanges and are definitely not rated for the 1600 kPa that your PSV is currently set at ...

 

[dcasto]

we are all assuming no superheated steam too.

I would seriously look at you requirements for steam pressure. Can you end users use lower pressure steam at a lower temperature, BUT with more avaialable energy in the form of the heat of condensation. I found in the past that operators would raise the steam pressure trying to get more duty to a reboiler, when if they lowered the pressure they would get more energy transfer for the same number of pounds per hour of steam.

 

[ericvd]

Thanks for all the inputs, I appreciate it. We have discussed with the Client the option of lower system pressure but they weren't up for it, unfortunately. So all flanges will now be changed to Class 300. I think its the best for the long term life of the plant. And personnell!

Cheers

 

[racookpe1978]

We've discussed flanges. Is the rest of the system safe?

Did you check the pipe schedules?

Equipment flanges? Instrument pressure ratings on attached sensors?

 

[ericvd]

Yes we've designed everything such that it is safe. I've also done a Ceasar model to verify that thier sched 10 piping is adequate. All valves will have to replaced though.

Thanks once again.