low temperatures at blowdown 2

[michael99]

Hi
I am designing a blowdown system for a gas export line topsides (modification to an existing plant). The calculations on hysis indicate we are 10 degrees below the minimum design temperature on the flare header.
I am using the detailed heat loss in the software as well.

Our piping have mentioned putting a peice of pipework for 2m immediatly downstream of the RO that is suitable for the very low temperature alas not sure how much pipe i should install. Another problem is the flare has fire protection on it. Does any one know where this philosophy can be found or other solutions

 

[25362]

Thread391-139300.

 

[rafcu]

Michael99,

I have the same problem, but, too late, a vent pipe is already damaged, the pipe was in CS: design temperature -20°C. The customer does not accept the HYSYS results, as the effect consider for the FO blowdown is just JT, (isenthalpic), and there is lower temperatures as kinetic energy is neglected by the isenthalpic simulation. I will suggest use LTCS 3 meters upstream the FO, but the design temperature for this material is -49°C, I hope that the material allowances allows lower temperatures for few minutes…to be followed…

25362 has given me a good advises you can analyse in thread391-139300.

Please if you find a solution advise me…

 

[michael99]

Thanks
I have put a -45 pipe spec all upstream of the system. Alas we also looked at the wall temperature upstream and this is fine. So relatively happy on the upstream section before BDV. Its the downstream section im more concerned about. I have considered putting a low temp high pressure from the BDV to the LO valve and ensure this meets the -55 condition. This should be about 6m. I have allowed a distance of 1m from the BDV to the RO. The existing Hp flare header is designed for -45 and im getting -55 exiting. Alas so near. I had some thoughts on the heat sink effect given i get this condition for the first four minutes. May be some of you could advise on this

In past i know we have has a similar problem on the flare on our other platforms in which we checked the material at the Hp flare and the metalagist check the material at the low temp condition, impact tests and had to review all the weld procedures. I was trying to avoid this route though.

 

[SJones]

When you say the flare header was "designed" for -45, do you mean that the designers actually determined -45 and then selected the material, or did they just select ASTM A333 Gr6 material and slap on -45 because that's what the material can go to? Take a look at ASME B31.3, Figure 323.2.2B and use the pressure that is around when the metal has reached -55 to determine the stress in the pipe wall; see whether the stress ratio gets you an allowable reduction in MDMT.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[michael99]

Hi
This platform is quite old. When i state the min design temp is -45 then they chose the pipe spec for carbon steel. The original blowdown calcs dont reach the -45 limit so they were OK. Originally a few years ago the system was allowed to run hotter. Nowadays we export the gas the temp came down.

I found out some info on the cold pipework lengths after the RO apparently it goes back to research conducted at imperial uni with Prof Richardson so im checking this out.