4 replies to this topic

[mykid]

Hi all,
Need some advice for specifying design conditions of a compression system.

normal condition: 95bara @ 40C.
Design condition: 120 barg @ 180C.
Corrosion allowance: CS+ 3mm

Based on the above condition, minimum pipe spec is 900#.

If this system is subjected to low temperature during blowdown, i.e. -25C, should we consider that as the minimum design temperature? If yes, it only valid at a much lower coincidental design pressure.

As advised by our piping pp, the current 900# CS pipe spec for 8" and above only allows for minimum design temperature of 0C due to the required thickness + corrosion allowance. If a lower design temperature is expected, LTCS is to be adopted.

My question is if the low temperature will only be seen during blowdown which corresponds to a low pressure, should we consider that as the minimum design temperature?
Should i put the design condition: 120 barg @ 0/180C or 120 barg @ -29/180C?

[Art Montemayor]

Mykid:

Please correct me if I am wrong in the following comments:

1) You are designing your piping class at 120 barg (1,740 psig) and 180 oC (356 oF); at these conditions, my ANSI / ASME B16.5 piping tables indicate that a flange classification of 900 lb. is good for 400 oF & 2,000 psig and is suited for your design service conditions. The carbon steel allowable stress is good down to -20 oF (-29 oC).

2) Therefore, your 900 lb. carbon steel pipe classification is good for your -25 oC resultant low temperature. Why do you state "it only valid at a much lower coincidental design pressure".

I don't agree with your statement, "the current 900# CS pipe spec for 8" and above only allows for minimum design temperature of 0C due to the required thickness + corrosion allowance". The temperature affects the allowable design stress for the carbon steel material – and this is valid within the range of -20 oF and +650 oF. The temperature does not affect the corrosion. Corrosion normally affects the pipe - not the flanges (which are the weakest link).

3) The design condition should be designated fully as 120 barg @ -29 to +180 oC.

4) What pressure piping / flange code or standard are you designing to? This may make a difference. What I design to is ANSI / ASME B16.5 & B31.3.

[shan]

Hi Mykid,

The piping design conditions are the anticipated worst conditions with some added safty factors (highest stress that the piping wall may be experienced) in all the operating cases. Therefore, the design temperature and design pressure should occur simultaneously. If the temperature value is from one process scenario and the pressure value from another scenario, the piping wall may never experience this kind of artificial mixed conditions. Therefore, I suggest the design conditions are [120 barg@180 oC / ??@ -29+(safty factor) oC] if you think both HP/HT conditions and ??P/LT conditions may be considered for the piping stress.

Regards

Shan

[JoeWong]

Just to add some previous experience...

I have seen piping class (carbon steel) the lower temperature limit is 0 degC. It limited by one of the component in the piping class...not very sure...

Mykid, can you check with your piping engineer, what is the limiter for our knowledge...

If not mistaken, there is another piping class which good upto -29 degC as mentioned by Mr. Montemayor...

[mykid]

Thank you all for replying.

Our piping engineer is referring to ASME B31.3, Table 323.2.2A which shows the minimum design metal temperature and thickness for carbon steel without impact testing. For corrosion allowance of 3mm minimum and design conditions as specified earlier, the thickness calculated is about 23.8mm, on that table the minimum temperature is shown as -1.7C based on Curve B.