Three different sets of Temp & Pressure on a Vessel's nameplate

[schaali1]

I'm trying to calculate Mean Allowable Wall Thickness (MAWT) of a Flare knock out drum (Design code of construction is ASME VIII Division 1 – W82). Peculiarly there are following three sets of design pressure and temperature values stamped on the Vessel's name plate as well as mentioned on its GA drawing.

Case I - 7.4 barg @ -79/90
Case II – 5.6 barg @ -79/199
Case III – 3.6 barg @ -79/224

The problem is if I take the worst case scenario meaning max pressure and temperature value of all three cases i.e. 7.4 barg @ 224 C, the required MAWT comes out to be more than the current nominal wall thickness of the vessel which essentially means it was not fit for service from the day 1. But when I calculate MAWT based on above individual cases separately, the required MAWT in all three cases is less than the nominal wall thickness of the vessel. My questions are

Does ASME VIII code allow that a Vessel's nameplate can have more than one set of temperature and pressure stamped?

Assuming worst case scenario, am I correct here in calculating MAWT based on max: pressure and temperature of all three cases?

Regards,
Schaali

 

[moltenmetal]

Correct me if I'm wrong, but a flare knock-out is usually connected to a flare, i.e. essentially an open pipe connected to atmosphere. All valves on the discharge, if there are any, are locked open. Is that correct? As such, it has no relief pressure and is not protected by a relief valve or rupture disc.

There may still be a credible scenario for over-pressure for a device, even if it is connected via a pipe directly to atmosphere. So the pressure drop expected through the piping to the flare tip, at the expected loading of material discharged into the flare drum during the worst anticipated upset condition, needs to be evaluated. In a large plant there may be several sets of relief event cases that need to be evaluated separately, each with its own expected relieving fluid temperature.

There is no problem stamping a vessel with several pairs of MAWP pressure and MAWT temperatures, and of course the MAWP will decrease with increasing MAWT. There IS a problem with setting a vessel's relief protection device at a pressure higher than its MAWP.

In cases where multiple MAWP/T combinations are going to be encountered, relief protection design has to take that into account.

 

[jamesl]

ASME code allows more then one set of pressure/temperature stamped on nameplate. See note 37 in UG-116.

In your case, 7.4 barg at 224 C is not a pressure/temperature set for this vessel. You don't pick the highest pressure and highest temperature from all three sets. Each MAWP has its own coincidental temperature.

 

[trottiey]

This is a double-post of thread794-291740

 

[chicopee]

About getting a copy of the manufacturer's data sheet from ASME for that wall thickness.