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Tuesday, February 5, 2008

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Have you ever experienced sizing Pressure Relief Devices in the following condition or one of the configuration in your plant was setup in the following way?

A heat exchanger with gas on shell side with low operating pressure of 1 barg (design pressure of 3.5 barg) is heated with High pressure steam in tube side at 40 barg (design pressure of 46 barg).


From earlier discussion on tube rupture assessment in "Criteria for Requirement of Pressure Relief Device for Tube Rupture", it is obvious that tube rupture scenario is credible and shell side shall be protected from heat exchanger tube rupture.

However, in the event of tube rupture, the relieving could be very complex. Why ?

In the event of tube rupture,
i) a complete sharp break will occur
ii) the sharp break may occur at tubesheet
iii) two orifice relieving high pressure steam and condensate to shell side (for simplicity)
One orifice is discharging high pressure steam and the other orifice is discharging high pressure condensate to shell side. The pressure relief device (PRD) may experience difference scenarios :

a) Pure hydrocarbon gas relief via PRD. The flowrate would be the volume discplacement by steam discharge through orifice + steam & condensate flash from condensate discharge via orifice

b) Steam mixed with hydrocarbon gas relief via PRD. The flowrate would be the volume discplacement by steam discharge through orifice + steam & condensate flash from condensate discharge via orifice. The mixtures of steam and hydrocarbon gas would varies.

c) Steam and hydrocarbon gas with condensate entrained relief via PRD. The flowrate would be the volume discplacement by steam discharge through orifice + steam & condensate flash from condensate discharge via orifice. The mixtures of steam, hydrocarbon gas and quantity of condensate would varies. Condensate in gas would varied from zero to 100% condensate.

d) Flashing condensate relief via PRD. The flowrate would be the volume displacement by steam discharge through orifice + steam & condensate flash from condensate discharge via orifice.

Above will results multiple complicated relieving scenario and it will be a great challenge to a PRD designer.

Apart, the discahrge of steam and flashing condensate into shell would be highly turbulence and potential leads to severe surging, vibration, support shaking, etc which may cause catatrophic failure of the heat exchanger. Surge analysis as well as detail study of liquid flowing dynamic may required to ensure above scenario will not occur.

Thus, the overpressure protection system against the tube rupture scenario for above configuration and condition may not as simple as just providing a pressure relief device on the shell. Additional measures shall be provided to eliminate potential of tube rupture scenarios.

Next post will discuss some measures may be taken to eliminate / avoid / minimize magnitude of complicated tube rupture scenarios.

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posted by Webworm, 3:55 PM

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