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Monday, February 11, 2008

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In earlier post "Complicated Tube Rupture Scenario... Nightmare in Sizing a PRD" related to complication of a tube rupture involving many relieving combination. This will be a great challenge to a Pressure Relief Device (PRD) designer to provide a correctly sized PRD. The original case study was :
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).
The are some ways may consider to avoid or minimize tube rupture complication. The following are some recommendations but they still require proper engineering and HAZOP analysis before implementation.

i) Use low pressure steam so that it match with or within low pressure side (LPS) maximum allowable working pressure, MAWP, in order to eliminate the tube rupture scenario. Read more related to tube rupture in "Criteria for Requirement of Pressure Relief Device for Tube Rupture" and "Two-third (2/3) rule or Ten-thirteen (10/13) rule ?".

ii) As discussed earlier, the complication is properly cause by condensate flashing across the ruptured tube (orifice), thus the effort is to minimize (avoid) the opportunity of condensate back flow. The condensate return line upto last valve can be a high design pressure section but it return to a low pressure condensate header where the operating pressure lower than Shell side 110% of design pressure. In the event of tube rupture, only minimum condensate will flash in to the Shell side (LPS) and there is NO continuous condensate reverse flow into the Shell side.

iii) Apply enhanced strength welding for all joints during design and fabrication of heat exchanger.

iv) Provide double Non-Return Valves (NRV) with difference technology on the condensate line to minimize the probability and flowrate of condensate back flow into the Shell side.

v) High integrity protection system - Provide vertical stand pipe (as shown in image) and provide High-High Level trip (LZHH) with two-out-of-three (2oo3) voting technology to shutdown and isolate entire condensate system. In the event tube rupture, condensate back flow would increase level in the vertical stand pipe until it reach the High-High level, the shutdown system will close SDVs on condensate line to avoid back flow.

vi) Provide Low-Low Pressure (PZLL) trip in the vertical stand pipe. In the event tube rupture, the pressure will lose very quickly (possible re-built again) and PZLL will be triggered and initiate shutdown of steam and condensate system.

vii) Provide High-High Temperature (TZHH) trip on the Shell side. In the event tube rupture, the steam flow into the Shell side will quickly increase the vapor temperature and trigger TZHH which will subsequently trip the steam and condensate system.

vii) Provide rupture disks (dual with High pressure alarm in between) instead of Pressure relief valve (which is reclosable type) on the Shell side. In the event tube rupture, the disc will rupture and there will less accumulation of pressure in the Shell side. Those any condensate flash into the shell side would have the highest chances of turn into steam.

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posted by Webworm, 12:38 PM


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