EI Tube Rupture Guidelines
Sudden tube ruptures in shell and tube heat exchangers operating at a high differential pressure in which the low pressure side contains an incompressible fluid and the high pressure side contains gas have been recognized to have potentially severe, even catastrophic, consequences. Furthermore, it has been hypothesized that conventional (i.e. spring loaded) relief devices may not have sufficiently fast response times to mitigate the effects of a sudden tube rupture. To address this, the installation of rupture disks directly on the low pressure sides of exchangers in these services has been recommended by industry practices in recent years; however, the precise definition of which sets of operating conditions require installation of rupture disks has been ambiguous in industry practices, and as a result, actual field installations vary widely, dependent on industry, age of facility, and geography.
The Energy Institute (EI) published the second edition of their “Guidelines for the Safe Design and Operation of Shell and Tube Heat Exchangers to Withstand the Impact of Tube Failure” in November of 2015, which builds upon the first edition originally published in 2000. This new revision, among other things, includes additional shock tube testing to provide data on relief device response time and pressure propagation, guidance on tube rupture failure modes, further investigation into the validity of dynamic analyses, and an industry survey of operators’ experience with tube rupture in exchangers.
The new guidelines also present the results of validated simulation work performed to model the impact of a sudden tube rupture on three typical exchangers: “small,” “medium” and “large”. The presented results, although qualitative, are enlightening. Peak incident pressures generated in the low pressure side from the rupture are much more severe for relatively small exchangers (those with low liquid volumes and smaller diameters), and can exceed the operating pressure of the high pressure side. For medium and large exchangers, the results are more attenuated, with peak pressures in the range of 0.2 to 0.6 times the high pressure side operating pressure. Furthermore, installation of rupture disks (versus slower acting conventional valves) provides much more attenuation of peak pressures for the smaller exchangers; the benefits of installing rupture disks to reduce peak pressures in the event of a tube rupture are much less pronounced for the larger exchangers.
In addition to the qualitative comparisons of the effects of a tube rupture on different exchangers, the guidelines also develop an equation to estimate the peak impulse pressure upon a tube rupture, in which the high pressure side contains gas and the low pressure side operates liquid full. With the exception of the propagation path, the input variables are readily available (density of fluid, gas specific heat ratio, velocity of sound in gas, wavespeed in liquid, etc…) and the equation lends itself to easy solution in a spreadsheet. These are used to calculate the gas impact induced initial step in pressure, which travels at the wavespeed through the low pressure side liquid and is then reflected off the ends of the exchanger shell. This reflection will typically cause a doubling of the initial step pressure, as most conventional relief devices will not respond quickly enough to the initial pressure wave to cause dissipation.
Although these new tools in the EI guidelines are not intended to replace more rigorous dynamic simulation of exchanger tube rupture, they do give the user the ability to relatively easily and quickly obtain estimates of the peak impulse pressure in exchangers. This can be very valuable, not only in the design phase, but also in evaluating potential risks in existing facilities and helping to prioritize these risks.