2 replies to this topic

To All - this is my first post, but I've been a frequent visitor and learned a lot from your comments. I'm hoping someone can give me some guidance as I am perplexed with the particular problem I'll attempt to describe below:

I've been tasked with evaluating the need for/sizing of pressure reliefs for a new process. The application is a horizontal reactor vessel with flange mounted bayonet heaters providing heat input (mounted on the bottom of the vessel). They are "stinger" type bayonets. I'm not sure of the correct terminology, but they are basically a tube within a tube with the outer sleeve being a corrosion resistant metal with stainless steel internal tubes. The steam enters through the SS tubes and condenses on the inside wall of the corrosion resistant metal and exits at the bottom. The vessel and exchangers are designed to 300 psig. Steam feed is max 150 psig steam. Operating pressure of the vessel is >150 psig. I have sized the vessel PSV, but am having a hard time determining the need for relief valves on the bayonet heaters. My questions are:

1) Does code consider this type of heat exchanger a shell/tube exchanger? (This should be fairly obvious, but I'm confused...)
2) If so, since the design pressure of the tubes = design pressure of the shell (reactor vessel) and is greater than maximum service pressure, is relief required? (ie Tube rupture not an issue...)
3) How do I treat fire case? External fire is governing contingency for the vessel PSV. In evaluating the heaters for external fire, the surface area exposed to the fire is very small (only the exposed steam chest), however the stingers are exposed to the vessel contents which will heat up during the fire to temperatures exceeding that of 150 psig steam. I assume I need to account for both sources of heat input in sizing the valve if it is required, if so, should I evaluate at the maximum relieving temperature I calculated for the reactor vessel?

I hope that I've given enough detail to get some opinions. I'm also sure that those opinions will lead to more questions on my part. Thank you in advance for your valuable advice....

[Nirav]

Would you please add a small sketch of the system you are talking about. Because it is required to understand the scenario.

Nirav, thank you for the reply. I should have posted a sketch initially as reading it back it is confusing. Attached is an elementary sketch in excel. I hope this clarifies the situation for you. I look forward to hearing your comments.

Attached Files

•  Bayonet_Heater.xls   18KB   58 downloads