Questions about high and low pressure heat exchangers
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Shell-and-tube heat exchangers whose design pressure on the low-pressure side is less than 80% of the design pressure on the high-pressure side； Regarding such heat exchangers, there are the following questions: 1. If the low-pressure side is not designed at 80% of the pressure specified for the high-pressure side, safety measures such as safety valves or burst discs should be considered for that side. However, in the case of water coolers, if a burst disc is installed on the water side, where should the debris be discharged to? After all, the process side involves flammable and toxic gases, right? 2. If the design pressure on the low-pressure side is increased to 80% of that on the high-pressure side, can it be assumed that the heat exchange tubes will not rupture? Does the corresponding pipeline design pressure need to be increased as well?
The design pressure of the corresponding pipeline should not need to be increased, right? It should be determined according to the pipeline grade table
Please ask an expert to answer! The strength calculation of the heat exchange tubes shows that they are satisfactory; the tubes will not burst regardless of the pressure in the shell side ； I’m not sure if this is correct – why is it necessary to design for safe explosion in the shell side? Stay tuned!
Shell-and-tube heat exchangers whose design pressure on the low-pressure side is less than 80% of the design pressure on the high-pressure side； Regarding such heat exchangers, there are the following questions: 1. If the low-pressure side is not designed at 80% of the pressure specified for the high-pressure side, safety measures such as safety valves or burst discs should be considered for that side. However, in the case of water coolers, if a burst disc is installed on the water side, where should the debris be discharged to? After all, the process side involves flammable and toxic gases, right? 2. If the design pressure on the low-pressure side is increased to 80% of that on the high-pressure side, can it be assumed that the heat exchange tubes will not rupture? Does the corresponding pipeline design pressure need to be increased as well? Answer: In heat exchanger design, when there is a significant difference between the design pressures of the tube side and the shell side – such as 80% as you mentioned – it is common to place the higher pressure on the tube side, as this helps to save materials. In the shell side, the pressure is low; if the design pressure is increased to match that of the tube side, it would be uneconomical. The design pressure should remain at the low value applicable to the shell side. The thickness calculated in this way should then be increased by a few millimeters – but by how much exactly? First, it is necessary to determine whether the medium in the shell side is compressible, that is, whether its compression ratio is high or low. If the ratio is high, less increase is needed; if it is low, more increase is required. As you mentioned here, for water, the compression ratio is low, so you need to increase it by a few MM. In a situation like yours, the water is in the shell side; a rupture disc is installed on the shell side (safety valves are generally not used, but one can be connected in series, as is the case with flammable and toxic gases). The installation requirements follow the standard guidelines. Why is this done above? Due to the high pressure in the tube side, if a tube bursts during heat exchange (although this is unlikely), the process gas will immediately enter the shell side. The lower the compression ratio on the shell side, the faster the pressure there will rise, and it’s possible that the shell side tube could burst as well. That’s why rupture disks are installed; generally, safety valves are not used because they have a delay in response, and in cases involving toxic gases, a series connection is necessary to ensure safety. It will just explode, it really will. There is no need to increase the design pressure of the pipeline, as rupture is also achieved using a device designed for this purpose, namely a burst disc. Once the rupture disc bursts, safety is ensured. It doesn’t affect the pipeline anymore. As for whether the heat exchange tubes will break or not, it depends on their respective design pressures. Even if you increase the pressure on the shell side, the tubes can still burst; it’s not possible to control whether this will happen. Can you adjust the pressure both upward and downward? It was then designed based on the pressure difference. I hope you are satisfied with the above answer. :P
It’s described in great detail! That’s why the safety factor for the tube side needs to be high, while the shell side is designed for low-probability scenarios ； If the tube bursts, an explosion disc failure will also occur in the shell side, because the medium is toxic! Right?
Because it is toxic, the burst disc and safety valve must be connected in series, so that a conduit can be used to discharge it to a safe location. If it is non-toxic and non-explosive, then a burst disc will suffice; the burst disc serves to prevent the shell side from rupturing in case of a pipe explosion.
I still don’t understand the purpose of series connection; how is the conduit connected, to the safety valve or to the burst disc?
There are two possibilities according to the specifications: one is the corrosiveness of your medium; if only a safety valve is installed, the material cost will be too high； Second, it requires complete leaklessness.
A burst disc is attached to the device (as required by the burst disc standards, so that no fragments are generated during rupture); a safety valve is then connected in series with it, and an air guide pipe is connected to the safety valve.
If it bursts, it’s unclear whether liquid or gas will be discharged? Are there any design specifications for high and low pressure heat exchangers in China?
If the high-pressure side is on the shell side and the low-pressure side is on the tube side, as is the case in some chemical processing units, is it still necessary to install relief facilities?