Calculation of external pressure verification for the heat exchange tubes in the heat exchanger of SW6
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It’s the most common type of shell-and-tube heat exchanger. The design parameters are: 1.0 MPa for the shell side and 0.6 MPa for the tube side; steam flows through the tube side, and copper T2 tubes are used as the heat exchange tubes. All other parameters meet the requirements, but the calculation for the external pressure that the heat exchange tubes can withstand is not satisfactory. According to the table, for copper tubes with an outer diameter of 19 and a wall thickness of 1.2, the allowable external pressure is only 0.15 MPa. This difference is quite significant. I found that the main issue lies in the value of the external pressure stress coefficient B: it’s 1.0 MPa for copper, while it’s over 100 for stainless steel tubes. This results in a difference of 100 times, which naturally leads to a lower allowable external pressure. Yet, I’m sure many people have used equipment with similar operating conditions. At my previous workplace, we handled countless sets of drawings and designs for such equipment. My current clients also say they’ve never heard of cases where copper tubes cannot be used – how do you guys carry out the calculations? Or is it not allowed to use copper tubes in conditions where the shell-side pressure exceeds 1.0 MPa? I remember that the older version of 151 did not specifically mention the external pressure verification for the heat exchange tubes. Although this has been added in the newer version of 151, is it appropriate to carry out the verification entirely in accordance with the chapter on external pressure vessels for 150, just like in SW6? I humbly ask the masters for their guidance
I discussed this with my colleagues. On SW6, the external pressure coefficient curve for copper tubes is based on the standards for copper pressure vessels. Unfortunately, these standards do not provide a curve for pure copper tubes at temperatures above 65°C (see Figure 5-1; there is only a curve for pure copper tubes at temperatures below 65°C, while Figure 5-2 shows curves for brass up to 170°C). The B value of 1 MPa given in SW6 – I have no idea how they arrived at that figure. I guess it might be based on the B value specified in equation 4-3 on page P13, but that value is applicable to situations where the A value is too small and thus falls on the left side of the external pressure curve. In the case of ordinary heat exchange tubes, the A value never ends up on the left side of the curve, so SW6’s approach seems completely arbitrary. Do you all have any insights regarding the calculation of external pressure for heat exchange tubes? Why don’t the standards for copper pressure vessels provide curves for pure copper at temperatures above 65°C? Is it because the standards do not recommend using pure copper in situations with external pressures above 65°C? Yet pure copper heat exchange tubes have been in use for many years, and situations with temperatures above 65°C are quite common. I’m not sure how others calculate these values; perhaps it would be better to simply omit the section on external pressure calculations, as if it didn’t exist
According to the definitions, the external pressure charts in JB/T 4755 are based on those from the ASME standards. I know very little about the ASME standards; could someone please provide me with the external pressure curve for pure copper pipes at 165°C as specified in ASME? Thank you very much
Using the corresponding ASME grade and performing calculations with PVELITE, the external pressure calculation for the heat exchange tubes can be satisfied.
The temperature on the external pressure curve for NFC-1 also reaches 65, and PVELITE is calculated using the B value at this temperature as well.
Sir, since I have not used PVLITE before, I would like to ask you whether it is possible to use PVLITE at a design temperature of 165° for piping. What is the value of B in the software? A friend showed me the external pressure curve for NFC-1 in ASME, and it indeed shows values only below 65°. Does that mean ASME also does not recommend using copper tubes in external pressure applications at temperatures above 65°C? I’m confused; I always thought that heat exchange tubes shouldn’t be simply used in conjunction with containers without further consideration