A question about storing high-pressure materials as low-pressure materials
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During the design process, it is common to encounter situations where high-pressure fluid is reduced in pressure and stored in low-pressure tanks, or multi-column distillation with intermediate pressure reduction steps. We know that most fluids experience a significant temperature drop after being throttled to reduce pressure; what we’re not sure about is whether you take this temperature drop into consideration during the design process. I also asked some seniors, but they seemed to think it wasn’t really a problem and hadn’t considered it during the design process. I’d like to hear what the experts on Haichuan think about this issue
What comes to mind is that in general engineering design, one first considers whether the temperature drop will cause changes in the fluid flow properties that remain within acceptable limits, and then examines whether the temperature drop has any adverse effects on the material of the piping equipment. The former generally does not have significant effects very often; it rarely causes the fluid to reach low temperatures close to its freezing point as a result of pressure reduction or throttling. At most, the viscosity increases, and in many cases this is simply ignored. The impact of the latter on the material is also rarely reduced to the lower temperature limit of the equipment or pipeline material, and can be ignored as well. However, in the case of corrosive media, it is necessary to take into account the negative effects of lowering the temperature; for example, if there is H2S and H2O present, a drop in temperature may lead to stress corrosion, and this cannot be ignored at that point. I wasn’t aware of it when others were designing; for our designs, regardless of the reasons for temperature increases or decreases, I will at least consider from a piping expertise perspective whether it might have an impact on the equipment and pipe materials. So I think it’s not that it’s completely ignored; it’s just that most of the time, when there’s no obvious negative impact, it gets overlooked.
Compared to rising temperatures, the adverse effects of falling temperatures are likely to be less severe, so they are not easily given attention.
Does a decrease in pressure at a regular control valve result in a temperature drop in the same way as a throttle valve? Or is the cooling effect not as significant as the throttling effect?
The temperature drop of control valves and throttle valves can be estimated if heat losses are ignored; it isn’t as significant as one might think.
The throttle valve effect is quite significant; for many fluids, a pressure drop of just a few kilograms can result in a temperature drop of over 10 degrees. I have simulated this using Aspen, and in fact, this is also the approach used in cryogenics. But it’s not clear whether control valves can achieve such a cooling effect, as throttling valves are generally not used in designs; instead, control valves are employed to regulate flow. It’s uncertain whether this temperature drop needs to be taken into account in practice, or how significant it will be
When it comes to the operation of throttling valve-based refrigeration, the temperature drop must be taken into consideration. Of course, if the medium that is throttled or depressurized in such systems is a refrigerant like liquid ammonia or Freon, the temperature drop resulting from depressurization or throttling cannot be ignored; otherwise, problems will arise. Apart from media such as refrigerants, the temperature effect caused by pressure drop is not significant. In fact, whether the temperature drop caused by the control valve is significant also depends greatly on the nature of the medium; each case must be treated individually. The throttle valves and control valves I’ve encountered before were mostly used with ordinary process media, and they were often overlooked: lol