Function of the flash tank
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This post was last edited by cf0532 on 2011-12-31 09:56. A flash tank achieves the instantaneous vaporization of a liquid based on changes in pressure. As shown in the figure above, it is a flash tank installed outside the methanol storage tank, connected behind the pump. Based on some information reviewed, it is believed that the high-pressure methanol in pipeline P-1124-A1A1-6'' does not normally enter P-1131-A1A1-2'' ； When its internal pressure exceeds the set value, a portion of the high-pressure methanol enters the P-1131-A1A1-2'' pipe and from there goes into the flash tank, where it is depressurized to become saturated methanol and corresponding methanol vapor at the pressure inside the flash tank. If that’s the case, may I ask: where is the set value located, and how is it set? And how is it controlled whether to allow flow into or not into the P-1131-A1A1-2'' pipe? Under what circumstances is pressure usually higher than normal? If not, then how?
It’s not clear from the P&ID diagram you provided how to control it. Personally, I think single-loop control can be achieved; by setting a PT at P-1124-A1A1-6'' and a PCV control valve at P-1131-A1A1-2'', it should be possible to accomplish this, and it can be implemented in the PICA controller. Abnormal pressure can occur behind the pump; for example, it might be the case that the valve downstream closes when the pump starts operating, and the pressure resulting from this closure can be quite high. In such situations, the flash tank plays a very important role by providing a safe means of pressure release. After all, methanol is a quite dangerous medium. However, it’s not clear from your diagram how to control it. Since the overall process is unclear, this is for reference only.
Judging from the diagram, it’s likely that the control in question needs to be operated manually; the 3/4\" wire is connected to a pressure gauge (or is it a safety valve?) ) There are no automatic control valves on line P-1131-A1A1-2″; it is likely that the manual valve will be used to control the amount of material entering the flash tank
1. There are no detection and control instruments for “above set value” in the diagram. There is only one manual valve, which must be operated by workers; but what parameters should the workers use to operate it? 2. Methanol flash tank: Since it involves high-pressure methanol flashing, methanol gas will inevitably be \"vaporized\"; if this gas is released into the atmosphere, won’t it cause poisoning among the workers on site?
Reply to 3# timwangwt: It should be a safety valve to prevent the pipeline from experiencing overpressure. Should its set value be less than the design pressure of the downstream equipment? It seems that there is no control valve; perhaps, after overpressure occurs, workers manually turn it on and off
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From the diagram, it appears to be a liquid drainage collection tank, not a flash tank; there is an SG on the inlet pipeline, which should be a sight glass. In the case of a flash tank, a pressure reducing valve is installed on the inlet pipeline to reduce the pressure from high to low, or a dual-valve system is used for manual pressure reduction.
The valve on pipeline P-1131-A1A1-2 is likely a safety pressure relief valve; it opens automatically when the pressure reaches the set value, and closes again after the pressure is released. A pressure level higher than normal can be caused by blocked pipes or closed valves, or it may result from the fluid inside the pipes reaching an excessive temperature after passing through a heating furnace or heat exchanger.
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This is clearly a liquid drainage collection tank, used probably to drain the methanol from the pipelines during system shutdown or maintenance.