Hi all,
I have some doubts I hope you can help me with.
The main scenario is this, there is a vessel at a certain pressure and then it flows to another vessel. The second is a flash vessel so it is expected to separate the flow into a vapor and a liquid stream. In PFD, in books or the like, I have seen this operation depicted as follows (Fig 1): there is a pipeline, then there is a valve, then there is a vessel with 2 outlet streams one from the top and one from the bottom, and that is pretty much it. However I would like you to clarify me what is really happening.
Since I have seen this depiction many times, in my mind there was the idea that the valve was a piece of equipment that somehow reduced the upstream pressure to the flash vessel pressure. Then, after reviewing some classes of fluid engineering I came across a formula that let us know how much pressure drop is having place in the valve Cv = Q (Sg /DP)^0.5, where Q is the volumetric flow, Sg, the specific gravity and DP the pressure drop across the valve. Then mixing this two pieces of information I thought that the change of pressure between the first vessel and the flash vessel was due to this pressure drop created in the valve. However, then, I thought it would be difficult for the valve to give exactly this much pressure drop equal to the differences of pressure between the first vessel and the flash vessel, lets say a 100 psi pressure drop.
Then I started looking for this on the internet but I couldn't find the exact answer I was looking for, yet, I found a video (Fig 2) that shows 3 valves, one in the entry stream to the flash vessel, one in the vapor stream and one in the liquid stream. They say that the valve in the inlet is used to control the inlet flow, the one in the liquid outlet stream is for controlling the liquid flow, and i guess the liquid level in the vessel, and the one in the vapor outlet is said to control the pressure of the vessel. So I thought maybe this is it, this is the way that the pressure from the first vessel (lets say 200 psia) changes to (lets say 100 psia) the pressure in the flash vessel. Now this arised another question on my mind. And is related to how this works, I am imagining that the tank is being filled, it gets to a desired level, then the liquid outlet valve is opened so it stabilizes and the vapor outlet valve is closed so the vapor is allowed to "build up" or be kept in the vessel, thus increasing the pressure, then the vapor valve is opened and then so the pressure is maintained. All of this meanwhile somehow the system reaches a steady state where the inlet mass flow equals the outlet mass flows. But then another question came to me, so if the pressure in the flash vessel is set, or controlled by the vapor in the vessel, then that pressure is kind of ... "moving" backwards upstream, so the pressure immediately after (downstream) the inlet valve should be almost the same as the one in the flash vessel. But then, why doesn't that pressure transmits up to the first vessel and reduces the pressure inside it to (100 psia). Do the valve "protects" the flow upstream from this effect on the pressure? or said in another way Do the pressure upstream the inlet valve is indeed 200 psia and downstream the valve is 100 psia? if so, why?
But then I was still looking after all this questions all mixed up in my head, and I found that there are some valves called pressure reducing valve, and backpressure valve that ingeniously have a set mechanical pressure inside the valve so i only opens when the desired pressure is below that set, kind of resisting, pressure.
So well, I hope you can clear this doubts. And tell me if there is indeed a valve that only by itself can reduce the pressure 100 psia, or if in reality there is a team of valves doing the work or it these pressure reducing or backpressure valves are used instead, or if something very different is occurring.
A friend of mine also have another question regarding flash tanks and it was that she thought of a flash tank as a big big big pipe it is connected to the pipeline upstream, so she was thinking if the change in area/volume from the pipe diameter to a vessel "diameter" had any effect on the pressure of the fluid and was this would be.
Thank you for your interest and feedback,
Usu~
Attached Files
Edited by Usu, 02 March 2022 - 08:11 PM.