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How much liquid remains in vessel?

How much liquid remains in vessel?

How much liquid remains in vessel?

I am rather new to the cryogenic liquid field, and I would like for someone to either confirm my understanding on this issue or straighten me out.

I work for a manufacurer's representative, supplying pressure control manifolds for medical gases to hospitals.  The manifold regulates the delivery pressure from a liquid oxygen vessel (1000 liter typical size).  The vessel has a "pressure building" regulator that delivers oxygen gas at approximately 300 psig to our manifold.  The manifold has two functions:  (1) to reduce the O2 pressure to 50 psi as required by the hospital gas distribution system;  and (2) to automatically switch the supply to a second liquid vessel upon depletion of the first.  An adjustable "switchover" pressure switch tells the manifold when to switch to the second vessel.  This switchover pressure is normally set at roughly 100 psig.  

I have a specific problem with one hospital that could be resolved by changing the switchover pressure from 100 psig to 200 psig.  I have discussed this possibility with my customer and received an objection based on his belief that this change will substantially increase the amount of O2 left in the "depleted" vessel.  

I believe that the pressure decrease in the depleted vessel only occurs after all of the liquid has been evaporated.  So the difference between the amount of O2 at 200 psig and  the amount at 100 psig is very small compared to the liquid capacity of the vessel.  Am I wrong in this understanding?  

RE: How much liquid remains in vessel?

well, you are right in saying that it would not make much of a difference in 200 psig or 100 psig left in the tank. But it also depends upon the size of the vessel. If the vessel size is too large then the amount of oxygen gas left in the vessel will be accordingly.

Antway, the amount of oxygen left in the vessel should not be of concern for the employer as that never gets wasted.

RE: How much liquid remains in vessel?

I'm not sure I agree.  If the LOX is saturated at a low pressure (<100 psig), then as liquid is pulled from the bottom, the gas space expands and drops in pressure roughly equal to the increase in gas volume.  Not until you drop the gas pressure to the liquid saturation point does all the liquid have to be gone to drop the gas pressure further.  Of course, the pressure builder would try to keep up with the increasing gas space volume.  Maybe the pressure builder can't keep up with the liquid withdrawal rate.  How quickly do they go through the 1000L?  Is the pressure builder all icy/snowy?  Maybe it's restricted.

RE: How much liquid remains in vessel?

Yeah, what your customer believes is true. defacto, the amount of Oxygen left out will be significantly higher if you switch over at 200psig rather than 100psig. Remember, the 100psig rate is give to restrict the manifold starving rather to get all the liquid out of the storage tank and it is a safety hazard too. You can convince your customer by telling whatever the liquid left out after changing the set point of the switchover to 200psig will be used up again once the tank is filled and moreover it will hlp u in recovering the gas generated during filling. Moreover, u should discuss the kind of problem faced by you and why at all u want to change the pressure setting???

RE: How much liquid remains in vessel?

If the 1000L vessels you are talking about are being filled on site then you lose nothing as the new fill is simply being added to the vessel and you are charged by meter for what leaves the truck.  If the unit is hauled away and then refilled at the gas supplier, the supplier charges you for the entire contents of a 1000 liter vessel each time one is delivered.  It is not uncommon for 10% of the "contents" to come back to the plant for re-filling.

Hope this helps - 6 months after the fact

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