Maximizing Flow From Liquid Vessels
A problem frequently presented to us by customers is maximizing flow from liquid vessels to achieve a higher vaporized medgas rate from their manifold. Typical oxygen or nitrogen liquid vessels will provide between 300 to 400 SCFH per vessel through the Use or Gas valve when the pressure building circuit is open. The problem being presented is although they are using two liquid vessels in their supply bank (as shown in figure middle left), they are not able to achieve or sustain flow rates of 400 to 500 SCFH. “If each liquid vessel provides 300 to 400 SCFH, my system should provide 600 to 800 SCFH from the bank of two vessels – correct?”
The answer is probably not. At least not without taking steps to ensure the ‘head pressures’ on the two vessels are always equal. Why doesn’t it work? And, what needs to be done to ensure the ‘head pressures’ of the two vessels are always equal? The reason it doesn’t work (provide 600 to 800 SCFH) with two liquid vessels connected is a simple matter of pressure differential. Inevitably, one of the two vessels pressure building regulator circuits is set at a higher pressure than the other, when the two vessels feed gas to the manifold header, the vessel providing Medgas at the higher pressure will close the check valve on the connection to the vessel providing gas at the lower pressure preventing it from supplying gas. Until there is enough pressure drop in the manifold header to defeat the pressure differential, the vessel providing gas at the lower pressure will be held in reserve.
As the bank is used, the two vessels will alternate providing gas to the manifold as the pressure builds in each vessel and then is relieved through feeding the manifold. What can be done to ensure the ‘head pressures’ of the two vessels are always equal? The answer is to add a pigtail connecting the vent valves of the two vessels and then to open the vent valves on both vessels. (As shown in red figure middle right) This will ensure the ‘head pressures of the two vessels will always be equal and the vaporizing coils of both vessels are used equally. If more than two vessels are used in the bank, a tee and additional pigtail(s) may be connected to all of the vessels making up the bank. (As shown in red figure bottom left).
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