Quote: (Originally Posted by
jorgey)
Hi,
I have read the article, a very interesting thema.- i would have some questions.-
the advantage of a compensated 1st. stage would be, that there is no depth limit for it? and the disadvantage the gas comsuption? and the no truly constant flow?
if the upstream should be at least twice the downstream, taking for a example a kiss classic (i think, it has a non compensated 1st stage), that it's recommended for no deeper as 75m, means that, that the 1st stage at them is setted to 17bar <(1+7,5)x2>? or more, because safer would be to take 2,5x ambient pressure, i think.- it would mean over 20bar.- is this right?
j o r g e
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jeorgey
It is true that, at a given surface IP and SEV flow setting, a compensated 1st stage allows a greater diving depth at the desired mass flow than an un-compensated 1st stage. However, there are also depth limitations, with uncompensated 1st stages that need to be observed. Take this simplified, idealized example:
1st Stage IP at the surface= 160 psig (175 psia) / 2 X critical CMF pressure ratio (based on abs. press. )= 88 psia, 165 fsw, 50 msw,
Below this critical 2X pressure ratio, the orifice starts to go sub-sonic, and the mass flow begins slowly to decrease, until at 175 psia, , 360 fsw, 110 msw, the upstream and downstream pressures are equal, and the mass flow stops entirely.
As indicated in the Article, equal MOD performance is achieved, by running the uncompensated systems at higher initial surface IPs to achieve the same SEV flow values as a compensated stage.
Do I understand correctly that the data you wanted to see illustrated graphically is how fast the mass flow falls off (rolls off) after the orifice critical pressure ratio falls below 2X and the mass flow rate becomes progressively lower. ??