Negative Pressure at Orifice Flange Exit
Negative Pressure at Orifice Flange Exit
(OP)
I am reading a negative pressure at the exit side of the orifice flange assembly even though the main reservoir (shown on the left in the attached image) and scavenge sump (not visible to the right)are vented.
Creating a larger vent in the scavenge sump did not affect the magnitude of negative pressure.
As shown there is a 1" T-valve connecting all three.
This phenomenon occurs only at higher temperatures (10F - 200F).
The speed is 3,600 RPM.
Does anyone have a suggestion on the source of the vacuum?
Creating a larger vent in the scavenge sump did not affect the magnitude of negative pressure.
As shown there is a 1" T-valve connecting all three.
This phenomenon occurs only at higher temperatures (10F - 200F).
The speed is 3,600 RPM.
Does anyone have a suggestion on the source of the vacuum?





RE: Negative Pressure at Orifice Flange Exit
RE: Negative Pressure at Orifice Flange Exit
The transducer is tapped into the orifice flange, which leads to the T-valve and then to the main reservoir and scavenge sump.
Initially I thought the scavenge sump was inadequately vented, creating the vacuum caused by the suction line from the scavenge. But after observing the magnitude of the negative pressure unchanged despite doubling the vented area, I am stumped.
RE: Negative Pressure at Orifice Flange Exit
you have orifice flanges that is clear but you are trying to mesasure gage pressure from the downstream flange tap.
are they weld neck flanges?
Flange size?
is a plate installed?
is the orifce assembly discharging into a sump or into return vessel? If so is it a submerged discharge.
I would start with a sketch of your piping arrangement
RE: Negative Pressure at Orifice Flange Exit
I have a pressure transducer at the upstream and downstream flange taps.
The flanges are screwed boss type and 1" size.
An orifice plate is installed.
As I attempted to explain, the orifice assembly discharges into a main reservoir AND a scavenge sump using a T-valve to control the flow ratio. In neither case is it a submerged discharge.
RE: Negative Pressure at Orifice Flange Exit
Flow rate , Beta ratio? Liquid property?
Type of orifice?
For orifice, if the flow is too high
you get cavitation.
RE: Negative Pressure at Orifice Flange Exit
The flow rate is 30.9 in^3/s.
The beta ratio is 0.24.
The specific gravity is 1, and the kinematic viscosity is 6 cSt.
The orifice is square-edged rather than sharp-edged through a 1/8" thick plate.
RE: Negative Pressure at Orifice Flange Exit
Good luck,
Latexman
RE: Negative Pressure at Orifice Flange Exit
Keep in mind a key detail mentioned earlier: The magnitude of the negative pressure decreases as the operating temperature decreases (and viscosity increases). So at about 0F the exit orifice pressure is what I had expected from the start: zero.
Someone in my office suggested that at higher speeds the fluid flowing through the T-valve may be lowering the pressure enough to create a vacuum (Venturi effect). Does this seem plausible?
RE: Negative Pressure at Orifice Flange Exit
I should add one more detail that might help. At temperatures below 0F the exit orifice pressure reads slightly positive values. Does that strongly suggest a constriction (the T-valve) is creating back pressure in this case (lower temperature, lower speed, higher pressure, higher viscosity)?
RE: Negative Pressure at Orifice Flange Exit
What about the effect of viscosity on the vena contracta?
Could it explain the downstream pressure variation on cooling?
RE: Negative Pressure at Orifice Flange Exit
while 1" piping is always a concern for orifice metering, but your meter d/p is soo large as to invite problems.
if you increase the bore size to 0.500 or so, the meter d/p drops to 64"wc at normal flow, and the net meter loss to 1.6 psi
RE: Negative Pressure at Orifice Flange Exit
and more importantly the reynolds number is below 10,000 so you really cannot get a good measurement with a traditional square-edged plate
RE: Negative Pressure at Orifice Flange Exit
why are yu taking so much drop across the plate?
RE: Negative Pressure at Orifice Flange Exit
RE: Negative Pressure at Orifice Flange Exit
the orifice may have been intended as a flow restriction to avoid use of a valve, if you do use a larger orifice, make sure your valve can handle the throttling service