orifice plate theory

[Jimbo2010]

I have a submersible body that’s about 3m tall and that takes in water from the bottom through three 120mm holes and allows the air to escape at the top through three 50mm holes. I am trying to work out the pressure inside the body, using standard orifice plate theory as described in BS EN ISO 5167 and just wanted to see if anyone has suggestions on how applicable this method is and if there other techniques that might be better suited?

Jimbo

http://www.engcosolutions.com

 

[hacksaw]

metering coefficients are for pipe mounted devices, and would not apply in your case.

 

[Jimbo2010]

Thank you Hacksaw, I am not looking to use this to take measurements just to find pressure increases as the fluid flows through the orifice. Could you elaborate further why it would not be applicable?

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[ione]

Pressure increase as fluid flows through the orifice??? It really works the opposite.

 

[Jimbo2010]

What im getting at is that I have two sets of orifices in series. The first set has a larger diameter then the second therefore I believe more volume of liquid is entering into the chamber then air is exiting as the object submerges. Therefore the air is compressing increasing pressure. I’m trying to calculate this effect.

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[Apakrat]

Ain't no way to ever get a pressure build up between flowing water and air caused by the orifice size stated above.

At 74th year working on IR-One2 - - UHK PhD - - -

 

[Jimbo2010]

The way I was initially looking at the problem was that the flow of water into the body is restricted at the outlet. Therefore a situation would arise where the air compresses stressing the body. As you can guess fluid mechanics is not my specialty!? There must be some way of calculating this problem?

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[Apakrat]

Does the air going thru the 50mm orifices enter a closed chamber or does it vent to atmosphere?

At 74th year working on IR-One2 - - UHK PhD - - -

 

[Jimbo2010]

Initially as the body is at the surface it vents into the atmosphere. As it sinks towards the seabed it vents into sea water at a pressure of depth + 6m

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[Apakrat]

To get a pressure differential across the air holes, determining how small the air holes have to be to restrict the volume of flow of air to be less than the existing volume of water entering thru the 120 mm holes during a specific time period?


At 74th year working on IR-One2 - - UHK PhD - - -

 

[1gibson]

Does it sink by itself, or is it forced down?

If it sinks by itself... The speed it moves will be partially determined by the rate air escapes (plug the holes, and it will reach some equilibrium depth unless it hits the floor first.)

Hard to imagine much of a pressure buildup in that scenario.

 

[Jimbo2010]

It sinks mechanically at a max rate of 4m/s. How would you propose to calculate the critical diameter of the holes to restrict air flow as suggested by Apakrat?

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[1gibson]

Calculate the flowrate of water going into the vessel assuming the highest pressure differential: vessel at ocean floor, atmospheric pressure inside vessel.

Then, you have a flowrate.

Do the orifice calc of the air out, using that same flowrate and ignoring any compression of the air.

The pressure drop across the orifice should equal the pressure that could be retained in the vessel... Right?

 

[Drexl]

I might be missing something here but are you trying to calculate if the vessel will collapse under the external pressure from not being able to stabilize the pressure fast enough? Are both holes open? You basically have a cylindrical vessel with one hole in the top and one in the bottom that is being pulled down at 4m/s?

 

[Jimbo2010]

Thanks 1gibson I will look into this.

I’m trying to calculate the failure mode for the vessel. This could come in the form failure through collapse due to external pressure or maybe pressure build up within the vessel. Either way you need to know what the internal pressure is correct? Yes both holes are open but the inlet is of greater size then the outlet.


Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[Drexl]

Hi,

If the vessel is sinking 4m/s, the pressure increases 0.4bar/s. In a worst case scenario you can calculate

Say your vessel can withstand an external pressure of 2 barg and have a volume V.
If we would forget to open all holes and drag the vessel down 20m we would reach its pressure limit. We now open the bottom holes and simulate 1 second, the external pressure would now be 2.4 barg.

In order to hold, the internal pressure would have to reach 0.4 barg in that same second. This equals a compression of the air according to p*V=p_2*V_2, or 1bara*V=1.4bara*V_2. V_2 = 0.71 V. Thus 0.29 V would have to be filled with water in one second or Vs = 0.29V/s (29% of the volume per second)

If you are able to get a flow of 29% of the vessel volume through the bottom holes with a differential pressure 2 bar (or whatever the vessel can withstand) you are ok with margin as the top holes would further help stabilize the pressure.

Unfortunately i realize at this point that this will not be the case and you will have to make a simulation or similar..
The simulation will be difficult as modeling the top hole will be complicated. You will have an underpressure in the vessel and water will try to get in and air bubbles out.

 

[Cockroach]

Try the American Gas Association, Report Number 3 (AGA Report 3) which is the standard for orifice metering in the transmission pipeline business. There is another specification under API, but it escapes me at the moment.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Apakrat]

Any idea or estimate of the vessels dry weight?

At 74th year working on IR-One2 - - UHK PhD - - -

 

[Jimbo2010]

Apakrat I do not have this to hand. How does it have relevance to the problem and I will see if I can obtain it?

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[Apakrat]

Seems to me that part of the solution to the issue involves buoyancy.

Without any air vent holes, the device will float at some level in the water with water inside covered with a calculable volume of compressed air at a calculable pressure.

Now You will have a known gas pressure to start working on an orifice size for controlled sinking of the vessel.


At 74th year working on IR-One2 - - UHK PhD - - -