Hello!
First of all, I would like to than the creator of this calculator and forum. Good work! I would recommend you to add some information about you as a creator as there seems to be slim to none information if I where to refer to this page.
Top my problem:
I'm doing some calculations about transporting Nitrogen gas from the surface of the ocean to a depth of approx. 50 meters. The flow is not decided but will be at least 2000 litres/min (atm) and the hose will be 1" 95% of the length and 1/2" in the end( Cheaper couplings). I will feed the hose with 20 bars relative pressure at the surface. I would like to hear your thoughts about using valves made for water flow for this application. I want to put a pressure regulating valve at the seabed but in order to get a fairly cheap valve I'm thinking of using a valve made for water flow. The thing is I'm not sure what difference the change of flow will do to the C_v/K_v/A_v value and how to determine the size of this valve.
I hope someone understands my concern and can reply to help me out!
best regards
Olaf
Comparing compressible and incompressible flows
Re: Comparing compressible and incompressible flows
Thank you for nice words about my work as I am very glad to hear it from time to time 
.
To properly dimension control valve - pressure regulator - you should know pressure in front of it and pressure after it that you try to achieve. Using water valve for nitrogen maybe can be appropriate depending on the material that is used for water valve. For gases like nitrogen it is common that due to high pressure reduction temperature of gas and the valve itself drops bellow zero and it can cause problems if material is not suitable for low temperature application.
On the other hand as nitrogen is not corrosive it should be ok to use the same valve as used for water and also nitrogen is not aggressive to regular materials used for sealing, so it also should be no problem. If you can find ductile iron body valve or steel cast valve, with NBR used for sealing it should be good.
. To properly dimension control valve - pressure regulator - you should know pressure in front of it and pressure after it that you try to achieve. Using water valve for nitrogen maybe can be appropriate depending on the material that is used for water valve. For gases like nitrogen it is common that due to high pressure reduction temperature of gas and the valve itself drops bellow zero and it can cause problems if material is not suitable for low temperature application.
On the other hand as nitrogen is not corrosive it should be ok to use the same valve as used for water and also nitrogen is not aggressive to regular materials used for sealing, so it also should be no problem. If you can find ductile iron body valve or steel cast valve, with NBR used for sealing it should be good.
Pipe flow calculations - since 2000
Re: Comparing compressible and incompressible flows
Hello!
Thank you very much for your fast response.
I know the pressure drop, it'll be between 20 and 15 bars. I haven't thought about the temperature dropping below zero, not sure how to calculate it. The gas control valve calculator could provide me with the answer, but I'm not sure how to determine the density if the flow is not constant.
The NBR sealing has already been thought of, but it's nice to see that you agree with that choice.
The valve is actually a differential pressure valve with an extrenal pressure of 1-5 bars. The outgoing pressure of the valve is to be kept approximately 0.5 bars above the external pressure.
Thanks for the help!
Olaf
Thank you very much for your fast response.
I know the pressure drop, it'll be between 20 and 15 bars. I haven't thought about the temperature dropping below zero, not sure how to calculate it. The gas control valve calculator could provide me with the answer, but I'm not sure how to determine the density if the flow is not constant.
The NBR sealing has already been thought of, but it's nice to see that you agree with that choice.
The valve is actually a differential pressure valve with an extrenal pressure of 1-5 bars. The outgoing pressure of the valve is to be kept approximately 0.5 bars above the external pressure.
Thanks for the help!
Olaf
Re: Comparing compressible and incompressible flows
Hello again!
I took a closer look at what kind of materials I could choose between with the valve and the possibilities are Red Bronze and Ductile Iron. The Red Bronze seems to have more suitable fittings for my application, what do you think about it as a material for Nitrogen gas?
Olaf
I took a closer look at what kind of materials I could choose between with the valve and the possibilities are Red Bronze and Ductile Iron. The Red Bronze seems to have more suitable fittings for my application, what do you think about it as a material for Nitrogen gas?
Olaf
Re: Comparing compressible and incompressible flows
For 15 bar nitrogen pressure at 15 C the density is:
rho = p/RT = 1500000 / 297*288 = 17.5 kg/m3
As this density is much lower than the density of water, the same control valve that has capacity for water of 2000 l/m will have higher capacity for nitrogen.
If you have 20 bar upstream pressure and pressure drop of 15 bar it will cause chocked flow on the valve. Also 15 bar pressure drop will induce roughly 7 C drop in temperature, so depending on the temperature on the start of pipe it will maybe drop bellow zero after the valve.
Bronze is suitable for nitrogen, but you must take care about the pressure class of valve to meet actual pressures that you have in installation (class 150 is required for 20 bar pressure)
rho = p/RT = 1500000 / 297*288 = 17.5 kg/m3
As this density is much lower than the density of water, the same control valve that has capacity for water of 2000 l/m will have higher capacity for nitrogen.
If you have 20 bar upstream pressure and pressure drop of 15 bar it will cause chocked flow on the valve. Also 15 bar pressure drop will induce roughly 7 C drop in temperature, so depending on the temperature on the start of pipe it will maybe drop bellow zero after the valve.
Bronze is suitable for nitrogen, but you must take care about the pressure class of valve to meet actual pressures that you have in installation (class 150 is required for 20 bar pressure)
Pipe flow calculations - since 2000
Re: Comparing compressible and incompressible flows
Hello again!
The control valve doesn't really have a capacity of 2000 l/m. It's more like (maximum) 12,5 m^3/h ~=200 l/m for water. BUT, the (minimum) 2000 l/m nitrogen gas is in normal conditions. With your calculations the ratio of density between water and nitrogen at given pressure is 1200/17= 70,5. I believe this would allow med to use even a smaller valve than the biggest.
I hope I have not embaressed myself with lack of knowledge...
Olaf
The control valve doesn't really have a capacity of 2000 l/m. It's more like (maximum) 12,5 m^3/h ~=200 l/m for water. BUT, the (minimum) 2000 l/m nitrogen gas is in normal conditions. With your calculations the ratio of density between water and nitrogen at given pressure is 1200/17= 70,5. I believe this would allow med to use even a smaller valve than the biggest.
I hope I have not embaressed myself with lack of knowledge...
Olaf
Re: Comparing compressible and incompressible flows
Due to nitrogen compressibility the choked flow conditions are crucial. For not compressible flow your approach is correct but due to possible high speed in valve and possibility of chocked flow in valve, this is not good way to think for compressible flow. Please can you once more check pressure drop in valve that you should have?
You can read more about chocked flow here:
http://www.pipeflowcalculations.com/pip ... d-flow.php
And about cavitation here:
http://www.pipeflowcalculations.com/pip ... tation.php
You can read more about chocked flow here:
http://www.pipeflowcalculations.com/pip ... d-flow.php
And about cavitation here:
http://www.pipeflowcalculations.com/pip ... tation.php
Pipe flow calculations - since 2000
Re: Comparing compressible and incompressible flows
The pressure drop is going to be about 15 bar. The valve will only be functioning no more than an hour at time.
I'm sorry to say that I'm not good at compressible flow, that's why I came to this forum.
I will still construct it and evaluate it after testing.
Olaf
I'm sorry to say that I'm not good at compressible flow, that's why I came to this forum.
I will still construct it and evaluate it after testing.
Olaf
Re: Comparing compressible and incompressible flows
Your results are very welcomed!
Pipe flow calculations - since 2000