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A Valve Controls Flow Or Pressure?


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#1 Alaaeldin55

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Posted 18 April 2021 - 01:46 PM

Hello. I have a hard time understanding how control valves affect pressure and flow. Say there is a pressurized air tank and attached to it there is a vent valve to the atmosphere. So, when I open this valve 20٪, only a small amount of air is allowed to flow out of the tank and I can feel that the coming air is at low pressure. On the other hand, if I opened the same valve 100٪ not only a greater amount of air flow is obtained but also air comes out at greater pressure. Am I understanding this right? That is a valve opening is proportional to fluid flow and pressure downstream the valve.



#2 Pilesar

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Posted 18 April 2021 - 04:32 PM

Keep at it! Fluid flow is not necessarily intuitive. A couple of basic principles: PV=nRT. Reduce the moles of gas in a fixed volume vessel and the pressure decreases. This allows modulating a control valve to adjust the vapor inventory to reach a targeted pressure.
The other principle to remember is Bernoulli's. Spend some time on Bernoulli's to try to understand it. There are good videos available on the internet. Here is link to a simple one: https://youtu.be/UJ3-Zm1wbIQ


#3 fallah

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Posted 18 April 2021 - 11:59 PM

Hello. I have a hard time understanding how control valves affect pressure and flow. Say there is a pressurized air tank and attached to it there is a vent valve to the atmosphere. So, when I open this valve 20٪, only a small amount of air is allowed to flow out of the tank and I can feel that the coming air is at low pressure. On the other hand, if I opened the same valve 100٪ not only a greater amount of air flow is obtained but also air comes out at greater pressure. Am I understanding this right? That is a valve opening is proportional to fluid flow and pressure downstream the valve.

 

Actually this valve controls the flow; supposing the pressure inside the tank is almost constant during the air release you can model the valve as a restriction orifice with constant upstream/downstream pressures hence constant differential pressure in which the higher beta ratio will lead to a higher passing fluid flow rate...



#4 Chemitofreak

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Posted 23 April 2021 - 01:51 AM

The control valve located on the vent to atmosphere is killing the pressure. when the valve opening is 20% there more pressure drop across the control valve, which leads to lower pressure and flow at the outlet. Similarly, when the valve is 100% open there is least resistance, hence the flow is high and the pressure drop across the valve is less which leads to higher outlet pressure. 

 

as suggested by Pilesar, you need to understand the basics of gas behavior and Bernoulli's equations.

 

Regards






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