Pressure variations after rapid pressurization

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In summary, the conversation discusses the physics behind rapid pressurization, specifically in a system with a piece of pipe, a closed valve, and a pressure indicator connected to a high pressure line. It is explained that when the valve is opened and quickly closed, the pressure indicator rises and then drops to a lower value. This is due to the transfer of gas taking place adiabatically, causing a decrease in temperature and pressure in the pipe. The effect is more significant when the valve is left open for a shorter period of time. It is also noted that this is a thermal phenomenon rather than a wave or impulse related one.
  • #1
CCG
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Why pressure falls after rapid pressurization
Hi all,

A question on the physics behind rapid pressurization (relatively high pressures e.g. 300 bar) and perhaps transients in systems. For simplicity, imagine that we only have a piece of pipe with an initially closed valve on one side and a pressure indicator on the other side. The current pressure in the pipe is atmospheric (~1 bar, zero over pressure). Further, on the other side of the valve I have connected a high pressure line. If I open and within very short time close the valve, the pressure indicator would first quickly rise and indicate same pressure as in the high pressure line. Just after closing the valve, the pressure indicator drops to a value less than the pressure in the HP line.

I the valve would remain open for a longer period of time before being closed, the effect can not be observed.

Could someone explain the physics behind this? Do temperatur variations during the pressurization have to do with it?

Many thanks!

C
 
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  • #2
What kind of timing and physical dimensions are we talking about?
 
  • #3
Hi!

Upstream the valve, we have about 200 bar (very large volume compared to the volume downstream valve)
Downstream valve roughly 2 meters of approx 10 mm diamater piping. This piping is closed (confined volume) with a pressure indicator.
 
  • #4
And timing, valve may be open for 0.5 s or so. The longer the valve is open, the less the effect. The pressure drops and becomes stable after approx 10 s.

Does Joule - Thomson have anything to do with it? The gas can be said to be throttled when the valve is opened and after a while (10 seconds or so). If there was a temp increase in the pipe, this is then perhaps cooled down to the ambient temp after 10 s and the pressure consequently drops (by 10-15% in this case)
 
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  • #5
Joule Thomson has something to do with this, but that is not the whole story. This effect would occur even if the air behaved like an ideal gas. And, for an ideal gas, the Joule Thomson coefficient is zero.

If we apply the open-system version of the first law of thermodynamics to this system, and assume that the initial transfer of gas takes place adiabatically, we obtain:
$$d(un)=h^0dn$$where u is the internal energy per mole of gas in the pipe, n is the number of moles of gas in the pipe and ##h^0## is the molar enthalpy of the gas in the high pressure line. If we integrate this equation (and neglect the initial contents of the pipe compared to the final contents), we obtain:
$$u=h^0=u^0+P^0v^0=u^0+RT^0$$where ##v^0##, ##P^0##, and ##T^0## are the molar volume, the pressure, and the absolute temperature, respectively, of the gas in the high pressure line. From the above equation, for an ideal gas, we have:
$$u-u^0=C_v(T-T^0)=RT^0$$where T is the temperature before final cooling begins. The solution to this equation is $$T=\gamma T^0$$where ##\gamma=C_p/C_v=1.4##. So, if ##T_0=293\ K##, T = 410 K = 137 C. And, if the high pressure line pressure is 200 bars, the pressure in the pipe after cooling would be 143 bars.

Of course, this is all for the case in which the air is treated as an ideal gas. The calculation would be a little more complicated when including the real-gas properties of air.
 
  • #6
CCG said:
Summary:: Why pressure falls after rapid pressurization

Hi all,

A question on the physics behind rapid pressurization (relatively high pressures e.g. 300 bar) and perhaps transients in systems. For simplicity, imagine that we only have a piece of pipe with an initially closed valve on one side and a pressure indicator on the other side. The current pressure in the pipe is atmospheric (~1 bar, zero over pressure). Further, on the other side of the valve I have connected a high pressure line. If I open and within very short time close the valve, the pressure indicator would first quickly rise and indicate same pressure as in the high pressure line. Just after closing the valve, the pressure indicator drops to a value less than the pressure in the HP line.

I the valve would remain open for a longer period of time before being closed, the effect can not be observed.

Could someone explain the physics behind this? Do temperatur variations during the pressurization have to do with it?

Many thanks!

C
It sesm analogous to an electrical transmission line, where wave will propagate down the pipe as a result of an impulse. When it bounces from the end, we can get a reversal of pressure. Once the initial wave dies down, due to friction, we see the steady pressure of the line.
 
  • #7
tech99 said:
It sesm analogous to an electrical transmission line, where wave will propagate down the pipe as a result of an impulse. When it bounces from the end, we can get a reversal of pressure. Once the initial wave dies down, due to friction, we see the steady pressure of the line.
He mentioned that the effect was present after 0.5 sec., and took 10 sec. to die out. How long does it take a pressure wave to travel at the speed of sound for 2 meters?

Did you read my answer?
 
  • #8
Chestermiller said:
He mentioned that the effect was present after 0.5 sec., and took 10 sec. to die out. How long does it take a pressure wave to travel at the speed of sound for 2 meters?

Did you read my answer?
Apologies, I missed that, it does look thermal.
 
  • #9
Thank you very much for the thorough explanation. I'll need some time to digest it and dust off my knowledge in physics from university which I haven't applied for quite some time but hopefully I'll understand it in detail.

And as pointed out by you, I think it is a 'thermal phenomenon' rather than wave and impuls related. I've tried it in other piping systems with different pressure indicators and pressure up to 300 bar as well. The pressure indicator(s) goes 'immediately' up to the high pressure upstream the valve and is very stable until the valve is closed (when it drops).

Again, thank you for the extensive answer. I might get back to you if there are details in the explanation which I don't manage to understand.
 
  • #10
CCG said:
Thank you very much for the thorough explanation. I'll need some time to digest it and dust off my knowledge in physics from university which I haven't applied for quite some time but hopefully I'll understand it in detail.

And as pointed out by you, I think it is a 'thermal phenomenon' rather than wave and impuls related. I've tried it in other piping systems with different pressure indicators and pressure up to 300 bar as well. The pressure indicator(s) goes 'immediately' up to the high pressure upstream the valve and is very stable until the valve is closed (when it drops).

Again, thank you for the extensive answer. I might get back to you if there are details in the explanation which I don't manage to understand.
How do your measurements compare quantitatively with this analysis for an ideal gas? Did you do any measurements of the gas temperature (or the pipe wall temperature)?
 

Related to Pressure variations after rapid pressurization

1. What causes pressure variations after rapid pressurization?

Pressure variations after rapid pressurization are caused by the sudden increase in air or gas pressure inside a confined space. This can occur when a container is rapidly filled with gas or when a sudden change in atmospheric pressure causes a pressurized system to expand.

2. What are the potential dangers of pressure variations after rapid pressurization?

The sudden changes in pressure can cause structural damage to containers or equipment, leading to leaks or explosions. It can also cause discomfort or injury to individuals who are exposed to these pressure variations.

3. How can pressure variations after rapid pressurization be measured and monitored?

Pressure variations can be measured using pressure gauges or sensors. These devices can be installed in pressurized systems to continuously monitor and record changes in pressure. Regular maintenance and calibration of these instruments are necessary to ensure accurate readings.

4. What are some ways to prevent pressure variations after rapid pressurization?

Proper design and construction of pressurized systems can help prevent pressure variations. This includes using materials that can withstand high pressures and incorporating safety features such as pressure relief valves. It is also important to follow proper procedures for filling and pressurizing containers.

5. Are there any regulations or standards for managing pressure variations after rapid pressurization?

Yes, there are regulations and standards set by organizations such as OSHA and ANSI that provide guidelines for managing pressure variations in different industries. It is important to follow these regulations and standards to ensure the safety of individuals and the proper functioning of pressurized systems.

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