Question for pressure engineer about cascading gas

In summary, the conversation discusses the process of "cascading" gas from a higher pressure vessel to a lower pressure vessel in order to reach a desired pressure. The calculations for the flow rate through a 1-inch pipe and the time it would take to fill the lower pressure vessel are mentioned, with an estimate of around 3 minutes provided by one of the participants. The type of gas being used (nitrogen) is also mentioned. There is a request for input from a pressure systems engineer on the reasonableness of the calculation and an admission that the speaker is not experienced with high pressure gases.
  • #1
steves1080
65
1
Say I had a high pressure vessel with a volume of about 1400 cubic feet and pressurized to 5,000 psig. Now say I had a lower pressure vessel with a volume of 1800 cubic feet (slightly higher) and pressurized to 2,000 psig. The goal is to "cascade" gas from the higher pressure vessel to the lower pressure vessel in order to get the lower pressure tank up to 2,700 psig. The gas would flow through a pressure regulator (with max output set point of 2,700 psig) fully opened up. The gas going into the lower pressure tank flows through a 1-inch diameter pipe and only about 30 feet in length. I've already done the calculations using the compressible mass flow rate equation for choked flow using ideal gas law assumptions and using the Cv of the regulator to determine the magnitude of the choke point. I neglected temperature, enthalpy, and pressure drop affects, and stuck to the basic physics behind it. But the answer I got for how long it would take to cascade that much gas was ~3 minutes. I am looking for an experienced pressure systems engineer to let me know if this sounds reasonable or if I need to do some more calculations. It would also be helpful to know what type of number to expect if this seems way off. For some reason I was expecting it to take much longer, but I don't have too much experience with high pressure gases. I've run my numbers multiple times, so I think I am correct.
 
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  • #2
I should note that the gas is nitrogen, so it's basically just air
 
  • #3
The flow of air through a 1" pipe will be relatively low in volume, and considering the size tank you have, there's quite a lot of it required to raise the pressure by 700 psi. Also consider that the pressure in the first tank will drop as it's volume is depleated by the flow to the second tank, thus reducing the flow rate. As an off the cuff guess I'd say 3 minutes sounds reasonable.

I haven't crunched any numbers, so take that for what it is.
 
  • #4
Thanks for the response. I did crunch some numbers, but the problem is that I have no way of gauging what seems reasonable. 3 minutes sounds possible to me, but on the other hand that it a lot of volume to fill that quickly.
 

1. What is cascading gas and how does it work?

Cascading gas refers to a system where multiple gases of different pressures are used in a sequential manner to achieve a desired pressure. This is often done to optimize the use of different gases and to ensure that the pressure does not exceed the maximum allowable limit.

2. How do you determine the appropriate gases and pressures for a cascading gas system?

The selection of gases and pressures for a cascading gas system depends on several factors such as the desired final pressure, the properties of the gases, and the equipment being used. A pressure engineer typically uses simulations and calculations to determine the most efficient and safe combination of gases and pressures for a specific application.

3. What are the benefits of using a cascading gas system?

Using a cascading gas system can offer several benefits, including cost savings, improved efficiency, and increased safety. By using different gases at different pressures, it is possible to optimize the use of each gas and reduce waste. This can also lead to improved efficiency in processes that require precise pressure control. Additionally, using a cascading system can reduce the risk of exceeding maximum pressure limits, which can help prevent accidents.

4. Are there any potential drawbacks to using a cascading gas system?

One potential drawback of a cascading gas system is the increased complexity and potential for leaks or malfunctions. This can be mitigated by proper maintenance and monitoring of the system. Another consideration is the cost of setting up and maintaining a cascading system, which may not be feasible for smaller applications.

5. How do you ensure the safety and reliability of a cascading gas system?

To ensure the safety and reliability of a cascading gas system, a pressure engineer must carefully design and monitor the system. This includes selecting appropriate gases and pressures, regularly inspecting and maintaining equipment, and implementing safety measures such as pressure relief valves. It is also important to follow industry standards and regulations for gas handling and pressure control.

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