Why does the location of valve in a gas flow system affect its DP and the flow rate? 5

[SilverRule]

Hey guys,

So in order to understand some things better, I'm playing around with different scenarios in a fluid flow simulation program.

I noticed that for a gas flow system (with constant inlet and outlet pressures, fixed pipe length, fixed diameter and fixed valve opening say @ 50%), the more downstream you place the valve the bigger the DP across it. After considering that velocity in a gas system increases along the pipe length, I understood how that would mean the DP across the valve would have to be higher the further you place it along the pipe length because a higher velocity would be going through it.

What I didn't understand was that the flow/inlet velocity are following the same trend: increasing along with the DP across the valve as it moves downstream. This isn't making sense to me because I thought that when a valve takes a higher DP (with the inlet and outlet pressures held constant), that means the flow/velocity would have to become lower because the available remaining DP is now lower.

Does anyone know what's going on here? Please let me know if you need any more details.

Thanks again!
SR

 

[Latexman]

Pipe upstream of the valve sees a denser gas, than downstream of the valve, because of the relatively large dP of the valve. If the valve is at the end, then ALL the pipe sees a denser gas. A denser gas at the same flow rate gives less dP, so at constant dP end-to-end flow rate must increase.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[SilverRule]

Perfect! Clear, concise and straightforward explanation. Exactly what I needed. Thank you so much!

A very relevant follow up:

Given these dynamics, how would you go about deciding where to place the valve within in a gas flow system if you could place it anywhere? What would be your hierarchy of criteria? Because clearly the location can have a significant impact on the process in terms of:
- what the flow rate is
- how much authority the valve has (how much dP it takes)
- how much the gas expands from inlet to outlet which affects how much it cools
- anything else I'm missing?

 

[katmar]

Usually the pipe sizing would be the controlling factor. For example, in a let-down station it is common for the downstream piping to be larger diameter than the upstream piping. In this case you would probably want to minimize the large bore piping and put the valve further downstream.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Latexman]

I'd also ask your instrument mechanics, where do we NOT want to put this CV, and where are the best locations, so you can bake that into the decision too. It's important to have maintainability.

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[LittleInch]

To optimise costs you want the lowest pipe size for the required flow.

For gas this means operating at the highest pressure.

It is quite analogous to electricity.

To transport electricity (gas)over long distances you use the highest possible voltage (pressure) to generate the lowest possible amps (ACTUAL flowrate) which uses the smallest size of cable (pipe).

So the same with gas - why reduce pressure unless you really have to. You don't which is why city gate stations exist and you gradually lower the pressure at the latest point you can.

If you have lots of regulators - better to lower the pressure through a big one and then let each supply have a much smaller, cheaper regulator.

Just don't mix up flowrate at standard condiiton (scf or scm) with actual flowrate in the pipe which is dependant on pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SilverRule]

Thank you all. Appreciate your responses.