Pressure drop vs volumetric flowrate 1

[Thuba]

l saw this article on LinkedIn, and applying this theory to practical, didn't add up. The way we reduce flowrate to a vessel, either for a liquid or steam is by restricting flow, increasing pressure drop and resulting in a lower flowrate downstream of the valve. ln this case it seems, for liquid it remains the same and for gas it increases!

 

[IRstuff]

l saw this article on LinkedIn, and applying this theory to practical, didn't add up. The way we reduce flowrate to a vessel, either for a liquid or steam is by restricting flow, increasing pressure drop and resulting in a lower flowrate downstream of the valve. ln this case it seems, for liquid it remains the same and for gas it increases!

Your drawing is of volumetric flow, mass is conserved, always, so regardless of the volume flow, mass flow is constant, and the volumetric flow is a consequence of mass flow as a function of pressure, temperature, and velocity

 

[pierreick]

Hi,
Consider the continuity equation, mass conservation
mass before valve = mass after valve
Pierre

 

[Thuba]

Hi,
Consider the continuity equation, mass conservation
mass before valve = mass after valve
Pierre

Thank you. yes l understand the principle of conservation of mass/ mass balance. But surely opening a feed valve to a vessel leads to more mass entering a vessel. Conversely, 2 flowmeters placed before and after a restricted valve will read differently.

 

[Thuba]

Your drawing is of volumetric flow, mass is conserved, always, so regardless of the volume flow, mass flow is constant, and the volumetric flow is a consequence of mass flow as a function of pressure, temperature, and velocity

Thanks for your reply. For liquid, the flowmeter (volumetric flowrate) reads the same regardless of the valve.

 

[LittleInch]

l saw this article on LinkedIn, and applying this theory to practical, didn't add up. The way we reduce flowrate to a vessel, either for a liquid or steam is by restricting flow, increasing pressure drop and resulting in a lower flowrate downstream of the valve. ln this case it seems, for liquid it remains the same and for gas it increases!

The article is poor, but what they are referring to is "actual" volume flowrate, not a volume relating back to a standard set of conditions. (Usually something like 15C, 101.625 kPa). When looking at gas volume flows, you always need to know at what pressure or temperature or make sure all readings are in scf or scm.

So in scf or scm, the gas flowrate would remain the same.

Same for liquid as actual volume can change with temperature and pressure so the best method is to refer to a volume flow standardised to a set pressure and temperature. Then you can compare the two figures much better.

Oh and liquids are not "incompressible", they are virtually incompressible. The difference may be small, but it's not zero. Most of the times you can ignore it, but when you have large volumes then it can become significant.

 

[Asisraja D]

I have also seen the post in linkedin and i guess it is little bit arguable because as per bernouli's principle the internal energy + kinetic energy + pressure energy is constant across the valves because when the fluid leaves from the valve it losses its pressure energy due to loss of co-efficient ( i guess it is a globe valve so KL value is 10) even the head loss occurs here when it leaves the valve it gains speed and it gains speed (kinetic energy) and the same flow rate achieves in the expense of pressure energy by the compensation of internal energy + kinetic energy. but it works for inviscid flow (non viscous, incompressible fluid) but i have no knowledge in experimental side.

 

[pierreick]

Thank you. yes l understand the principle of conservation of mass/ mass balance. But surely opening a feed valve to a vessel leads to more mass entering a vessel. Conversely, 2 flowmeters placed before and after a restricted valve will read differently.

a) I think you need to go back to your textbook and review the continuity equation.
b) You also need to review the equations related to pressure drop for liquid and Gas.
c) As I said in my first reply, stick to mass flow or actual volumetric flows, forget about standard flow which are source of errors and are useless to calculate head loss.

CheCalc ‐ Fluid Flow

Calculation for pressure losses in pipes and fittings for a variety of fluid flows.

checalc.com

Don't mix up steady state and dynamic state.
Good Luck.
Pierre

 

[Snickster]

As little inch indicated the flow being measures is actual cfm which is the actual cfm flow at a given pressure and temperature, The measured flow upstream is 430 cu ft / min at 250 psig and some temperature. Flow through the valve produced a pressure drop to 140 psig but no temperature drop since the flow through valve is friction flow where although pressure drops, enthalpy or total energy remains same, and therefore temperature remains same. Using the ideal gas equation the downstream volume increase can be determined as follows"

P1V1=mRT=P2V2 Where P1V1 is upstream pressure and volumetric flowrate and P2V2 is downstream

Since mRT is constant, where m is mass flowrate, R is universal gas constant and T is temperature

P1V1=P2V2 or V2=P1V1/P2 in absolute pressure units

(254.7/154.7)(430)=736

 

[GBTorpenhow]

Flow through the valve produced a pressure drop to 140 psig but no temperature drop since the flow through valve is friction flow where although pressure drops, enthalpy or total energy remains same, and therefore temperature remains same.

Mr. Joule and Mr. Thomson disagree. Throttling is not ideal gas behavior.

 

[Thuba]

Thank you guys. Now l understood the principle. Yes throttling a valve definitely reduces flowrate. Mass is conserved, as throttling a valve increases pressure drop/back pressure hence flowrate upstream a valve will decrease too. For a pump the pump curve will shift left, as the pump head will increase and Q decrease as valve is throttled.

 

[Snickster]

Mr. Joule and Mr. Thomson disagree. Throttling is not ideal gas behavior.

Of course the J/T effect but I am not going to get that detail with someone just trying to understand the basics.