Liquid static head vs. dynamic head?

In summary, the conversation discusses predicting flow based on a known static head and the effect of pressure drop on flow rate. The Darcy-Weisbach equation is mentioned as a way to calculate pressure drop, which is proportional to the square of the fluid velocity. The concept of static head vs. dynamic head is also explained. The conversation also touches on the difference in flow rates between a head tank source and a throttled external source.
  • #1
pushpole
3
0
It's been a looooooong time since I delved into fluid properties.

I'm trying to predict flow based on a known static head. I have a water column which will result in 6 psig of static head. The height of the water column will not change during the flow. I know the downstream piping will flow at 51gpm with a 125 psig source throttled to 5 psig and 40 gpm when throttled to 3 psig.

I don't need very many significant digits!
Thanx for any help.
 
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  • #2
Hi pushpole,
I posted a tech paper that applies here:
https://www.physicsforums.com/showthread.php?t=204473

Note equation 1 (Darcy-Weisbach) equation, shows that pressure drop in a pipe (dP) is proportional to the square of the fluid velocity. Note also it is proportional to friction factor f, which will also change with fluid velocity. Luckily in your case, the friction factor won't change too much if dP doesn't change much (ie: from 5 psi to 6 psi) so you might just ignore that. The result is that dP varies only as a square of the flow. So if you know the flow is 51 GPM with a dP of 5 psi, then the flow would be 56 GPM with a dP of 6 psi. Note also that using a pressure drop of 3 psi results in a calculated flow of 40 GPM, just as you've suggested, so the friction factor variation is seen to be too small to be significant.
 
  • #3
I understand your response. Great link for my future reference.

I'm having a little trouble with the concept of static head vs. dynamic head. If I start flowing water out of the head tank (static pressure of 6 psig). The pressure should drop at the tank outlet/pipe inlet as PE is converted to KE. How would I compare this to my measured stable dynamic readings?

Thanx again!
 
  • #4
Not too sure what you're asking here. I assume you have a tank at some pressure, say 6 psig, and a pipe coming out that dumps to atmosphere somewhere. You open the valve and get a flow out which you've measured when the tank is at 5 psi and 3 psi. I believe that's the scenario you have.

If that's the case, then the pressure you read on a pressure gage which is oriented perpendicular to the flow velocity will read the static pressure. If you had a bunch of these lined up along your pipe, you'd see the tank at 6 psi and as you went along, you'd see a drop in pressure in each of the gages until it finally read zero. The only caveat to this is if the pipe goes up or down in elevation or if the pipe diameter changes, in which case pressure will change according to Bernoulli's equation in addition to the irreversible pressure losses predicted by the Darcy Weisbach equation. See equation 16 in the paper.

If you're wondering how the pressure will vary if your gage is parallel to the flow, that's a different story, but I don't think you'll need to concern yourself with that.

Hope that helps.
 
  • #5
Thank you for the help. What I'm trying to reconcile in my limited brain is the difference I would see in flow based on the source.

You are correct in your assessment of the basic layout. The uncertainty I'm trying to reconcile is the demonstrated flow capability using the throttled external source, the 5 psig and 3 psig readings (taken perpendicular to flow downstream of the throttle valve) and the static head tank source.

head tank pressure = 6psig = all Potential Energy
observed flow= 5 psig @ 51 gpm = Kenetic Energy + Potential Energy
A conversion of 1 psig from PE to KE resulting in 50 gpm flow through a 1.5" pipe seems like alot. However, if the numbers support the observations I will certainly accept it.

Bottomline,
Can I expect the head tank to perform like the throttled 125psig source?

Thanx again!
 

What is the difference between liquid static head and dynamic head?

Liquid static head and dynamic head are two different ways of measuring pressure in a liquid system. Liquid static head measures the pressure at a specific point in the system, while dynamic head measures the overall pressure throughout the system.

How do you calculate liquid static head?

Liquid static head can be calculated by multiplying the height of the liquid column (in feet) by the specific gravity of the liquid and the gravitational constant (32.2 ft/s²).

What factors can affect liquid static head?

The height of the liquid column, the specific gravity of the liquid, and the gravitational constant are the main factors that can affect liquid static head. Other factors may include temperature, density, and viscosity of the liquid.

What is the significance of liquid dynamic head in a pumping system?

Liquid dynamic head is important in a pumping system because it represents the total energy required to move the liquid through the system. This includes both the static head and the frictional losses in the piping system.

How can you convert between liquid static head and dynamic head?

To convert from liquid static head to dynamic head, you can use the Bernoulli's equation, which takes into account the velocity and elevation changes in the system. To convert from dynamic head to static head, you can use the inverse of this equation.

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