9 Replies Latest reply on Aug 26, 2013 1:57 PM by Jared Conway

# Gauge Pressure vs Total Pressure

If I have an existing pipe flow with a real world pressure gauge (gage) reading of 56 psig and I want to convert it to an inlet Total Pressure boundary condition;  would it be correct to calculate as follows:

Total Pressure = Pstatic + dynamic pressure

--> Pstatic = Gage Press ( 56 psig) + 1 Atm (14.7 psi)

--> lets state that dynamic pressure is basically negligible (.0002 psi)

then --> Total pressure = 71.700 psi ?

In addition, if I have "Pressure Potential" turned ON would that change "Total Pressure"  BC input

• ###### Re: Gauge Pressure vs Total Pressure

I meant:  Total Pressure:  70.700 psi

• ###### Re: Gauge Pressure vs Total Pressure

Daniel,

I've never done compressible flow in SolidWorks, so maybe somebody else can help you better, but generally speaking.

psig = psia - 1atm = psia - 14.7psi

Total pressure is constant and is the pressure where v = 0, i.e., in a reservoir. If the pressure gage is at a point where the velocity is negligible, then it is measuring the total pressure. You would usually convert it to psia before using it.

If the gage is at a point where the velocity is non-negligible, then I think you'd get total pressure from isentropic flow relations. I'm not sure how to do it from just the one measurement, though. I think you need more information.

• ###### Re: Gauge Pressure vs Total Pressure

Thanks Mike!  learn something new everyday ...

Here is the answer that I am receiving from Tech Support regarding Pressure Potential turned on for initial setup conditions:

Flow Simulation does not have direct hydrostatic capabilities. The “Gravity” option is primarily used for natural convection problems to account for the heated air rising in the correct direction and velocity.

• ###### Re: Gauge Pressure vs Total Pressure

In addition:  I think you would really want to apply an inlet static pressure and not a total pressure. This is assuming that your pressure gauge reading is measuring static pressure and not total pressure. So I would apply an inlet static pressure opening of 70.7 PSIA (56 PSIG + 14.7)

• ###### Re: Gauge Pressure vs Total Pressure

Total pressure and pressure potential are pretty different subjects.

What are you trying to do in flow? How did those topics come up?

I think your explanation of the calculation make sense but I'm not clear on what you are trying to do with it.

• ###### Re: Gauge Pressure vs Total Pressure

Without getting into too much detail ... the project is an existing Slurry Distribution Loop System that delivers slurry to the Semi-Tools via flow controllers located on a 2nd floor.  The slurry is pumped up ~34 ft from the basement.  The goal is to conduct a fluid study of the current design, understand its internal flow behavior(s) i.e. turbulence, vortices, etc. and look for opportunities for improvements.  Below is an elevation view.  The FAB engineers tell me that the gauge pressure (at the inlet supply line) is 56 psig and the flow rate is 1.25 L/min. That is how these topics surfaced ... • ###### Re: Gauge Pressure vs Total Pressure

How is total pressure and gravity related to the problem?

Flow works in absolute pressuresfor reference.

• ###### Re: Gauge Pressure vs Total Pressure

What would be nice if I could run the Flow Simulation using only outlet/inlet flows and have it solve the pressures required to obtain these flows but in my experience have not been able to do it.

So I am requiring to input a pressure (Total, Static or Environ) to perform the analysis.  If I had a Pitot tube; I could then obtain a total pressure but I don't --> our facility has a pressure gauge installed in the system.  I am just trying to ensure I could correlate the pressure inlet either static or total to run the analysis correctly (based on the facility's gauge pressure reading).  All other BC are outlet flows (m3/sec) .

The next question does Flow Sim have hydrostatic capabilities on dynamic internal flows? If so would that change the way I have to input the initial (total or static) pressure input...?

The slurry density is:  1059.11 kg/m3

Delta height:  9.144 meters (30 ft)

Gravity: 9.81 m/s2

Pressure:  94995.09 Pa (13.77 psi)

If it is confusing then let me know ... your thoughts at this point. • ###### Re: Gauge Pressure vs Total Pressure

Hi Daniel, i'm really not following. Sorry. If you put a flow rate in at one end and a pressure opening at the other, the output will be the pressure difference across the system. If you put pressures are both end that result in a difference, the result will be a flow rate.

something maybe to consider. if you were going to perform this calculation by hand, what would be the conditions you'd have on both ends of your control volume? that might help better define how you want things to be in flow.

and as for hydrostatic pressures, yes, they are used in internal flow problems. see the other post you brought up. if you make a tube that is vertical, the pressure will be different on one side vs another.