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Static pressure difference in different ID pipe tees?

Static pressure difference in different ID pipe tees?

Static pressure difference in different ID pipe tees?

(OP)
Can someone explain what kind of pressure difference I would expect to see on a gage on a pipe tee (1" NPT pipe size) if I had an original pipe tee ID of .815" and changed it to a lower pressure tee with a smaller wall thickness which makes the ID 1.160"?

The gage is located on the non straight through leg of the tee.

I know static pressure increases as pipe size increases and static pressure decreases as velocity increases... so is there a way to calculate the expected difference? is it directly proportional to size difference? thanks

 

RE: Static pressure difference in different ID pipe tees?

Static pressure won't change.
Dynamic pressure change is related to velocity difference, which can be related back to the difference in area.  V2 = V1/A1*A2, pressure reduction would be (V2-V1)^2/2/g

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?

(OP)
its air as a medium.... should have mentioned that... thanks

RE: Static pressure difference in different ID pipe tees?

Same-same.   

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?

(OP)
I thought that total pressure = static pressure + dynamic pressure...

total pressure remains constant so when dynamic changes doesnt static have to adjust accordingly to some degree?

thanks for the help so far... I just need to connect the dots

 

RE: Static pressure difference in different ID pipe tees?

(OP)
Pstatic1 + .5(density)(velocity1)^2 = Pstatic2 + .5(density)(velocity2)^2 right?

so if Pstatic1= 100psi and V1=3ft/sec and v2=2v1

then Pstatic2=86.5psi

am I missing something?

 

RE: Static pressure difference in different ID pipe tees?

You must know the density of air at 100 psi(a or g?) and at (What's the temperature °R?)
http://www.engineeringtoolbox.com/air-properties-d_156.html
say @ 20°C = 1.205 kg/m3 = apx. 0.075 lbs/ft3

calculate stagnation pressure of 3 fps
= ρ*v2
= 0.075 pcf / 32.174 ft/s2 * (3 fps)2
= 0.00234 * v2
= 0.00234 * 3 2
= 0.0211 psf
psi
= 0.0211 / 144
pressure change at low velocities will be very very small

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?


Since there is no change in the flow conditions in the main 1-in pipe, why should there be a change of pressure readings?

RE: Static pressure difference in different ID pipe tees?

Is there flow in the branch from the tee to the gauge?

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Static pressure difference in different ID pipe tees?

Pressure change between the two possible assemblies, one with ID 1= 0.815" and ID2=1.160" is due to the change in ID, hence the area changed, resulting in a velocity change.  Venturi effect, but looking at the difference between velocities in two assemblies, not two diameters in one assembly.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?


The only change has been apparently done on a dead end pressure gage, therefore no change in flow conditions.

RE: Static pressure difference in different ID pipe tees?

No. In a dead end, you'd have no velocity at exit, so in that case the complete stagnation pressure would add to the static pressure to reach the total energy grade line.  

If total energy is assumed to remain the same and flowrate is assumed to remain the same, density same ...
P1/den + V1^2/2/g =  P2/den + V2^2/2/g
P2  = P1 + ((Q/A1)^2/2/g - (Q/A2)^2/2/g) * den

In any case, the dP caused by 3 fps dV isn't going to be noticable on my pressure gauge.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?

(OP)
sorry just got back to my computer... I was way off on the velocity... the flowrate is 70scfm so velocity in the first pipe tee is 324ft/s and the second is 160ft/s if I did the math right...

second thing is I see where you got that density from on the chart but isnt the density at 100psi and lets say 70F is approx the temp in our shop = .585lb/cf which I get from the chart on the same website that relates air density to temp and pressure

If I use those numbers above for psi=density*(v^2) I get 3.22 psi for the larger dia and 13.24 psi for the smaller dia

I am also confused on what pressure the gage is reading... I thought pressure gages only read static pressure... so by figuring out the stagnation pressure are you figuring out what pressure to theroetically subtract from the total pressure to get what you would see on the gage? so in this case it would be approx 97 and 87 psi?  

RE: Static pressure difference in different ID pipe tees?

Got me.  You do need v at flow conditions.  Take into account if it is psia or psig too.  At 100 psia or g, compressibility is almost equal to 1, so you can probably neglect that effect.

density should be (gauge + atmos) psia divided by atmos
so if 100 is psig, then (100+14.73)/14.73 * 0.00234 = den2
Correct for temperature if you need to do so too.

I wasn't expecting 10 psi difference, is it really that much?

 

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?

had some more time.
0.075pcf /32.174 ft/s^2 * (14.73+100)/14.73 * (3fps)^2 = 0.163 psf
correcting mass density for 100 psiG
0.00233 slugs/ft3 * (14.73 + 100)/14.73 * (3fps)^2 = 0.163 psf

still looks pretty small.
  

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: Static pressure difference in different ID pipe tees?

Your velocity calcs are way off. A flow of 70 SCFM of air is equal to a mass flow of about 0.09 lb/s.  At 20 C and 100 psig the density of dry air is 0.59 lb/ft3. This makes the velocity in the 0.815" pipe 42.1 ft/s and 20.8 ft/s in the 1.16" pipe.

Doing calculations like this in customary units is almost a guaranteed way of getting the wrong answers, unless you are doing this on a daily basis and have the units and conversions at your fingertips.

The formula for velocity head is
h = v2/(2g)
where h is the head in feet of flowing fluid
v is the fluid velocity in ft/s
g is the earth's gravity

If you want the answer in pressure units you multiply by the fluid density and the earths gravity

P = ρv2/(2)
BUT if you still have the velocity in ft/s and the density in lb/ft3 then the pressure is in poundals per square ft

If you want P in psi you have to divide by gc to convert poundals to pounds force and by 144 to convert square ft to square inches

i.e.  P = ρv2/(2*144*gc)

To get the difference between your two conditions, and assuming constant density

ΔP = ρ x ( v12 - v22) / (2*144*gc)

= 0.59 x (42.12 - 20.82) / (2 x 144 x 32.2)
= 0.085 psi

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: Static pressure difference in different ID pipe tees?

0.085 psi is back to more like what I expected.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

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