6 replies to this topic

[mido]

it can be easy question but its important question for me.

if we have gas flowing in 12 in pipe and then the same quantity flow in 24 in pipe

is pressure increase or decrease in 24 in pipe

logic say it decrease but i know from bernouli equation that pressure direct proportional with diametrer that mean according bernouli pressure increase

can anyone discuss this

thanks for care

[chenblue]

Hi,

I don't know which equations u used.
Actually, the pressure drop for pipe flow because of viscous dissipation.
Let's assume the flowrate of fluid doesn't change in pipe, then we wanna know what the pressure drop wanted changes from 12 in pipe to 24 in pipe?
u got a "pressure drop not changing" result if ur equ don't consider the viscous term,
and u got a "pressure drop decreasing" result if ur equ do consider the viscous term.

[katmar]

This is one of those topics that is easier to discuss when we have concrete numbers to deal with, so let me make up an example.

Lets say we have a gas with a density of 5 kg/m3 flowing in your 12" line at a velocity of 30 m/s. The velocity head is therefore (5x30x30/2) Pascal = 2250 Pascal or 2,25 kPa. Now, when this gas gets into the 24" section the velocity drops to 7,5 m/s (assuming that the density is still 5 kg/m3). The velocity head is now (5x7,5x7,5/2) Pascal = 141 Pascal or 0.14 kPa. The change in velocity head is potentially recoverable as static pressure, so if there were no other losses the static pressure from the 12" section to the 24" section would increase by (2,25 - 0,14 =) 2,11 kPa. This is the result calculated by Bernoulli.

Unfortunately there is no way of transitioning from the 12" section to the 24" section without incurring some friction and drag losses. If the two were connected by a sudden increase (i.e. a step change) or by a typical pipe reducer the k factor would be about 0,52 (relative to inlet velocity) and so the loss in the transition would be (0,52 x 2,25) = 1,17 kPa. The net increase in static pressure from the 12" section to the 12" section would therefore be 2,11 - 1,17 = 0,94 kPa.

A gradually tapered conical section is more efficient than a sudden increase or pipe reducer, and with an included angle of 20 degrees the K value would reduce to 0,24 making the losses (0,24 x 2,25) = 0,54 kPa. Now the net increase in pressure drop is greater than before i.e. it is 2,11 - 0,54 = 1,57 kPa, getting us closer to the ideal of Bernoulli.

[fallah]

The net increase in static pressure from the 12" section to the 12" section would therefore be 2,11 - 1,17 = 0,94 kPa.

I am sure you mean:..... 12" section to the 24" section ......

[mido]

katmar&chenblue

thank u for care with my message but i want to know something static pressure is absolute pressure of gas that measure with pressure gauge or pressure transmitter?

[katmar]

@mido - yes that is correct. The static pressure is the pressure that would be measured by a pressure gauge where the connection to the gauge is perpendicular to the direction of flow. On the other hand the stagnation pressure is the pressure that would be measured by a pitot tube pointing directly into the flow. In the pitot tube the flow is brought to a halt (i.e. made stagnant) and the velocity head is converted to pressure head. The stagnation pressure is therefore the sum of the static pressure and the velocity head.

@fallah - thanks for picking up my typo.

[mido]

katmar
thanks at last i can say that when diameter increase pressure increase
is that true????