flow in a pipe 3

[sree555]

What is the relationship between the pipe or duct diameter
and the maximum flow allowed?

 

[zdas04]

Interesting question for a mechanical engineer. I'd say that the relationship is described by the Navier-Stokes equation. You can probably find it in your Fluids book or do a Google search.

David

 

[mbeychok]

sree555:

There is no equation, formula or relationship that will calculate the "maximum flow allowed" in a pipe.

For a given fluid, given pipe diameter, given pipe length and given flow, there is a specific pressure drop. Then it becomes a matter of:

(1) Can your system tolerate that pressure drop? If not, then you need a larger diameter.

(2) Also, will the linear velocity of the fluid in that given pipe diameter lead to erosion ot some other undesirable effect? If so, then you need a larger diameter.

As a very broad generality, these linear fluid velocities are usually found to be acceptable: about 7 feet per second for liquids and about 40-70 feet per second for gases.



Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[sailoday28]

Structural and environmental effects should also be considered. High velocities can cause vibrations and noise.
A big consideration is the economic impact. Higher pressure might require a thicker wall. High velocity may case errosion and hence a thicker wall.

 

[Artisi]

Although everything said so far is correct, I disagree that there is in fact a theoretical but impractical "maximum flow rate" thru a pipe(line), a somewhat pedantic argument but worth mentioning.

As the head loss in a pipe(line)increases as the square of the flow rate IE, double the flow increases the head by a factor of 4, a point is reached where irrespective of what pressure you apply to the flow - no appreciable increase in flow will ensure - so, at some point prior to this the maximum flow is reached.

However, good engineering practice is to analyse the flow/ head loss/costs of the pipe line /the pumping system and the operating costs for the most economical solution.

Naresuan University
Phitsanulok
Thailand

 

[25362]

When dealing with steam visit

thread378-98039​

 

[sree555]

How to reduce the total dissolved salts in water for industrial usage? For example the water with 7000ppm TDS and a total hardness of 2000ppm as to be reduced to 2000ppm TDS and the total hardness to 600ppm? How to go about it withot Reverse Osmosis?

 

[sailoday28]

Artisi,
Continuing with your reasoning, what happens (const friction factoran non flashing -incompressibleflow of course) as the pipe diameter increases? Head decreases in the ratio of D^3.
Regards

 

[Artisi]

I think you will find that as pipe diameter increases head decreases as D^5

Naresuan University
Phitsanulok
Thailand

 

[25362]

It seems to me all depends on the value of the friction factor f.
The general formula states that

[Δ]P[sub]f[/sub] is proportional to f (L/D)(V[sup]2[/sup])​

and

f = f (Re, [ε]/D)​

[ε]/D is the rugosity ratio expressing the pipe's surface relative roughness; Re is the Reynolds number.

When looking at the Moody diagram one gets the impression that

[Δ]P[sub]f[/sub] ~ k[sub]1[/sub]/D[sup]4[/sup]​

for highly turbulent flows (Re>10[sup]6[/sup]) in smooth tubes ([&epsilon;]/D <0.000001), with f about inversely proportional to Re.

For pipes with typical rugosities, at high Re (fully developed turbulence), the friction factor is about constant, and

[&Delta;]P[sub]f[/sub] ~ k[sub]2[/sub]/D[sup]5[/sup]​

 

[sailoday28]

Artis
Thank you for the correction.
Perhaps we could continue the discussion with the effect of the friction factor also varying (turbulence not controlling).
In fact, if D gets to large we are entering two dimensional flow.
Regards

 

[ddkm]

I was going to start a new thread, but this problem was so similar I've decided to post here.....

Given a certain pipe size and a certain fluid, is it possible to STATE a maximum volumetric flowrate that can pass through the pipe?

My view is this:

Max vol flowrate = (Pi)(D^2)/4 multiply with v(max)

where v(max)= maximum velocity


Example: Given a pipe diameter of 1 inch (i.e. 25mm), then my max vol flowrate would be 4.4m3/hr if I assume max velocity to be 2.5m/s for a normal flow.

Comments???



---engineering your life---

 

[ddkm]

If the above "method" is correct, how do we "assume" or "determine" the maximum fluid velocity in a pipe?

I've assumed about 2.5m/s for a liquid stream (Newtonian) and this is equivalent to about 8.2 ft/s.

Some of you have mentioned a range of:
a) 7 ft/s (by Milton)
b) 2-7 ft/s (from another website, can't remember)

but what is actually the "maximum" velocity we can use?

---engineering your life---

 

[Ashereng]

There is no equation, formula or relationship that will calculate the "maximum flow allowed" in a pipe.

Some plants have specified "maximum flow rates" in their piping specs.

Aside from that, the limitations come from other consideration such as costs (higher pressure results in more expensive pipe - thicker walls, instruments, valves, vessels, etc), degradation of product - bio field, as well as noise and erosion as stated above.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ddkm]

Ash, what if it's gravity flow, example a weir or overflow line from a tank? How do we determine the max flow to enable proper discharge?

---engineering your life---

 

[WebWizz]

a good website to calculate this is

http://www.engineeringpage.com

The line sizing routines show the maximum velocities that are guidelines from practice. Based on capacity a line size will be provided.

Good luck

 

[Ashereng]

Velocity in a pipe is independent of whether the fluid is moving due to gravity or a pump.

In our pipeline system, the fluid velocity stays pretty much between 3-4 ft/sec.

I don't have any experience with a weir (open channel flow). We don't do a lot of that.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[ddkm]

WebWizz, thanks. I used that web link and it gave a similar answer to my own calculations. The problem I have is on the value of the liquid velocity to be used, i.e.:
1) What is the normal range to be fixed?
2) Why do we fix it that way (or how is it determined)?


Ash, this is also what I thought, that "typical" velocities in a pipe would be independent of the source of the flow (i.e. gravity or pump). And at far higher volumetric flowrates (given the same pipe size), we end up with far higher velocities, which then give rise to other flow problems (i.e. fluid moving at velocities beyond the "typical" range). Correct me if I'm wrong.

The example I'm having is not an open weir/channel, but an overflow line through gravity.

---engineering your life---

 

[Ashereng]

ddkm,

If you want to flow higher flow rates, you need a larger pipe to keep velocities in the "typical" range.

Other problems at higher velocities are increased pressure costs, pressure losses across valves, loss of flow measurement accuracy if you are outside the instrument's range.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[cvg]

"Velocity in a pipe is independent of whether the fluid is moving due to gravity or a pump."

note that maximum flow rate in gravity flow is limited by the force of gravity, however, with a big enough pump and strong enough piping, you can achieve much higher flow rates then you would get using gravitational force alone.