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flow in a pipe
3

flow in a pipe

flow in a pipe

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

RE: flow in a pipe

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

RE: flow in a pipe

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)
.

RE: flow in a pipe

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.

RE: flow in a pipe

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

RE: flow in a pipe

(OP)
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?

RE: flow in a pipe

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

RE: flow in a pipe

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

Naresuan University
Phitsanulok
Thailand

RE: flow in a pipe


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

ΔPf is proportional to f (L/D)(V2)
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
ΔPf ~ k1/D4
for highly turbulent flows (Re>106) in smooth tubes (ε/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
ΔPf ~ k2/D5

RE: flow in a pipe

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

RE: flow in a pipe

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---

RE: flow in a pipe

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---

RE: flow in a pipe

Quote (mbeychok):

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?

RE: flow in a pipe

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---

RE: flow in a pipe

a good website to calculate this is 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

RE: flow in a pipe

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?

RE: flow in a pipe

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---

RE: flow in a pipe

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?

RE: flow in a pipe

"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.

RE: flow in a pipe

cvg, agreed that the max flowrate is limited by the gravity force.

In my case, I'm sizing for an overflow line, in the event some malfunction results in overfill of the tank. As such, my question was what is the BASIS to determine the size of the oveflow piping to enable sufficient throughput for the oveflow case?

---engineering your life---

RE: flow in a pipe

We usually consider the feed line size to determine tank overflow size eg. if the supply line is 6" we choose 8" for overflow. Simply becouse becouse an 8" line can can accomodate any supply at any rate from a 6" line.
Thanks

RE: flow in a pipe

Milton,

From your reply back in Oct last year:  


"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."



My Questions:

a) What do you mean by "can the system TOLERATE the pressure drop"?  Are you referring to the source pressure, example a pump, whereby it is insufficient to cater for the pressure drop?

b) If in an overflow situation (by gravity), example a tank filled to the brim, then overflows out through overflow line, how do we determine the allowable pressure drop?

---engineering your life---

RE: flow in a pipe

ddkm,

in a) you could add - when the pipe bursts

in b) the pressure drop is limited by the head you can tolerate in the tank. You want to leave some safety factor in the ullage before the tank overflows, so this fixes the physical height of the liquid and therefore the pressure available.

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

RE: flow in a pipe

Katmar,

a) LOL

b) Basically, the overflow is a pipe outlet near the top of the tank and we WANT the liquid to overflow through this overflow pipe. However, it must be sufficiently large so that the liquid does continue "moving up" and flow through the roof.

You mention the physical height of the liquid, but isn't this applicable for a gravity flow at the BOTTOM of the tank? In my case, the overflow is right near the TOP. Will this be the same?

Thanks.

---engineering your life---

RE: flow in a pipe

Sorry, typo error. I meant:  

...so that the liquid DOES NOT continue "moving up"....

---engineering your life---

RE: flow in a pipe

ddkm,

This type of overflow, near the top of a tank, is usually designed to run part-full. Have a look in Perry 5th Ed page 5-43 for a description and design method. If you don't have a 5th ed look for "Drain pipes" in the index.

If you are not designing for part-full then you can allow a small head above the pipe. If you are working with water and you can allow the level to go to 200 mm above the overflow pipe, then you would design for 2 kPa pressure drop. Plus you would have to analise the piping from the overflow to the ground. Depending on its length and diamter you may get some pressure recovery - i.e. a syphon. However, this is working very close to the bleeding edge and unless there was a very strong reason not to, I would design an overflow for part-full flow, and I would ensure that the vetical section was self venting.

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

RE: flow in a pipe

OI was a little impressed with the amount of answers, but will add one anyway.

In case of liquid gravity flow, the static pressure of the liquid will be equal to the total pressure drop of the pipe. So in this case the pressure drop that is a result of the piping diameter and fittings etc are in balance with the pressure available by the liquid column (rho * h * h). The exit k (value 1) should be used to cater for the dynamic head loss of the fluid. Can be calculated iteratively using a calculation such as on www.engineeringpage.com (my favourite)

In case of pumping application the general approach is one of economics. Pumping costs energy and therefor money. The generally used velocities represent practical values that keep you out of trouble and balance operating (the energy) against investment (buying and installing the kit) costs.

True maximum capacities in technical terms do exist. For compressible flow in pipes the speed of sound is the limiting velocity.

Hopes this contibutes

RE: flow in a pipe

How to calculate NPSH and Total Head for a new Water pump (Q=14 ML/D) and pipe diameter 12"(D=0.3m)?

RE: flow in a pipe

isaac289 (Mechanical) 28 Jun 06 3:37  
How to calculate NPSH and Total Head for a new Water pump (Q=14 ML/D) and pipe diameter 12"(D=0.3m)?

Suggest you start a new thread for this if you want answers.

Naresuan University
Phitsanulok
Thailand

RE: flow in a pipe

Still looks like nobody has figured out the absolute maximum velocity of fluid in a pipe is the velocity of sound in that fluid under flowing conditions.

I doubt that sonic velocity is practical for the remainder of the questions, so for those I would recommend that you concentrate on optimizing pipe size and wall thickness expense vs powering expense for iterations that result in the required flows, inlet and outlet pressures, then apply any minimum or max velocity limitations or others as dictated by your engineering judgement.  This usually gives the most appropriate velocity, as well as pipe diameter and driver power needed to operate it.

RE: flow in a pipe


From another angle: for particular nonconductive liquids in pipes, when entering storage tanks at submerged levels, and to ensure safe (short) static electricity relaxation times, a maximum of 7 m/s (or even lower) has been repeatedly quoted.

RE: flow in a pipe

Right, there's always someone who doesn't want to blister a tank from jetting product into it from a velocity nozzle, etc.  Formulas are no substitute for thinking about what you're doing.  Personally I have found that hydraulic simulation programs don't give you a lot of information about how long its NOT going to take before you cut the gizzards out of a control valve from all the dirt in the oil.  Not everything is in the numbers.

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