Pipeline Velocity Calculations

[jonathanfp123]

Hi, I am new to the forum so forgive me if i have put this thread in the wrong location.

I am looking for some help and advice on a calculation i have transposed in order to help size cooling lines on a diesl firewater pump package.

The calculation takes the pipe size in inches, and the pipe velocity limit in m/s and provides the maximum allowable flow in L/S (messed up i know but that's just the units i have to work with haha!)
Anyway, this is what i have got:

=((((2*0.0254)^2)*5)/1.274)*1000 2 = the pipe size 5 = the volocity limit.

(Apologies for the layout, i grabbed it out of my excel spreadsheet)

I originally started with the volocity calculation v=1.274q/d^2 and transposed to get to my formula, adding in sections to change units along the way.

Does my transposition look correct or is there something i am missing? I ask because we have a pipeline velocity slide rule from a supplier of ours and it appears to give quite different values to those that my calculation has provided:

pipe size my calc sliderule
1/2" 0.63 0.975
3/4" 1.42 1.7
1" 2.53 2.75
1 1/4" 3.96 4.8
1 1.2" 5.7 6.5
2" 10.13 10.8
2 1/2" 15.83 15.2
3" 22.79 23.5
4" 40.51 32.25
6" 91.15 91
8" 162.05 160

I accept that the values read off of the sliderule are not going to be absolutely accuraute but there is quite a considerable difference on some of the pipe sizes. (ie the difference betweenthe values for 1/2" equates to around 2.5 m/s on the slide rule)

Basically i am after a sanity check. Is my calculation correct, and the sliderule wrong; or is there a flaw in my calc somewhere?


Additionaly, a colleague has a comment on another formuala of mine for calculating pipe size, given velocity and flow rate. He has suggested that maybe i should consider temperature or density for my calculations. Will this have much of an effect on the velocity/pipe size/ flow rate, or is any efefct negligable.

If they do need to be included, could anybody suggest a transposition of the velocity formula to include for this?


Apologies for rambling on, i hope i've not been too hard to understand!
Thanks

 

[BigInch]

Convert all your pipe diameters to meters. Use the inside diameters, not outside diameters.

v = Q/A = units of m3/s/m2
A = pi()/4 * d^2 "d" is in meters, [π] is in ... [π]
v = 4/pi() * Q * d^2

If Q is in m3/s and d is meters, that's the end of it.
If Q is in l/s and d is meters, divide by 1000

No more math problems please. Math is some other website.

Independent events are seldomly independent.

 

[rconner]

Such calculations are of course heavily influenced by the actual inside diameter (ID) plugged in for the type of pipe involved. Are you sure the "calculator" and your calculations are based on identical assumptions in this regard? [In any case, if your maximum velocity is 5 m/sec, the second or third columns don't seem out of the ball park.]

That being said, mixing English and metric units in the same calculations, without thorough explanation/depiction is admittedly at least confusing.

 

[jonathanfp123]

Hi BigInch,

Thanks for your reply, i think i understand where you're coming from.
However, i don't believe that my problem was a maths related one. I am 99% confident in the formula i used i just wanted clarifcation that i hadn't missed something important, such as density or temp considerations, as mentioned in the latter part of my post. I fail to see that a maths advisor would know much about the minutii of a fluid mechanics formula.

Many thanks
Jonathan

 

[IFRs]

You appear to be doing math only - the problem is fully constrained. If however you are attempting to determine the maximum design velocity in a pipe, there are many other factors including but not limited to pump power, flow and head, inside pipe roughness, density and viscosity of the fluid, corrosion issues, the ability of the pipe to withstand the pressure, etc.

 

[BigInch]

If there is a change in pressure, or temperature and your fluid is a gas, then you messed up by not considering pressure, temperature and density.

Your basic formula is v=Q/A. Even math guys get it.
Oh I see where you're coming from. Math guys never include the units.

Independent events are seldomly independent.

 

[jonathanfp123]

Hi Rconner, thanks for your response.

Both my calculation and the slid rule work from the nominal diamter of the pipe in question, thus being the ID
I shall try converting all the various parts into one unit form beforehand ans then calulcating again to see if i still get the same differences.

Addittionaly, do you know iw any consideration of temperature or density is required when calculating the required pipe size from a given velocity limit and flow requirement?

Many thanks
Jonathan

 

[jonathanfp123]

Hi IFR, BigInch,

Basically, i wrote a calculator that takes a user specified flow requirement, and velocity limit will provide a pipe size required to meet such requirements. We use two materials of pipe, 316SS and 90/10 cu/ni, with maximum velocity limits of 5 and 3.6 m/s respectively, determind by the engineering powers that be here. my line manager asked me to include a box for max rate of flow in whichever pipe size, so that the user can see how far their required flow is from the maximum flow of the pipe size the claculator selected. A colleague then asked if i had taken into accound the temperature and density of the fluid being piped, as we pump fresh and sea water, direct from source. This is what led me to your good selves. . Can someone suggest how i would include allowance for temperature and density?

Thanks
Jonathan

PS didn't mean to sound ratty in my previous response BigInch!

 

[Artisi]

simply get a chart of friction losses / pipe diameters and start from there, it's not rocket science.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[BigInch]

Water isn't all that compressible with pressure. The density varies with temperature some though. If your seawater temperature doesn't freeze, or boil, and unless you're selling it for $1000/m3, density variations probably won't make much difference. Density vs temperature charts for water are pasted all over the internet. I'm sure you can find one.

Independent events are seldomly independent.

 

[cvg]

first of all, throw the slide rule and charts away. no self respecting engineer uses those any more in the digital age. secondly, unless the fluid is compressible (and fire flow pumps pumping water would indicate negligble) than Q=VA as previously posted by big inch is your formula and friction, heat, density and everything else do not apply.

 

[rconner]

Just one further comment -- if you are trying to get numbers to be meaningful, "right" or consistent to multiple decimal points, I would not assume that all types or schedules of pipes meet what you literally refer to as, "the nominal diameter of the pipe in question, thus being the ID". The actual ID of many schedules of many types of pipe is in fact seldom precisely equal to "nominal", and some are a good ways from it. This is why I made the comment I did. I will repeat, are you sure the developers of the rule assumed exactly the same OD, ID or schedule as you did in your calculations (I guess the rule could conceivably be correct, at least for the specific pipe or whatever they are promoting or most used to?)

That being said, in this age of e.g. ExCel spreadsheets I agree with cvg that there is little need for slide rules that were once very common (as long as we have electrical power, and that is not knocked out!), as the effective ID to determine velocity from flow is simply OD - 2(wall thickness) in the case of unlined pipes or OD - 2 (wall plus lining thickness) in the case of internally coated or lined pipes.

As for me and my house, I will at least keep some rules in the drawer.

 

[jonathanfp123]

Hi Gents,

Thank you all for your replies, very helpful indeed!

RConner, that was my exact thinking in digitising this calculator, once written, its far easier to do it onexcel, with much less chance for user error. And i see what you mean, we may consider a 2" nb to be exactly 2", whereas the sliderule may consider it to be 2.05" for example.

BigInch, cvg, I'm very inclined to aggree wholeheartedly with you. The application is fire protection on O&G rigs so unless the disastrous happens, the system will only ever be run for 1 hour per week. As such, i think you're right that any effects of heat and desnity etc will be wholly negligible.

Many thanks
Jonathan

 

[LittleInch]

Jonathon, I don't think you've understood what rconner is saying. Nominal size or nominal bore is exactly that NOMINAL. Up to 12", nominal size IS NOT ID. Just type in pipe size od and you'll get all the info you need for a look up table in excel if you want to use nominal size as an input, but you also need schedule or wall thickness.

2" od is actually 2.375"od,
4" is 4.5", 6, 6.5"
8, 8.625
10, 10.75
12, 12.75
14,14

As you are using stl, which is often quite thin, your ID is almost certainly bigger than the nominal size.

Especially at the low sizes, it will make a big difference but there is no excuse for getting it wrong.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[jonathanfp123]

Hi LittleInch,

Thats crystal clear now! I used the correct values for various schedules and now my figures are much more correlative with those of the sliderule. Time to banish it the back of the drawer!

Thanks guys!!