10 replies to this topic

[E-Tantoy]

Sir,

Good day.

I am doing a hydraulic calculation to check the pressure balance and flow distribution in a closed loop cooling water system. I am using the as built isometric drawings for the pipe length and the K-Factor method for the pressure loss calculation in the line.

I have attached a simple diagram to further illustrate my queries.

For my question, assuming:
a. distance from point 1 to point to 2 is the same for Figures A and B
b. Figure A tee main and pipe has the same schedule number and the tee is standard (non-reducing tee)
c. flow rate from point 1 to point 2, for figures A and B, is the same
d. no other fittings in between point 1 and point 2 for both figures A and B.

I checked one website (http://www.freecalc.com/fricfram.htm) and compared the difference if I specify one piece of "tee-flow through" for a length of 100m and resulted in a 0.13psi difference in pressure loss (with all inputted data to be the same for both calculations made: 11275GPM, 100m of 24" schedule 40pipe).

Question #1: If the branch line is closed and no flow goes to the branch line (as shown in figure A), is it just the same as the flow through a straight pipe (as shown in "Figure B" in terms of total friction loss? Or, expressing my question in another way, do we expect to have the same pressure at point 2 for both Figures A and B?

Question #2: If the pressure at point 2 in Figure A is different in Figure B, what causes this difference, considering the assumptions stated above, if we are only considering the flow from point 1 to point 2? (The online calculation showed this difference)

In addition to the investigation made on the online calculation, I also noticed that in some hydraulic calculation software (that we are using), there are several options for a tee: tee-through (or run), tee-branch. It seems that different considerations are made in the k-factor depending on the flow in the tee. I am not sure if it would really matter I specify straight pipe instead of specifying "tee-flow through".

Your point-of-view on this will be appreciated. Thank you very much.

Edited by E-Tantoy, 19 August 2012 - 08:34 PM.

[Bobby Strain]

Does it really matter?
Bobby

[kkala]

As Bobby Strain is understood to note, difference in ΔP of 0.13 psi is insignificant. However, you may want to clarify the matter for gaining knowledge.
In applications I have assumed that "tee through" means just straight flow without directional changing. For instance, a pipeline (where flow occurs) is connected to another pipe, the latter having a valve closed (for the moment). The tee at connecting point is "tee through" in this case.
Tee-branch means that the flow changes direction, usually by 90 degrees. The above tee would be "tee-branch", if above pipeline were isolated downstream and the valve of the pipe were opened.
Perry (Fluid dynamics, Table of "additional frictional loss for turbulent flow through fittings and valves") gives K=0.4 for tee along run (branch blanked off), versus K=1.0 for tee branching flow. This is expected to be so due to directional change.
Nevertheless the diagram to illustrate the queries has not been found, can you send it attached to another post? Just go to "more reply options" to realize attachment.

[pravin164]

Where is your attachment.

[E-Tantoy]

Sorry, I didn't notice that the attachment was missing. Anyway, I have attached the file below.

Attached Files

•  Tee Flow.xlsx   11.28KB   350 downloads

[E-Tantoy]

@Bobby Strain,

For my example, it is insignificant and maybe it would not really matter but I just want to know how, as experienced by others, it is considered. I may do the conservative way (to always include the tees causing additional pressure drop) or eliminate it, either way is possible.

@Kkala
Thank you, Sir. I may be digging too much on the details but, as you mentioned, it is just for me to understand the approach (and to gain knowledge).

The tee K=0.4 for tee along run (branch blanked off) is due to what reason if there is no directional change in flow? it seems that it is only just the normal losses to a line due to the physical length of that tee RUN) , is it correct?

[ankur2061]

The tee K=0.4 for tee along run (branch blanked off) is due to what reason if there is no directional change in flow? it seems that it is only just the normal losses to a line due to the physical length of that tee RUN) , is it correct?

E-Tantoy,

Tees are defined as "straight run" (what you call as branch blanked off) or as "branched run". Tees which have connections for instruments such as pressure gauges or transmitters are called "straight run" whereas "branched run" tees are where the flow splits or divides. Your question is that why there is a K=0.4 value for branched run tees. While I cannot provide you the mathematical basis of K=0.4, it certainly is clear that any kind of break in any straight length of pipe will cause turbulence at the point of the break (the tee junction) which will cause more frictional loss compared to a straight pipe of the same straight length. This additional friction loss is thus accounted by the head loss coefficient K for different kinds of pipe fittings including tees.

Regards,
Ankur.

[kkala]

1. P2 is expected slightly lower in Figure A
2. This is due to additional frictional ΔP along the path of figure A, incurred by the tee-through.

Even though this tee-through case does not change flow direction, some pressure loss due to it has been measured experimentally (lower than in the case of tee-branch). Apparently flow finds extra friction in meeting the pipe branch (even blanked off), so some more energy is lost.
By the way, having used mentioned software I arrived at a ΔP difference of 0.5 psi (post No 1). This does not effect the above.

Edited by kkala, 20 August 2012 - 04:34 AM.

[katmar]

There is an awful lot of misinformation floating around about the pressure drop through Tees used as runs. Fortunately, even though the relative values given by different sources vary widely, in absolute terms the pressure drops are small and it is all covered by Bobby Strain's comment "Does it really matter".

The K value of 0.23 which is used by the FreeCalc site you linked to probably comes from Page A-29 of the old (pre-2010) version of Crane TP-410. BUT this value is intended for threaded Tees which would never reach the size of 24". I guess the Perry values quoted by kkala come from a similar source. In the original publication (Chem Eng, April 2001) of his 3-K method Darby gave parameters that would result in a K value of about 0.04 for a standard flanged or welded Tee under your flow conditions. Darby revised this value up to about 0.13 when the errata were published for his book. Hooper's 2-K model was also published with errors in the original publication (Chem Eng, Aug 1981) and revised to be similar to Darby's values in the "Piping Design Handbook" edited by McKetta.

The Hooper values are for standard Tees which have well radiused curves from the run to the branch. For a straight stub-in (as you have drawn) Hooper gives values that are basically zero for turbulent flow.

As to the reason for the pressure drop - I suppose you can think of it as a sudden (small) expansion followed by a sudden (small) contraction.

If I was doing this calculation I would probably use a K value of around 0.15, which gives an equivalent length (L/D ratio) of about 12 - equal to just less than 7 meters in your case. It probably doesn't really matter in this case.

[E-Tantoy]

Thank you very much for all the responses.

Although the effect of the tees is not that so significant in terms of pressure balance, it's just for additional knowledge since I am doing many hydraulic calculations and I always encounter this "tee-through/run" or "tee-branch". It helps me understand better how the calculation works for a given software and how to consider each component of a given system to arrive at a reasonable result (with confidence). As my superior always told me...always keep the curiousity alive.

Thank you ankur2061, kkala and katmar for the explanations.

[beeone]

Hi

 

I'm a student doing a final paper that includes calculating the pressure losses. I chose Darby's 3-K method and I have a really stupid question. What does r/D mean in charts in tee section. I totally get R/D at elbows, but here I don't get it. A d/D would be understanding, but r/D doesn't.

 

And if is not too much trouble, I'm in dilemma what to choose threaded or flanged if all fittings are welded to the pipes.

 

Please help me, because my professor is a little old school and didn't heard for this method yet so he hasn't got a clue, either.