Pressure loss in tee

[jcc0411]

I'm new at this and I'm a recently graduated engineer so my knowledge about Fluid Mechanics is just the basic. I'm in charge or a project where I have to design a gravity flow system to feed water to a spraying machine.

The idea is to have an elevated tank with the water and let it flow by gravity throught a hose. It has to feed 2 different spraying machines...my questions are:

what causes a greater pressure loss, having one hose with a tee to split the flow? or having 2 different hoses at the bottom of the tank?

Which is better, locating the hose at the bottom of the tank? or in the wall at the lowest part of the tank?

And, does the water level in the tank affect the flow, or only the height at which the tank is located?

I hope you can help me! thanks,

José C.

 

[BRIS]

Jose
I think you might be being a bit optimistic when you say your fluid mechanics is "just basic" (sorry).

1) The head driving the flow is the difference in level in the tank and the pressure head at the spraying machine. It makes no difference whether you tap in to bottom or the side of the tank.

The level in the tank obviously affects the flow!! The flow will reduce as the level drops. (flow is propoortional to H^0.5 where H is the presure head).

2) The pressure loss in the pipeline is related to the length of the pipeline, the velocity squared, the pipe roughness and the area of pipe in contact with the water. For the same flow two 1" hoses will give roughly the same head loss as one 1.5" hose.

3) The head loss at the tee will be about 1.2 x V^2/(2g).

So which solution gives you the most head loss depends on the diameter and length of the hoses.

If you are considering a single diameter for both solutions then the twin hose solution will give less head loss.

brian

 

[MachinaMan]

Hi Jose

Brian is right on this one but, I would consider taping in to the side of the tank so that you don't get the debris from the bottom of the tank in your lines. As for the pressure (head) loss in a tee versus 2 pipes, it is totaly dependant on the lenth and the diameter of the pipes. Good Luck.

Rene

 

[patrickraj]

Hai BRIS,
I disagree with your view that tap location make no difference.
In the case of side tapping, a 90deg bend is must, this will incur pressure loss and flow will be low. If tapping is taken from the bottom of the tank, there is no need for 90deg bend, correspondingly pressure will be less and marginal higher flow will be obtained.

 

[jcc0411]

Thanks for your posts, I'm preparing a document about gravity-fed systems, like the one I'm designing.
What are some characteristics of this kind of systems? and what are some recomendations about the do's and dont's so the pressure drop wouldn't be so big?

and what are some of the main causes of pressure drops in any kind of systems?

Thanks,
José

 

[BRIS]

patricraj

True but one assumes that he is coming off the tank with flexible hose and therefore there will be no 90 degree bend. Hence there will be no difference in head loss whether the tapping is from the side or bottom. Tapping from the side will avoid debris as noted by frenchcanuk.



jcco411

Pressure drop in the pipeline is proportional to velocity squared. In addition there are head losses due to additional turbulence at each fitting: bend, tee, entrance etc. The head loss is also proportional to v squared. (k x V^2/(2g) (SI units). There are many published tables giving K values for various fittings.

Thus to reduce pressure drop you need to reduce the velocity in the pipeline (increase the diameter) and reduce the the number of fittings.

Obviously a smooth pipe material has less head loss than a rough material.


brian

 

[25362]

If you'd like some info on the subject of loss coefficients read "Correlate Pressure Drop through Fittings" by Ron Darby
(Texas A&M University). This is an article that appeared in the Chemical Engineering issue of July 1999, under the Design heading.

 

[lilliput1]

Make sure you find out the minimum required flow and pressure through the spraying machine. It may require a minimum NPSH. Then make a table of each possible pipe size increment and equivalent length of pipe to represent fitting losses. Find the loss for each size and determine the required tank height for each. Select that which will fit within the space constraints. Provide adequate safety factor to make sure it works. Make sure the tank is vented.

 

[zdas04]

There have been some very good suggestions above. One additional thing to keep in mind is that "hoses" ain't "pipe". The work done by the fluid on the hose eats up available energy faster than the same length of hard pipe. The friction component is similar (if you get the roughness correct), but the hose jumping around increases the pressure drop measurably.

You can mitigate this by shifting to hose as late as possible.

The discussion above about the location of the tap being important may lead you to a sub-optimum decision. Once you get a flow rate worked out, calculate the Reynolds Number. My guess is that this will show you to be in Laminar flow. The difference between going through an elbow and not going through an elbow in Laminar flow is an infinitesimal difference in dP. Pick your location based on the propensity of the fluids for solids accumulation and on ease of overall fabrication.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[friartuck]

Basic information is required.

a. Flow rate?
b. Pressure at outlet.
c. Distance between tank and outlets.
d. Any changes in direction.

Basically it will be a 'balancing' exercise, or re-iteration of calculations.

Shower nozzles have pre-determined flow rates set against head available. eg a shower nozzle may produce 0.15l/sec of water spray at 1Bar(g) pressure, so you will then know that the tank neeeds to be at least 10m high to give the 1bar plus an allowance for pipe and fittings losses.

You might make an allowance of say 0.2 bar for friction losses in the pipework and from the required flow rate and from the fittings you have, you can work out that the pipe size is 'x'. In addition, the head loss of the fittings will require you to lift the tank accordingly.

You could approach the problem another way and design your own 'low loss' nozzle to reduce the tank height although using pre-determined shower outlets requires less guess work.

Connections to the tank should be at the side to prevent debris from clogging up the pipe. (Ours fill with bird poo if the lid drops off)

As regards the one pipe with 2 tees or 2 separate pipes. You could size either system to give you what you desire. One pipe however will have the characteristic that when both outlets are operating, the combined common pipe flow will increase and the pressure drop will increase in a squared proportion. The 2 pipe option will remain unaffected by either outlet being on or off so a constant flow is achieved.

Friar Tuck

Friar Tuck of Sherwood