can head loss be greater than pipe length?

Question

mechanical enginnering, fluid dynamics, Thanks for your help

Answer 1

D-W Formula for,

Head loss due to friction in piping network

=(friction factor*length*velocity^2) divided by (2*g*inside diameter of pipe)

So, from above formulae and my practical experience ,head loss can not be greater than pipe length in a fairly designed piping network.

Answer 2

The term "head" refers to the height that a column of the fluid (usually water) can be pushed by a particular pressure. In the mechanical energy equation, it has the units of pressure/(sp. wt.). The dimensions are: pressure is F/L^2, and specific weight is F/L^3. Therefore P/sp. wt. has the dimension of length. But it's actually related to pressure, not a physical length. Head loss is due to frictional loss from the pipe surface, and from the "K" values of the valves, fittings, etc. A short pipe with a lot of valves, reductions/expansions, etc, could certainly have a head loss greater than the physical length of the pipe.

Answer 3

Yes. Head loss generally refers to frictional velocity loss expressed as an equivalent pressure loss. Frictional head loss can approach but not equal the head value at the input end of the pipe, and contrary to the first answer will not completely stop the flow. Per the ref, it is directly proportional to the length of pipe, the square of the fluid velocity, and a term accounting for fluid friction called the friction factor, and thus would be zero if flow were stopped.

Answer 4

There once was an engineer that requested a large number of 50 foot lengths of 3/4" i.d. hose. I asked him the purpose and he said "I am going to run them to a point I need water". I suggested, being a pipefitter by trade, that it would not work. He proceded to string out several hundred feet of hose. The water supply was the city water supply, about 40 lbs. or so. The valve was turned on and several hours later a very small trickle came from the hose.

Answer 5

Yes. It is possible.
If the head loss is greater than the pipe length equivalent, then there will be no flow.

Answer 6

The pressure drop (pressure change) of the 3" diameter would be lower, since the diameter is in the denominator of the equation. Therefore, your pressure would be maintained better in the larger diameter so the pressure would be higher at the exit (the pressure drop would be greater in the smaller diameter, but the larger diameter will have a higher outlet pressure; you asked 2 different questions). This is of course assuming you have the same initial pressure. The larger diameter would outlet at a higher flowrate too. Q=Velocity*Cross-sectional area, where the cross-sectional area=pi*r^2