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?
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
"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?
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.
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?
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.
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
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.
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.
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.
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.
