Hydraulics - side weirs as flow distribution in a channel

[hobeex1]

I am trying to calculate how flow will be split over 4 side weirs coming off a common channel. The channel is a feeder channel to 4 identical process tanks (stopping just after the last weir), and the aim is to design the system such that as equal a flow split as possible is achieved between the 4 tanks fed from it.

I am trying to convince someone that, although there may be some variation in discharges over the 4 weirs, it can be kept to a minimum by selecting the correct channel cross section and weir heights, thereby eliminating the need for a distribution chamber. I'm hoping to keep the weir heights equal if possible.

I'm at the edge of my hydraulic knowledge with this one, so I'd appreciate any advice on whether I'm correct in my assumption, and how I can prove it.

 

[hobeex1]

Sorry guys - selected the wrong forum

 

[bimr]

There is no way to get an equal split if you are coming down a deadend channel and you desire to take flows off the side with weirs. If you tried to put adjustable weirs on, you would be able to precisely split the flow at a certain channel velocity, but then you would have to readjust the weirs at each change in channel velocity. It may be workable if you have a constant channel flow.

A better solution would be to come to the end of the channel, go under a velocity dissipating baffle wall and then have the flow go upward over the 4 weirs. This may be what you are referring to as a distribution chamber.

 

[Ussuri]

It is possible to use a flow splitter chamber to split flows equally to four process units with a weir arrangement. You need to arrange your design so that the velocity in your chamber is zero and you have sufficient hydraulic gradient in your upstream pipework to develop a head in the chamber and back up the system. You can do this by a low level entry into the chamber and set your weir height above this.

If you have a velocity of any kind in the chamber your backwater profile will vary with distance, which will vary the head over each weir and consequently the discharge.

Couple of potential problems, depending where you are in the process, silting up and solids deposition may be an issue. And you do require a reasonable hydraulic gradient.

A useful document on side weirs is 'Hydraulic Design of Side Weirs' By RWP May, BC Bromwich, and CE Rickard.

 

[rcooper]

hobeex,

I believe you can get close to an even split between the four process units. If the channel is constant cross section, as the water passes each weir the level will increase slightly. If you assume there is no energy loss in the main body of water as the flow goes past a weir, the total energy up stream equals the total energy down stream. The dynamic energy (v^2)/(2g), however, has reduced because the flow rate has reduced, so the gravitational potential energy must increase to make up the difference.

Now, if you taper the channel to as sharp a point as practicable as the flow rate reduces down the channel, the velocity is maintained because the cross sectional area is reducing as the flow is reducing, hence you get approximately equal level down the channel and even split between the tanks. By opening up the point of the taper you may be able to eliminate some of the frictional effects of the channel, again improving distirbution.

The distribution will be improved if you use as narrow a weir as possible and as low a velocity on the feed channel as practicable.

A channel distribution system will never be as good as a traditional distribuation chamber and is cignificantly worse if one process unit is taken off line, but is commonly used where the accuracy of distribution is not critical.

Another option is to use discharge flow control. You could put a flow meter and flow control valve on the discharge from each process unit. One system I have looked at using but have not found a sutiable application yet is to put an ultrasonic level meter mounted on a weir penstock at the discharge from the process units. The ultrasonic level sensor moves with the penstock. One ultrasonic is the master flow measurement and the others are slaves, controlling their penstocks so the water in each process units are at the same level above the weir of the outlet penstocks.

 

[bungayman]

hobeex1 - I not sure if Hoi would give many marks for getting your Flow Distribution tutorial done for you on a forum.

bimr - I think you need to do the Engineering Hydraulics module that hobeex1 looks like he's doing.

As rcooper says, taper the channel width, plus you can taper the channel invert, and vary the weir lengths. The flow distribution can be solved in terms of; Downstream Surface Level, Downstream Liquid Velocity, and Upstream Total Energy.