Low flow through 4" flume 1

[jvoisin]

I have an alum dosing pump connected to to an ultra sonic flow meter which reads the flows through a 4" flume in a manhole.

My peak flows are about 3 gpm, and the a 4" palmer bowlusflume effectively operates in the flow range of 5-55 gpm. I am concerned that the flow measurements will not be accurate which will result in improper alum dosing. Are there alternative technologies to measure low flow situations like this?

Thanks

 

[bimr]

A magmeter would work, but the magmeter application would depend on the physical layout of your project. Magmeters installations require a full pipe. I would expect that you would also have problems with the accuracy of your metering pump.

What are you trying to do?

 

[jvoisin]

Thanks Bimr.

We have a private sewage system for a campground. Sewage flows via gravity to a series of septic tanks. In order to meet regulatory objectives, we are required to reduce phosphorus to to 1.5mg/L.

We are proposed a intermediate clarifying tank that will be put inline after the septic tank which will house the mixing of alum and sedimentation. Treated water will flow out of the clarifying tank via a weir which flows to a flow metering chamber that houses the ultrasonic flow meter and flume. This flow information is relayed back to the alum dosing pump so it can adjust dose based on flow through the system.

 

[cvg]

v notch weir might work better

 

[bimr]

I agree with cvg, your best option is the v-notch weir.

 

[jvoisin]

I found some information on the internet:

In talking with customers about weirs, we run into two common misconceptions. The first misconception is that weirs can read down to zero – that there is no minimum head. The second is that weirs are more accurate at these flows than flumes. Unfortunately, both assumptions are incorrect.

While a V-notch weir can read relatively low flows (a 22-1/2º V-notch can read down to 3.990 gpm [0.2518 l/s]), they cannot accurately read flow below this. The problem is the need for the flow over the weir crest to spring free. Below 0.2-feet [6.10 cm] there is insufficient head to ensure that the flow springs from the crest at all times. Flow may either alternate between between clinging to the downstream weir face and springing free or simply cling to the downstream weir face at all times.

Unfortunately for the proponents of all things weir, the 0.4-foot HS flume is accurate down to 0.0718 gpm [0.0045 l/s] and with a head of 0.12-inches [0.30 cm]. Even the 1-inch [2.54 cm] Parshall goes lower than the lowest V-notch weir both in terms of flow rate (1.460 gpm [0.0921 l/s]) and head (0.60-inches [1.52 cm]).

http://www.openchannelflow.com/blog/article/weirs-how-low-can-you-go#sthash.VukfCkdb.dpuf

 

[jvoisin]

look like a h type flume will work:

http://www.openchannelflow.com/products/flumes/h-type

similar to v notch I suppose.

 

[bimr]

There is very little equipment suitable for accurately measuring such a small flow, especially since the water is not clear.

Another alternative may be to construct a pumped system. Install an effluent pump in a small sump. Energize the alum pump when the effluent pump is operating so that alum is added proportional to flow.

When the system is installed, you can measure the flow from the effluent pump with a bucket and stopwatch. Then you can set the flowrate from the alum pump.

You can probably install the pumped system for what the ultrasonic transmitter costs.