Posted on March 18, 2013
This article in not intended to discuss the causes of foam (other than an overview); but instead is intended to provide practical solutions that can be applied to flumes and weirs to mitigate the effects of foam on obtaining accurate flow readings.
Foams: From the Top
Foam can be broken into two general categories: chemical foams and biological foams.
Chemical foams tend to be white, fluffy, and collapse readily when touched. Chemical foams can typically be reduced (or eliminated) by the addition of defoaming or antifoaming agents. Defoaming agents are fed at the foam generating spot to knock down and control foam, while antifoaming agents are fed before foaming is present to prevent foam from forming.
In general, biological foams tend to be off white to brown, dull, sticky, and slimy. Biological foams are normally due to the presence of FOG (Fat-Oil-Grease) or hydrocarbons. Biological foam does not respond well to typical chemical foam control products as they are usually developed for industrial (e.g. surfactant, latex polymers, etc.) foams. In industrial effluent applications (e.g. bakeries), proteins and starches can also cause foams.
Foam is the bane of ultrasonic flow meters, plain and simple. Without dissipation of the foam or modifications to the meter type or installation, accurate flow measurement will be impossible. Ultrasonic meters work by transmitting a pulse of sound and measuring the time for the echo to return (read more here). When foam is present, the meter senses the top of the foam and not the water’s surface. Level (and flow) readings are high and can vary quickly as the foam moves under the sensor’s face.
Changing from an ultrasonic to a bubbler or submerged probe type meter may be a solution as these meter types are unaffected by foam (reading the differential pressure of water – to which foam’s contribution is negligible). Keep in mind, though, that each of these meter types have their own drawbacks: FOG and biological growth can great affect bubbler meters, while submerged probes are also affected by FOG and rapid temperature fluctuations (e.g. laundries). Both meter types will also require retrofitting the primary device (flume or weir) with mounts to accommodate the new meter’s sensing element.
Modifying the Flume / Weir
Modifications to the primary device can be more tricky than changing out a flow meter or adding defoaming / antifoaming agents, but they can be done.
If an ultrasonic meter is being use, and the meter cannot be changed / chemical agents are not effective, a stilling well may be the solution. Stilling wells are round, vertical chambers connected to the main flow of a flume or weir through an inlet. The inlet draws from the junction of the primary device’s sidewall and floor. So long as flow covers the stilling well inlet, foam can be greatly reduced (or eliminated). Also, a stilling well offers a convenient point at which to add chemical defoaming agents for spot control.
Remember that when applying stilling wells:
- For flows with high sediment / debris loads maintenance may become an issue
- Stilling wells are not recommended for sanitary flows due to clogging
- During freezing weather, low flow conditions may cause water to freeze in the stilling well or its inlet
- Stilling well lag can affect flow readings when the well is too far away
Where space and conditions allows, stilling wells are suitable for either flumes or weirs.
Underflow baffles are vertical baffles that extend from above the surface of the water and down towards the floor of the primary device. Foam is stopped upstream of the point of measurement and the flow after the baffle is typically quiescent (and less likely to generate foam). When used to control foam, it is important that the baffle plate extend high enough about the water’s surface to ensure that the foam does not overtop the baffle plate. The foam capture area upstream of an underflow baffle plate is a convenient point at which to add chemical defoaming agents. For more information on the use of baffle plates in weir installations read more here.
Underflow baffles are present in all factory standard weir boxes from Openchannelflow and extend to the top of the weir box. Underflow baffles are not commonly used with flumes due to the sensitivity that flumes exhibit to variances in flow conditions just stream of the point of measurement.
Image: MJK North America