Dampening Excessive Upstream Turbulence / Energy in Parshall Flumes

Openchannelflow offers a number of different flow conditioners to help to normalize the flow before it enters a Parshall flume. Some of the conditioners are in-flume accessories, while others are installed upstream of the flume itself.

Upstream

For applications where the flume is installed below grade either in a fiberglass Packaged Metering Manhole (PMM) or a concrete manhole / vault, Openchannelflow offers Energy Absorbing Manholes (EAM).

An Energy Absorbing Manhole is placed immediately upstream of the flume manhole and serves to disrupt the flow stream and dissipate excess energy. After fully disrupted / dissipated the flow, it is guided out of the Energy Absorbing Manhole and into the downstream pipe so that when it enters the flume manhole / vault, it is smooth and subcritical – two key elements for accurate flow measurement.

fiberglass energy absorbing manhole upstream of a packaged metering manhole

Energy Absorbing Manholes work well on sanitary, industrial, and mixed flow streams. For industrial customers where space constraints limit upstream straight runs, or where higher than desired slopes are exist, Energy Absorbing Manholes can create the conditions necessary for accurate pretreatment flow measurement.

In-Flume

For in-flume conditioning, the inlet end adapter can be lengthened to allow for the installation of baffle plates, flow tranquilizing racks, and energy absorbers.

Smaller Parshall flumes – typically 1-3-inches in size – can have perforated baffle plates installed in the inlet end adapter. The baffle plates are perforated in a staggered pattern and, used in groups of two or more, help to break up poor inlet velocity profiles. Constructed of 1/4-inch PVC sheet, the plates rest in side / bottom cavities and can be easily removed should they ever become clogged or if the flume needs more flow capacity. Baffle plates are not normally retrofitted into existing applications as the recesses for the baffle plates are molded into the sidewalls / floor of the inlet end adapter – although surface mounted baffle plates are possible.

Baffle plate installed in the inlet of a flume

Larger Parshall flumes – 6-inches and above – require more robust flow conditioning. For these sizes, the perforated plates used above tend to flex under the force of the incoming flow and reinstallation under flow conditions can be difficult. As a result, Openchannelflow offers tranquilizing racks to condition flow in larger Parshall flumes.

flume tranquilizing rack for energy dissipation and flow conditioning

Tranquilizer racks are vertical racks that are slid down into the channel or end adapter upstream of the flume. Like the perforated baffle plates above, tranquilizer racks are typically used in groups of two or more, with the first rack disrupting the flow and the second rack straightening it out before it enters the converging section of the Parshall flume. Tranquilizing racks are fabricated from 304 or 316 stainless steel and are secured to the flume or upstream channel through the use of bolts or screws through the top mounting plate. When more than one tranquilizing rack is used, the Parshall flume inlet end adapter usually has to be lengthened to accommodate the installation of the racks.

In-flume energy absorbers are somewhat different than those used in Energy Absorbing Manholes. In an Energy Absorbing Manhole, the energy absorber is a flat plate set perpendicular to the flow stream. Flow hits the plate, is disrupted, and then rolls around the sides of the plated and then is smoothed and directed out of the manhole by the cheek walls. No attempt is made to minimize upstream splashing or surface turbulence as the flow is piped out of the manhole.

flume mounted energy absorber upstream of tranquilizing racks

For an in-flume energy absorber, surface turbulence is more of a concern as the energy absorber is installed upstream of the Parshall flume’s converging inlet section. While still a baffle plate perpendicular to the flow, the in-flume energy absorber is usually designed to with a top splashguard to reduce turbulence as the flow stream is disrupted.

Like perforated baffle plates, in-flume energy absorbers are intended to slide down into cavity molded into the sidewalls of the inlet end adapter. Also, they are typically used in conjunction with tranquilizing racks – with the energy absorber disrupting the flow and then the tranquilizing racks conditioning it prior to it entering the flume. As a result, the inlet end adapter is usually lengthened to accommodate the flow conditioning accessories.

Remember that in-flume flow conditioning accessories usually are not suitable for use on flows with a sanitary component or ragging solids. Other fine solids, such a powder coating waste, may pass through in-flume flow conditioners, although this is not guaranteed. Further, the potential need to remove and clean the flow conditioners generally precludes their use in Packaged Metering Manholes unless the inlet end adapter can be moved so that it is inside the manhole barrel.