Parshall flume installation

Where the channel is wider than the inlet of the Parshall flume, wing walls should be formed to smoothly direct the flow into the flume. The inlet wing walls should be of a constant radius and should end tangent to the inlet walls of the flume. When connecting to inlet piping, observations have shown that the pipe should be straight and without bends for at least 15 pipe diameters.

Where a Parshall Flume must be set above the floor of a channel, a rise:run slope should be formed into the flume. Slopes greater than this should be avoided as they can cause turbulence as the flow separates at the junction of the ramp and the inlet of the flume. When flow out of the Parshall flume is returning to a channel or pipe, the EPA recommends that the channel be straight and unobstructed for throat widths — although flow spilling freely off the end of the flume can eliminate this requirement.

Unless the site can accommodate this change in elevation naturally, either the downstream channel will need to be modified not always possible or the flume will need to be elevated above the channel floor. If the flume is to be elevated above the channel floor, it is important to make sure that the upstream channel banks are high enough to contain the increased depth created by the installation and elevation of the flume.

This change in upstream depth may necessitate that a flume larger than otherwise might be selected be used in order to reduce the increase in upstream depth cause by the installation. Depending upon the flume size selected and the downstream conditions, the possibility of submergence may also require that a Parshall flume be elevated. While this may seem obvious, the site should be readily accessible by operators.

For remote sites, this may mean access by car or truck; for plants it may be locating the flume where piping or other equipment. Even in good installation, the flume is still need to accessed periodically as part of regular maintenance procedures, to calibrate a secondary flow meter, or to take a visual flow reading.

In smaller Parshall flumes the cross-sectional width of the flume at the point of measurement can be quite small so access and natural lighting can come into play when trying to make manual readings or flow meter calibrations.

In siting a Parshall flume, some thought should also be given to the flow meter that will be used in conjunction with the flume and its particular requirements. Flow meters should be located as close as is practical to the flume so that one operator can both read level in the flume and then calibrate the flow meter.

Industrial applications are notorious for having a disconnect when it comes to flume and flow meter locations. Submerged flow conditions are usually avoided to allow use of the standard discharge tables and depth measurements at Ha only. Small Parshall Flume Installation When setting the flume in concrete, pay attention to the following points: The flume must be installed at the elevation specified by the engineer.

The flume must be installed with the flume's crest The flume must not float out of position due to grout pressure. The flume's internal dimensions must not be distorted due to grout pressure. Very small Parshall flumes can be set in place as follows: All-thread rods can be embedded in the concrete below the flume, with the rods aligning with the anchor clips on the flume's exterior.

Either grout the rods in place below the flume or drill holes and epoxy them in place. After the all-thread rods are firmly anchored in place, the flume can then be locked in place and leveled by using a nut and washer on both sides of the anchor clips. The flume can then be grouted in place. The first pour of grout should just cover the bottom of the flume and allowed to set before additional pours are made.

The finished surface should be sloped toward the flume so that any water will drain back into the flume. Large Parshall Flume Installation The positioning procedure described above can also be used to secure larger flumes in the correct position. Large flumes have large flat bottoms that require particular attention during installation. The buoyant forces of grout can cause a flume to float during installation. This usually results in less than optimum dimensional accuracy at the metering station.

The anchor clips on the flume exterior are designed to secure the flume against the grout once it is cured. They will not prevent a flume from floating during installation. Consideration should be given to the following suggestions concerning flume installation. Channel or vault should be designed with adequate clearance at the sides to allow grout placement and worker access for chaining during installation.

The more non-standard Parshall flumes you use, the more time consuming the process, which can cause big problems for time sensitive operations. One of the biggest advantages of using a Parshall flume in your water management system is the inclusion of a drop floor, which makes this flume adept at handling flows with varying rates. Unfortunately, this advantage is also one of the Parshall flumes biggest weaknesses depending on the needs of your water management system.

Because of the rapid change in elevation of the drop floor, it is extremely difficult to install Parshall flumes in existing channels. While this flaw can be overcome with a little preparation, it can add extra cost and time to flume installation projects, which many water management operations simply cannot afford. Perhaps the biggest drawback of Parshall flumes has to do with the previously mentioned issue of scale models. As discussed, smaller Parshall flumes are not made to scale because there is only one standard Parshall flume size.