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Frequently Asked Flume Questions
Question: What are the advantages and disadvantages of the various type of flumes?
Answer: Palmer Bowlus flumes and their close cousins the proprietary Accura-flo Low Flow flumes are the most convenient for measuring flows in pipes, particularly sewer applications. They are also relatively insensitive to downstream conditions and may be used as in line structures in pipes as flat as 0.25 % slope. Trapezoidal flumes are quite similar to Palmer Bowlus flumes and as the name implies lend themselves well to use in trapezoidal channels such as irrigation ditches and canals. They can be used to measure flows in pipes provided that an adequate transition from the circular to the trapezoidal cross section is provided. Trapezoidal flumes do not have a raised sill and therefore pass sediment extremely well. Like the Palmer Bowlus they are insensitive to downstream conditions. Cutthroat flumes are less familiar to most users. Their properties are similar to the Trapezoidal flumes. Like the Trapezoidals they do not have a raised sill and pass sediment well. They are rectangular in cross section and therefore are best used in rectangular channels. The Parshall flume is the most widely recognized flume type it features good resolution over a wide range but achieves this at the cost of a higher head loss than the flumes mentioned above. The Parshall is more sensitive to downstream conditions and care must be taken to set the flumes crest high enough to avoid an excessive submergence ratio. The Parshall is an excellent choice for installation in rectangular channels with sufficient depth to contain the somewhat higher heads generated by this flume. Parshall flumes are frequently seen at the head works to a sewer treatment plant. The Parshall is also the flume of choice for most applications in natural channels. The H-flume was developed by the US Forest service to measure watershed runoffs. The flume must be set so that the discharge is free. As a consequence the head loss is even greater than the Parshall. The V shaped throat allows the flume to resolve a wide flow range and so it is a good choice for seasonal flows that vary widely. The H flume is also excellent at passing sediment.
Question: How do I select the best flume for my application?
Answer: A number of factors go into the selection of the primary element in an open channel system. Your first step is to evaluate and select a site. The shape, size, slope and condition of the channel or pipe are important factors to make note of. It is also important that the channel be straight for a relatively long distance upstream of the point of measurement. Also note whether the flume will be installed in a section of channel or at the end of a channel with a free drop off. Once you have picked a site you need to estimate the range of flow to be measured. The best flume choice will then be the flume that meets the following criteria:
- Select a flume whose inlet cross section is compatible with the channel. This will minimize the need for costly transitions.
- Select a flume with an energy threshold high enough to operate correctly at the velocities and flow rates expected. Velocity is a function of channel slope and roughness. Generally flumes work best in channels with slopes between one half of one percent and two percent slope. Rougher channels will allow a flume to tolerate slightly higher slopes, while smoother channels will allow for slightly shallower slopes.
These first two criteria will put you into a group of flumes - Parshall, Palmer Bowlus, Trapezoidal etc. For example if you have a six-inch pipe flowing partially full at 1% slope, you would consider a Palmer Bowlus type flume because its "U" shaped inlet cross section is the most compatible with a round pipe. Consideration of the slope would confirm the choice of a Palmer Bowlus because this slope will produce velocities compatible with the Palmer Bowlus flume. You would also have narrowed the field to the six-inch size Palmer Bowlus flume because it matches the pipe size you have.
- Confirm that the flume selected has sufficient resolution to measure the anticipated minimum flow accurately.
- Confirm that the flume selected has sufficient capacity to measure the maximum expected flow.
For example if the maximum anticipated flow were 250 gallons per minute this would exceed the capacity of the 6-inch Palmer Bowlus flume. It would also be a red flag indicating that the pipe may be a bit small for the flow rate. Some options would be:
- Replace a section of pipe with an open channel and install a 3-inch Parshall flume, taking care to set the Parshall flume high enough to avoid excessive submergence caused by back up in the downstream pipe.
- Change the pipe to an 8-inch diameter pipe and use an 8-inch Palmer Bowlus.
- If you are at the end of the pipe with a free drop consider using an H-type flume.
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Question: What is the minimum distance of "straight channel" required upstream of a flume?
Answer: There is no hard and fast rule governing the amount of upstream straight run of channel. Any run of channel that produces smooth streaming flow with a uniform velocity distribution prior to reaching the flume is long enough. How long this channel will need to be depends to a large extent on how extreme the disruption in the channel is, for example water will return to normal flow conditions more quickly downstream of slight change in pipe diameter than after a 90 degree elbow. If your flow appears to be smooth and streaming, free of white water and visible surface boils at the point being considered for flume installation then the upstream channel is adequate. If you are designing a channel for a new installation a safe bet is to allow an upstream distance of straight channel equal to 25 times the maximum head anticipated at the flume. This much distance will allow the flow to overcome the effects of even extreme conditions. When that much distance is unavailable it may be necessary to modify the shape of the channel or add baffling to dissipate the energy.
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Question: What does the term submergence mean in the context of metering flumes?
Answer: The term 'submergence' as it relates to flumes does not mean that the flume is completely under water. Instead we talk about a submergence ratio. This is the ratio of tail water depth to the depth at the flumes upstream head measuring point relative to a horizontal reference plane. Submergence ratios are of particular concern with Parshall flumes. Parshall flumes can only tolerate submergence ratios of 50-70% depending on the size of the flume. For the Parshall the depths are both referenced to the crest of the flume. Palmer Bowlus, Trapezoidal and Cutthroat flumes are designed to tolerate higher submergence ratios and submergence is rarely an issue with these flumes. H-flumes have almost no tolerance to submergence and are usually installed with a free discharge.
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Question: Isn't the Parshall flume more accurate than other types of flumes like the Palmer Bowlus?
Answer: This is a persistent myth, the Parshall is better known than some of the other flume types and has a wider apparent range than flumes like the Palmer Bowlus. The wider apparent range is somewhat misleading and invites an apple and oranges type of comparison. The Parshall is intended for use in a truly open channel. On the other hand the Palmer Bowlus flume is designed to be used with a closed pipe that is flowing partially full. The range of the Palmer Bowlus is therefore limited by the capacity of the pipe. In situations where Parshall flumes are to be installed in channels fed and drained by closed pipes there range is also limited by the pipes capacity. Generally one flume is no more accurate than another is. The narrow throat section of a flume will produce a predictable change in depth in the upstream portion of the flume provided that the energy present in the flow stream is less than the energy threshold of the throat of the flume. Since energy is conserved it is possible to predict very accurately the depth that the water will take for a given flow rate. The when comparing two flumes, the flume showing the water the smallest cross section will produce the largest change in depth for a given incremental change in flow and hence have the best resolution.
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Question: What accuracy can I expect with an open channel flow system?
Answer: Flumes that are correctly manufactured and installed, are intrinsically accurate. A particular throat geometry will consistently produce a given change in depth at a given flow rate. The real issue is not accuracy but resolution. Therefore if resolution is expressed as a percentage of flow rate, flumes will measure higher flows with greater resolution than lower flows. For a given flume and flow meter combination it is not at all difficult to resolve flows to within +/- 5 % of rate over the range of 10 - 100% of maximum capacity. If one is careful setting up and calibrating the depth sensor, it is possible to achieve resolutions better than +/- 2% of flow rate over the same range. Below ten percent of maximum flow resolution of flow rate drops off quickly due to the fact that flow through a flume is not a linear function of depth.
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Question: What is the lowest flow rate I can measure with open channel flow techniques?
Answer: As a practical matter it is difficult to resolve open channel flows below about 5 gallons per minute. Most depth sensing techniques have a built in resolution of about +/- 0.01 feet and the most sensitive systems can only resolve depths to around +/- 0.005 feet. In addition the relationship between depth and flow through flumes begins to deviate from the theoretical rating when there is less than 0.1 foot of head available in the flume. When we apply these two facts to the smallest flumes in the various families of flumes we consistently come up against a practical lower limit around 5 gallons per minute.
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Question: What type of open channel flow meter is best to use with a metering flume?
Answer: There are a number of meters on the market that can be categorized by the type of sensor used to measure the depth. In general all of them work equally well with most flumes. The only exception being that some over head ultra-sonic transducer have a bean spread that is too wide for some of the very smallest flumes. Even this problem can be overcome by using a stilling well connected to the flume to provide a larger target. Flow meter selection is more dependent on the nature of the flow stream than on the flume itself. Bubbler systems have excellent resolution but can be clogged by debris, oil and grease. Overhead ultra-sonic transducers have the advantage of not contacting the flow stream but can have problems in applications with a lot of foam on top of the water. Capacitance probes, submersible pressure transducers and even float operated mechanical devices all have their niche. CSI manufactures a wide variety of mounting accessories to accommodate the various types of sensors.
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