Flow measuring techniques will vary depending on the application flow type. There are two basic types of flow systems; closed channel, and open channel. A Closed Channel can be described as water flow through a completely filled pressurized pipe. Flow measurement is typically performed by inserting a mechanical meter, venture meter, magnetic meter within the pipe. A typical example of a closed channel flow is a city potable water line that is metered with a turbine meter.
The second type of flow type, Open Channel, is best described as, water that flows with a “free surface” typically in a non-pressurized (atmospheric) pipe or channel. Examples are rivers, irrigation/drainage ditches, canals, and for sanitary sewer. The most practical method for Open channel flow measurement is accomplished by the use of a hydraulic structure; flumes and weirs. These hydraulic structures enable flow calculation by measuring the water depth at a single point. And by using the structure’s associated equation or table, the flow rate can be calculated.
Open channels are used to conduct liquids in most sewer systems, sewage treatment plants, industrial waste applications, and irrigation systems.
There are three methods for automatically measuring open channel flow:
- Hydraulic Structures
- Area Velocity
- Slope-Hydraulic Radius
The most common method of measuring open channel flow is the hydraulic structures method. A calibrated restriction inserted into the channel controls the shape and velocity of the flow. The flow rate is then determined by measuring the liquid level in or near the restriction.
The restricting structures are called primary measuring devices. They may be divided into two broad categories–weirs and flumes.
A weir is an obstruction or dam built across an open channel over which the liquid flows, often through a specially shaped opening. Weirs are classified according to the shape of this opening. The most common types of weirs are the triangular (or V-notch) weir, the rectangular weir, and the trapezoidal (or Cipolletti) weir. The flow rate over a weir is determined by measuring the liquid depth in the pool upstream from the weir.
Weirs can be simple and inexpensive to build and install. Common materials of construction include metal, fiberglass and wood. However, they represent a significant loss of head, and are not suitable for measuring flows with solids that may cling to the weir or accumulate upstream from it.
Measurement of the flow rate in an open channel flume or weir can be performed manually by reading a single level measurement and calculating, or by the means of an automatic flow meter. The most common open channel meters are the Ultrasonic, Bubbler, & Pressure Transducer. The Ultrasonic Meter measures the time required for an acoustic pulse to travel from a transmitter to the liquid surface (where it is reflected) and returned to a receiver.
The Bubbler Meter consists of a bubbler tube that is anchored in the flow stream at a fixed depth, then. The tube supplies a constant bubble rate of pressurized air. The air pressure required to maintain the bubble rate is measured; this pressure is proportional to the liquid level.
The Pressure Transducer consists of a sealed pressure transducer submerged in the flow stream at a fixed depth. The pressured measured by the transducer is proportional to the liquid level.
A flume is a specially shaped open channel structure that temporarily modifies the flow through the channel to enable the measurement of the flow rate. Specifically, the flume restricts the channel area and/or changes the channel slope, resulting in an increased velocity and a change in the level of the liquid flowing through the flume structure. The flow rate through the flume can be determined by measuring the liquid depth at a specified point in the flume, and using the flume’s associated equation (head-flow rate relationship).
There are several popular flume types; Parshall, Palmer-Bowlus, and Manhole Flume. The most common flume is the Parshall Flume. The flow rate through a Parshall flume is determined by measuring the liquid level one third of the way into the converging section. Parshall flumes are designated by the width of the throat, which ranges from one inch to 50 feet. The throat width and all other dimensions must be strictly followed so that standard discharge tables can be used. Also, note the drop in the floor of the flume, which makes it difficult to install a Parshall flume in an existing channel.
Another popular flume is the Palmer-Bowlus Flume is designed to be installed in an existing channel with minimal effort. The flow rate through a Palmer-Bowlus flume is determined by measuring the liquid depth at a point one-half pipe diameter upstream from the flume throat. Palmer-Bowlus flumes are designated by the size of the pipe into which they fit. Standard sizes range from four to 42 inches. The dimensional configuration is not rigidly established for each flume size. However, a Palmer-Bowlus flume with a trapezoidal throat with a flat bottom has emerged as the standard design for circular pipes.
The Manhole Flume is a unique variation of the Parshall Flume. An advantage of the Manhole Flume is that it designed for easy installation in standard manholes and existing pipe sewer lines.
Flumes can be more expensive than weirs. However, flumes result in a lower head loss and are self-cleaning, requiring less maintenance than a weir.
[MANHOLE FLUME IMAGE]
The Area Velocity method is an open channel water measurement design that does not require the installation of a weir or flume. Instead, it can be used directly in a new or existing pipe channel. The area velocity method calculates flow rate by multiplying the area of the flow by its average velocity. This is often referred to as the continuity equation, Q=AxV.
[AREA VELOCITY IMAGE]
For convenience, most area velocity flow meters use a single sensor to measure flow rate. Doppler ultrasonics is used to measure average flow velocity, while an integral pressure transducer measures the level in the channel. The flow meter converts this level into the area of the flow based on the size and shape of the channel.
The main advantage of the area velocity method is that it can be used to measure flow under a wide range of conditions.
- Open Channel
- Full Pipe
- Reverse Flow
Various resistance equations are used to estimate flow rate based on measurements of the water surface slope, cross-sectional area, and wetted perimeter over a length of uniform channel. The most popular of these equations is the Manning formula:
K A R2/3 S1/2 Q = _____________ n
where: Q = flow rate A = cross sectional area of flow R = hydraulic radius (cross sectional area divided by wetted perimeter) S = slope of the hydraulic gradient n = roughness coefficient based on channel material and condition K = constant dependent upon units
[SLOPE-HYDRAULIC RADIUS IMAGE]
The cross-sectional area A and the hydraulic radius R are calculated based on the liquid depth, and the size and shape of the channel. The slope S is often estimated based on installation drawings of the channel. The roughness coefficient n is selected from standard references based on the material of construction of the channel, and its condition.
Given the size, shape, slope and roughness of the channel, an open channel flow meter can calculate flow rate using the Manning formula based on a measurement of the liquid depth. The Manning formula is not as accurate as the hydraulic structures and area velocity methods, but it can provide sufficient accuracy in some applications. In addition, no weir or flume is required.