Water Flow Metering and Measurement

water flow measurement devices
Water flow measurement device comparison (click for larger view)
Water is commonly measured and sold in volumetric measurements, which allows for lower-cost metering options. The specific measurement technology chosen will depend on a number of factors including, but not limited to, current design, budget, accuracy requirements, resolution, minimum flow rate, potable versus non-potable (or at least filtered versus non-filtered water), range of flow rates, and maximum flow rate.

Volumetric water measurement can be broken down into three general operating designs:
  • Positive displacement
  • Differential pressure
  • Velocity

Positive Displacement – Nutating-Disk Flow Meter

Nutating-disk flow meters are the most common meter technology used by water utilities to measure potable-water consumption for service connections up to 3-inch. The nutating-disk flow meter consists of a disk mounted on a spherically shaped head and housed in a measuring chamber. As the fluid flows through the meter passing on either side of the disk, it imparts a rocking or nutating motion to the disk. This motion is then transferred to a shaft mounted perpendicular to the disk. It is this shaft that traces out a circular motion – transferring this action to a register that records flow.

There are a variety of differential pressure devices useful for water metering; two of the more common devices include orifice flow meters and venturi flow meters.

Differential Pressure – Orifice Flow Meter

The orifice element is typically a thin, circular metal disk held between two flanges in the fluid stream. The center of the disk is formed with a specific-size and shape hole, depending on the expected fluid flow parameters (e.g., pressure and flow range). As the fluid flows through the orifice, the restriction creates a pressure differential upstream and downstream of the orifice proportional to the fluid flow rate. This differential pressure is measured and a flow rate calculated based on the differential pressure and fluid properties.

Differential Pressure – Venturi Flow Meter

The venturi flow meter takes advantage of the velocity-pressure relationship when a section of pipe gently converges to a small-diameter area (called a throat) before diverging back to the full pipe diameter. The benefit of the venturi flow meter over the orifice flow meter lies in the reduced pressure loss experienced by the fluid.

The velocity measurement technologies described in this section include the turbine flow meter, vortex-shedding flow meter, and ultrasonic flow meters.

Velocity – Turbine Flow Meter

A multi-blade impellor-like device is located in, and horizontal to, the fluid stream in a turbine flow meter. As the fluid passes through the turbine blades, the impellor rotates at a speed related to the fluid’s velocity. Blade speed can be sensed by a number of techniques including magnetic pick-up, mechanical gears, and photocell. The pulses generated as a result of blade rotation are directly proportional to fluid velocity, and hence flow rate.

Velocity – Vortex-Shedding Flow Meter

A vortex-shedding flow meter senses flow disturbances around a stationary body (called a bluff body) positioned in the middle of the fluid stream. As fluid flows around the bluff body, eddies or vortices are created downstream; the frequencies of these vortices are directly proportional to the fluid velocity.

Velocity – Ultrasonic Flow Meters

There are two different types of ultrasonic flow meters, transit-time and Doppler-effect. The two technologies use ultrasonic signals very differently to determine fluid flow and are best applied to different fluid applications. Transit-time ultrasonic flow meters require the use of two signal transducers. Each transducer includes both a transmitter and a receiver function. As fluid moves through the system, the first transducer sends a signal and the second receives it. The process is then reversed. Upstream and downstream time measurements are compared. With flow, sound will travel faster in the direction of flow and slower against the flow. Transit-time flow meters are designed for use with clean fluids, such as water.

Doppler-effect ultrasonic flow meters use a single transducer. The transducer has both a transmitter and receiver. The high-frequency signal is sent into the fluid. Doppler-effect flow meters use the principal that sound waves will be returned to a transmitter at an altered frequency if reflectors in the liquid are in motion. This frequency shift is in direct proportion to the velocity of the liquid. The echoed sound is precisely measured by the instrument to calculate the fluid flow rate.

Because the ultrasonic signal must pass through the fluid to a receiving transducer, the fluid must not contain a significant concentration of bubbles or solids. Otherwise the high frequency sound will be attenuated and too weak to traverse the distance to the receiver. Doppler-effect ultrasonic flow meters require that the liquid contain impurities, such as gas bubbles or solids, for the Doppler-effect measurement to work. One of the most attractive aspects of ultrasonic flow meters is they are non-intrusive to the fluid flow. An ultrasonic flow meter can be externally mounted to the pipe and can be used for both temporary and permanent metering.

For more information on any flow application, visit http://www.teco-inc.com or call (504) 833-6381.


Natural Gas Flow Metering and Measurement


Natural gas is a hydrocarbon gas mixture consisting primarily methane, but includes a host of other chemical components. Accurate natural gas flow measurement usually requires the measurement of the fluid’s temperature and pressure in addition to flow. Additional constraints on natural gas measurement may include the physical space available or possibly configuration and weight of the metering system. Some of the fluid metering technologies require specific lengths of pipe, both upstream and downstream of the meter for proper function.

Before any technology decisions are made, discussions with equipment vendors and/or design engineers are recommended to ensure proper technology selection and installation design.

Depending on the application, flow rate, installation access, and desired accuracy, there are a number of technology options for natural gas metering. In general, measurement of natural gas volumetric flow rate is represented in standard cubic feet per hour (scfh) or per minute (scfm). The actual mass of gas flowing past a point of measurement changes with its temperature and pressure. Density changes resulting from temperature and pressure differences can result in differences between the energy content of similar volumes of the gas. To equalize the effect of density variations when metering gas, conditions are referenced against standard temperature and pressure conditions, hence standard cubic feet (scf) instead of actual cubic feet (acf). Gas flowmeters must compensate for density differences between standard conditions and actual conditions to accurately define standard flow rates.

The most common volumetric gas metering devices fall into one of the following categories:
  • Positive displacement
  • Differential pressure
  • Velocity 
In most applications, gas flowmeters are installed downstream of pressure regulation devices and the meters are then calibrated to that pressure. Natural gas meters may include options for temperature and pressure compensation.


POSITIVE DISPLACEMENT


A positive displacement meter functions by the fluid physically displacing the measuring mechanism and this displacement becomes the metered value. Of relevancy to natural gas measurement, the two predominant technologies are the diaphragm meter (most common) and the rotary meter. In each case, the volume of gas for measurement physically impinges on a measuring element (flexible diaphragm or rotary blower) to increment a recording dial or other output. The primary advantage of positive displacement flow meters is there are no straight-run piping requirements to establish a flow pattern that can be accurately metered. The primary disadvantage of positive displacement meters is higher pressure drops experienced across the meter at peak flow rates.


DIFFERENTIAL PRESSURE


There are multiple types of differential pressure meters: orifice flow meter, venture flow meter, and annubar flow meter. All differential pressure meters rely on the velocity-pressure relationship of flowing fluids for operation.

Orifice Flow Meter 
The orifice element is typically a thin, circular metal disk held between two flanges in the fluid stream. The center of the disk is formed with a specific-size and shape hole, depending on the expected fluid flow parameters (e.g., pressure and flow range). As the fluid flows through the orifice, the restriction creates a pressure differential upstream and downstream of the orifice proportional to the fluid flow rate. This differential pressure is measured and a flow rate calculated based on the differential pressure and fluid properties.

Venturi Flow Meter
The venturi flow meter takes advantage of the velocity- pressure relationship when a section of pipe gently converges to a small-diameter area (called a throat) before diverging back to the full pipe diameter. The benefit of the venturi flow meter over the orifice flow meter lies in the reduced pressure loss experienced by the fluid.

Annubar Flow Meter
The annubar flow meter (a variation of the simple pitot tube) also takes advantage of the velocity-pressure relationship of flowing fluids. The device causing the change in pressure is a pipe inserted into the natural gas flow.


VELOCITY


There are multiple types of velocity meters: turbine flow meter, vortex-shedding flow meter, and fluid oscillation flow meter. Velocity meters determine fluid flow by measuring a representation of the flow directly. Because the fluid’s velocity is measured (i.e., not the square-root relationship to determine velocity as with differential pressure meters), velocity meters can have better accuracy and usually have better turndown ratios than other meter types.

Turbine Flow Meter
A multi-blade impellor-like device is located in, and horizontal to, the fluid stream in a turbine flow meter. As the fluid passes through the turbine blades, the impellor rotates at a speed related to the fluid’s velocity. Blade speed can be sensed by a number of techniques including magnetic pick-up, mechanical gears, and photocell. The pulses generated as a result of blade rotation are directly proportional to fluid velocity, and hence flow rate.

Vortex-Shedding Flow Meter
A vortex-shedding flow meter senses flow disturbances around a stationary body (called a bluff body) positioned in the middle of the fluid stream. As fluid flows around the bluff body, eddies or vortices are created downstream; the frequencies of these vortices are directly proportional to the fluid velocity.

Fluid Oscillation Flow Meter
A fluid oscillation flow meter uses sensor technology to detect gas oscillations, which corresponds to the flow rate through the meters internal throat design.

For more information on any flow measurement requirement, visit Thompson Equipment (TECO) at http://www.teco-inc.com or call 800-528-8997 for immediate service,