TURBINE FLOW METER
CONSTRUCTION AND WORKING
Turbine flow meter is used for measurement of liquid, gas at very low flow rates. It works on the basic principle of turbine which consists of a multi bladed rotor (turbine wheel) mounted at right angles to the axis of the flowing fluid. The flowing liquid imparts a force on the blade which causes the rotation of the rotor. At steady rotational speed, the speed of the rotor is directly proportional to the fluid velocity and hence the volumetric flow rate. The speed of turbine wheel is picked up by a magnetic pickup coil which is fitted outside the metering housing.
The turbine flow meter is actually a pulse generator that consists of a precision turbine wheel mounted on bearings inside a housing, which is connected to a pipe and magnetic pickup coil. The velocity of the fluid flow imparts a force to the turbine blades; which rotate at a speed that is proportional to the flow rate. As each turbine blade passes the magnetic pickup coil, one alternating current pulse can be totaled, subtracted, or manipulated by a digital technique into an analog output signal. This type of flow meter can handle only clean fluids, although modified types can also handle slurries.
As each rotor blade passes the magnetic pickup coil, it generates a voltage pulse which is a measure of the flow rate and total number of pulses gives a measure of total flow rate.
The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to have all the flow traveling around it.
The turbine wheel is set in the path of a fluid stream. The flowing fluid impinges on the turbine blades, imparting a force to the blade surface and setting the rotor in motion. When a steady rotation speed has been reached, the speed is proportional to fluid velocity.
Turbine flow meters are used for the measurement of natural gas and liquid flow. Turbine meters are less accurate than displacement and jet meters at low flow rates, but the measuring element does not occupy or severely restrict the entire path of flow. The flow direction is generally straight through the meter, allowing for higher flow rates and less pressure loss than displacement-type meters. They are the meter of choice for large commercial users, fire protection, and as master meters for the water distribution system. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system. Turbine meters are generally available for 4 to 30 cm ( 1 1⁄2–12 in) or higher pipe sizes. Turbine meter bodies are commonly made of bronze, cast Iron, or ductile iron. Internal turbine elements can be plastic or non-corrosive metal alloys. They are accurate in normal working conditions but are greatly affected by the flow profile and fluid conditions.
Fire meters are a specialized type of turbine meter with approvals for the high flow rates required in fire protection systems. They are often approved by Underwriters Laboratories (UL) or Factory Mutual (FM) or similar authorities for use in fire protection. Portable turbine meters may be temporarily installed to measure water used from a fire hydrant. The meters are normally made of aluminum to be lightweight, and are usually 7.5 cm (3 in) capacity. Water utilities often require them for measurement of water used in construction, pool filling, or where a permanent meter is not yet installed.