As waterflood operations increase in size and number, there becomes an ever increasing effort on the part of operators to obtain equipment that will permit more efficient utilization of manpower and thereby reduce the cost or operation. The use of automation techniques is gaining acceptance in this field.
One or the basic instruments necessary to a waterflood project is a flow meter, the function of which is to measure the amount and/or rate of water injected. Turbine type flow meters are one type currently being used for this purpose. This paper presents information regarding turbine meters and how these devices are being used in conjunction with automated projects.
Metering injection water presents several specific problems that are not normally encountered in other liquid metering applications. One of these is the highly corrosive properties of many of the waters used. Another is the presence of sand fines which is commonly encountered. Usually, it is desirable to install individual flow meters at each wellhead, and this makes it necessary to travel considerable distance to obtain meter readings.
The use of turbine flow meters has presented a means of overcoming these problems. Corrosion problems can be solved by proper selection of materials of construction. Turbine meter clearances are large enough to permit the passage of sand fines without detrimental effects. Output signals from turbine meters can be transmitted to centrally located stations to facilitate accumulation of data. Flexibility of read but equipment permits design to fit the requirements of any project or the desires of any operator. Central read-out stations combined with remote operated actuators make a trip to the wellhead necessary only in emergencies.
The turbine flow meter [see Fig. 1] consists of a helically bladed rotor supported on bearings in the flow stream. Flow through the meter causes the rotor to turn at a speed proportional to the fluid velocity. The magnetic pickup mounted outside the flow stream contains a permanent magnet imbedded in a coil of wire. The rotor blades passing through the magnetic field induce alternating current in the coil with one pulse generated by each blade. The frequency of these pulses is a measure of the fluid velocity through the meter, and since the cross sectional area is a constant value, this represents volumetric flow rate. The total number of pulses generated during a given time interval represents the accumulated total flow that has passed through the meter during that time.
The output signal created resembles a sine wave with peak to peak amplitude between 30 and 1,000 millivolts. This signal can be transmitted directly using two-conductor shielded cable for considerable distance. The use of amplifiers permits signal transmission to any desired distance.
Turbine flow meters are calibrated by pumping a known volume of water through the meter at a constant flow rate and registering the number of pulses produced. The meter constant or calibration factor is obtained by dividing the pulses generated by the volume throughput. The flow rate range of the meter is the interval over which the calibration factor remains constant. Fig. 2 illustrates a typical turbine meter calibration curve.
Turbine meters contain no packing or seals and can easily be constructed to withstand high pressures. Corrosion problems are overcome by using corrosion resistant materials such as stainless steel, aluminum-bronze, nickel and tungsten carbide. They can be installed vertically or horizontally and are suitable for bi-directional flow.