1 PETROLEUM ENGINEERING DEPARTMENT FLOW LOOP EXPERIMENT EXPERIMENT #T-3 FLOW METERING2 OBJECTIVES - To study the concepts of flow metering. - To study the principles of different flow meters THEORY Flow metering is one of the most essential parts of any production operation. Even small errors in measuring the flow on a major pipeline will cause a major loss to either the seller or the buyer. Thus, it is in everyone's interest to meter the flow with the highest accuracy. Unfortunately, the techniques of metering are not always 100% accurate. Variations exist between the different techniques used. These techniques will have their optimum accuracy at different operating conditions and flow levels. Each flow meter is based on different operating principles: Mass Flow Meters (Micro Motion Flow Meter) In this meter, the mass of the fluid is measured directly under the assumption that the mass of the fluid is a property, which can be considered to be independent of changing fluid composition and environmental conditions. This is considered to be an advantage over other flow meters that measure fluid volume, since fluid volume is highly dependent on the composition and the environment. The Micro Motion meter operates by application of Newton's second law of physics, force equals mass times the acceleration (F = ma). It uses a tube that is vibrated up and down by a magnetic coil. Fluid is flowing through the tube. Fluid moving at the inlet side of the tube will be accelerated, while fluid moving at the outlet side will be decelerated. The mass of the fluid will resist the acceleration and the deceleration and will therefore exert a force on the tube. The force from the accelerated fluid will have the opposite direction of the decelerated fluid, but will be of the same magnitude. These two opposite forces will cause a twist on the tube. The angle of twist will be linearly proportional to the mass rate of the flowing fluid. When the tube reverses the direction, the accelerations, forces, and the tube twist in the reverse direction. The twist angle is again proportional to the mass flow rate. The twist is measured as time differences by two sensors, one on the right side and one on the left side. These two signals are analog voltage signals and will then be converted into standard analog and frequency outputs capable to be read and interpreted by standard data acquisition equipment. This meter is widely applicable for measurement of liquid flows. Application to gases is more limited in that the density of low-pressure gases is often insufficient to accurately operate the unit. Advantages of this unit, besides direct mass flow output, includes wide range of operating conditions, and applicability to non-homogeneous fluids.3 Figure 1. Mass flow meter (coriolis flow meter) Positive Displacement Meters The definition of a positive displacement meter by ASME states that the fluid to be measured has to pass through the meter in successive and more or less completely isolated quantities, by alternately filling and emptying compartments or cavities of fixed volume. Common types of displacement meters are: Bellows (Diaphragm), Geared or Lobed Impeller (Rotary), Sliding or Rotating Vane (Vane) and Liquid Sealed Drum (Wet). These meters are designed to measure the volumes of the fluids. An example is the Roots Positive Displacement Meter, which uses the rotary positive displacement operating principle. This system has two impellers; the bottom impeller rotates in a counterclockwise direction toward the horizontal position, fluid enters the space between the impeller and the cylinder. When bottom impeller reaches the horizontal position, a definite volume of fluid is contained in the bottom compartment. As the impeller continues to turn, the volume is discharged. Concurrently, the top impeller rotating in the opposite direction has trapped a definite volume in its horizontal position, confining another known volume of fluid. The process is repeated four times for each complete revolution of the impeller shafts. Flow of fluid creates the rotation movement of the impellers. The volume of fluid passing through the displacement meter could be calculated from the rotation speed of the impellers and the known volume between the cylinder and the impellers. However, the common method of calibrating a displacement meter is to flow a known volume of fluid through the meter and measure the rotation frequency. An important thing to remember is that the displacement meter will measure the in-situ volume. It is therefore necessary to correct for pressure and temperature when measuring gas. Liquid can in most cases be considered to be incompressible.Figure Turbine Meters The Turbine meters use a rotating turbine to measure the fluid flow. The performance of turbine flow meters is based on two basic assumptions: first, the rotor rotation varies linearly with the average fluid velocity and second, the volumetric flow rate is proportThe turbine is kept in a continuous rotation by the fluid streamdisplacement meter, the turbine meter does not break the stream into nominally discrete segof a secondary element, which is essentially a revolution counter, the meter measures the total distance of travel of the fluid past the primary device. With a known crossto units of total volume that has passed. Alternately, turbine speed can be converted into flow rate readings. In other words, the motion of the rotor vanes is sensed by a pickelectrical voltage signal. The frequency of the pulsingthe pulses relates to the total flow. The relationship between the number of pulses produced by the turbine flow meter and the volume of fluid passing through it is called the meterdetermined by individual calibration. The common method of calibration is to displace a known volume of fluid through the turbine over a known period of time.The advantage of the turbine meter is the temperature range. The turbine meter can be converted to measure mass flow by addition of compensating equipment. The disadvantage is that the turbine meters tend to be inaccurate at low flow4 Figure 2. Positive displacement meter ine meters use a rotating turbine to measure the fluid flow. The performance of turbine flow meters is based on two basic assumptions: first, the rotor rotation varies linearly with the