Flow rate -- Summary / QUIZ

We started this section off by observing that flow rate could be measured either as mass flow rate or volume flow rate. We also observed that the material being measured could be in solid, liquid, or gaseous form. In the case of solids, we quickly found that this could only be measured in terms of the mass flow rate. However, in the case of liquids and gases, we found that we have the option of measuring either mass flow rate or volume flow rate. Of these two alternatives, we observed that mass flow measurement was the more accurate.

Before proceeding to look at flow measurement in detail, we had a brief look at the differences between laminar flow and turbulent flow. This taught us that the flow rate was difficult to measure in turbulent flow conditions and even in laminar flow at high velocities. Therefore, as far as possible, the measurement was made at a point in the flow where the flow was at least approximately laminar and the flow velocity was as small as possible.

This allowed us to look at flow-measuring instruments in more detail. We started with mass flow and observed that this could be measured in one of three ways-conveyor-based methods, Coriolis flowmeter, and thermal mass flowmeter. We examined the mode of operation of each of these and made some comments about their applicability.

Moving on, we then started to look at volume flow rate measurement and worked progressively through a large number of different instruments that can be used. First, we looked at obstruction devices. These are placed in a fluid-carrying pipe and cause a pressure difference across the obstruction that is a function of the flow rate of the fluid. Various obstruction devices were discussed, from the commonly used inexpensive but less accurate orifice plate to more expensive but more accurate devices such as the Venturi tube, flow nozzle, and Dall flow tube.

After looking at flow obstruction devices, we looked at a number of other instruments for measuring volume flow rate of fluids flowing in pipes, including the variable area flowmeter, positive displacement flowmeter, turbine flowmeter, electromagnetic flowmeter, vortex shedding flowmeter, and, finally, ultrasonic flowmeters in both transit time and Doppler shift forms. We also looked briefly at several other devices, including gate-type meters, laser Doppler flowmeter, and thermal anemometer. Finally, we also had a brief look at measuring fluid flow in open channels and observed three ways of doing this.

We rounded off our discussion of flow measurement by looking at intelligent devices. We observed that these bring the usual benefits associated with intelligent instruments, including improved measurement accuracy and extended measurement range, with facilities for self diagnosis and self-adjustment also being common. This led on to some discussion about the most appropriate instrument to use in particular flow measurement situations and applications out of all the instruments covered in the section.

We then concluded the section by considering the subject of flowmeter calibration. These calibration methods were considered in three parts. First, we looked at the calibration of instruments measuring mass flow. Second, we looked at the calibration of instruments measuring the volume flow rate of liquids. Finally, we looked at the calibration of instruments measuring the volume flow rate of gases.

QUIZ

1. Name and discuss three different kinds of instruments used for measuring the mass flow rate of substances (mass flowing in unit time).

2. Instruments used to measure the volume flow rate of fluids (volume flowing in unit time) can be divided into a number of different types. Explain what these different types are and discuss briefly how instruments in each class work, using sketches of instruments as appropriate.

3. What is a Coriolis meter? What is it used for and how does it work?

4. Name four different kinds of differential pressure meters. Discuss briefly how each one works and explain the main advantages and disadvantages of each type.

5. Explain how each of the following works and give typical applications: rotameter and rotary piston meter.

6. How does an electromagnetic flowmeter work and what is it typically used for?

7. Discuss the mode of operation and applications of each of the following: turbine meter and vortex-shedding flowmeter.

8. What are the two main types of ultrasonic flowmeters? Discuss the mode of operation of each.

9. How do each of the following work and what are they particularly useful for: gate-type meter, jet meter, Pelton wheel meter, laser Doppler flowmeter, and thermal anemometer.

10. What is an open channel flowmeter? Draw a sketch of one and explain how it works.

11. What instruments, special equipment, and procedures are used in the calibration of flowmeters used for measuring the flow of liquids?

12. What instruments, special equipment, and procedures are used in the calibration of flowmeters used for measuring the flow of gases?

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