310201i Variable Area Flowmeters, Weirs, Flumes And Flow Switches

Question 1

True or false
When the flow rate through a variable area flowmeter increases, the differential pressure also increases

Answer 1

False
Differential pressure remains constant as the flow increases

Question 2

List the three main forces that act on a variable area flowmeter float

Answer 2

1. Flow force
2. Buoyancy
3. Weight

Question 3

What is the major disadvantage to the variable area flowmeter?

Answer 3

Any change in density or viscosity affects the accuracy of the flowmeter

Question 4

True or false
In a variable area flowmeter, the fluid composition must not differ from the fluid the flowmeter was designed for because different fluids have different densities

Answer 4

True

Question 5

List three methods of ensuring the float in a variable area flowmeter remains centered during operation

Answer 5

1. Slotted float head
2. Moulded alignment grooves
3. Guided rod

Question 6

What are the straight pipe requirements for installing a variable area flowmeter and why?

Answer 6

There are no straight pipe requirements because the upstream piping configurations do not affect variable area flowmeters

Question 7

What modifications must be made to a variable area flowmeter to have it function in a horizontal piping system?

Answer 7

Spring load the float within the tube

Question 8

Water hammer can cause damage to variable area flowmeters
What is water hammer, and what causes it?

Answer 8

Water hammer is when liquid flow suddenly stops and sudden pressure is created
This occurs with quick-closing and solenoid operated valves

Question 9

What is the flow rate and accuracy for an industrial grade variable area flowmeter that measures a gas flow?

Answer 9

Question 10

Weirs are classified according to the shape of their opening
List three common weir designs

Answer 10

1. Rectangular
2. Trapezoidal (or Cipolletti)
3. Triangular (or V-Notch)

Question 11

What is the general relationship between fluid height and flow rate using a rectangular weir with lateral contraction?

Answer 11

A 1% change in flow rate rrsults in a 0.7% change in fluid height

Question 12

In a paddle flow switch, if the paddle length decreases, the flow necessary for switch activation increases or decreases?

Answer 12

Increases

Question 13

When choosing a paddle flow switch for a process application where low rates can be expected, what is the most suitable design?

Answer 13

The folding circular paddle switch

Question 14

In a thermal slow switch, what happens to the resistance of the reference RTD sensor as the flow rate increases (assuming no change in fluid temperature)

Answer 14

The resistance of the reference sensor stays the same

Question 15

Describe the operation of a thermal flow switch

Answer 15

Operates on the principle of heat transfer
Consists of two separate RTD’s
One RTD is the reference RTD that measures the temperature of the fluid where the probe is immersed
The second RTD is self-heated at a constant power, this creates a temperature difference between the two RTD’s
The reference sensor ensures that a change in process temperature has no effect on the accuracy of the measurement
Varying the setpoint changes the power to the heated probe
The RTD’s in the sensor probes are part of a wheatstone bridge sensing circuit
The voltage output from the wheatstone bridge circuit feeds electronic circuits within the flow switch that are responsible for changing the output contacts of the flow switch

Question 16

Ehat are the four basic shapes that rotameter floats come in?

Answer 16

Question 17

What are the three major components of a variable area flowmeter?

Answer 17

1. The float
2. The metering tube
3. The scale