Question

In a flow control loop in a process for which you are specifying a flow sensor, it is undesirable to insert any sensing device into the actual flow because of interference with the flow. Because of the corrosive nature and high pressure and temperature of the flow, only stainless steel pipes can be used anywhere in the system. The liquid is non-conductive. Recommend a flow sensor, explaining its operating principle.

Answer 1

Solution:

Given that for the measurement of the flow, the sensing device should not be inserted into the flow loop as it will lead to interference. Thus the flow sensor to be used should be non invasive type of flow sensor.
Also, the setup states that the flow is travelling through a stainless steel piping system and the liquid is non conductive.
Since the liquid is no conductive electromagnetic type of flow sensors cannot be used.
Thus, one of the possible type of sensors that can be used is ultrasonic flow sensor which is non invasive in nature and can detect high temperature and pressure flow of liquids with good accuracy.

Operating Principle:
Ultrasonic flowmeters use sound waves at utrasonic frequency to determine the velocity of a fluid flowing in a pipe.
There are 2 types of ultrasonic flowmeters: (a) Doppler Type (b) Transit Time Type

(a) Doppler Effect Type:

A Doppler effect type flow sensor has a unit mounted on the pipe consisting of a transmitter and a receiver. At no flow conditions, the frequencies of an ultrasonic wave transmitted into a pipe by the transmitter and its reflections received at the received from the fluid are the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid moves faster, the frequency shift increases linearly. The transmitter processes signals from the transmitted wave and its reflections to determine the flow rate.

(b) Transit Time Type

Transit time utrasonic flowmeters have 2 transducers (transduced 1 and 2 shown above) placed opposite to each other at an angle with the flow axis of the liquid. Transit time ultrasonic flowmeters send and receive ultrasonic waves between transducers in both the upstream and downstream directions in the pipe. At no flow conditions, it takes the same time to travel upstream and downstream between the transducers. Under flowing conditions, the upstream wave will travel slower and take more time than the (faster) downstream wave. When the fluid moves faster, the difference between the upstream and downstream times increases. The transmitter processes upstream and downstream times to determine the flow rate.