Fluid Flow Measurement Q&A–(Part 1)

In this series I will try to sum up the main concepts, techniqus and standards used for flow measurement in industrial applications. Will not get in deep with math & physics .. just the main ideas/keys.

I depend mainly on the following 2 references:

1 - Lessons in industrial instrumentations by Tony Kuphaldt, which is a critical reference for any I&C and Automation Engineer.

2 - Measurement and Control Basics - Copy on google books

Hope this series to be useful ..

Q1. What are the main technologies used for Flow Measurement?

Above picture includes most of the types and technologies used in flowmetering. We could sum it up in the following:

• Basically there are 2 main types of flowmetering, Volumetric flowmetering (number of fluid volumes passing per unit time) and Mass flowmetering (number of fluid mass units passing per unit time). Both had many techniques branched into many flowmeters.
• Pressure based flowmeters: They are based on generating differential pressure in the flow stream using flow element .. And then measuring this DP, using it to calculate Q (volumetric Flowrate). Many flow elements could be used in this technique.
• Velocity based flowmeters: They are based on direct measurement for the fluid velocity, using it to calculate the Q (volumetric Flowrate). Many technologies are using this technique (Turbine, Vortex, Magnetic and Ultrasonic)
• Variable area flowmeters: Will be discussed later.
• Positive Displacement flowmeters: Will be discussed later.
• Mass Flowmeters: They are measuring directly mass flowrate.

We will talk later in detals about the basics of each flowmeter, its concept, pros and cons.

Q2. What is "Custody Transfer"?

“Custody transfer” is a measurement application where a fluid product is bought or sold by its mass. In simple words .. someone is selling, and someone is buying fluid quantities as part of a business transaction. Sometimes both buyer and seller operate and maintain their own instruments/equipments, while in other instances calibration and validation is done by third party.

Flowmetering accuracy here is mandatory (It is the money), so measurement is to be done in molecular quantity (M) not just volume (Q), which is why pressure and temperature compensation is mandatory for the "Custody transfer".

Q3. How to implement Pressure & Temperature compensation in the flow loop?

P&T compensation is needed when to use volumetric flowmeters only, while Mass flowmeters (Coriolis for example) is direct measuring the Mass.

The following photograph indicates an orifice plate with a differential Pressure Tx used to measure the flow of natural gas in a large turbine engine, while Temperature Tx is installed after it in the downstream:

As simplified in the left side .. Signals from TT & PDT are gathered and processed together as a final P&T compensated flow signal for the Mass flowmetering.

An alternative to multiple instruments installed on the pipe line, Multivariable Tx indicated below is used:

This approach enjoys the advantage of simpler installation over using multi-instruments.

Q4. Which flowmeter types are standardized for Custody Transfer?

American Gas Association (AGA) formalized the use of the following flowmeters for custody transfer application:

AGA report #3 is for “Orifice Flowmeter”

AGA report #7 is for “Turbine Flowmeter”

AGA report #9 is for “Ultrasonic Flowmeter”

AGA report #11 is for “Coriolis Mass Flowmeter”

AGA reports formalized the equations and regulate the usage of Pressure & Temperature compensation with the first 3 flowmeters, while Coriolis as mentioned is measuring Mass directly.

In the next part we will discuss the basic math & physics for flow measurement and will start with the DP flowmeters ..