A flow quality/ quantity indicating transmitter, abbreviated as FQIT, is an instrument that is used for measuring the amount of a liquid and a gas in two pase flow. The term Flow Quality is a deviation from the ISA Standard 5.1 which states that "Q" is for Quantity. Flow Quality Indicating Transmitter can also be expressed as an Analyzing Indicating Transmitter. Both Flow Quantity and Flow Quality are discussed here.
A flow quantity indicating transmitter is also known as a totalizer. A totalizer is installed for custody transfer purposes or other purposes where the total lifetime flow is of concern. For security purposes, a flow totalizer cannot be reset. FQITs are installed on LACT units, household water and water meters and where gas is purchased for use at a cogeneration facility or steam generator.
Two Phase Flow
The measurement of the quality of a two-phase flow is very important when designing and operating heat exchangers, turbines, steam engines, pumps and piping systems. In heat exchangers that operate at saturated conditions, such as a steam generator condenser, the performance is characterized by the state of fluid entering and leaving the heat exchanger. In order to quantify the state of the fluid at saturation conditions, the inlet or exit quality must be known. For example, in a boiler lower than expected outlet quality is an indication of poor heat transfer performance of inefficient steam separation equipment. In turbines, high moisture content at the turbine inlet can cause turbine blade erosion as well as reduced system efficiency.
A throttling calorimeter utilizes the principle of Joule-Thompson Expansion. A throttling calorimeter works by taking a portion of the wet steam and throttling it to a lower pressure, usually atmospheric and the temperature is measured as the steam becomes superheated in the calorimeter. Since this process is isenhalpic, the enthalpy stays the same at high and low pressures. Knowing the upstream pressure and enthalpy, the quality can be calculated.
Meters for measuring water quality can range from a hand held device to complete industrial installations. Water quality meters can display one or more of the following properties of the water:
- Total Dissolved Solids
- Oxidation-reduction Potential