TURBINE GAS FLOW METERS

Introduction

TGM gas turbine flow meter is a new type precision measuring instrument which has high accuracy and good reliability. It can measure all kinds of single-phase gas and be widely applied to measure gas in oil, chemical, industry, metallurgy, electric power and combustion gas in cities.
TGM gas turbine flow meter has good measuring performance both at low pressure and high pressure and could be applied to measure rapid flow rate gas especially the accurate measurement of the natural gas.

Structure

The fluid is rectified and accelerated in the deflector ahead (or the flow conditioner), when it enters the meter. The turbine vanes that make an angle of direction of the fluid begin to swirl after it overcome rub moment and fluid obstruction moment. The angular frequency is proportional to the volume flow rate within a certain scope of fluid velocities.
The pulse signal is derived from the coil terminals of the magneto-sensor according to principle of electro magnetic induction; the frequency of the signal is proportional to the fluid volume flow rate. The signal is delivered to the flow accumulator after having been amplified, filtered and shaped.  Then the accumulator displays the gas total volume on the LCD screen. The structure and principle are as shown in figure.

1. Volume corrector         
2. LCD screen
3. Output signal channel
4. Magneto sensor
5. Two-stage rectifier
6. Turbines
7. Oilier (optional)
8. Shell
9. Rear deflector

Principles

The flow accumulator consists of detect analog channels of temperature, detect data channels of flow rate, microprocessor unit, driver circuit of Liquid Crystal Display (LCD), interface of the output signal and other auxiliary circuit. Multi-channel signals from sensors are transformed and sent to microprocessor to calculate according to the gas equation. The results can be displayed locally and transmitted to remote hosts.

Gas equation can be defined as follows:  
Where;
Vo – Volume under standard conditions (m3);
V -- Volume under operation conditions (m3);
Ps – Pressure setting in the instrument
To – Absolute temperature under standard conditions (293.15 and it can be changed to 273.15k if necessary)
T – Absolute temperature of the medium measured (273.15 +t)K
t – Temperature of the medium measured which is detected by the temperature sensor or setting in ten instrument

Precision Grade

   Using the high repeatability of the turbine flow rate sensor can correct five segment coefficients of the instrument, and then the precision grade in the scale is 1 grade and 1.5 grade. Without correcting five segments coefficients, the precision grade is according to the ISO9951 international standard.
1.0 grade: 0.2Qmax~ Qmax: ±1%;    Qmin~ 0.2Qmax: ±2%
1.5 grade: 0.2Qmax~ Qmax: ±1.5%;  Qmin~ 0.2Qmax: ±3%

Pressure Loss Of The Flow Meter

The pressure loss relates to the turbine’s running, the friction in the pipe and the speed and director of the liquid. Pressure loss ΔP can be calculated by the following formula under the operating conditions:
               
Where:
ΔPQmax -- pressure loss of the biggest flow rate under standard conditions, when the medium is dry air (at 20’C, 101.325kPa, =1.205kg/m3).
o: medium density under standard conditions (20’C, 101.325kPa) (kg/m3)
Pa – local atmospheric pressure (kPa)
Pg – pressure of the flow meter at the measuring point (kPa)
Po – standard atmospheric pressure (101.325kPa)
To – absolute temperature under standard conditions (293.15k, and it can be changed to 273.15k, if necessary)
Tg – absolute temperature of the medium under operating conditions. (273.15+t)k
Q – flow rate under operating conditions (m3/h)
Qmax – the biggest flow rate under operating conditions (m3/h)