Why steam flow measurement need temperature and pressure compensation

The purpose of steam flow meter with temperature and pressure compensation is for when the volume deviate the actual flow compensation will correct the result well. Steam in gas status, the density varies with the changes of temperature and pressure itself. If don’t have the compensation will cause a huge error. Take a example, saturated steam in 1Mpa, when pressure drop to 0.8Mpa, density will reduce 82% compared with default value, if flow meter without temperature and pressure compensation like differential pressure flow meter the reading value will increase 10.4%, for vortex will increase 21.9%. Apparently, it’s essential to have the temperature and pressure compensation.

There is one problem we meet when for steam measurement, for example superheated steam may transfer to saturated steam due to heat loss during long way delivery or even turn to liquid-vapor phase. Is there any problems if still deal with superheated steam method? Saturated steam will decompression treatment then to use it but after that will it also in saturated steam status?

Steam density calculation 

 The main mission of temperature and pressure compensation is to get the density after detected the two values. As we know for saturated steam the pressure and temperature are related, we only need to track one factor to know the density, pressure priority or temperature priority. But for superheated steam the temperature and pressure are independent variable we need to know both the value to sure the density.

For saturated steam we can check the table to know the density

saturated steam
Temperature Pressure Compensation priority
Measured value Measured value Pressure
Measured value Default value Temperature
Default value Measured value Pressure
Default value Default value Pressure

The pressure of saturated steam has one-to-one correspondence with the temperature, that is if we know one of the factor we can know another, and vice versa.

With the known the pressure or temperature of steam to compared with the data on steam table. if the steam temperature or pressure value are higher than saturation temperature or pressure will define as super heated steam, if values are equal it can define as saturated steam.

What kind of compensation should be used for saturated steam measurement?

The pressure of saturated steam has one-to-one correspondence with the temperature so for saturated steam the results is same whether to take pressure or temperature compensation. Temperature compensation is more convenient and an economical way, because the costs of PT100 resistor is more cheaper than pressure transmitter. From the accuracy of the two kinds way it determined by the accuracy of pressure transmitter and PT100 resistor. We’ll analysis this influence from the example:

Under saturated steam status, pressure at 1.0Mpa, temperature at 184.1°C, ignore the error from the temperature and pressure installation, PT100 in A level, error of temperature measurement is ±0.59°C, we check the density with the measured data and we found the density error is ±0.072kg/m3, this results worse than the same 0.5 level pressure transmitter but better than 1.0 level pressure transmitter brings. Apparently, the accuracy of by pressure compensation is higher than by temperature compensation.

 Influence by fluid phase transition

Vapor phase to liquid phase

It’s often to see superheated steam to saturated steam when superheated steam after long-way delivery even part of steam condensation to water droplets. Will this water droplets influence the measured results?

For example: Under superheated steam, flow rate; qm, presume 10%qm condensate to water qml, and qms is the steam,

The meter with temperature and pressure compensation and we can know the density under steam status is ρs=5.68kg/m3, and density of water droplet is ρL=882.47kg/m3, the volume is

OL= volume of water droplets

OS=volume of dry steam

Rv=the proportion of dry steam volume to wet steam

We get Rv=99.93%,

The influence of when select Vortex meter when for steam measurement

Vortex flow meter is to measure the vortices when passing by the shedding body and also can track the density of medium by temperature or pressure compensation so steam contain vapor will not influence the results.

The influence of when select Orifice flowmeter when for steam measurement

qm= mass flow kg/s;

C= Outflow coefficient;

β= Throttle aperture and pipe inner diameter ratio, d/D

= Coefficient of expansion;

d= Throttle aperture diameter;

=differential pressure Pa;

ρ1=inlet fluids density kg/m3 of Throttle aperture;

Superheated steam will turn to critical saturation state or super saturation due to heat loss. Technically flow meter measurement don’t have huge error in critical saturation state from formula (7), according to steam pressure table can checked ρ1 is equally to actually density. But when it turn to super saturation state the situation will become complex.

We know steam is dry(X ≥95%) can assume there just single phrase fluid in the pipeline, for flow measurement only to use temperature and pressure for compensation. According to formula 7, ρ1 is the density of actual wet saturated steam and its value are higher than in critical saturation state and the dryness of steam the higher of density. With the pressure value we tracked the density is in critical saturation state is smaller than actual density so the results of mass flow becomes negative error. When steam dryness is lower(X<95%), we assume there two phrase fluid in the pipeline, measured results will be huge if fluid stratified flow.

Wet saturated steam become superheated steam

In situation, pressure greatly reduced and fluids when the fluid is in adiabatic expansion the wet saturated steam will becomes superheated steam.

(i) Process of phase change

The droplets in wet saturated steam its temperature and pressure in balance state, once pressure reduce and lower than balance pressure, droplets will evaporation and evaporation will reduce the temperature of medium meantime. If temperature lower just a bit or temperature are high before evaporation it all makes the temperature to reduce the limit related to the new pressure correspond saturated , a new balance will become. The steam still in wet saturated state. If pressure reduce a lot and temperature in very low before evaporation then the temperature after evaporation still higher than the temperature that new pressure correspond saturated, the steam becomes superheated steam.

(ii) Effect of evaporation on flow measurement

Above evaporation we will get two results, the first one will not influence the measured results, but if after evaporation the steam becomes to superheated steam and flow meter placed after pressure reducing valve, we’ll analysis the results from below points:

First, users already take into consider the situation of steam changes to saturated steam or superheated steam and take temperature and pressure for compensation, then above changes will not influence the measurement. Second, just design as saturated steam and use pressure compensation then above changes bring small error that is compensation error caused by the difference in density corresponding to the difference between the superheated steam temperature and the saturation temperature. Third,  design as saturated steam and use temperature compensation, use the superheated steam temperature to check the density on saturated steam the results will quite different.

For example, in a chemical factory, the boiler system designed pressure as 1.0Mpa,

In the system, the flow meter for measured the flow placed after the valve and consider as saturated steam with temperature compensation. After pressure reducing, the droplets in main pipe after evaporation makes the temperature higher than saturated temperature in superheated steam, the temperature and pressure collected in site as above display, with this data we checked density is ρ2=3.4528kg/m3, and with t2, P2 we known superheated steam density is ρ2’=2.2718kg/m3,

the mass flow measured error is 24.36%.

In this case, if we take pressure for compensation, under P2=0.42Mpa, the density is ρ2″=2.7761kg/m3, then compensation error is 2.1%.

Solutions:

(i) Placed the flow meter in front of the pressure reducing valve;

(ii)If no possible to take solution, then we can use temperature and pressure compensation by adding more pressure transmitter;

(iii)If the pressure reducing valve in good stability, we can set a constant value and use temperature and pressure compensation in PLC or system.

Conclusion

1, For saturated steam measurement, can use temperature or pressure for compensation, but use pressure compensation the accuracy will higher.

2, Superheated steam becomes to saturated steam will not influence the vortex meter but for orifice flowmeter the results will be different.

3, Supersaturated steam becomes to superheated steam, the measurement error is small if take pressure compensation but take temperature compensation will bring large error. The way is to choose a correct install point and by right compensation way can reduce or eliminate the error.