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What we do not measure we cannot control. Steam flow meters with ability to compensate for change in density due to changes in dryness fraction of steam, provide for acute measurement of utility flows.

In most plants, steam accounts for about 40% of the utility costs with the balance being shared between electricity and water. Monitoring the utility streams allows one to be aware of deviations in the consumptions. This allows controlling the deviation the moment it occurs, thereby reducing the energy loss. Often in many industries the utilities go unmonitored.

Nowadays several plants have taken the initiative to monitor their utility consumptions. Most steam meters however measure the volumetric flow of the steam flowing through them. Because steam is a compressible fluid, reducing its pressure reduces its density and vice versa. A change in density of steam alters the mass flow. Therefore it is important to know the density of steam to be able to convert volumetric flow of steam to mass flow.

Volumetric flow rate = velocity X area                              |    Mass Flow = Density x Volumetric Flow

With a change in the dryness fraction of the steam the density of the steam flowing also changes. Thus as compared to dry saturated steam, the mass flow of wet steam will be higher and a meter not capable of recognizing the change in density of steam and adjusting the readings accordingly will over read for the mass flow of steam.

Illustration:

Consider orifice type steam flow meters that are calibrated for dry steam being used on wet steam.

 

 

Parameters

 

 

System pressure

 

10

 

bar g

Meter calibrated at dryness fraction for dry saturated steam

 

0.98

 

 

Actual system dryness fraction for wet steam

 

0.80

 

 

 

 

 

 

 

Indicated flow

 

2033

 

kg/h

Actual flow

 

2250

 

kg/h

Meter UNDERreads by

 

10

 

%

As we seen in the absence of density compensation, the accuracy of monitoring the utility is greatly impacted.