STEAM METERING

Energy management of steam system is facilitated when steam becomes a metered service just like the other energy carrying services. Measurements starting in the boiler house, is essential if savings are made.

Fuel consumption is fairly easy to monitor, and until recently steam was less monitored. Even when steam was metered, the measurements were not always accurate as they can be now. Steam meters generally measure volume flow rate and translate this into mass flow rate with knowledge of steam density. Most steam meters currently installed are calibrated for use on one pressure. Using them on other pressures or on systems where pressure varies can lead to inaccurate readings. Modern steam meters have inbuilt density compensation meaning they give equally good accuracy at all steam pressures within their operating range. Some even have the capability of recording energy usage as well as steam flow. It is preferable to have clean steam through the meters and hence strainers, separators should be fitted upstream. Meters should be checked from time to time and most manufacturers offer recalibration service.

Steam metering is carried out in boiler house as well as where it is essential to be costed accurately. Meter reading provide the only positive proof of the success of scheme or improvements introduced to save steam. They also give useful information of plant performance, fouling of heat transfer surfaces and the malfunction of steam traps.

The parameters that should be selected for flow metering are:

1. Accuracy- 

Accuracy is used to describe the closeness of a measurement to the true value. Accuracy can be given in two types; measured value and Full scale deflection

Measured value:

It is the value that can be measured from the total range of reading values in the measuring system. The readings are made between fixed limits.

A percentage accuracy of ±1% of actual flow and indicated flowrate of 1000 Kg/hr, the ‘uncertanity’ (error) is between 1010 and 990 kg/hr. With the indicated flowrate of 100 kg/hr, the figure now become 101 and 99 kg/hr.

Full scale deflection:

It is the deflection between the maximum and minimum possible reading value indicated by the instrument. This means the accuracy is specified as a percentage of the maximum flowrate that the meter can handle. If we assume that 1000 kg/hr is the maximum flow or full scale reading and we consider a flowmeter with accuracy of ±1% F.S.D, then at 1000kg/hr the error is again 1010 to 990 kg/hr. But at 100 kg/hr, the uncertainty is now between 110 (100+10) and 90 (100-10) kg/hr.

2. Repeatability- 

The repeatability of a meter is its ability to indicate the same value for an identical flow rate on two or more successive occasions.

3. Density compensation-

Most Steam Meters measures the volumetric flow of the steam flowing through them.

Because steam is compressible fluid, reducing its pressure necessarily decreases its density, and vice-versa. Temperature & Pressure of Saturated Steam can be co-related with each other.

4. Turndown Ratio-

Turndown: The effective range of an instrument over which an instrument meets the specified accuracy requirements. It is the range of the flowrate over which the meter will work within the accuracy and repeatability tolerance given. If flowmeter works at maximum flowrate of 1000 kg/hr and minimum of 100 kg/hr, then dividing the maximum by the minimum gives a turndown of 10:1. The true turndown of a flowmeter is calculated based on that minimum flowrate up to which its accuracy specification is met. Practically the turndown vary between 6:1 to 10:1.

5. Rangeability: This is the capability of a flow-measuring device to operate between the minimum and maximum flow range within an acceptable tolerance. Range ability is generally expressed as a ratio of the maximum flow to the minimum flow.

6. Ease of Installation

7. Installation & Maintenance Costs