Steam Engineering Principles and Heat Transfer
Contents
Measurement of Steam Consumption
Methods of measuring steam consumption, from the very basic to sophisticated flow metering, are explained in this tutorial.
By a steam flowmeter
The use of a steam flowmeter may be used to directly measure the steam usage of an operational item of plant. This may be used to monitor the results of energy saving schemes and to compare the efficiency of one item of plant with another. The steam can then be costed as a raw material at any stage of the production process, so that the cost of individual product lines may be determined.
It is only in comparatively rare cases that a meter cannot measure steam flow. Care should be taken, however, to ensure that the prevailing steam pressure is considered and that no other calibration factor has been overlooked.
Steam flowmetering is discussed in detail in Block 4.
By a condensate pump
A less accurate method of estimating the steam consumption is by incorporating a counter into the body of a positive displacement pump used to pump condensate from the process. Each discharge stroke is registered, and an estimate of the capacity of each stroke is used to calculate the amount of steam condensed over a given time period.
A purpose built electronic pump monitor can be used which enables this to be carried out automatically, converting the pump into a condensate meter. The electronic pump monitor can be read locally or can return digital data to a central monitoring system. If the pump is draining a vented receiver, a small allowance has to be made for flash steam losses.
By collecting the condensate
Steam consumption can also be established directly, by measuring the mass of condensate collected in a drum over a period of time. This may provide a more accurate method than using theoretical calculations if the flash steam losses (which are not taken into account) are small, and can work for both non-flow and flow type applications. However, this method cannot be used in direct steam injection applications, humidification or sterilization processes, where it is not possible to collect the condensate.
Figure 2.7.3 shows a test being carried out on a jacketed pan. In this case an empty oil drum and platform scales are shown, but smaller plant can be tested just as accurately using a bucket and spring balance. This method is quite easy to set up and can be relied upon to give accurate results.
The drum is first weighed with a sufficient quantity of cold water. Steam is then supplied to the plant, and any condensate is discharged below the water level in the container to condense any flash steam. By noting the increase in weight over time, the mean steam consumption can be determined.
Although this method gives the mean rate of steam consumption, if the weight of condensate is noted at regular intervals during the test, the corresponding steam consumption rates can be calculated. Any obvious peaks will become apparent and can be taken into account when deciding on the capacity of associated equipment. It is important to note that the test is conducted with the condensate discharging into an atmospheric system. If the test is being used to quantify steam consumption on plant that would otherwise have a condensate back pressure, the steam trap capacity must relate to the expected differential pressure.
Care must also be taken to ensure that only condensate produced during the test run is measured. In the case of the boiling pan shown, it would be wise to drain the jacket completely through the drain cock before starting the test. At the end, drain the jacket again and add this condensate to that in the container before weighing.
The test should run for as long as possible in order to reduce the effect of errors of measurement. It is always advisable to run three tests under similar conditions and average the results in order to get a reliable answer. Discard any results that are widely different from the others and, if necessary, run further tests.
If the return system includes a collecting tank and pump, it may be possible to stop the pump for a period and measure condensate volume by carefully dipping the tank before and after a test period. Care must be taken here, particularly if the level change is small or if losses occur due to flash steam.