Main steam flow rate of boiler
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Our company has two boilers: one with a capacity of 20 tons and the other with a capacity of 35 tons. Differential pressure flow meters are used to measure the main steam flow rate in these boilers, and the main steam outlets of both boilers are connected to the steam distribution cylinders. When one furnace with a capacity of 235 tons is in operation, the flow meter shows stable readings. When the other furnace with a capacity of 20 tons starts to generate steam, the main steam flow rate of the 35-ton furnace increases by about 2 tons. The boiler’s control settings remain unchanged, but the feedwater flow rate is not constant and varies. This is because once both furnaces are running, the pressure rises from 0.6 MPa to 0.9 MPa, and the temperature also increases slightly. It seems that the amount of coal and feedwater supplied remains unchanged. According to the law of conservation of mass, the steam flow rate should not increase. Please, experts, provide some explanations.
1. To measure steam flow, in addition to the differential pressure measurement, temperature and pressure compensation are also necessary to obtain a more accurate result ； 2. As you mentioned, when both boilers are in operation, both pressure and temperature increase; as a result, the properties of the steam change, that is, its density changes. Even if the same pressure difference is measured, the flow rate of the steam is actually different. 3. Another thing to consider is whether the steam you produce is saturated steam or superheated steam. If it is saturated steam, compensation can be done using either only the temperature or only the pressure ； If it is superheated steam, temperature and pressure compensation must be used. 4. If you simply use compensation modules within DCS to fix the temperature and pressure, that is, to perform calculations using a fixed density, then when the temperature and pressure change, the actual density also changes, which can lead to measurement errors. You can consider it comprehensively.
We used the temperature and pressure compensation feature. What’s needed mainly is an expert analysis of the impact of system changes on flow rate when two boilers are in operation.
If the water supply volume remains constant, won’t the steam flow rate also remain constant? I don’t think so. When the temperature and pressure of the generated steam increase compared to the original conditions, and it is then adjusted to match the steam parameters of the designed operating conditions, even with the same amount of water supplied, the flow rate will definitely be higher than that under the designed operating conditions. If the original poster does not intend to discuss the errors that occur in instrument measurements, they can go to the boiler area or the process area, where there may be colleagues who have a better understanding of the entire boiler system.
It is recommended to use vortex flow meters for steam measurement
Yes, steam measurement is not done using vortex flowmeters (for any steam measurement, temperature and pressure compensation is essential). All steam in our company uses this type of flow meter; differential pressure flow meters have too many factors to consider.
Is there a problem with the main steam outlets of your two boilers and their connection to the steam distribution cylinders? Don’t doubt that the flow meters are the issue.
Have you been paying attention to the drum water level??