The furnace negative pressure gauge always shows positive pressure
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Colleagues, the negative pressure gauge for the boiler’s furnace in our facility keeps showing positive readings. We have replaced the transmitter, but that didn’t help. We considered the possibility that the pressure tap pipe was too long, so we installed the gauge directly at the site, but this also didn’t resolve the issue. Upon inspection, there was nothing wrong with the pressure tap pipe; it was a differential pressure type gauge, with one end exposed to the atmosphere. To prevent interference from wind, that end was covered with cotton, but the gauge still occasionally showed positive readings of +40~50 Pa. This problem occurs only on one side; it has happened on both sides at times, but not simultaneously. What should we do to address this issue?
Are there any process photos (what processes)? It could be due to improper adjustment of the exhaust fan control or other reasons
This post was last edited by tiger2010 on 2020-9-6 at 12:11. Your image is a bit blurry; did the furnace stop working due to the vacuum gauge? :o Consider other furnaces to identify the cause, analyze historical trends in conjunction with process parameters, and determine whether it is a problem with the instruments themselves or an issue with improper adjustment of the process parameters
1. Negative pressure in the furnace is very important; some systems even have interlocks in place. Therefore, it is abnormal for there to be positive pressure, whether due to issues with the instruments or the process itself. 2. The measurement range for the furnace negative pressure is very limited; it is recommended to use a differential pressure transmitter with a small range but high precision (such as ROSEMOUNT 3051CD0, with a range of (-7,46 to 7,46 mbar)). 3. Instrument installation is important; it should be located near the pressure tapping point, with the piping slope facing upward. 4. The pressure introduction method on the negative pressure side, that is, the atmospheric side, is also crucial; otherwise, factors such as strong winds can cause significant fluctuations in the measurements. API551 on the effect of wind on negative pressure measurements: Wind surrounding the furnace also interferes with these measurements. If the draft reference point is located on the upwind side of the furnace, the wind causes the measured atmospheric pressure to increase. The deviation can be as much as 50 Pa (0.20 in. Hg at 20°C), which is twice the desired value for draft control. If the draft reference point is located on the downwind side of the furnace, the wind results in a pressure there that is lower than the actual atmospheric pressure; the draft measurement on the downwind side can vary by up to 76 Pa (0.30 in. Hg at 20°C). According to the World Meteorological Organization, the error caused by wind can reach up to 300 Pa (1.2 in. Hg at 20°C). Stable draft measurements can be obtained by using a piezometric pressure fitting or a barometric pressure port connected to the atmospheric side of the draft measuring device. Nevertheless, wind blowing across the furnace stack can still cause variations in draft, but these represent actual changes in furnace operation. As a result, it may be necessary to filter the measurement signal.
This is under normal conditions; yesterday, for a while it was at +40Pa
Sometimes I wonder if the flue dampers aren’t adjusted properly, but it seems that the process is manual – once set in a certain position, they stay there. Now I’m wondering if the pressure gauge tube has become damp
Thank you! Let me take a look; the instrument has been installed nearby, and the side facing the atmosphere has also been sealed with cotton, so it should not be affected by strong winds. I’ll keep an eye on it for now
Looking at this process flow in the diagram, the fan located between the combustion air fan and the exhaust fan – I’ve never seen such a process before； Are the colors of the three fans in the image red both when they are running and when they are stopped? :o
Is it possible that there is indeed a positive pressure? A pressure of just a few dozen pascals can easily fluctuate; if the combustion in the furnace is unstable and the exhaust fan isn’t adjusted in time, it’s normal to have a slight positive pressure. It would be useful to plot the negative pressure curve – the normal negative pressure value could be set around -100, and then observe the situation again.
They are the exhaust fan and the supply fan respectively; the smaller one in the middle is the return fan. Red indicates operational status, while green indicates stopped status.