A orifice plate flow meter is used to measure steam, and it still registers flow even when the boiler is shut down. Please explain. Thank you

In an orifice flow meter, the differential pressure is in an inverse square relationship with the flow rate; even small changes in differential pressure can result in significant changes in flow rate. As a result, the range ratio of orifice flow meters is low, typically 3:1. The error increases as the flow rate falls below 30% of the full range, which is why the parameter of small-signal rejection was introduced. Check how much traffic there is, and see if small-signal rejection has not been enabled.

Just perform a small-signal rejection, and that’s it. This is a basic question regarding configuration

The normal operating differential pressure is 3.25 Kpa, with instantaneous values around 65 T. The boiler has been shut down for a day; the differential pressure across the orifice plate is around 1.87, with instantaneous values around 39 T. How can this be considered a minor signal issue?

Under normal operating conditions, the differential pressure is 3.25 Kpa, with instantaneous values around 65 T. The boiler has been shut down for a day now, and in that time the differential pressure across the orifice plate was around 1.87, with instantaneous values around 39 T. After noticing this issue, I went to the site and did two things: first, I opened the balance valve to check the zero point of the differential pressure meter, which was -0.01, indicating no problem; second, I drained the water from the condenser tubes. After allowing some time for condensation to occur, I reopened the valve, and the differential pressure reading was around -0.02, with no instantaneous variations. After another night, the differential pressure was around +0.06, with instantaneous values of 9 T. This falls within the acceptable range of precision. Our question now is: why is there still a differential pressure of 1.87 after the boiler has been shut down for a day? What could be the cause? Please explain this for us. Thank you

Is the condensate in the two pressure tubes the same?

Well, first, both the condensation tank and the pressure guide pipes are made of stainless steel, so it’s not possible to visually determine whether the liquid levels are the same. Second, before the plant was shut down, the orifice plates were showing normal readings, and no one touched them after that either; it can be said that the liquid levels were the same during normal operation. (There is no issue with the condensation tank being at the same level.) So, can it be assumed that the liquid levels remained the same after the plant was shut down as well? Assuming the liquid level is the same, where does the differential pressure value come from? Our analysis suggests that the liquid level changed after the furnace was shut down, and it is this difference in liquid levels that results in the differential pressure. So the question arises: what caused the change in liquid level? We are unable to uncover this mystery, so we turn to you for help. Thank you

The reasons are estimated to be: inconsistent lack of condensate water in the pressure guiding tube； There may be air in the pressure guide tube ； The pressure guide tube may be blocked (it became unobstructed after flushing it). In the first two cases, normal operation is restored automatically after the boiler has been running for a while. Personal opinion, for reference only.

It seems that experienced meter technicians all say that the orifice plate flowmeter is most accurate within a range of 30% to 80% – this is probably also based on experience. As for what you mentioned, where the boiler is shut down and there’s still a reading, but everything is fine once the boiler is started again, then you don’t need to worry about it

Replace the flow meter – the original domestic orifice plates and vortex flow meters had issues such as an inability to measure low flow rates, sensitivity to vibrations, and a limited range. By using an imported professional steam flow meter, you might want to consider SpiraTech’s flow meters

Since it’s not possible to visualize it, I would like to ask how to verify it