Problems with orifice plate flow meters
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Ladies and gentlemen, I have a orifice plate flow meter in use, with 250°C saturated steam as the medium being measured. We suspect that the indicated flow rate is too low, and it remains stable at a constant value over time with little variation. The specified full-scale pressure difference per the specification sheet is 25 KPa, while the measured pressure difference shown by the DCS is 26.26 KPa. Is this beyond the range? Is this the reason for the inaccurate steam flow measurement? The parameter settings are as shown in the attached figure.
First, use the three-valve assembly to check whether the differential pressure transmitter can return to zero; then examine whether there are any leaks in the pressure guiding tube at the negative pressure side of the differential pressure transmitter. Confirm that the output value is indeed as high as indicated. In my opinion, there is a problem with the design of the orifice flow meter, and it is necessary to contact the manufacturer to have it redesigned and replaced.
Yes, based on the information you provided, the pressure difference of 26.26 Kpa shown by the DCS does indeed exceed the full-scale pressure difference of 25 Kpa specified in the specifications. Exceeding the range indeed leads to inaccurate measurements by the flow meter, and this could be one of the reasons why the flow reading on the steam flow meter is low. It is recommended to check or replace the flow meter, or adjust the system parameters, to ensure that the measured pressure difference remains within the specified range. .
1. The range is 25 KPa, but 26.26 Ka was measured (is this the pressure difference?) Check whether the orifice plate is measuring the pressure difference accurately, the installation method, and whether the pressure tapping tube is filled with condensate. 2. Should the upper and lower limits of the flow rate range here not correspond to the actual flow rate range? In the calculation sheet, 0-25 KPa corresponds to flow rate values of 0-25, right? Here, temperature and pressure compensation has been taken into account in the calculations.
I took another DCS photo; the flow rate was measured at 26.26 Kpa. My understanding is that this represents the pressure difference measured by the orifice plate. Please take another look
The range is 25 Kpa; the DCS shows 26.26 Kpa. What does the field differential pressure transmitter show? Find out about the operating conditions to confirm whether it has exceeded the designed range of use.
The value of 21.7 is the result after voltage stabilization compensation for the topmost image, while 26 is the final value obtained. The large deviation is mainly due to the design pressure of 5.3 and the actual pressure of 3.5, I guess. You think the flow rate is too low; I think the range setting in the control system is incorrect – since the differential pressure is at its full scale, the flow rate should be quite high. Check the calculation sheets and design documents for the flow meter to see what the corresponding differential pressure range is at 25 KPa, and is the unit KG/HR? There are still two issues: one is that the differential pressure is 26.25 Kpa, so the instruments need to be checked; the other is that this configuration must correspond to the design documents and calculation sheets of the instruments.
The ranges of the field instruments and control systems must be consistent. The actual operating conditions deviated from the designed conditions. All of these need to be adjusted.
The flow rate through the orifice plate is related to the measured differential pressure, which is equal to the difference between the squares of the upstream and downstream flow velocities divided by the density of the medium (with temperature and pressure compensation). Calculate the flow rate by combining the diameters of the upstream and downstream pipes. It doesn’t have a direct relationship with this 25, right? Is there a pressure difference limit of 25? Where does the data for that instantaneous pressure difference 26 come from?