Is the value calculated by the orifice flow meter on site, after temperature and pressure compensation by the DCS, accurate?

First, one needs to look at the calculation sheet for the orifice plate to find out what the operating conditions specified in it are, and then use the formulas to make the conversions

1. The gaseous state requires pressure stabilization compensation； 2. Verify whether the configuration settings of the on-site instruments are consistent with those specified in the design documents ； 3. It can be calculated based on the actual operating conditions and design conditions

Within the normal range of ratio values, the orifice plate provides accurate parameters (such as gas composition), the transmitter offers high precision, and the discharge coefficient of the throttling device is highly repeatable, thus ensuring high accuracy as well

. Since the design temperature and pressure are not specified, it’s difficult to say for sure whether it’s accurate or not. This flow rate refers neither to the hydrogen flow rate at a pressure of 0.2 Mpa nor to the flow rate at the design pressure. It is the flow meter that uses the density of the medium under operating conditions as the result of calculations based on design temperature and pressure. When the temperature and pressure of the gas under operation deviate from the design values, the indicated value will deviate in the positive or negative direction relative to the design conditions (referring to the standard conditions corresponding to the same indicated value) ； However, at this point, since the actual amount required by the process must meet the standards under standard conditions, the indicated value will deviate in the positive or negative direction relative to the design conditions – that is, the amount indicated when the process achieves the same operational parameters. The above two concepts are a bit confusing (with opposite directions of deviation); only this much can be described at present, and it requires careful consideration. Therefore, the result of \"converting to flow rate under standard conditions\" here is highly dependent on the way it is compared with the compensation algorithm. Specifically, it is only possible to determine whether temperature and pressure compensation is necessary by comparing the process consumption after it is put into use; this depends on the tolerance level for errors. When the deviation from the design is small or stable, the operator can consider a certain indicator to be normal (regardless of the deviation) and thus avoid performing temperature and pressure compensation. If it is necessary to always know the flow rate under standard conditions, then it must be done. .

Thermal and pressure compensation is necessary; one can write formulas manually or directly use the compensation blocks available in the DCS, to ensure that the parameter settings are accurate. If it’s still not allowed, then other factors must be at play, such as whether the pressure guiding tube is blocked or not Is the transmitter calibrated? Each site is different; it’s necessary to analyze the specific situation.

A differential pressure flow meter used for measuring gases must be equipped with temperature and pressure compensation

Did you learn the ideal gas equation in middle school? Will you use the temperature and pressure compensation formula? If you can do both, I suggest you calculate it manually; for DCS, it is likely necessary to manually set the temperature and pressure compensation.

This post was last edited by qugd on 2022-12-27 at 11:39. The precision of orifice plate differential pressure flow meters limits the accuracy of this flow measurement method; moreover, during gas measurement, factors such as temperature, pressure, medium density, and medium viscosity have a significant impact as well. When initially designing a system that uses such flow meters, it is necessary to take into account changes in temperature, pressure, and the composition of the fluid. Specialized orifice plate calculation software is used to determine factors such as the flow range, pressure loss, the accurate measurement range, and the differential pressure setting (range). By selecting the appropriate parameters for the orifice plate as well as the compensation parameters, more satisfactory measurement results can be achieved. Choosing orifice plate differential pressure to measure gas flow means that, even with temperature and pressure compensation, the precision will not be very high. For use in process measurement, it is sufficient if it can provide some reference value; there is no need to demand too much from a method that is not inherently precise, as this will only cause unnecessary frustration.

Middle school knowledge is not sufficient, as the behavior of fluids and flow measurement are far beyond what can be addressed with the physics knowledge taught in middle and high school. Although there are some simple adjustments, for measuring gas flow, it is better to use professional formulas and parameters for calculation.

This operating condition involves an ideal gas, so an ideal gas correction can be applied. The reason for the inaccurate data is that, I assume, he isn’t making the necessary corrections based on the operating conditions. With such large discrepancies after those corrections, it’s either an error in the calculations using the orifice plate, or an error in his corrections.