How is the differential pressure signal ultimately converted into mass flow rate for display on the DCS?
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We all know that the throttling device measures a differential pressure signal; the field transmitter takes the square root of this differential pressure signal (in some cases, the square rooting is done on the DCS), and then converts it into a 4–20 MA signal that is sent to the DCS. So the question is: how does the DCS convert this signal into the mass flow rate that is displayed? If it’s a linear relationship, multiplying by a coefficient will suffice; but is there really any relation between this mass flow rate and the density of the medium? If the density of the process medium changes, will this mass flow rate change as well?
When fluid passes through a throttling element, a pressure difference is generated. According to Bernoulli’s equation, this pressure difference is proportional to the square of the flow rate; thus, once this pressure difference is measured, it indicates the flow rate.
Generally, flow meters are calibrated for temperature and pressure; they should have correction factors that take into account changes in temperature and pressure
According to Bernoulli’s equation, a flow meter measures volumetric flow rate; to obtain the mass flow rate, it is necessary to multiply by the density of the medium. The 4–20mA signal output by the flow meter corresponds to the range you set on the transmitter; similarly, the range set on the DCS should be consistent with that of the flow meter. (You can also set the volumetric flow rate on the transmitter, while setting the mass flow rate on the DCS.) There is a linear proportional relationship between the flow value and the current value. Since you use a fixed density multiplied by the volumetric flow rate, the mass flow rate calculated will be inaccurate if the density of the process medium changes.
The differential pressure signal also needs to go through a standardization process; the range of the differential pressure gauge for each throttling device corresponds to the current signal, with the minimum range corresponding to a 4 mA signal and the maximum range corresponding to a 20 mA signal. This ensures consistency. The same principle applies when expanding the range of the flow meter. Additionally, flow meters have requirements regarding the cutoff at low flow rates; the smaller the value, the larger the value obtained after taking the square root
If it is a liquid, a mass flow meter can be used, as it already takes the actual fluid density into account for corrections.
Changes are definitely necessary; someone who works with DCS needs to make the appropriate adjustments to the internal parameters.
Density compensation is required to obtain the actual flow rate