Pressure Temperature Compensation Flow Measurement
The orifice is commonly used to monitor liquid, gas, and steam flow. The opening creates differential pressure (DP) across the plate, which a DP-type transmitter detects. The comparable flow is then squarely rooted from the differential pressure. Square rooting can be done on the side of the DP-type flow transmitter or on the side of the control system.
When we operate the plant at the required operating conditions as calculated by the flow element sizing calculation, the flow output is valid. In practice, key characteristics, such as design temperature and design pressure, cannot be maintained during the operation in the industry.
When the pressure/temperature changes, the density changes, causing the volume to change, and the DP transmitter to fail to detect the change. Pressure and temperature have a significant impact on the volume of gas/steam recorded. This is when pressure and temperature.
When we have DP type flow elements such orifices, venturis, pitot tubes, and so on, and Gas or Steam as a service application, this form of compensation is appropriate. A volumetric gas flow at specified conditions is converted into an equivalent volumetric gas flow at base conditions using pressure and temperature (PT) compensation.
Density measurement is not commonly employed; instead, molecular weight is used. The typical orifice has been completely transformed by PT (P&T) compensation. Because the inaccuracy effects of the other variables at the operating circumstances are adjusted, the resultant figure for flow is more accurate. Absolute temperature and pressure are used in the PT calculations.
Pressure Temperature Compensation
The final configuration will resemble the P&ID diagram below.
In general, the pressure transmitter should be put upstream of the flow element and the temperature transmitter should be installed downstream so that the velocity profile in the flow element is not distorted. Nowadays, a Multi-variable transmitter is usually used to avoid a configuration like the one described above, which includes complete DP, Pressure, and/or Temperature measurement as part of the transmitter itself.
Separate RTD / Thermocouple for temperature measurement is an option available in Multi-variable transmitters. Vortex transmitters with integrated pressure and/or temperature adjustment are also available and can be used in Steam/Gas applications.
To improve clarity, various cases are simulated in Conval sizing programme. Simulated pressure and temperature changes are compared to corrected outputs. This is to understand why PT compensation is so important!!
Take the case of H2 Gas, where the orifice is calibrated/designed for a DP of 100 mBar, an input pressure of 35 bar, an input temperature of 300 K, and a flow of 30,000 m3/hr, with an orifice Beta ratio of 0.52034 obtained from Conval Software output.
Now, using reverse calculation, we change the input pressure from 35 to 30 bara while maintaining the orifice bore fixed. The calculated flow output is 27,790 m3/hr. Assume that density measurement is not available.
The following equation can be used to calculate the resultant compensated flow due to pressure changes.
30,000 *SQRT(30/35) = 27,774 m 3 /hr Flow Compensated
As a result, a 5 Bara reduction in pressure compared to the design condition resulted in 7.3 percent less flow in this example (when you have pressure compensation in place.)
Let’s see what happens if the temperature is raised from 300 to 325 degrees Fahrenheit while keeping all other factors constant. The calculated flow output is 28843 m3/hr.
The following equation can be used to calculate the resultant adjusted flow due to temperature changes.
30,000 * SQRT(300/325) = 28,823 m3/hr Flow Compensated
As a result, a 25 K increase in temperature resulted in a 3.8 percent decrease inflow in this sample.
If we consider the temperature and pressure changes in this example in relation to the design condition, the flow output is 26,719 m3/hr.
The following equation can be used to calculate the resultant compensated flow due to changes in pressure and temperature.
30,000 * SQRT((30*300/(35*325)) = 26,684 m3/hr Flow Compensated
There is a difference in flow output computed by Conval and equation because Conval software estimates the actual properties of H2 gas based on the operating conditions. However, the above-mentioned equation is valid to use.
Similarly, if the molecular weight (MW) changes with respect to the design, flow compensation can be determined using the equation below.
The flow compensated can be computed using actual temperature, pressure, and molecular weight, among other things.
The molecular weight can be determined using a lab test or an internet analyzer.
Pressure values shall be in Bar(a), Temp shall be in K.
DP-based flow = flow estimated from the square root of the actual DP reading
The flow element sizing sheet can reveal all of the design inputs/reference conditions. Actual parameters will be included in the control system’s analogue input signal.
Euler’s equation of continuity and Bernoulli’s principle are used to get the equation illustrated above. The derivation of this formula is outside the scope of this article.
It’s critical to send the correct flow compensation equation based on the process requirement (Pressure compensation/ Temperature compensation / P&T compensation / P, T & MW Compensation), as well as your design parameters and ranges, to the control system vendor so that this flow compensation calculation can be implemented inside the PLC/ DCS.
Exercise for you
Calibrated Flow Range 0-5000 Kg/hr, DP type flow transmitter calibrated at 0-2500 mmH2O. Design Pressure 40 Bar(a), Design Temperature 375K, Calibrated Flow Range 0-5000 Kg/hr.
What will be the P&T compensated flow output if your field pressure transmitter reads 35 Bar(a), your temperature transmitter reads 340K, and your DP FT reads 1250 mmH2O?
3473.06 Kg/hr is the correct answer.