![]() Derivation of this formula is out of the scope of this article. The equation shown above is derived from Euler’s equation of continuity and Bernoulli’s principle. Actual parameters will be part of the analog input signal to the control system. Pressure values shall be in Bar(a), Temp shall be in K.įlow-based on DP = flow calculated from square rooted actual DP readingĪll the design inputs/ reference conditions can be known from the flow element sizing sheet. Resultant compensated flow due to change in pressure can be derived by the following equation Assume that density measurement is not available. The flow output calculated is 27,790 m 3/hr. Now, we take reverse calculation, keeping the orifice bore fixed, and change the input pressure from 35 Bara to 30 Bara. Let’s take an example of H 2 Gas where in Orifice is calibrated/designed for DP of 100 mBar, Input Pressure 35 Bara, Input Temp 300 K, Flow is 30,000 m 3/hr, where orifice Beta ratio found from Conval Software output is 0.52034. This is to know how PT compensation really matters !! Example Calculations Pressure and temperature changes are simulated and compensated outputs are compared. Various examples are simulated in Conval sizing software to get better clarity. Vortex transmitter is also available with integrated Pressure and/ temperature compensation which can be suitable to use in the Steam / Gas application. There are options available in Multi-variable transmitters like separate RTD / Thermocouple for temperature measurement. Nowadays, a Multi-variable transmitter is commonly used to avoid such an above arrangement which has compete DP, Pressure, and/ Temperature measurement as part of the transmitter itself. In general, the pressure transmitter installed at the upstream side of the flow element, and the temperature transmitter installed downstream so that it does not distort the velocity profile in the flow element. The complete arrangement will look as per the following P&ID figure. The PT calculations are based on absolute temperature and pressure. This produces a resultant value for flow that is more accurate because it is compensated for the error effects of the other variables at the operating conditions. PT (P&T) compensation is a complete makeover of the traditional orifice. Pressure and temperature (PT) compensation convert a volumetric gas flow at specific conditions into an equivalent volumetric flow at base conditions.Ĭommonly, density measurement is not used, another approach is to measure Molecular weight (online/offline) and then compensate with design molecular weight. This type of compensation is applicable when we have DP type flow elements like Orifice, venturi, pitot tube, etc, and Gas or Steam as a service application. This is when we require pressure, temperature compensation where in actual operating conditions will be used. The volumes of gas/steam measured are highly affected by pressure and temperature. Once the pressure/temp varies, density will be affected due to which Volume will change and the DP transmitter cannot detect the density changes. In the industry, practically, these parameters like design temperature and design pressure cannot be maintained throughout the operation. Flow output is valid when we operate the plant at the specified operating conditions as per flow element sizing calculation. Square rooting can be done either at DP type flow transmitter side or at the control system side. This Differential pressure then square rooted to convert into the equivalent flow. The orifice produces differential pressure (DP) across the plate and it is sensed by DP type transmitter. The above result tells us that if the cross-sectional area changes, then the velocity of the fluid must change to keep the flow rate constant. The fluid speeds up when entering a narrower section and slows down when entering a wider segment of the pipe.Īlthough, the volumetric flow rate, or current, and the fluid velocity are both related to the rate at which the fluid moves, these quantities describe different fluid properties.The orifice is normally used for flow measurement of liquid, gas, and steam. For flow rate as:Īpplying the continuity equation, which states that current must remain the same in a steady-state fluid system, between the narrow and wide regions of the pipe system in Figure 5.3.3, we find that: ![]()
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