How to decide LRV and URV of Flow transmitter (differential pressure transmitter)


#1

in specification of DPT LRL is 0 inH2O and URL is 500in H2O, how to decide lrv and urv , we use orifice for creating DP, maximum dp created when gas flow is about 200 inH2O but not fixed it may differ time to time, flow value changes with change in urv(changed by HART) like at 300 inH2O flow different and at 500 inH2O flow different, i’m not able decide fixed value, is there any calculation plz advice ?


#2

An orifice flow-meter is a design flowmeter. It is designed and manufactured to create a certain differential pressure according to flow-rate under certain static pressure and temperature. You need to find the calculation sheet of the orifice plate and enter the Dp value created at the max flow value (Qmax) to Dp transmitter under square-root linearization. Then you can set the dcs to Qmax value for correct reading. If you don’t have the calculation sheet, you have to measure the Beta value (inner radius / line size) of the orifice and use a software to calculate the Dp value by using your actual process medium, temperature, static pressure but with estimated flow rates.


#3

I understand that the d.p. produced by your orifice plate varies widely. In order to decide the URL and LRL of the transmitter, it is important to know the total variation of d.p under different flow conditions. The transmitter range should cover the lowest and the max flow d.p. The LRL will of course be 0, but the URL depend on the max flow d.p…


#4

DP-Transmitter is the secondary measuring element which its sensor converts the sampled representative process variable by taken from a primary element said to be Orifice. Here,

1.)The Transmitters has two pressure ports(HP and LP) across which the DP gets created and then the Transmitter Converts this DP to Electrical signals which is used in Automation.

2.)Orifice creates the DP by Blocking the flow across it. This DP plays the measure role for the measurement. DP varies in square function of Flow. So small change in Flow make the DP big by squaring it.

3.)In your case it is gas (Compressible/ expansive )service, so every surrounding & other process parameters(mainly temp. & Pressure) plays a role in variation of the Flow rate. So calculate the DP at lowest flow rate & lowest temp for LRV and at highest flow rate & highest temp. for URV along with other factors as per the specified formula. Balance all combinations of Flow/temperature will fall between your LRV and URV.

4.) Since the linear range of sensor fall between appx. 35% to 65% of the measuring range , chose LRL and URL such that LRV= LRL+35% of span and URV=URL-35% of Span. Manufacture has to select the transmitter with sensor of measurement range accordingly which will be higher than the LRL/URL. But they will calibrate the instruments as per the LRL and URL in the software of the transmitter which will automatically take care your LRV and URV.