Differential pressure transmitter measurement circuit failure

　　Differential pressure transmitters often have various faults during use. Among them, the measurement circuit of the differential pressure transmitter is caused by the pressure pipe, which is a typical fault and a common problem. Next, the editor will take the pressure pipe failure as an example to analyze the measurement circuit failure of the differential pressure transmitter.

　　1. The pressure pipe is blocked:

　　During instrument maintenance, it is common for the positive and negative pressure pipes to be blocked due to untimely discharge of the differential pressure transmitter pressure pipe or dirty or sticky media.

　　When the actual flow rate decreases from before F to after F, the static pressure in the pipeline also decreases accordingly, and the decrease value is set as P0; at the same time, when the actual flow rate drops to after F, the P-value will also increase due to the decrease in the flow velocity of the fluid in the pipe, and the increase value is set as P0'.

　　That is: △P=(P+-P0)-(P-+P0′). At this time, the output value of the transmitter should decrease.

　　2. Positive pressure pipe leakage:

　　In fact, when the leakage is very small, it is difficult for process operators or instrument maintenance personnel to find it due to various reasons. It will only be found when the leakage is large and there is a large error between the measured flow and the actual flow. At this time, even if the actual flow increases, it is always △P after leakage <<△ P before leakage, F after leakage <

　　3. Balance valve leakage:

　　Assume that the pressure before leakage is P1 and the pressure after leakage is P2, P1= P1+- P1-, F1 is the transmitter output value before the balancing valve leaks, and F2 is the transmitter output value after the balancing valve leaks.

　　We assume that the fluid flow rate in the pipeline does not change when analyzing, and let the leakage pressure be PS.

　　Then: The static pressure of the positive and negative pressure pipes after leakage is:

　　P2+= P1+-PS，P2-= P1-+ PS

　　P2= P2+- P2- = P1-2 PS，

　　According to the relationship between differential pressure and flow rate, F2 is obtained

　　4. Measurement error of the transmitter when the gas flow pressure pipe is filled with liquid:

　　Assume that the pressure at the positive pressure pipe tapping point is P0+, the pressure at the negative pressure pipe tapping point is P0-, the pressure at the positive end of the differential pressure transmitter is P1+, and the pressure at the negative end of the differential pressure transmitter is P1-.

　　P0= P0+- P0-

　　P1= P1+- P1-

　　Under normal measurement:

　　P0 = P1

　　Assume the flow rate under normal measurement state is F, then F=K

　　Here K is a constant coefficient.

　　Assuming the density of liquid water is ρ, when the liquid accumulation height of the positive pressure pipe is h+ and the liquid accumulation height of the negative intelligent star pressure pipe is h-:

　　P1+= P0++ρgh+

　　P1-= P0-+ρgh-

　　P1= P1+- P1-= (P0+)+(ρ h+)-( P-+ρ h-)= P+ρ (h+-h-)

　　The transmitter output is:

　　F=K

　　When h+>h-, the differential pressure actually measured by the transmitter increases and the output flow signal increases.

　　When h+ is here, due to the pressure taking method of the positive pressure catheter, as time goes by, h+ gradually becomes greater than h-, and the measured flow rate also increases.