Differential Pressure Meter Signal Distortion Cause

In the standard specification for the installation of differential pressure flowmeters, the laying of pressure pipes and the installation of meters are generally referred to simply, that is, there should be no distortion in the differential pressure signal. In fact, it is very difficult to truly achieve undistorted transmission of differential pressure signals. There are facts to prove that: when the differential pressure flowmeter has just stopped, or when the valve on the pipeline with a differential pressure flowmeter primary device is conditionally closed, and the flow through the flowmeter has been confirmed to have dropped to zero, from the specific flow In terms of readings, there is not much true zero. The main reason for zero drift is mostly the distortion in the differential pressure transmission process.

The transmission distortion of the differential pressure signal causes the differential pressure signal received by the differential pressure transmitter and the differential pressure signal generated by the differential pressure flowmeter to be unequal, thereby causing additional errors.

Differential pressure signal transmission distortion includes steady-state value distortion and dynamic distortion. In steady flow conditions, there is only a steady-state value distortion. Under pulsating flow conditions, there may be both steady-state value distortion and dynamic distortion. Distortion of the steady-state value may be caused by the following causes: the lead of the pressure guiding tube is not reasonable; the shut-off valve is not properly set; the height of the condenser is not equal; the height of the spacer fluid is not equal; the gradient of positive and negative pressure is not reasonable; The medium density is not equal;

(1) The error caused by the unreasonable introduction of the pressure guiding pipe and improper setting of the shut-off valve is most likely to occur in the differential pressure steam flow meter and should be installed strictly according to the installation standard specification.

When a constant-velocity differential pressure flowmeter is used to measure the flow of low-pressure gas that may precipitate condensate, a horizontally-positioned needle-type shut-off valve may cause the condensate to block the gas path inside the valve, thereby generating a differential pressure signal distortion.

(2) The error caused by the unequal height of the condenser, the two condensers on the positive and negative pressure tubes should be symmetrical and the installation height should be equal, which may make the heights of the two condensers equal in height, because the difference in the liquid height is 1mm. It will introduce 10Pa differential pressure distortion. However, it is not uncommon for two condensers to have unequal mounting heights at the site of use, and some even turn the condenser upside down, which does not function as a condenser at all.

The positive and negative pressure ports of the elbow flowmeter are open in the outer ring of the elbow and the other in the inner ring of the elbow. When the diameter of the elbow is large, it is easy to cause unequal heights due to the differential pressure of the elbow flowmeter. The signal is inherently small, so the differential pressure signal transmission distortion is not too great from the absolute value of the differential pressure, but the relative error caused by the flow measurement is considerable.

(3) The error caused by the unequal height of the spacer fluid level. The isolators in the pressure-inducing pipeline are used to isolate the corrosive medium from the differential pressure transmitter using the spacer fluid. When the spacer fluid is just filled, the level of the spacer fluid in the two isolating containers can be the same through the balancing valve of the three-valve manifold. However, after running for a period of time, due to leakage of the spacer fluid or accidental opening of the balancing valve during operation, the height of the spacer fluid may not be equal, thereby introducing an additional differential pressure to cause the differential pressure signal to be distorted.

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