For orifice plate flow meters, if the flow measurement is inaccurate, why can adding an isolation fluid correct it?

In orifice plate flowmeters equipped with an isolation fluid, the pressure guiding tubes coming out of the orifice plate are filled with this isolation fluid. The level of the isolation fluid in the positive and negative pressure guiding tubes is the same, so no pressure difference is created, which has no impact on flow measurement； Gas flow rate is measured without an isolating fluid; if there is condensed liquid in the pressure guide tube, the static pressure of this liquid will cause measurement errors.

For example, is an isolation fluid injected into the pressure transfer pipe from the very beginning for liquefied gas, or can it be used without such an isolation fluid? For example, if the flow meter is inaccurate, should one drain the liquid directly without using any isolation fluid in order to clear the pressure guiding tube and remove the liquefied gas?

It’s the differential pressure; can the differential pressure remain the same even if the media are different? Isolation fluid and liquefied gas are two different things

Remove the pressure generated by the isolation fluid; this does not affect the measurements

In essence, a orifice plate flow meter is still a differential pressure flow meter; an isolating fluid is used to transmit pressure, and although the pressure remains constant, the internal design of the flow meter contributes to improved accuracy.

What is measured is the differential pressure; what does it have to do with the isolation fluid? The inaccuracy is due to an issue with the level of the condensate

A orifice plate flow meter measures flow based on the pressure difference at the orifice in the meter; the use of an isolating fluid has little impact on its measurements. If no isolating fluid is used, the pressure tubes on either side of the meter may become clogged, leading to pressure buildup and inaccurate readings.

Applying isolation fluid can prevent the medium from clogging the pressure measurement pipeline.

This post was last edited by Pandora on 2017-10-18 at 15:00. First of all, please observe your flow meter carefully. Orifice plates and differential pressure transmitters are used to measure flow rate. The pressure tapping points of the transmitter are at the same level, the isolation tank is at the same level, and the positive and negative diaphragms of the transmitter are at the same level. Whether an isolation fluid is used in the tank or not, there is no differential pressure at either end of the transmitter. The transmitter measures the pressure at the pressure tapping point as well as the pressure of the isolation fluid. Although there is pressure from the isolation fluid, the positive and negative pressures of this fluid cancel each other out (they are supposed to be at the same level). So what is obtained in the end is still the difference in pressure between the positive and negative pressure sampling points. The tank isolation fluid does not improve accuracy. The function of the isolation fluid is to prevent direct contact between the pipeline medium and the diaphragm box. Some high-temperature, highly corrosive media can damage the transmitter diaphragm box. For example, if it’s about measuring the flow rate of steam. Before the isolation fluid tank was in place, the diaphragm box came into direct contact with steam, which could damage the transmitter. Then, the steam will gradually condense into water, and the levels of condensed water in the positive and negative pressure pipes may differ (although over time they will end up at the same level). So at that time, the instruments couldn’t give accurate readings either. So, first, use a isolation fluid – this serves to protect the instruments and also reduces the measurement errors of those instruments.