Gauge line effects and DP transmitter response to flow pulsation through orifice plate

Abstract

Flow pulsation is associated with pressure oscillation at the orifice plate taps. The actual pressure signals are usually distorted by the gauge line and the differential pressure (DP) transmitter, resulting in inaccurate flow rate metering. In this study distortions of the pressure difference across the orifice plate and of mass flow rate by the gauge lines and DP transmitters were quantified experimentally. It was found that the flow rate error caused by a moderate pressure oscillation can reach ± 5%, while 2% could be attributed to measurement errors. Two models were used for the numerical simulation of the gauge line effect but their application is restricted by oscillation frequency, gauge line length and expected accuracy. It was found that the gauge line together with the transmitter internal volume forma resonator which can amplify or attenuate the actual oscillating pressure signal measured across the orifice. Furthermore, it was demonstrated that gauge line fittings and the valve manifold which contain abrupt changes in the internal diameter alter the mean differential pressure. It is therefore recommended that very short gauge line and a constant-diameter tubing, fittings and valve manifolds, together with a DP transmitter having a small chamber and high-frequency response, could minimize the pressure oscillation errors if square root correction is properly executed.