The effects of swirling flow on the measurement accuracy of a 152 mm (6 inch) orifice meter are presented in this paper. The test was conducted in a low-pressure air flow calibration facility with swirls generated by an axial vane-type swirler. The flow fields inside the orifice meter were characterized by a multiport pitot-static probe traversing across the meter run. The orifice meter performance was compared with reference critical flow nozzles.
The salient features of the observed flow fields compared quite closely with swirling flows generated by two close, out-of-plane elbows reported in the literature. The axial velocity profile was shown to be generally flatter than the fully-developed one. The swirl angle measurements delineated a secondary flow pattern of solid-body rotation. Without a flow conditioner in the line, the orifice meter was found to under-measure the true flow rate in a swirling flow. The deviation in flow measurements increased with increasing swirl angle. The tube bundle flow conditioner was very effective in removing swirls in the flow but not rectifying the deficient axial velocity profile. Based on the collected swirling flow data, a method was proposed to estimate possible swirl-induced flow rate errors for orifice meters in field use.