Clamp-on Ultrasonic Meters As Diagnostic Tools

Clamp-on ultrasonic flow meters are attractive to the gas industry because they are capable of providing portable, non-invasive gas flow measurement. This article describes the results of a Pipeline Research Council International (PRCI) funded project that addressed the ability of a clamp-on ultrasonic meter (CUSM) to measure distorted flow profiles.

The test approach was to traverse a single ultrasonic transducer pair around the perimeter of the pipe in sufficiently small increments to measure the flow field at a given pipe cross section independent of the amount of flow distortion present. Detailed profile measurements at the same locations were also made with a wedge probe. The intent of this testing was to determine the accuracy with which CUSM measurements, performed with sufficient spatial fidelity, can be used to provide a reference flow rate for in-situ meter proving.

The tests were conducted at Southwest Research Institute’s Metering Research Facility (MRF) in the High Pressure Loop (HPL) using transmission-grade natural gas (i.e., approximately 95% methane gas mixture) at nominal operating conditions of 800 psia and 70°F.

The 8-inch piping was arranged as shown in Figure 1 with a single 90-degree elbow upstream of a 139D (where “D” is the nominal pipe diameter) long, straight pipe run. The flow upstream of the elbow was conditioned using a Sprenkle flow conditioner followed by 43D of 8-inch pipe. Clamp-on meters were installed 12D and 126D downstream of the elbow. These locations allowed an existing velocity profile measurement spool to be installed such that the velocity probe could measure flow at the same axial location (downstream of the elbow) as the clamp-on meters.

Figure 1. Piping Configuration For The Meter Tests.

Data were collected simultaneously from the test meters and from the MRF HPL critical flow nozzle bank, which served as the flow rate reference. Five binary-weighted critical flow nozzles in the HPL nozzle bank were previously calibrated in-situ, at different line pressures, against the HPL weigh tank system (Park et al., 1995).

The volumetric flow rate reported by each ultrasonic meter was acquired through the use of the meter’s frequency output function. Each meter produced a pulse train where the pulse frequency was proportional to the flow rate computed by the meter. The pulses were counted using the MRF data acquisition system (DAS) and combined with pressure, temperature, and gas composition measurements to compute a mass flow rate that could be compared to the MRF reference flow rate. Although not discussed here, other meter-reported parameters, including speed of sound, path status, etc., were recorded using software from each meter manufacturer.
Pipe measurements were made at each of the meter installation locations. The outside diameter measurements were made using calipers, while the wall thickness measurements were made using an ultrasonic thickness gage. The diameters used to set up the meter electronics were based on the measurement of the pipe circumference (with a flexible tape measure) and the average measured wall thickness.