On on-site commissioning of metal tube flowmeters

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Commissioning Report for Metal Tube Flow Meters
Based on customer feedback, there were significant variations in the measurement accuracy of the metal tube flow meters. Commissioning was carried out on-site, and now the adjustment of the float components in these meters has been completed. The following adjustments were made according to the conditions at the site:

1. The flow meters were installed in accordance with standard requirements (with straight sections before and after them). Measurements were taken using an electronic scale. Comparing results with those obtained without the front straight section showed that the instantaneous flow rate remained unchanged, while the cumulative flow rates differed slightly. The test results are as follows:

| Meter No. | Flow Rate | Time | Cumulative Flow | Measurement Accuracy |
|------------|-----------|------|------------------|-----------------------|
| 1 | 325 kg/h | 2 min| 9.1 kg | 8.67% |
| | 350 kg/h | 2 min| 9.9 kg | 8.83% |
| | 350 kg/h | 2 min| 9.62 kg | 10.20% |
| 2 | 330 kg/h | 2 min| 9.6 kg | 7.00% |
| | 350 kg/h | 2 min| 10.0 kg | 8.30% |
| | 350 kg/h | 2 min| 9.8 kg | 9.30% |
| 3 | 300 kg/h | 4 min| 18.68 kg | 3.30% |
| | 380 kg/h | 3 min| 17.56 kg | 4.80% |
| | 300 kg/h | 4 min| 18.24 kg | 4.40% |
| 4 | 600 kg/h | 2 min| 17.04 kg | 8.88% |
| | 600 kg/h | 2 min| 17.4 kg | 7.80% |
| | 550 kg/h | 2 min| 16.08 kg | 6.76% |
| 5 | 600 kg/h | 2 min| 16.12 kg | 11.64% |
| | 600 kg/h | 2 min| 15.96 kg | 12.12% |
| | 600 kg/h | 2 min| 15.79 kg | 12.60% |
| 6 | 625 kg/h | 2 min| 20.08 kg | 2.26% |
| | 600 kg/h | 2 min| 18.9 kg | 3.30% |
| | 610 kg/h | 2 min| 19.3 kg | 3.10% |
| 7 | 350 kg/h | 2 min| 12.04 kg | 1.87% |
| | 360 kg/h | 2 min| 12.4 kg | 2.00% |
| | 350 kg/h | 2 min| 12.04 kg | 1.87% |

From these data, it can be seen that the seven metal tube float flow meters exhibit good repeatability, but their accuracy levels are relatively high. To determine whether the flow meters indeed have inaccurate measurements, a second testing method was employed.

2. A mass flow meter was connected in series with the metal tube float flow meters on-site, along with flow restrictors for testing purposes. First, the instantaneous flow rate was adjusted using the mass flow meter; the difference between the two readings was significant, around 100 kg/h at a flow rate of 600 kg/h. Then, the instantaneous flow rate was adjusted using the metal tube flow meters, with similar results.

3. Flow meters of the same model but with different serial numbers were used at the same location. Under conditions of the same valve opening and instantaneous flow rate, the indicated instantaneous flow rates varied.

Based on these three experiments, and assuming that the supplier has already calibrated the flow meters, the following conclusions can be drawn:
1. The accuracy levels obtained under laboratory conditions differ significantly from those measured on-site for some of the flow meters.
2. The mechanical precision of the currently used flow meters is not high enough, resulting in variations in the instantaneous flow rates indicated by the floats under the same operating conditions.
3. When using analog gauges, it is difficult to achieve precise control through manual valves, as the resolution of the scales and the instability of the instantaneous flow rates lead to significant errors in the measurements.

Based on this analysis, it was decided to calibrate each flow meter on-site using a mass flow meter, record the corresponding flow values, and remanufacture the dials. These calibrated meters were then tested on-site, with error levels not exceeding 3.5%. Subsequently, all flow meters used in this process had their dials remanufactured and replaced, meeting the user’s requirements to a large extent.