3.5KW-5.5KW MPPT High Frequency Inverter
3.5KW-5.5KW High Frequency Inverter(MPPT)
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Rotary piston flowmeters are a type of volumetric flowmeter known for their reliable operation, high measurement accuracy, and minimal pressure loss. They perform well even when the viscosity of the medium exceeds 50 cp, as changes in viscosity do not affect the measurement accuracy. These meters are ideal for measuring both instantaneous and cumulative flow rates of liquids that do not contain solid particles. However, after prolonged use, many of these devices require repairs, and calibration data often shows that they have gone out of tolerance. This article explores the measurement principle, structural composition, and root causes of errors to identify effective correction methods.
**Measurement Principle Analysis**
The rotary piston flowmeter operates based on the continuous filling and emptying of a fixed-volume measuring chamber (V1 + V2). The core components include a measuring chamber with a cover, a rotating piston, and a partition plate. The slotted piston has two vertical pins, while the partition plate is radially positioned between the inlet and outlet. The measuring chamber pin aligns concentrically with the guide ring. As the fluid flows through the meter, the slot of the piston moves linearly along the diaphragm, and the piston pins rotate around the guide ring. Since the volume of the measuring chamber remains constant, the instantaneous flow rate depends solely on the rotational speed of the piston, while the total flow is determined by the number of revolutions.
**Measurement Error Generation and Correction**
**1. Errors Caused by Changes in the Measuring Chamber Volume**
During long-term use, fluctuations in pipeline pressure can cause the piston to rotate too quickly, leading to impacts on the partition plate and bending of the diaphragm. This can result in misalignment of the piston, causing severe wear on the inner walls of the measuring chamber, the piston's end faces, and the partition plate. To restore normal operation, the measuring chamber and piston must be ground, which alters the internal dimensions and, consequently, the volume of the chamber.
Before grinding:
V = (V1 + V2) = [(D² - d²) + (D1² - d1²)]
After grinding:
D increases, d decreases, so the new volume becomes:
V' = [(D² - d²) + (D1² - d1²)] + ΔV = V + ΔV
This change leads to a deviation in the measured flow, increasing the error over time. To correct this, two main approaches are used:
**(a) Gear Replacement Based on Calibration Data**
Many flowmeters use gear mechanisms to transmit signals. By replacing the paired gears according to the calibration results, the measurement error can be corrected. For example, if a flowmeter originally calibrated at 0.5% accuracy now shows a -1.27% error, the gear ratio can be adjusted to compensate. This involves identifying the original gear markings, calculating the actual error, and selecting new gears that match the desired accuracy.
**(b) Coefficient Adjustment**
Modern control systems such as distributed control systems (DCS) and KMM regulators allow for coefficient adjustments. By modifying the gain factor (k), the measured value can be corrected to reflect the true flow. For instance, if the flowmeter reads -1.05% after recalibration, the system can apply a correction factor to adjust the displayed value accordingly.
**2. Errors Caused by Temperature-Induced Density Changes**
Temperature significantly affects the density of liquids, which in turn impacts the accuracy of volumetric flow measurements. While pressure has little effect on liquid density, temperature variations can be substantial. For example, ethylene glycol’s density changes with temperature, leading to discrepancies in volume flow readings when the mass flow rate is kept constant.
If the mass flow rate is fixed but the temperature drops, the volume flow will increase, causing the meter to show an incorrect reading. In such cases, the only effective correction is to adjust the setpoint of the volume flow, not the meter itself or the gear/coefficient method. This ensures that the process requirements are met accurately.
In conclusion, understanding the sources of error in rotary piston flowmeters—whether due to mechanical wear or temperature-induced density changes—is crucial for maintaining accurate measurements. Implementing appropriate correction methods, such as gear replacement or coefficient adjustment, helps restore the meter’s performance and ensures compliance with process requirements.