Rotary piston flow meters are the best instruments for liquid material assessment and management because they have the advantages of low lower flow limit, wide range, high accuracy, simple structure, reliable operation and lower price than volumetric flow meters of the same specifications. They are also conducive to the in-depth development of energy-saving work.
It can be directly installed in trains, tractor generator sets, and ships navigating inland waterways to measure the fuel consumption of various power machines, as well as various loading and unloading meters and pipeline liquid flow meters. It is welcomed by energy metering departments, scientific research departments, transportation departments, vehicle and ship manufacturers, and various liquid-using fields. It is currently the ideal fuel consumption meter that can be applied to a large number of transportation vehicles.
Working principle of rotary piston flow meter
A rotary piston flow meter is a type of volumetric flow meter. It operates on the principle that the piston and the metering chamber remain in a tangential, sealed state. It has a fixed eccentric metering element, the piston. Under the influence of a pressure differential, a torque is generated on the piston, causing it to rotate eccentrically. The number of piston rotations is proportional to the fluid flow rate. By recording the piston rotation ratio through a counting mechanism, the total fluid flow rate can be measured.
The rotary piston flow meter measures flow based on the principle of continuously filling and emptying the measuring chamber (V1+V2) (see Figure 1). The measuring mechanism consists of a covered measuring chamber, a rotary piston, and a partition. The slotted piston has upper and lower piston pins, and the partition is radially installed between the inlet and outlet. The measuring chamber pin is concentric with the guide ring. During measurement, the slot of the slotted piston moves linearly along the partition, while the piston pin moves circumferentially along the guide ring. Since the volume of the measuring chamber is fixed, the instantaneous flow rate of the medium flowing through the measuring chamber depends only on the rotational speed of the rotary piston, and the cumulative flow depends on the number of rotations of the rotary piston.
Instantaneous flow rate (Q) = (V1 + V2) / Rotational speed (V) Cumulative flow rate (Q) = (V1 + V2) / Number of rotations (n)
From the measurement principle, we can see that the accuracy of the measurement, and whether it meets or reaches the required precision, mainly depends on whether the volume of the measuring chamber changes. As long as the volume remains constant, measurement errors are unlikely to occur. In other words, the root cause of measurement errors is a change in the volume of the measuring chamber.
Generation and correction of measurement errors
Shanghai Kanghui's U-How® rotary piston fuel flow meter has developed a set of methods for correcting flow meter errors in its measurement and testing services for fuel consumption in large industrial enterprises such as oil fields, ports, and transportation.
1. Errors and corrections arising from changes in the volume of the measuring chamber
During operation, pressure changes within the process pipeline can cause excessive instantaneous rotational speed of the rotary piston in rotary piston flow meters. This can lead to impacts from the piston groove against the baffle plate, causing the baffle plate to bend. During maintenance of these meters, it was found that malfunctions caused by baffle bending or even breakage result in the rotary piston not rotating along its original trajectory, causing severe wear and even scratches and grooves on the inner and bottom walls of the measuring chamber, the upper and lower end faces of the piston, and the inner and outer walls. In such cases, it is necessary to replace the baffle plate and then grind the measuring chamber and piston to allow the rotary piston to rotate freely within the measuring chamber and resume its measuring function. However, after multiple maintenance and grinding operations, the volume of the measuring chamber will change (see Figure 2).
Before grinding: V = (V1 + V2) = [(D2 - d2) + (D12 - d12)]
H—Depth (height) of the measuring chamber;
V—Total volume; V1—External volume;
V2—Content Volume.
After grinding, the following dimensions will change.
D↗, D1↗ (increases), d↘, d1↘ (decreases), then
V'=（V1+V2）=[（D2-d2）+（D12-d12]+△V=V+△V
V'—Total volume after grinding; △V—Volume change before and after grinding.
If the volumetric flow rates corresponding to V and V' are Q and Q' respectively, the reference error for deleting this table is:
As the number of maintenance and grinding cycles increases, the δ value will become increasingly larger, exceeding the accuracy class range of the meter, making its measurement accuracy unable to meet the needs of production processes. To restore its measurement accuracy and regain higher measurement precision, this measurement error can be corrected through the following two methods to meet the accuracy requirements.
(1) Correction is made by replacing the matching gears using the calibration data after overhaul.
As we all know, many flow measurement instruments use gear mechanisms for transmission, and the gear ratio determines the proportional relationship between the indicating mechanism and the measuring mechanism. We achieve the correction by changing this proportional relationship.
For example, a rotary piston flow meter with an accuracy of 0.5 class, after multiple repairs and grinding, was recalibrated, and the calibrated indication error value was -1.27%. The correction is now achieved by replacing the mating gear, as follows:
a. Disassemble the transmission gear part of the signaling mechanism, remove the transmission gear, and read the markings on the gear. If the measuring mechanism gear is marked "1-74", the indicating mechanism gear is marked "1-125".
b. By referring to the table, we find that the indicated error of the original paired gear has been compensated to be 0.29%, and then we calculate the actual error of the flow meter.
Actual error = Compensated indication error + Calibration error = 0.29% + (-1.27%) = -0.98%
c. Refer to Table 2 to find the matching gear with a compensation value of -0.98%. The measuring mechanism gear is marked "1-60" and the indicating mechanism gear is marked "1-110". Find the matching gear with the mark, install it, and recalibrate to achieve the required accuracy.
(2) Correction is performed by multiplying by a coefficient k.
Currently, many factories use distributed control systems (DCS) and instrumentation systems such as KMM digital controllers. These systems have the function of adjusting and modifying the coefficients of variable parameters. We utilize this coefficient adjustment method to correct errors.
For example, a rotary piston flow meter with an accuracy of 0.5 and a flow range of 0.6~6 m3/h, after multiple repairs and grinding, has its relative indication error recalibrated to -1.05%. Based on the given data, Table 3 and Figure 3 are obtained.
Table 3
Based on Table 3 and Figure 3, we can see that:
Before maintenance and calibration:
Ratio coefficient (k) = tɡα = Calibration value / Indicated value = 1
Post-repair calibration:
Ratio coefficient (k') = tgα' =
Based on the calculated ratio coefficient, the measured signal from the field is corrected in the control loop of the distributed control system, or in the variable parameters of the KMM controller, to obtain the actual indicated value and cumulative value.
Actual indicated value = Measured value (PV value) × Ratio factor (k'), that is:
Q'=Q·K'
2. Errors and corrections caused by changes in medium density due to changes in medium temperature.
It is well known that many volumetric flow meters are affected by temperature in terms of flow measurement accuracy. This effect can be mitigated to the greatest extent possible through compensation and selection. However, many production plants, especially chemical plants, have high flow requirements, and chemical reactions are conducted using mass flow rate rather than volumetric flow rate. The relationship between mass flow rate (M) and volumetric flow rate (Q) is: M = ρ·Q (ρ is the density of the medium). If the density of the fluid medium changes while the volumetric flow rate remains constant, the mass flow rate will change accordingly.
For liquid media, pressure has a negligible effect on density. However, the effect of temperature on density cannot be ignored. To achieve the goal of measuring mass flow rate using a rotary piston flow meter that measures volumetric flow rate, we have made some attempts. Below, we take ethylene glycol, a chemical raw material, as an example, and discuss the influence of temperature on volumetric flow rate measurement based on the temperature-density relationship of ethylene glycol.
For example, a rotary piston flow meter with an accuracy of 0.5 and a measurement range of 0.6~6 m³/h is set to control a flow rate of 2.5 m³/h at a medium temperature of 180℃. If the temperature changes to 165℃, the volumetric flow rate will change while the mass flow rate remains constant, resulting in a deviation of the indicated flow rate from the set value, indicating an out-of-tolerance reading.
The volumetric flow rate at 180℃ is 2.5 m³/h, and the density of ethylene glycol is 986 kg/m³. Therefore, the controlled mass flow rate at this temperature is:
M = ρ·Q = 986 × 2.5 = 2465 kg/h
At 165℃, if the controlled mass flow rate is to be 2465 kg/h, and the density of ethylene glycol is 1000 kg/h, then the corresponding volumetric flow rate is:
Therefore, due to temperature changes, the error caused by volumetric flow rate is obvious when the mass flow rate remains constant.
Relative error of indication =
Since this error is caused by external factors (temperature) in the rotary piston flowmeter and has a significant impact on the process, and chemical reactions are only related to the mass (or moles) of the substance, but not its volume, to ensure a constant mass flow rate, it is necessary to consider correcting for volumetric flow rate errors when there are significant temperature changes. This correction cannot be achieved through the rotary piston flowmeter itself, nor by changing the proportioning gear or multiplying by a coefficient k; it can only be corrected by changing the setpoint of the volumetric flow rate. In the example above, changing the flow rate setting from 2.5 m³/h to 2.465 m³/h will achieve the correction and meet the process's measurement requirements.
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