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Analysis of Sealing Problem of Measuring Chamber of Rotary Piston Flowmeter

Rotary piston flowmeter is a positive displacement flowmeter. The isolation and sealing of the inlet and outlet of its metering chamber is an important link related to whether the metering is accurate and stable. The seal is formed by the contact between the rotating piston and the inner and outer cylinder and the diaphragm, and the design of the top notch profile in contact with the diaphragm and for the swing of the piston is particularly critical. During operation, the profile should not only ensure that the piston moves according to the correct track without interference, but also ensure that the notch profile is in contact with the diaphragm, so as to isolate the inlet and outlet chamber and form a reliable seal.

Structure and working principle of measuring chamber of rotary piston flowmeter

As shown in Fig. 1, the piston 7 whose top surface can accommodate the position gap of the diaphragm during movement is located in the annular area composed of the inner wall of the outer cylinder 1 and the outer wall of the inner cylinder 6. The inner and outer walls of the piston are in contact with the outer and inner walls of the inner and outer cylinders respectively, so that the piston center o 'has a offset e from the inner and outer cylinder center O. it is not difficult to see that the outer and inner radii R 1, R 2 of the rotating piston, It has the following relationship with the inner and outer radii R, R 3 of the outer and inner cylinder:

Piston wall thickness Î´= R 1 - R 2 offset e = R - R 1 piston outer radius R 1 = (R R 3 Î´) / 2. There are radial diaphragms between the inner and outer cylinders to separate the annular area.

The radial dimension of the annular area I = R - R 3. The length of the diaphragm L 1 within R is slightly greater than L. the thickness of the diaphragm is B. the distance from the diaphragm to o point L 2 = R - L 1

In this way, the annular area is divided into two crescent shaped inner and outer cavity metering chambers by the piston wall and diaphragm. Under the action of inlet liquid pressure, the piston center o 'rotates counterclockwise around o along the arrow, and the rest of the piston swings in the annular area. Its inner and outer cylindrical surfaces are in contact with the inner and outer cylinders in turn, and the metering chamber continuously moves in a circle to transport fluid. The piston center o' rotates around o for one circle, and the inner and outer metering The cavity rotates for one cycle at the same time, and the liquid is transported and discharged once. The volume sum of the internal and external metering cavity of the crescent shape is the volume flow of the liquid transported and discharged by the rotation of the piston center for one cycle, which is a measurable constant, which is:

Where, H - height of metering chamber

According to the number of revolutions n of the piston, the amount of liquid flowing under the number of revolutions n can be measured:

Analysis of sealing problems of flowmeter metering chamber

1. Sealing principle

From the above analysis of the structure and working principle of the flowmeter, it can be seen that when the piston works, the outer circular surface of the piston, the inner circular surface of the outer cylinder and their mutual contact form the outer metering chamber; while the inner circular surface of the piston, the outer circular surface of the inner cylinder and their mutual contact form the inner metering chamber. When the piston moves, a gap is required at the contact between its top surface and the diaphragm to ensure The movement is not interfered. More importantly, the notch profile should always be in contact with K near the center of the diaphragm to ensure the sealing of the inner metering chamber; otherwise, the liquid input by pressure from the inlet will not be transported through the metering chamber, but directly flow into the outlet through the notch, which may lead to inaccurate metering or failure of the flowmeter.

2. Analysis and design calculation of piston top notch profile equation

1) As shown in Figure 2, when the rotary piston works, its center o 'rotates counterclockwise around o, and the included angle between the OO' line and the initial position is Î±, with Î± The polar coordinate parameter equation of the top notch profile is derived. For the convenience of design and processing, the polar coordinate system is consolidated on the top surface of the piston. Let the origin of the coordinate system be o ', the initial position of the polar axis be the symmetry line o' B of the notch profile, and the clockwise direction is the positive direction of the phase angle Ï†ã€‚

When 0 Â°â‰¤ Î± When â‰¤ 180 Â°, the right half profile equation of the notch is deduced as:

E, B, L 2 and R 1 are known as described above.

2) When 180 Â°â‰¤ Î± â‰¤ 360 Â°, due to symmetry, the- Î±, - Ï† Replace in equation (1) Î± , Ï† 180 Â° â‰¤ Î± Polar equation at â‰¤ 360 Â°

3. Example calculation

Raw data: r = 27 mm, R 1 = 23 mm, R 2 = 21 mm, R 3 = 17 mm, l 1 = 11 mm, B = 2 mm

Calculated parameters e = R - R 1 = 4 mm, L 3 = 16 mm Î± Value from small to large to R and Ï† The corresponding values of are shown in Table 1.

Machining of profile

Convert equations (1) and (2) into rectangular coordinate equations, as shown in Figure 3, and the conversion formula is as follows:

(3) equation is still Î± It is difficult to directly obtain the functional relationship y = f (x) of Y and X according to the given parameter equation Î± Values obtain the corresponding values of X and Y (see the table above), take their corresponding coordinate points as nodes, and establish a piecewise equation of y = f (x) by using Newton equal difference interpolation or spline interpolation; Based on this equation, a program can be programmed to complete the machining conveniently on NCN wire cutting machine.

Other measures to improve sealing reliability

1) The leakage of liquid at the gap on the top surface of the piston mainly occurs in the inner metering chamber, so as to shorten the communication time between the inner metering chamber and the inlet as far as possible, so as to reduce the time process of pressure. When designing the shape and position of the inlet, make the inlet close to the inner cylindrical surface of the outer cylinder as far as possible.

2) Similarly, the shape and position of the liquid outlet shall be arranged to communicate with the metering chamber as soon as possible.

epilogue

1. For the dynamic seal of the metering chamber of rotary piston flowmeter, in addition to carefully considering the dimensional relationship between the cylinder radius and the eccentricity of the piston radius, so that the inner and outer cylindrical surfaces of the piston are tangent to the cylindrical surfaces of the inner and outer cylinders respectively, the contact seal between the gap on the piston top surface and the diaphragm, i.e. the profile design of the gap, should also be carefully considered.

2. The notch profile design of the piston top surface should not only meet its normal movement without interference, but also consider the dynamic sealing problem there. Therefore, the notch shape is not random, but carefully designed, as shown in Figure 3.

3. The piston top notch profile can be designed and calculated according to the formulas (1) and (2).

4. The node fitting interpolation equation can be obtained by (1), (2) and (3) parameter equations, and the machining can be completed on the NC wire cutting machine.

5. The flowmeter is a portable device produced by Shanghai Kanghui and used by drivers for fuel flow detection and verification.

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