Establishment of the hottest piston type gas flow

2022-08-13
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Abstract: This paper introduces the principle, work flow, structural characteristics and functions of the gas piston device, mainly introduces the design idea and technical indicators achieved. It can be seen from the index that the development of the device is successful, which has accumulated experience for further work

I. characteristics of gas piston device

the standard volume value is accurate: the standard volume is only related to the descending distance of the piston and the diameter of the cylinder block, and there are few related influencing factors

good repeatability: in principle, its repeatability is only related to the change of cylinder block area, the repeatability of grating measurement and the deformation of sealing ring, and these influencing factors are very small

large internal pressure: the balance pressure in the cylinder can be adjusted by filling the cylinder with compressed air. The maximum internal pressure of this device can reach 200KPa (gauge pressure 100KPA)

stable flow: since the flow value is the product of the piston running speed and the cross-sectional area of the cylinder block, the piston is driven by the servo motor, and the rotation speed of the servo motor is easy to adjust and control, so the flow can easily reach and stabilize at the set flow point during the test

pressure stability: for flow meters with a proportional relationship between flow and differential pressure, pressure stability can be achieved in a short time by relying on the self leveling relationship between flow and differential pressure during the test

II. Working principle

1 Working principle

as shown in the figure, when the device works, the piston moves downward to discharge the gas in the cylinder through the pipeline and flowmeter. When the piston moves downward at a uniform speed, the pressure in the cylinder gradually reaches a stable value after an upward process. It is considered that the temperature, pressure and volume of the gas do not change in this process. Because the inner diameter of the cylinder block is known, the volume value of the gas discharged by the piston running a certain distance can be calculated. Take this as the standard volume flow and compare it with the indicated value of the flowmeter to detect the flowmeter

2. System composition

piston type gas flow device is mainly composed of servo motor, reducer, ball screw, grating, piston cylinder, piston, temperature measurement system, pressure measurement system, pipeline and inlet and outlet valves, computer data acquisition and control system, etc

3. Volume flow calculation method

calculate the volume value of gas discharged by piston descent: vs = a * H (1)

where: a - cylinder block area, m2

h - effective lowering height of piston, m

it is considered that the temperature and pressure of gas in the cylinder remain unchanged, so the volume of gas discharged by the piston is the cumulative flow value of gas through the flowmeter

that is: VP = v s (2)

the instantaneous volume flow of gas through the flowmeter is: QS = VP/T (3)

where: QS - mass flow given by the standard device, kg/s

t - test time, s

III. mechanical structure

the core part of the piston type gas flow device is the piston and cylinder block. It is the piston cylinder and piston that form the enclosed space in the cylinder body, which provides accurate volume for flow measurement. During operation, the piston moves along the cylinder axis in the cylinder. On the premise of ensuring sealing, the product of the internal sectional area of the piston cylinder and the moving distance of the piston is the standard cumulative flow given by the device. Therefore, the design of the device is mainly centered on ensuring the sealing between the piston and the piston cylinder, the geometric tolerance of the piston cylinder, and the dimensional tolerance index, combined with the performance price ratio and other factors under domestic machining conditions

the mechanical structure of the device is mainly composed of cylinder block, piston, stroke measurement system, deceleration and lead screw system, sealing system, base and reinforcing rod, inlet and outlet gas pipelines and valves

1. The overall structure is stable and reasonable

according to the structure and operation process of similar devices, the piston cylinder should be placed vertically, which has the following characteristics:

a the cylinder barrel is evenly stressed and not easy to deform

b if the sealing oil in the cylinder seeps and accumulates, it will not affect the standard volume value

c the dead weight of the piston will strengthen the pressure in the cylinder to a certain extent

d is conducive to the smooth operation of the piston and is not easy to produce crawling phenomenon at low speed

since the motor and transmission system are located above the piston cylinder, in order to increase stability and system rigidity, three additional supporting rods are added, which are connected with the tray on the upper part and the base on the lower part

2. Piston cylinder

the total volume in the cylinder is calculated by the design flow. Considering the diameter height ratio of piston cylinder, the size of piston cylinder is determined as: Ф 200mm × 800mm, of which 100mm above and below the cylinder barrel is a non working area. During processing, it is mainly to ensure the geometric tolerance of the non cylindrical degree of the inner circle of the cylinder barrel within 600mm in the middle of the cylinder barrel

① determination of cylinder wall thickness

the wall thickness of the cylinder should meet the requirements of cylinder strength and rigidity under normal working conditions. The wall thickness of the cylinder block is calculated from the allowable radial and circumferential stresses and strains of the cylinder block. Get the wall thickness of the cylinder block δ= 8.6mm。 Therefore, the wall thickness of the cylinder block is 10mm

② calculation of volume deformation of cylinder block

through calculation, it can be obtained that under the maximum working pressure of 0.2MPa, the volume deformation of cylinder block is 0.0019%. Therefore, the influence of cylinder volume deformation under maximum pressure can be ignored

③ processing requirements

in order to ensure the reliability of the seal between the inner surface of the cylinder barrel and the sealing ring and effectively reduce friction, there should be higher requirements for the non cylindricity and surface finish of the inner surface of the piston cylinder barrel

3. Piston and sealing system

① determination of piston size

the working process of the device is completed by the piston moving at a uniform speed along the axial direction in the piston cylinder barrel. In this process, the piston needs to run smoothly, that is, when the piston runs in the cylinder barrel, it should always be vertical relative to the axis of the cylinder block, and there should be no distortion or shaking

the shaking of the piston in the cylinder block will not only cause uneven operation, but also cause the life plug to jam in serious cases

the compression amount of the sealing ring is 0.2mm. To ensure that the relative position of the piston and the axis of the piston cylinder changes little during the movement, the piston thickness B should be increased. However, if the piston thickness increases too much, the overall height of the device will be increased. Here, b=120mm is selected

② sealing structure

during the operation of the device, the gas in the cylinder can only be discharged through the air outlet. Because the piston and the cylinder body have relative movement, there must be a good sealing structure to make there is no leakage between the piston and the piston cylinder. There are three sealing rings on the piston, and between the two sealing rings is a sealing oil groove. The oil in the oil groove is connected to the oil level indicator outside the device through the positioning guide rod. The indicator is equipped with an alarm for photoelectric oil level measurement. When the sealing oil leaks, the oil level decreases. When the gas in the cylinder enters the oil tank, the oil level rises or bubbles are discharged. When the oil level changes beyond the warning position or bubbles are discharged, the alarm will be sent out

4. Servo motor, reducer, lead screw and guide rod

servo motor, reducer and ball screw use integrated finished products. The ball screw can provide good positioning and running stability. In order to ensure the analysis and manufacturing of piston running speed and the stability of air and space vehicle, the motor, transmission system and ball screw need to have good stability. The motor uses servo motor, and the reducer is turbine scroll rod

5. The dust ring of the base actuator adopts a double lip seat, air inlet and exhaust pipes and blowdown holes

the bottom surface of the base is a conical surface, and the lowest end is a blowdown hole. The upper edge of the air inlet and outlet pipes are higher than the conical surface to prevent oil or other dirt from entering. There are two temperature measuring holes and one pressure measuring hole on the bottom

two ball valves with good sealing are installed on the air inlet pipe, and a compressed air inlet is connected between the two ball valves. When measuring the flow meter with pressure loss greater than 500pa, the pressure in the cylinder can reach the predetermined value first, so as to reduce the balance time required for the test. The front end of the intake pipe is equipped with a gas filter, which is two-layer metal filter with 60 mesh and 120 mesh to prevent dust from entering the cylinder. There is a ball valve on the exhaust pipe, and a flange connection port is connected with the measured flowmeter. Under the base is a base plate, on which there are three foot horizontal adjustment screws. During installation, adjust the cylinder block to the vertical position

IV. electric control

1 Hardware configuration

the electrical control cabinet is mainly composed of five parts: electrical layer, controller layer, computer layer, instrument layer and drive layer. The functions of the electrical layer are: power switch and motor power off; The main functions of the controller layer are: controller power switch, motor control, manual speed signal, etc; The computer layer is used to place the computer host; The instrument layer is used to place thermometer, pressure gauge and grating digital display meter; The drive layer is used to place motor drivers, DC power supplies and other equipment

2. Software configuration

flowmeter verification program is a measurement and control software written under visiuai basic5.0, using VB5.0's own access database

v. technical indicators

1 Technical index

flow range: (0.009 ~ 0.6) m3/h

volume flow uncertainty: 3. Spring testing machine should be relatively important for a production-oriented enterprise Improper use may cause experimental errors. Only after strict experiments can products be produced in large quantities and then put on the market. Therefore, the use requirements of spring testing machine should be controlled in detail U=0.05%, k=2

repeatability: better than 0.05%

working pressure in cylinder: absolute pressure 200KPa

working temperature: (10 ~ 30) ℃

tightness: maximum relative leakage of the device: 0.005%/min

2 Function

computer data collection:

temperature in the cylinder (10 ℃ ~ 30 ℃, ± 0.01 ℃)

pressure in the cylinder (95kpa ~ 200KPa, ± 10Pa)

temperature in front of the flowmeter (10 ℃ ~ 30 ℃, ± 0.01 ℃)

piston descent height (0 ~ 600mm, ± 0.005mm)

test time (30s ~ several different working principles of digital sensor of electronic universal testing machine 1800s, ± 0.001s)

pulse signal input (10Hz ~ 10kHz, counting 999999 ± 1)

current signal input (0 ~ 10mA, 4mA ~ 20mA, ± 0.001ma)

voltage signal input (0 ~ 5V ± 0.001V)

control: it is divided into manual and automatic:

automatic: after setting parameters such as flow, stroke or test time, test times, input mode, press the test start key, the piston runs, and the system automatically judges that the pressure is stable before starting the test. It can be stopped in an emergency halfway, and the piston stops after touching the limit switch

Manual: manually control the start and end of the test, control the operation of the piston, etc

calculation: the average temperature in the cylinder

the volume represented by the descending height of the piston V

is converted to the volume before the flowmeter by the state equation V

and the corresponding flowmeter performance calculation

report: the verification record is automatically generated and stored in the access database

alarm: oil level alarm, collision, lower limit switch motor stop

VI. conclusion

1 the gas piston flow device has the advantages of high stability and good repeatability, and has the functions required by the flow standard device

2 the internal pressure of the gas piston flow device can reach up to 200KPa, meeting the calibration needs of high-pressure loss flowmeter, especially sonic nozzle

3 the principle of the flow produced by the device is constant flow, and then the constant pressure is achieved by the characteristics of the fluid itself, so the adjustment of the flow point is easy to control

4 it is believed that this device will play an important role in the flow transmission system, especially in the aspect of comparison and equivalence between devices

Author: China Academy of Metrology (end)

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