The pressure gauges and vacuum gauges used in the oil reservoir pumping system are all spring tube pressure gauges. They are mainly composed of spring tube, transmission mechanism, indicating mechanism and case. The spring tube is a hollow tube bent into a circular arc with an oval cross-section. One end of it is welded to the stem of the pressure gauge and communicates with the medium under pressure. The other end of the tube is a closed free end. Under pressure, the free end of the pipe is displaced. Within a certain range, the displacement is linear with the measured pressure. The transmission mechanism is mainly composed of a sector gear, a center gear, and a hairspring. The transmission mechanism is also called the movement. The main function is to amplify the elastic deformation of the spring tube, and convert the linear displacement of the free end of the spring tube into the arc-shaped rotation displacement of the meter pointer. The indicating mechanism includes a pointer, a dial, and the like. The main function is to indicate the elastic deformation of the spring tube by turning the pointer so that the pressure value is read on the dial. The case is also called a stand. Its main function is to fix and protect the above three parts and their components.
Spring tube pressure gauge structure and working principle as shown. The spring tube 2 communicates with the connector 7, and when the pressure is drawn from the connector 7, the cross-sectional area of ​​the spring tube changes from an ellipse to a circle, and at the same time the bending of the spring tube becomes smaller, the tube is slightly straightened, and the free end is displaced. This displacement is converted into the rotation of the sun gear shaft by the transmission mechanism consisting of the pull rod 5, the sector gear 9 and the sun gear 10, and the indicator 3 is driven to rotate on the gear shaft t, so that the pressure value can be read from the dial 6. The balance gear 10 is provided with a hairspring 11 at the lower end, one end of the hairspring is fixed on the sun gear shaft, and the other end is fixed on the struts 13 between the upper and lower clamp plates 4 and 12 . The function of the hairspring is to eliminate the clearance when the sun gear meshes with the sector gear and is measured. The pointer is returned to zero after the pressure is removed.
Based on the different requirements for the accuracy of the measured pressure, the different pressure values, and the different characteristics of the temperature, properties and use environment of the measured pressure medium, the pressure gauges should be properly selected, used, installed and maintained. In this regard, the following issues should be noted.
Pressure gauges are only allowed to be used without damage, with certified seals and without exceeding the test cycle conditions.
When selecting a pressure gauge, consider the measured pressure values ​​and characteristics. Under normal circumstances, under a fixed or evenly varying load, it should be used within the range of 1/33/4 of the instrument scale, and within 1/31/2 of the scale of the instrument under oscillating or fluctuating loads. Pump pressure gauges are usually used within this range.
Pressure gauge spring tubes have soldered or brazed connections, as well as threaded connections. In general, when the temperature of the measured medium is lower than 180T: when soldering is used, and when the temperature is higher than 1801:, a pressure gauge connected with brazing or threaded connection may be used.
When in use, the pressure gauge should be installed vertically in an easy-to-observe, easy-to-maintain position.
The pressure gauge should be used within the range of -40t:~60t for the measured medium and ambient temperature. When the ambient temperature exceeds 25T:, an additional 0.04% additional temperature error will be generated. The pressure gauge installation position is perpendicular to the pressure measurement point. He is a university chemical engineering major, lecturer, engaged in professional research and teaching of oil storage and transportation, and has published more than 30 papers in domestic journals.
When it is larger, the liquid column difference correction should be performed. The correction value is equal to the pressure value generated by the liquid column in the catheter that is vertically different from the pressure measurement point. When the instrument is higher than the pressure measurement point, this correction value is added; when the instrument is lower than the pressure measurement point, this value should be subtracted.
Pressure gauges in use should be tested regularly according to the different conditions of use. The maximum verification period is no more than six months.
In the past, the scale unit of the old-style pressure gauge was kg//cm2, and the scale unit of the vacuum gauge was mmHg. Now the scale units of the new pressure gauge and the vacuum gauge are all MPa, and the conversion relationship between them must be grasped.
Figure j spring tube pressure gauge structure indicating a machine base; 2 - bullet magazine; 3 - pointer; 4 on the splint; 5 * connecting rod; 6 - dial; 7 - joints; 8 - joint screws; 9 a sector gear ;10-central gear; 丨丨 a hairspring; 12 ─ lower splint; 13 pillar 2 "two tables" role 2.1 According to the "two tables" readings estimate pump head In the chestnut pipeline installation, a The discharge valve is installed on the pressure gauge and vacuum gauge is installed on the suction population. The readings of the pressure gauge and vacuum gauge can be used to understand the operation of the pump. The readings of the pressure gauge and vacuum gauge have the following relationship with the head of the crucible: the difference between the A/7 pressure gauge and the vacuum gauge, W; the reading of the pm pressure gauge, the reading of the pM vacuum gauge, MPa; the severity of the liquid being transported, N/w3;v flow rate at the chest discharge outlet, wA; w* pumping population flow rate, wA; formula (1), when the pressure gauge measurement point is higher than the vacuum gauge measurement point, A/z is positive, Instead, it should be negative. When the pressure gauge and vacuum gauge are installed at the same height (such as Y oil), that is, AA = 0. When the suction pipe diameter of the pump is equal to the discharge pipe diameter, the relationship between the pump head and the reading of the pressure gauge and the vacuum gauge may be as follows: Said: In actual work, when estimating the lift of the pump, Equation (2) 2.2 can be used to judge the fault of the pipeline according to the changes of the “two tablesâ€. According to the basic knowledge of hydraulics, Bernoulli's equation can be used to obtain the calculation formula of the two readings. : The resistance loss of the road;/, The installation height of the discharge pipe; The resistance loss of the hm discharge pipe.
From Equation (3) and Equation (4), it can be seen that when the pump installation system does not change, that is, the vacuum gauge reading! It is proportional to the resistance loss of the suction pipe; the reading of the pressure gauge is proportional to the resistance loss hffia of the discharge pipe. In this way, as long as the pipe's resistance loss changes during work, it will inevitably be reflected in the vacuum gauge and pressure gauge readings.
Let's analyze the change of the “two-meter†reading when there is a pipe failure.
2.2.1 After the centrifugal pumping pipe is blocked, the resistance loss of the suction pipe increases, and the system flow will decrease. It can be known from formula (3) that the vacuum gauge reading will increase; and the resistance of the discharge pipe will increase. The loss is reduced due to the decrease of the system flow. It can be seen from equation (4) that the reading 1* of the pressure gauge will also become smaller.
After the discharge line is blocked, the resistance loss of the discharge line increases, and the system flow will decrease. It can be seen from formula (4) that the reading of the pressure gauge will increase accordingly: while the resistance loss of the suction pipeline is due to system flow Decrease and decrease. It can be seen from equation (3) that the reading/* of the vacuum gauge will also decrease.
After the discharge pipe is broken, the resistance loss of the discharge pipe is reduced and the system flow rate is increased. The reading of the pressure gauge will be reduced by Equation (4), and the resistance loss of the suction pipe will increase due to the sudden increase of the system flow. It can be seen from equation (3) that the vacuum gauge reading/* will also increase.
After the above three kinds of faults appear, the system flow will change, and the flow rate of the suction pipe and the discharge pipe will also change. The influence of the meter and pressure gauge readings, but compared with the impact of changes in the pipeline resistance loss on the “two tables†readings, the speed head changes have little effect on the “two tables†readings and may not be considered. .
It is worth noting that the changes in the readings of the "two tables" when the suction pipe is blocked and the discharge line is broken are the same, that is, the vacuum gauge reading is larger than the normal value, and the pressure gauge reading is smaller than the normal value.
However, chestnuts in the work of the risk of rupture of the discharge pipe is greater than the risk of inhalation pipe plugging, therefore, when the vacuum gauge reading is found to be greater than normal, and the force meter reading is smaller than the normal value, should first consider the discharge pipe Break this situation. In addition, when chestnuts are airy, vacuum gauges and gauge readings are smaller than normal, unstable, and even drop to zero in storage and transportation technology. When the pump generates cavitation, the readings on the “two tables†are extremely unstable, especially vacuum gauges, which are high, low, and severely zero.
2.2.2 Volumetric pump volumetric pump When a pipeline fault occurs, the change of the vacuum gauge and pressure gauge is generally the same as that of the centrifugal pump except that the speed is reduced and there is gas in the pump. The analysis is as follows: When the blockage is not serious, the vacuum gauge reading increases, because the pump flow rate does not change, so the pressure gauge reading does not change. When the blockage is severe or even completely blocked, the vacuum gauge reading increases and the gauge reading drops to zero. Because the traffic is greatly reduced at this time. Even stop flow.
When the discharge line is blocked, the gauge reading rises, because the pump flow rate does not change, so the vacuum gauge reading does not change; when the blockage is serious and the pressure exceeds the safety valve control pressure, the safety valve opens.
The pressure gauge only drops.
When the discharge pipe bursts, the gauge reading suddenly drops. Vacuum gauge readings are generally unchanged because the flow rate does not change.
3 Conclusion It needs to be pointed out that in order to use the "two tables" reading change to judge the pipeline fault, it is necessary to understand the pressure gauge and vacuum gauge readings during normal operation. Only when you know that you are normal can you distinguish between abnormalities.
At the same time, it should also be noted that it is not possible to be confined to conclusions based on only a single phenomenon. Instead, it should be based on the specific circumstances at the time and use the specific knowledge of the learned knowledge and practical experience to analyze it.
No, it is not. As long as we understand the significance of the “two tables†readings and can skillfully use the changes in “two tables†readings to judge pipeline failures and solve practical problems, it will be clear that the role of the “two tables†should not be ignored, but should be carefully selected. Maintain them and make full use of them.
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