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Common misoperations and solutions in pump experiments

Posted by: steel world 2021-10-19 Comments Off on Common misoperations and solutions in pump experiments

Abstract: Summarize some common wrong operations and solutions in pump test. The correct operation procedure that should be adopted in the performance test is also introduced by taking the open bench manual test as an example. Key words: pump test operation method The pump is a mechanical product with a large quantity and a wide range, which plays a wide and important role in various sectors of the national economy. effect. The quality of pump products must be inspected by tests. Accurate test results are the basis for judging whether the pump products are qualified or not, and accurate test results are the basis for correct test operations. It can be seen that correct test operations are extremely important for pumps. Significance. In the usual tests of the pump factory, it is found that some testers have made many common misoperations in the test. Because of these misoperations, the measured test results cannot accurately reflect the true performance of the product, which is either exaggerated or degraded. The actual quality of the product has caused undue losses to pump manufacturers and users. Now take the open test bench manual test as an example to introduce some common misoperations and solutions in the test.
  1. Common misoperations and solutions
    1.1 The reference of the pump is inaccurate, resulting in inaccurate calculation of the epitope difference The benchmark is defined as: the horizontal plane passing through the center of the circle drawn by the outer end of the impeller blade inlet, for multi-stage pumps, the first stage impeller is the benchmark, for vertical double-suction pumps, the upper blade is the benchmark, as shown in Figure 1. .

    The benchmarks of common pumps are as follows: IS, IH, IR, IT, S, Sh, Y. D, DG and other horizontal pumps are based on the horizontal plane where the centerline of the pump shaft is located. DL, LD, LG and other vertical multi-stage pumps The datum is the horizontal plane at the point of the largest diameter of the inlet side of the first-stage impeller blade.

    Figure 2 For this type of pump, due to the poor standard of the pump, the pump head adopts the following calculation formula:

    H is the total head of the pump, m where is the pressure of the pump outlet pressure gauge, MPa P is the density of the pumped medium, kg* g is the acceleration of gravity, m/s2 Z, .2 is the distance from the center of the outlet pressure gauge to the liquid surface, m V is the flow velocity at the pressure side section of the pump outlet u, M/s
    1.2 ignores the calculation of the velocity head, which leads to increased test errors. For pumps with small flow, high lift, and small difference in the inner diameter of the pump inlet and outlet flanges, because The speed head value accounts for a small proportion of the total head and has little effect; however, for pumps with large flow, low head, and large differences in the inner diameter of the pump inlet and outlet flanges, the proportion is quite large, as shown in Table 1. , The efficiency index is significantly reduced, and each test point must be calculated, otherwise the correctness of the test data will be affected. The following formula can be approximated when calculating:

  1.3 Water leakage at the vent hole of the orifice flowmeter Sometimes a little water leaks at the vent hole of the flowmeter, which did not attract the attention of the tester. The leakage is considered to be small and has little effect on the flow rate. In fact, it will bring large errors to the actual measurement results. The orifice flowmeter measures the flow rate by measuring the pressure difference between the two sections. Although the leakage is small, it has a significant impact on the measured pressure value of the side flow section, so it also has a significant impact on the flow measurement result. Take measures to prevent water leakage at the air release valve.
  1.4 Orifice flow, the gas at the meter is not exhausted During the test, some test operators did not exhaust the gas due to insufficient knowledge when venting the orifice flowmeter. The design of the orifice flowmeter is designed according to the liquid-filled medium, not according to the gas-liquid two-phase design. In the liquid transportation of the test, a small amount of gas has a greater influence on the measured flow rate. Someone has done a test. Because the gas is not exhausted, the flow measurement error has reached more than 20%. Therefore, in the test, we must pay attention to the deflation problem at the orifice flowmeter and exhaust the gas here.
  1.5 Improper selection of test instruments During the test, due to improper selection of test instruments and meters, errors have been brought to the test results. The following specific situations are common: (1) The accuracy of the test instruments is not enough. The instruments and meters are not in accordance with GB3216. The standard requires selection, the accuracy of the instrumentation is relatively low, and the error increases. For example, the B-level test bench is equipped with the C-level standard. (2) The measuring range of the test instrument is inappropriate, too large or too small. The solution is to choose strictly according to the GB3216 standard. (3) The use time of test instruments exceeds the valid time of calibration.
  1.6 Pre-rotation at the inlet of the pump should be corrected. Sometimes during the performance test, pre-rotation occurs at the inlet of the pump in the small flow area, which makes the test data inaccurate and the performance curve produces a hump; what is more, it will make the performance test of the pump. If it cannot go on, the large flow cannot be adjusted. In this case, the test data measured under the pre-spinning condition should be calibrated according to the method stipulated in the GB3216 national standard; for the situation where the performance test cannot go on, a suitable pump can be selected and connected in series with the pump under test. Before that, experiment again.
  1.7 Improper selection of cavitation test method For pump products with different cavitation performance, different appropriate test methods should be adopted. Improper methods will bring great errors. Nowadays, it is more commonly used in the industry to adjust the opening of the gate valve to change the suction resistance. This force-one method is easy to cause cavitation first because the internal structure of the gate valve affects the flow state. In some cases, the cavitation test has a large error. This has little effect on pumps with a large NPSH value, while the impact on pumps with a small NPSH value is very obvious. For this type of pump cavitation For the test, it is recommended to use a closed test bench to test by changing the pressure on the suction liquid surface (that is, the method of vacuuming). The test of pumps with large flow and head can be automatically controlled by a computer. The pump test with small flow and head must be manually adjusted. The data collection of each working condition point must have sufficient stabilization time until the flow state is stable before collecting data.
    1.8 Unreasonable installation and selection of the suction pipeline will affect the test accuracy. The specific conditions are as follows: (1) The suction pipeline is not well-sealed and the suction pipeline is leaking. The air leakage here is not easy to find. The reason is that the suction pipeline is under negative pressure during the test operation. The solution for air leakage from the outside to the pipeline is: the suction pipeline can be pre-installed, and the pressure test is performed to check the tightness. After the installation is qualified, do not disassemble easily without special circumstances. (2) The size of the suction pipe is not properly selected. One situation is that the inner diameter of the suction pipe is smaller than the inner diameter of the pump suction port flange; the other is that the length of the steady flow section of the suction pipe straight pipe is not selected according to the standard requirements, and the inner diameter of the suction straight pipe section should be selected to match the pump suction. The flange inner diameter is the same specification, and the length of the suction straight pipe section must be greater than or equal to 12D (D is the inner diameter of the pump suction flange). (3) The structural installation of the suction pipeline is unreasonable, and the phenomenon of gas trapping occurs. The straight pipe section of equal diameter should be selected in accordance with the GB3216 national standard.
    1.9 Inappropriate test data processing, resulting in large errors in the results (1) Some test operators often, in order to save trouble, when calculating the critical cavitation margin, directly interpolate between the two operating conditions on the test record data. have to. This is inaccurate. In the case of cavitation, the flow state is quite bad, and the fluctuations in flow and pressure are relatively large, so the collected data error is relatively large. If the test operator staff condition adjustment procedures Incorrect, it will bring greater error to the test results. The correct way to deal with it is to draw the H-NPSH performance curve based on the test record data, and obtain the critical cavitation margin on the curve according to the specified drop value of the head. (2) Some testers often ignore the conversion of NPSH when the actual speed is different from the standard speed when processing the test data. When the difference between the actual speed and the standard speed is small, the error is correspondingly smaller, but in some cases, the difference in speed cannot be ignored. For example, the specified speed of the tS pump is 2900r/nine in, but the actual measured speed is higher. As shown in Table 2, this error has exceeded the standard specified 5.3% (C-level standard) error.

  2. Operation procedure of pump test
上面介绍了所遇到的一些常见的误操作,但实际试验中还存在一些其它方面的错误.为了准确地进行试验,还必须严格地按照操作程序进行试验,以避免或减少试验中的错误,现以开式台手动试验为例介绍一下试验操作程序。    性能试验的操作程序:    2.1由库房提取待试泵,对待试泵进行盘车及其它外观检查,发现异常现象应及时向有关技术人员提出,停止继续工作,通知钳工进行维修或另做其它处理。    2.2仔细查看泵的标准性能参数,根据性能参数进行试验的准备工作.    (1)确定试验方法及试验场地.    (2)选择合适的仪器仪表试验设备。    (3)准备试验的流量单及振动、噪声、泄漏量的允许标准值等。    2.3合理进行安装联接    应注意的几个重点部位:    (1)吸入吐出管路的联接.    (2)测试仪表的联接.    (3)联轴器的找正。    (4)安装基础的联接.    2.4对扭矩仪进行调零(如用扭矩仪进行功率转速测量的话)。    2.5对泵进行灌水或抽真空,检查系统内是否漏气,若有水封、冷却水(高温试验)等应打开给水阀门,同时为泵的起动做好准备。    2.6关闭仪器仪表(出口压力表打开),起动泵。    2.7打开仪器仪表,将工况点调至设计点。    2.8对系统进行放气.放气的位置:单管差压计、流量计部位、双管差压计、出口压力表处等。若是填料密封,应调节填料的松紧程度。    2.9进行运转试验,运转时间参见标准,同时观察泵的运转情况.    在泵的运转中,应仔细做到:耳“听,、眼“看”、手“摸”。    “听”—随时注意听泵和电动机运转声音是否正常。    “看”—经常注意观看测试仪表的读数是否正常及轴封的泄漏量是否合适.    “摸”—用手摸轴承、轴封处和电动机的温度是否正常。    计算泵的实测性能参数,对比测试数据与标准参数,若差异较大,应分析检查原因。    2.10测试泵的振动、噪声、泄漏量。    2.11将泵工况调至较小的稳定工况点进行放气,准备开始进行性能试验(离心泵的试验)。    2.12调至待测工况点,稳定后对各性能参数同时进行记录。    对离心泵由小流量向大流量方向调节。    对混流泵、轴流泵由大流量向小流量方向调节。    2.13最后一个工况点记录完毕后,将工况调至设计点。    2.14对试验记录进行定性的检查,若发现某点有异常情况应重新进行该点试验.    2.15进行汽蚀试验,具体情况略,见有关文献。    2.16关闭仪器仪表,停泵。    2.17按有关标准对试验数据进行整理井绘制性能曲线。    2.18再次检查试验结果,若发现试验不符合有关规律,应分析检查原因。    2.19试验结果无误,通知试验人员拆泵。    拆泵顺序:    (1)仪器仪表的联接部分。    (2)联轴器的串螺(指橡胶塞销串)。    (3)泵的进出口管路。    (4)泵、扭矩仪、电动机的地脚压板。    2.20放掉泵内积水,封闭泵的进出口法兰,将泵及其它设备放回原位。    2.21出具正式的产品试验报告,包括试验人员、校核人员的签字及单位盖章等.
  3. Conclusion
  The test of pump products is a job involving a variety of professional knowledge, such as pump professional knowledge, electrical knowledge, computer knowledge, test knowledge, etc. It is not easy to be truly familiar with this task. Matters need to be explored, accumulated, and mastered by experimenters. The above are some of the problems and understandings I encountered in my work, which are for reference only. Please criticize and correct any improprieties.

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