Performance assessment indicators of Roots oil pump and analysis of reasons for tripping

（1） The performance evaluation index of Roots oil pump is not accurate enough to use pumping speed as the main performance evaluation index. It cannot truly reflect the quality level of Roots oil pump. It is recommended to use zero flow compression ratio as the performance index to evaluate the pumping capacity of Roots oil pump

From the surface, the pumping speed and zero flow compression ratio of Roots oil pumps are two completely different concepts, seemingly unrelated performance indicators. But in reality, they are not only performance indicators that represent the suction capacity of Roots oil pumps, but also closely related to each other. Compared with pumping speed, the zero flow compression ratio can truly reflect the design and manufacturing quality of Roots oil pumps, and has unparalleled advantages in testing and application. Therefore, we believe that the zero flow compression ratio should be used as an evaluation index for the pumping capacity of Roots oil pumps

Generally, the Roots oil pump can only work when matched with the front stage pump, so its pumping speed is closely related to the front stage pump. Roots oil pump, liquid ring pump, rotary vane pump, slide valve pump, piston pump, etc. can form various types of vacuum units, and various ratios can be used according to different needs, so the vacuum units formed have different pressures and pumping speeds. Therefore, the so-called Roots oil pump pumping speed is actually the pumping speed of the vacuum unit. It largely depends on the type and size of the front stage pump, that is, the pressure and pumping speed of the front stage pump. Although the quality classification standard for Roots oil pump products specifies the model of the front stage pump, due to the standard for front stage pumps (rotary vane pumps, slide valve pumps), the geometric pumping speed of the pump is (1.1-1.2) nominal pumping speed, so the measurement error of pumping speed alone reaches 2%

The change in ambient temperature will inevitably have an impact on pumping speed measurement, especially when the vacuum degree is high. It indirectly affects pumping speed by affecting the vacuum degree of the preceding pump. This point has been elaborated on in the author's previous article "Discussion on Performance Evaluation Indicators of Roots Oil Pump - Pressure". The "Roots Oil Pump Product Quality Grading" standard specifies the use of the pre pump recommended in the "Roots Oil Pump" standard when assessing pumping speed. This is not a big problem for Roots oil pumps below ZJ-300, but a big problem for Roots oil pumps above ZJ-600. One of them is the pre stage pump recommended by the standard, which includes both Roots oil pump and rotary vane (slide valve) pump. It is not necessarily required by the user, but is only designed and manufactured for measuring pumping speed, specifically for a vacuum unit, or even a very complex unit. For example, when testing a ZJ-10000 Roots oil pump, it is equipped with front stage pumps such as ZJ-2500, ZJ-600, and H-150. On the other hand, such a matched unit can only test pressure and pumping speed, and cannot measure large allowable pressure differences and zero flow compression ratio curves, because the Roots oil pump, as a front stage pump, cannot withstand the pressure differences generated during testing. In this way, in order to test the larger allowable pressure difference and zero flow compression ratio curve, and to equip a front stage pump, another set of units is manufactured, which is less desirable for Roots oil pump manufacturers

(2) Analyze the reasons for the tripping of the Roots pump

After using the Roots pump for a period of time, various faults will occur to affect the performance of the equipment. Among them, the phenomenon of equipment tripping is one of them. To solve this problem, we need to correctly analyze the reasons for equipment tripping and develop a reasonable solution.

1. Electrical reasons

2. Motor reasons

3. The outlet pressure fluctuates and the inlet resistance is high

An instantaneous increase in outlet pressure or outlet resistance can also lead to instantaneous overcurrent. The high import resistance causes negative pressure at the suction port, leading to overcurrent; Small export gas consumption can also cause overcurrent:

① Liquid level fluctuation

② Abnormal or stuck check valve at the outlet of the outlet valve

③ The outlet waterproof backflow device is abnormal

④ The fan flow rate is too high, resulting in an increase in pressure

⑤ Large pressure difference, i.e. high back pressure or inlet pressure

⑥ Inlet filtration blockage, outlet pipeline obstruction or blockage

4. The root pump's own reasons

① Check if the components and runout of the coupling are abnormal

② Check the clearance between each rotor; Collision and friction (jamming) of rotating parts

③ Check the bearings at both ends of each rotor and check if there is any outer race phenomenon of the ball bearings

④ There are pollutants and impurities on the impeller, causing the gap to be too small

⑤ Gear wear causes large side clearance

⑥ The gear is not firmly fixed and cannot keep the impeller synchronized

⑦ Bearing wear leads to increased clearance

⑧ The oil level is too high

The above are the reasons that caused the Roots pump to trip. Based on the above introduction, we can have a good understanding of the causes of equipment failures, and then adopt relevant methods for treatment and solution, in order to restore the use of the equipment and ensure its working effect