When the main pump of the Roots pump set is in operation, the vacuum level cannot be increased; however, the vacuum level can be increased when the main pump is not used. Why is that?
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Previously, the Roots pump system consisted of an oil-free piston-type pre-pump coupled with a Roots pump as the main pump. As part of recent safety improvements, belt-driven systems were no longer allowed, so screw vacuum pumps were used as pre-pumps instead. However, for various reasons, the pumping rates of the pre-pump and the main pump are not well balanced: for example, the pumping rate of the pre-pump is 120 L/s, while that of the main pump is 150 L/s. When the pre-pump is started, the vacuum level drops rapidly; however, once the main pump begins to operate, the vacuum level reaches -0.09 MPa and then remains almost constant. When another pipeline is connected to isolate the main pump, the vacuum level becomes normal, reaching -0.099 MPa. I’ve never figured out what the principle is! I seek help from fellow sailors! !
It is recommended to provide a flowchart along with the relevant parameters marked for easier discussion.
The equipment manufacturer didn’t provide any explanation either; since it was a modified unit, the screw pump manufacturer said that there was nothing wrong with their screw pumps and thus ignored the issue
I’ve reviewed a lot of information, and theoretically it seems that there is no explanation available yet. But I’m wondering: could it be that the vacuum level at the front end of the main pump is low, causing the Roots pump to draw back flow – in other words, the inlet has become the outlet! Is it possible?
There is no such possibility, unless you check whether the Roots vacuum pump is rotating in the correct direction or not. Secondly, you need to determine whether the motor of the Roots vacuum pump is of 2-stage or 4-stage type
Check the mechanical seal of the main pump; there should be a leak!
Take photos of the pump’s nameplate and the nameplate of the motor connected to the pump and take a look
According to the operating procedures, a combination of a front pump rotor pump and a main pump rotor pump is used. Based on the structural principles of rotor pumps, they do not have inlet and outlet valves like piston-type vacuum pumps; therefore, rotor vacuum pumps cannot be used in series. Instead, the front pump operates until a certain level of vacuum is achieved, after which the main pump is turned on while the front pump stops working. Adjust the bypass valve to achieve process stability.