Why are the control valve and vent valve installed in front of the valve?

First, you need to understand the function of backwashing in the control valve assembly. 1. Before the device is turned on, there is no material in the pipeline; therefore, you cannot fill it from back to front – it should be filled from front to back. So the function of backwashing is to remove the air from the pipeline ahead of the valve assembly. 2. After the device is operating normally, open the manual valves before and after the control valve fully. If there are impurities in the material flowing through the pipeline, it is necessary to turn on the backflow discharge system. Since the flow rate is controlled by the control valve, under normal conditions the control valve is not fully open. If your backflow is located behind the control valve, then it is possible that some impurities may flow out of the valve assembly along with the fluid. It’s just my personal opinion; I’m not sure if it’s clear enough, so it’s for reference only.

The original poster above has already explained very clearly the function of the backflow valve. I have a personal suggestion: it would be helpful if you could find a schematic diagram of this valve, as that would resolve the issue. The backflow valve needs to be placed in front of the valve in order to remove all impurities or liquids from inside; if it is placed behind, it won’t be possible to eliminate all such substances. Therefore, I suggest you take a look at its schematic diagram.