Design of the backup pump to prevent backflow

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A certain project design specification includes the following requirements: To prevent backflow, the following devices should be installed at the pump outlet. When the pump outlet pressure is: P < 4.0 MPa: one check valve; 4.0 < P < 8.0 MPa: two check valves of different types (rod or shaft-operated type, with impact-free types being the most suitable). P > 8.0 MPa: Two types of check valves (rod or shaft-operated, with impact-free types being the most suitable), along with a valve on the main pipe that shuts off automatically in low-flow conditions as a safety measure. If there is a backup pump for the same model, the following measures should be considered at the pump inlet to prevent hazards caused by backflow,... The backflow amount is determined based on the following principles: a check valve is installed at the pump outlet, and the backflow amount is calculated as 90% of the normal flow rate. There are two check valves at the pump outlet; the reverse flow rate is calculated as 10% of the normal flow rate. Question: 1. 4.0 < P < 8.0 MPa: Two types of check valves (rod or shaft-driven type; a non-impact type is most suitable). At this point, has anyone designed it with two check valves? “Blow-off type with rod or shaft, impact-free – it seems to be a translation; what is the common term for this? 2. \"The backflow rate shall be determined in accordance with the following principles: A check valve is installed at the pump outlet, and the backflow rate is calculated as 90% of the normal flow rate.\" There are two check valves at the pump outlet; the reverse flow rate is calculated as 10% of the normal flow rate. " How should this clause be understood and implemented?