What is net positive suction head？

What is net positive suction head? What is suction head? What are their respective measurement unit and their represented letter?

Reply: When the pump is working, the liquid will generate gas at the inlet of impeller because of certain vacuum pressure. The evaporated gas bubble will cause corrosion on the impeller and other metal surfaces under the impact and movement of liquid particles and thus damage the impeller and other metals. Here, the vacuum pressure is called vaporized pressure, and net positive suction head refers to surplus energy over the vaporized pressure that the liquid of unit weight has at the suction inlet of the pump. Its unit is meter and it is represented as (NPSH) r. The suction head is the necessary NPSHΔh：that is, the vacuum level of the liquid allowed by the suction of pump, it is also means the installed height of the pump allowed and its unit is meter.
　
Suction head= standard atmospheric pressure (10.33 meters) - net positive suction head- safety limit (0.5m)
The standard atmospheric pressure can press the vacuum height of the pipeline for 10.33 meters.
Example: The necessary net positive suction head of a pump is 4.9 meters, please figure out the suction headΔh.
Solution: Δh= 10.33-4.0-0.5=5.83m