Start here and refer to the appendix. NFPA 20, 2016
4.21 Water Flow Test Devices.
There is a huge difference between North America and Europe on that topic.
I know that test headers are widely used in the US/Canada to test fire pumps (and thus pouring large quantities of water directly outside the facility). Probably because many installations are directly fed by city water supplies (the water will be lost anyway, even if a booster pump has been provided).
In Europe, the practice is to have a dedicated flowmeter with water returning to the water tank (to save water and money).
My personal point of view is that a flowmeter is more convenient (cost friendly as less water is spoiled and more practical to have a value as the flow is indicated directly on the device and you don't need to use pitot tubes and make calculations, each time with a possible margin of error).
But this is not mandatory.
What is mandatory is to have a way top test the pump up to 150% of its rated flow (via a flowmeter or a test headers).
Have you ever read NFPA 20and if it is the case , you shall not miss the Flometer requirement in the Fire Pump for testing purpose
I have always designed them into the by-pass for High Rise booster Fire Pumps.
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