In relay pumping operations, how many engines can be hooked in series?

In relay pumping operations, it is indeed possible to hook two or more engines in series. The purpose of relay pumping is to transport water over long distances when the supply source is not accessible to the fire scene. By using multiple engines in series, the water can be pumped from one engine to the next, maintaining pressure and flow as it travels from the source to the destination, which is especially useful in scenarios where the distances are extensive or terrain is uneven.

Using only one engine would limit the capacity and distance water could be pumped effectively. Having four or five engines, while theoretically possible, is not commonly the standard limit in practice and is not necessarily a requirement. Therefore, the correct understanding is that any number beyond one, such as two or more, can be utilized in a relay system for effective water supply.