What device protects the pump from debris and scale?

A strainer is the device that protects the pump from debris and scale. It functions by filtering out unwanted particles and contaminants from the liquid being pumped. By preventing these materials from entering the pump, the strainer helps maintain the efficiency and longevity of the pump, ensuring that it operates smoothly without the risk of clogs or damage caused by solid materials.

Other options serve different functions: a check valve is used to prevent backflow in a piping system; a pressure gauge measures the pressure of the fluid within the system, allowing for monitoring and adjustments; and a flow meter measures the flow rate of the fluid, providing vital information for system management. None of these options provide the filtration capability that a strainer offers, making it the correct choice in this context.