Which type of pump does not feature an automatic pressure relief characteristic?

The correct answer is the piston pump, as it operates by moving a piston within a cylinder to create pressure. This mechanism can generate high pressures for specific applications, but it does not inherently include an automatic pressure relief feature. Piston pumps often require external regulation or a manual pressure relief valve to prevent over-pressurization.

In contrast, centrifugal pumps utilize an impeller to move fluid and typically rely on the flow of the liquid to maintain operational pressure. If the system's pressure exceeds a certain level, these pumps can experience a drop in flow rather than dangerous over-pressurization.

Diaphragm pumps use a flexible diaphragm to create a pumping action, and they often include built-in mechanisms that function similarly to pressure relief, allowing fluid to bypass when pressure builds excessively.

Roller pumps also do not have an automatic pressure relief feature, but they are designed for specific flow rates and can handle variable flow applications effectively. Each of these pumps functions differently and is suited to different applications, but the piston pump stands out as the one that requires additional measures to manage pressure, making it the answer to the question.