Which of the following is NOT true about positive displacement pumps?

Positive displacement pumps are characterized by their ability to move a specific volume of fluid with each cycle, leading to a consistent flow rate regardless of the discharge pressure (as long as the pump operates within its design limits). This is a fundamental principle that differentiates them from other types of pumps, like centrifugal pumps, which may experience fluctuations in flow rate based on pressure changes.

The statement regarding their operation in various applications is also true. Positive displacement pumps are versatile and can be found in different industries, handling everything from viscous fluids to slurries.

The assertion about having a maximum efficiency limit is also accurate since, like any mechanical system, they are governed by principles that dictate performance parameters, including efficiency ratings.

However, the idea that positive displacement pumps always require a variable flow rate is not true. In fact, they are often used where a consistent flow is needed, such as in metering applications. Their design allows them to generate a constant flow rate, making them suitable for processes requiring precise control over fluid volume, rather than being limited to applications needing variable flow.