California Wastewater Treatment Operator Practice Exam

Which of the following is true about a positive displacement pump?

1. It must never be started with the discharge valve closed

2. It should always be started with the discharge valve closed

3. It does not require priming before operation

4. It is most effective for high-flow applications

The correct answer is: It must never be started with the discharge valve closed

The statement that a positive displacement pump must never be started with the discharge valve closed is accurate. This type of pump operates by trapping a fixed amount of fluid and forcing it into discharge lines, creating a rigid volume of liquid movement. If the discharge valve is closed when the pump is started, the pressure can build up quickly to excessive levels, potentially causing damage to the pump, piping, or other connected equipment. Therefore, ensuring the discharge valve is open before starting the pump is critical for safe operation. Additionally, positive displacement pumps are generally not best suited for high-flow applications. Instead, they excel in delivering consistent flow at relatively low to moderate flow rates and pressures. This characteristic distinguishes them from centrifugal pumps, which are more effective for high-flow scenarios. While priming might not always be necessary for specific types of positive displacement pumps, it is commonly required for others, contradicting the idea that they inherently don't require it.