Operating Characteristics of Positive Displacement Pumps

Positive Displacement Pumps

• The main principle of operation for positive displacement pumps is that they capture a fixed volume of fluid and force it out of the pump chamber, creating pressure.

Operation and Safety Precautions

• If the discharge valve of a positive displacement pump is inadvertently closed, the pump will continue to generate pressure, leading to a potential explosion or damage.
• Positive displacement pumps must be equipped with a pressure relief device in the discharge line to prevent over-pressurization.
• The pressure relief valve in the discharge line serves to prevent over-pressurization and potential damage by releasing excess pressure.

Characteristics

• Positive displacement pumps differ from centrifugal pumps in that they have very small clearance between the impeller and housing.
• Positive displacement pumps can develop considerable discharge pressure and vacuum at their suction inlet due to their ability to create a tight seal.
• They are considered self-priming because they can create a vacuum at the suction inlet, allowing them to draw in liquid.

Priming and Startup

• It is advisable to manually prime rotary positive displacement pumps before starting to ensure they are full of liquid.
• Running pumps when full of liquid helps to prevent damage and ensures proper operation.
• After starting a pump, it's essential to monitor potential issues by checking the pump's performance.

Shutdown and Maintenance

• During the priming process, the air vent should be closed when liquid appears at the vent.
• Before removing a lockout from a positive displacement pump, ensure it is safe to do so.
• One of the post-start checks that should be performed for all pumps is to monitor suction and discharge pressures.
• Monitoring suction and discharge pressures is crucial to detect potential issues and prevent damage.

Routine Checks

• A routine check that should be performed during scheduled rounds for all pumps is to monitor temperature, pressure, and vibration.
• When shutting down a pump, the discharge valve should be closed first.
• Signs of shaft misalignment in a pump, such as vibration or noise, should be corrected as soon as possible.
• It is essential to ensure the coupling is secure and does not have excessive play before starting a pump.