A suction line is a pipe, hose or tube that delivers a fluid to the suction or inlet side of a pump or compressor. These lines supply fluids for chemical processing, refrigeration systems, and vacuum pump applications. They also provide fluids key to oil and water well drilling. Suction lines can deliver any product that a pump or compressor can handle, including gases and petroleum products. Well drilling mud or other liquid-solid mixtures may also be transported in this way, along with adhesives and epoxies.

Oil drilling makes use of suction line piping to supply drill mud to the rig. Drill mud is a mixture of clay and water that's used to cool and lubricate the drill head, and to remove waste material from the drill hole. A mud pit or mud tank holds the drill mud, and a suction line is connected from the mud tank to the mud supply pump to supply pressurized mud to the drill assembly.

Chemical processing may involve the pressurization and movement of gases or liquids. The gas compressor or liquid pump takes material from storage or an earlier processing step through a suction line, discharging the fluid to a downstream processing step or storage. Many liquid chemicals can vaporize easily if positive pressure is not maintained on the pump, so a suction tank may be installed prior to the line to ensure adequate pressure and supply.

Refrigeration and air conditioning systems also use a suction line to deliver refrigerant gas from the evaporator to the compressor. Most refrigeration systems circulate a mixture of refrigerant liquid or gas, and a liquid lubricating oil for the compressor. Reciprocating compressors can be damaged by excess liquid returning through the line, so an accumulator is normally added to trap liquid and provide a more constant liquid flow.

Design of suction lines involves both a determination of required flow, and adequate pressure drop to prevent starvation of the pump or compressor. The viscosity or thickness and the weight of the fluid being pumped are key variables for suction line design. Often, design engineers are concerned with net positive suction head. This means having enough positive pressure of fluid at the pump suction to prevent the formation of gas bubbles, called cavitation, which can damage a pump.

Pump suction can be compromised if the supply point for the pump is lower in elevation than the pump, or if the suction lines are too small. Also of concern, a large number of fittings or bends can cause pressure to drop. Incorrect sizing may also compromise suction. Maintenance of suction line devices such as filters or traps is needed to prevent clogging and starving the pump.