The displacing pumping action activated by changing the volume of a given space causes the fluid to be moved physically. This can be achieved using a piston and cylinder arrangement with a reciprocating motion or a rotating unit using vanes or screws.
The diagram shown below represents a reciprocating pump with positive displacement to demonstrate its operating principle. It’s a double acting pump, meaning that fluid is admitted from either side of the piston for being drawn and discharged alternatively. When the piston moves up, suction is caused beneath it and the liquid gets drawn in while the valve arrangement ensures that the discharge valve doesn’t open during the process. When the piston moves down, the suction and discharge operations take place on the opposite sides.
An air vessel is generally provided in the discharge pipeline to reduce pressure variations during discharge. With the rising discharge pressure air in the vessel is compressed and it expands when the pressure falls. Therefore, energy at peak pressure is stored in air and sent back to the system with the falling pressure. reciprocating boiler feed pumps are not provided with air vessels as they could let in the air in the de-aerated water.
The suction and discharge valves must be left open while starting the pump. Ensure that none of the valves in the discharge is open; else the relief valve could lift or damage the pump on its starting. Though the pump is self priming, wherever possible it should be filled with liquid before starting to minimize wear and risk of seizure. An electrically driven pump runs erratically for a short while till it draws liquid. A pump working on steam would need the normal draining and warming before the steam gets admitted gradually.
Why Is Relief Valve Here?
The relief valve, provided between the pump suction and discharge chamber to safeguard the pump, ought to be operated with the valve of the discharge line kept closed.
As it’s the moving components that need maintenance, they need to be inspected during overhaul. The piston of the pump and its rings as well the cylinder should be thoroughly checked. Ridges are formed at the ends of the travel path of piston and these should necessarily be removed. You may also need to ground or resurface the discharge or suction valves.