Reciprocating Pump

Positive displacement pump, reciprocating pumps may be defined as pumps that operate using a back and forth, straight-line motion, like Piston Pumps,

Reciprocating Pumps may be defined as pumps that operate using a back and forth, straight-line motion. Frequently, reciprocating pumps are driven by air or steam operated power pistons which are connected to the pump by connecting rods. Some applications require reciprocating pumps to be driven by internal combustion engines or electric motors.

Reciprocating Pumps have two sections; the pumping section which actually does the pumping of the fluid and the driving section which provides the force required to operate the pumping section. The pistons or plungers in reciprocating pumps move at relatively slow speeds. If more fluid needs to be pumped, larger cylinders or more cylinders can be used or the speed of the pump can be increased.

Piston Pumps

One of the most familiar reciprocating pumps is the piston pump. The main components of this pump are cylinder, piston, piston rings, suction valve, discharge valve, packing, and pump casing.

The piston moves back and forth in the cylinder. Each complete movement of this piston along the cylinder length is called a stroke. Movement of the piston toward the driving section of the pump is called the backstroke.

A forward stroke is movement of the piston away from the driving section of the pump. A full stroke is the movement of the piston from one end of the cylinder to the other other end and back to its original position.

Starting with the piston forward against the valve end of the cylinder, the piston moves away from the valve end on the backstroke, creating a suction which opens the suction valve and allows fluid to enter the cylinder.

On the forward stroke, the piston moves toward the valve end of the cylinder and presses against the fluid, causing the suction valve to close and the discharge valve to open.

Under pressure from the piston, the fluid leaves the cylinder through the discharge valve. At the end of the forward stroke, the piston is back in its original position and the cycle begins again.

A pump which pumps fluid only on one side of the piston is called single-acting.

These pumps produce an uneven flow by discharging the fluids in spurts. To overcome this limitation of single-acting pumps, double-acting pumps are used. These pumps are designed so suction and discharge are accomplished on each forward stroke and backstroke.

Double acting pumps have both suction and discharge valves on each end of the cylinder. This allows fluid to be pumped on both sides of the piston.

Sometimes when a large amount of fluid needs to be pumped or a more uniform flow is the required, multi-piston pumps are used.

When pumps have two pistons, they are referred to as duplex pumps, three piston pumps are triplex, etc. The strokes of these multi-piston pumps are mechanically timed to provide the smoothest discharge possible.

It is not uncommon for properly timed pumps to produce an uneven pumping action. To compensate for this, a surge chamber or dampener may be installed on the discharge to smooth out the discharge pressure.

Piston pumps are sealed internally and externally against fluid leakage. Internally, the pistons are fitted with piston rings which prevent fluid leakage between the piston and the cylinder wall. Externally, packing is used to fill the space where the piston rod penetrates the casing. This section of the casing is referred to as a stuffing box.

The pliable packing material can be tightened around the piston rod by adjusting the nuts on the gland follower; how ever, some leakage is required for lubrication of the packing.

Plunger Pumps

A plunger pump operates the same as a piston pump except for a few important differences. Discussion of plunger pumps will be centered on these differences.

First, in piston pumps, the piston fits close to the walls of the cylinder with piston rings sealing the space between the piston and cylinder wall. The piston moves totally within the cylinder. In plunger pumps, the diameter of the plunger is much smaller than the diameter of the cylinder, and the plunger moves outside the cylinder as well as inside.

Second, in piston pumps, the piston displaces most or all of the fluid from the cylinder during the forward stroke. In the plunger pumps, the plunger only displaces a part of the fluid.

Plunger pumps are often used for high-pressure service or when the fluid being pumped could cause a piston to stick or scoring of the cylinder.

Diaphragm pump

Another type of reciprocating pump is the diaphragm pump. This pump gets its name from the flexible diaphragm used to displace fluids.

The diaphragm is usually made of a flexible, rubber-like material often covered with a thin metal disc where the connecting rod is attached.

This metal disc spre the force from the connecting rod over the diaphragm so holes won’t be punched in it by the action of the connecting rod.

The diaphragm extends across the widest part of the pump cylinder, completely sealing one half of the cylinder from the other half. This makes diaphragm pumps ideal for pumping corrosive or abrasive fluids.

The diaphragm is connected to a motor-driven eccentric or cam by means of a connecting rod. As the wheel rotates, the connecting rod pushes and pulls on the diaphragm, causing the diaphragm to bulge towards and then away from the suction and discharge valves on successive strokes.

In operation, when the eccentric rotates and the connecting rod pulls the diaphragm away from the suction and discharge valves, a suction is created in the lower half of the pump cylinder. This suction opens the suction valve and draws fluid into the cylinder.

As the eccentric rotates farther, the connecting rod moves down and pushes on the diaphragm. The pressure exerted by the diaphragm on the fluid closes the suction valve and opens the discharge valve, forcing the fluid out through the discharge piping.

As the connecting rod moves all the way down, the discharge stroke is complete and the suction stroke begins. Each complete revolution of the eccentric results in a full stroke of the pump.

• [accordion]
• 1. Reciprocating Pump Animation
• 
• 2. Handbook of Pumps and Pumping by Brian Nesbitt
• 3. PLP E-4-2003 , Reciprocating Pumps-3rd Ed-Rosen
• 4. PLP E-3-2003 , Centrifugal Pumps-3rd Ed-Rosen
• 5. ISO 13710-2004 Reciprocating Positive Displacement Pumps
• 6. BS EN ISO 13710-2004 Reciprocating Positive Displacement Pumps