Reciprocating Pumps Calc

Pump ED 101
Positive Displacement Pumps
Part I – Reciprocating Pumps
Joe Evans, Ph.D http://www.pumped101.com

There are many pump designs that fall into the positive displacement category but, for the most part, they can be nicely divided into two basic groups. The reciprocating group operates via pistons, plungers, or diaphragms while rotary pumps use gears, lobes, screws, vanes, andperistaltic action. Their common design thread is that energy is added to the pumped fluid only periodically where, in dynamic pumps, it is added continuously. PISTON & PLUNGER PUMPS The piston pump is one of the most common reciprocating pumps and, prior to the development of high speed drivers which enhanced the popularity of centrifugals, it was the pump of choice for a broad range of applications. Today, they are most often seen in lower flow, moderate (to 2000 PSI) pressure applications. Its close cousin, the plunger pump, is designed for higher pressures up to 30,000 PSI. The major difference between the two is the method of sealing the cylinders. In a piston pump the sealing system (rings, packing etc) is attached to the piston and moves with it during its stroke. The sealing system for theplunger pump is stationary and the plunger moves through it during its stroke. Reciprocating pumps operate on the principle that a solid will displace a volume of liquid equal to its own volume. The figure to the right is that of a generic double acting piston pump. If we were to remove the two valves at the left hand side of the figure and replace them with an extension of the cylinder, we wouldhave a single acting pump. The single

acting pump discharges water only on its forward stroke while the double acting pump discharges on its return stroke as well. During the suction stroke (right to left) the single acting pump’s discharge valve closes and allows fluid to enter the cylinder via the suction valve. When the piston changes direction (reciprocates) the suction valve closes andwater is discharged through the discharge valve. In the double acting pump, the same sequence occurs during both strokes and almost twice as much fluid is discharged per unit time. Pressure The head created by a centrifugal pump depends upon the velocity it imparts to the fluid via its impeller. Therefore, for any given impeller diameter and rotational speed, head will be some maximum, unvaryingamount. Not so for reciprocating pumps. Although they will have a maximum operating pressure rating, the maximum pressure (P) attained depends upon the application. Against a closed discharge valve, pressure is limited only by the capability of the driver and the strength of the materials employed. Only the “breaking point” of some component will limit discharge pressure. Therefore some form ofpressure relief must be supplied if an application is capable of exceeding the pressure rating of the pump. Capacity The capacity (Q) of a single acting piston or plunger pump is proportional to its displacement (D) per unit time. The displacement is the calculated capacity of the pump, assuming 100% hydraulic efficiency, and is proportional to the cross sectional area of the piston (A), the length ofits stroke (s), the number of cylinders (n), and the pump’s speed in rpm. In gallons per minute it is: D = (A x s x n x rpm) / 231 In the case of double acting pumps the cross sectional area of the piston or plunger is doubled and the cross sectional area of the piston rod (a) is subtracted. Again, in gallons per minute D is: D = ((2A - a) x s x n x rpm) / 231

In real life the theoretical capacityof a piston or plunger pump is tempered by several factors. One is known as slip (S). The major component of slip is the leakage of fluid back through the discharge or suction valve as it is closing (or seated). It can reduce calculated displacement from 2 to 10% depending upon valve design and condition. Increased viscosity will also adversely affect slip. Another important factor that affects...