Editor's Note: This is the fourth in a series of five articles based on the Hydraulic Institute's new Positive Displacement (PD) Pumps: Fundamentals, Design and Applications e-Learning course. To read the previous article, click here. To read the next article, click here.
Last month, we introduced Module 3 of the course, which focused on rotary pumps. In this issue, we will focus on Module 4, reciprocating pumps. For the reciprocating pump family, this article will provide an overview of their characteristics, types, application analysis and pump selection process.
Reciprocating Pump Overview
Since the third century B.C. when an Alexandrian named Ctesibus built a basic hand pump that could transfer water, reciprocating pumps have played a significant role in the development of the modern world. Today, reciprocating pumps have developed into technically advanced machines capable of delivering more than 40,000psi of fluid pressure.
Reciprocating pumps comprise a major segment of the positive displacement technology category. Reciprocating pump designs can pump a full range of liquid types including low viscosity chemicals, high particle content slurries and high viscosity materials. Given this wide operating range, they are often the technology of choice for difficult applications.
One main difference between rotodynamic and reciprocating pumps is that for a given speed, the rate of flow of a rotodynamic pump can be varied from zero flow to a maximum flow. Conversely, reciprocating pumps will have a constant flow for a given speed. Pumps that belong to the reciprocating pump family have several common operating characteristics, including a constant fluid delivery per stroke and mechanical trapping of fluid using suction and discharge valves.
Inherent to their reciprocating motion, these pumps also typically produce pulsation. Consideration for additional devices to reduce pulsation, such as pulsation dampeners or attenuators, may be needed for some applications. Additionally, as with many PD pump types, systems may require overpressure relief protection. Efficiencies of reciprocating pumps vary widely across the category due to driver types and mechanical configurations.
Types of Reciprocating Pumps
Four common types of reciprocating pumps-power pumps, power diaphragm pumps, air operated diaphragm pumps and air operated piston pumps-are reviewed in the reciprocating pump module.
Power pumps are reciprocating machines where plungers or pistons are driven within a valved cylinder by a power end. The power end (see Figure 1) converts the rotary motion of a motor-electric, air or hydraulic or diesel engine-into reciprocating motion by means of a crankshaft, connecting rods and crossheads. The liquid end (see Figure 2) connects to the power end and contains the plungers, packing, fluid chambers and valves.
The reciprocating motion of the plunger in concert with the other liquid end components can develop more than 40,000psi of fluid pressure or more than 4,000gpm of fluid flow. These pumps typically have one, two, three, five, seven or nine connecting rods and crossheads that drive an equal amount of fluid plungers. Configurations of an odd amount of cylinders are preferred to reduce pulsation. Pulsation is generated by the oscillating pressure of each fluid chamber from suction pressure to discharge pressure. The oscillating pressure, cycling at 50- to 500-rpm, is a popular driver of pump failures, but these pumps are constructed robustly to resist fatigue.
It is important to control pressure with power pump systems. As the pump injects the displaced liquid into the discharge system, the pressure is increased. The pressure will continue to increase until it meets the requirement of the system. However, if the system pressure requirement is not controlled, the pressure will continue to build up until something ruptures in the system or the pump or the driver of the pump stalls out. Power pumps should be equipped with a pressure relieve device to prevent the over-pressurization of the system beyond its recommended limits.
Power pumps are typically used for low viscosity chemicals, oils, high pressure cleaning, ore slurries, drilling mud, reverse osmosis, saltwater injection, hot oil applications, blow out preventers and subsea applications.
Air operated diaphragm pumps combine the reciprocating motion of a flexible membrane, called a diaphragm, with check valve mechanisms to transfer fluid. These pumps typically have two diaphragms connected to a reciprocating shaft in which one side of the diaphragm is in contact with the liquid being pumped and the other side is in contact with the compressed air. The reciprocating motion is created by an air motor that alternates pressurization of the two diaphragms. Fluid pressure is usually equivalent to the supply air pressure of the pump; however, amplification devices allow specialized pumps to operate at pressures up to three times the supply pressure.