4 pump types for difficult fluids

In brief:

• There are four common types of reciprocating pumps - power pumps, power diaphragm pumps, air-operated pumps and air-operated piston pumps.
• Power pumps convert a rotary motion into reciprocating motion that can develop more than 40,000 psi discharge pressure or deliver more than 4,000 gpm.
• Diaphragm pumps have no rotating seals, are physically compact, require no electricity, cab self-prime, run dry and pass entrained solids, which makes them ideal for corrosive and abrasive fluids, hazardous environments, where space is limited, and portability is required.

This is the fourth in a series of five articles based on Hydraulic Institute’s comprehensive e-learning course, “Positive Displacement Pumps: Fundamentals, Design and Applications.”

Overview

Since the third century B.C., when Ctesibius built a basic hand-operated pump that could transfer water, reciprocating pumps have played a significant role in the development of civilization. Today, reciprocating pumps have developed into technically advanced machines capable of delivering more than 40,000 psi of discharge pressure.

Reciprocating pumps comprise a major segment of the positive displacement technology category. Reciprocating pump designs handle a full range of liquid characteristics, including low viscosity chemicals, high particle content slurries and high viscosity liquids. Given this operating range, they’re often the technology of choice for difficult applications.

The main difference between rotodynamic and reciprocating pumps is that for a given speed, a rotodynamic pump’s flow rate can be varied from zero to a maximum. Conversely, reciprocating pumps have nearly constant flow for a given speed, regardless of pressure. Reciprocating pumps have several common operating characteristics. This includes a constant fluid delivery (volume) per stroke and the mechanical trapping of the fluid by means of suction and discharge valves. The reciprocating motion typically produces pressure pulsations and additional devices to reduce pulsation, such as pulsation dampeners or attenuators. Additionally, as with many PD pump types, systems might require overpressure relief protection. Reciprocating pump efficiency varies widely across the category and is related to driver type and specific mechanical configuration.

Reciprocating pumps types

The four common types of reciprocating pumps reviewed here are:

• Power pumps: piston, plunger and direct acting (steam)
• Power diaphragm pumps, mechanically or hydraulically actuated
• Air-operated pumps: double diaphragm pumps
• Air-operated piston pumps

Power pumps are reciprocating machines in which a power end drives plungers or pistons within a valved cylinder. The power end (Figure 1) converts a motor’s rotary motion into reciprocating motion by means of a crankshaft, connecting rods and crossheads. The liquid end (Figure 2) connects to the power end and contains the plungers, packing, fluid chambers and valves. The plunger’s reciprocating motion in concert with other liquid-end components can develop more than 40,000 psi discharge pressure or deliver more than 4,000 gpm.

Figure 1. The power end converts a motor’s rotary motion into reciprocating motion by means of a crankshaft.

These pumps typically have 1, 2, 3, 5, 7 or 9 connecting rods and crossheads that drive an equal number of fluid plungers. An odd number of cylinders is preferred to reduce pressure pulsations. The oscillating pressure in each fluid chamber generates pulsations that range from suction pressure to discharge pressure. This pressure oscillation, cycling at 50 CPM to 500 CPM, is a leading contributor to pump failures; however, these pumps are constructed robustly to resist fatigue.

Figure 2. The liquid end connects to the power end and contains the plungers, packing, fluid chambers and valve.

It’s important to control the pressure in power pump systems. As the pump injects the displaced liquid into the discharge system, the pressure increases. The pressure continues to increase until it matches the system requirements. If the system pressure isn’t controlled, the pressure continues to increase until something in the system ruptures, either in the pump or the driver stalls out. Equip power pumps with a pressure relief device to prevent the overpressurization beyond recommended limits.

Power pumps typically are 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 valves to transfer fluid. Typically, these pumps have two diaphragms connected to a reciprocating shaft or connecting rod in which one side of each diaphragm is in contact with the liquid being pumped and the other side is in contact with the compressed supply of air or gas. An air motor pressurizes the two diaphragms alternately to produce the reciprocating motion. The process fluid pressure typically is equivalent to the supply air pressure; however, amplification devices allow specialized pumps to operate at pressures as much as three times the air supply pressure.