Pump Operation

The pump is the heart of the high pressure cleaning system. Like the human heart, most pumps are built to last for a lifetime. Many different types of pumps are available but only a few of these pump configurations develop sufficient pressures at relatively low flows and are priced economically enough for widespread use in the high pressure cleaning equipment industry.

All pumps in common use in the industry operate according to the same manner, moving water with the action of a piston or plunger in a cylinder. The pump applies force to water to create flow. Plungers or pistons moving in the pump’s cylinders apply this force to the water. The process is roughly the reverse of the action of an automobile’s internal combustion engine. The engine uses piston movement to turn a crankshaft. The pump uses crankshaft movement to drive pistons, which move the water in and out of the pump.

Flow, Not Pressure
Pumps are referred to as creating pressure but that’s not technically correct. A pump actually creates a flow of water. This flow is generally measured as a volume of water moved during a certain period of time, that is a specified volume of water moving at a specified speed. Flow is usually expressed in gallons per minute (gpm). In some cases – either steam cleaning applications or chemical injection – flow may be expressed in gallons per hour (gph). Cleaning systems produced in other countries may have their flow rated in liters per minute (lites / min).

Note on Conversion:
A liter is slightly more than one quart (1.06 quart so four liters will be a bit more than one gallon). Liters per minute can be converted to gallons per minute by multiplying the liters per minute figure by 0.264. Gallons per minute can be roughly converted to liters per minute by multiplying the gallons per minute figure by 3.79.

The smaller the opening this flow must pass through, the faster the water must move to ensure that the same volume passes through the smaller opening in the same time that it moves through the larger opening. What is generally referred to, as pressure is the force exerted on the water. That pressure is stored as energy in the water flow. When the water flow passes through a restricted orifice such as a pressure nozzle, it gains velocity. This velocity makes the fluid better able to perform work. The more force exerted against the water, the faster it will move through the nozzle restriction.

When we speak of a pump’s ability to produce a certain pressure at a specified flow, what we really mean is that the pump is capable of moving that volume of water through a particular orifice at the specified rate in gallons per minute. Pressure is generally referred to in pounds per square inch (gpm). Equipment manufactured outside of the United States may have pressure expressed in bar. One psi is roughly 14.5 bar. A system producing 1000 psi will be rated at 69 bar.

The Pump In Action
The simplicity of pump action can be demonstrated with a theoretical one-cylinder pump. Although this example is simplified, each cylinder in a multi-cylinder pump works exactly the same way as the one cylinder model. For example model we will use a single cylinder closed at one end. A plunger runs the length of the tube or cylinder. Two valves are set at the closed end of the cylinder or manifold. These are called check valves. They control the direction water flows through the pump.

These check valves are like entrance and exit doors that only open in one direction. One of the check valves opens only inward. This is the inlet check valve. Fluid flows through this check valve into the pumping cylinder. The other check valve will only swing outward. This check valve lets water escape from the cylinder. It is the outlet check valve. Water coming from the other direction will actually press the valve more tightly closed. These check valves allow water to move through the pump in only one direction. Every pump cylinder must have an inlet; an outlet, inlet and outlet check valves and a plunger or piston.

Suction & Compression

Each pump operating cycle consists of two pump strokes: a suction stroke pulling water into the cylinder and a compression stroke pushing that water out of the cylinder. The suction stroke pulls the inlet check valve open and sucks water into the cylinder. The compression stroke presses the water back out of the cylinder. This outward movement of water pushes open the outlet check valve and closes the inlet check valve. In each stroke of the pump, the same force that opens one check valve holds the other check valve closed.

Every time the crankshaft turns a full revolution, the cylinder completes two strokes, a suction stroke and a compression stroke. Pump crankshaft rotation speeds for pumps used in the industry range from less than 700 to 3500 revolutions per minute (rpm). Speeds may be higher or lower as well but most pumps in the industry fall in this range. Rotation speeds of 1725 and 3450 rpm are common for direct drive pumps used with electric motors.

This rapidly alternating series of suction and compression strokes moves water through the system with the pressure applied by the compression strokes. When a pump has more than one cylinder, each cylinder is joined in a manifold, which allows water flow to and from each cylinder. Water enters the cylinders from an inlet chamber and is expelled from each cylinder into an outlet chamber of the manifold and then out of the pump. In some pump types the inlet and outlet manifolds are separate components.

Pump Components

The standard plunger pump is divided into two main sections, the crankcase and the pump head. The crankcase contains the crankshaft and connecting rods and oil supply. Although the moving parts are lubricated in this section, it is commonly referred to as the "dry" side of the pump.

The pump head, where the work of moving water goes on, is called the "wet" end. This section of the pump houses the plungers and check valves and includes the manifold. The manifold, or manifold assembly in pumps with split manifolds, in fact, is usually referred to as the pump head. The terms manifold and head are often used interchangeably through such usage is not entirely correct.

The plungers move back and forth in cylinders between the head and the crankcase. An oil seal keeps oil from leaking out of the crankcase. The pump packing keeps water in the wet end of the pump. The check valves are one-way doors, which direct water flow through the cylinders.

Remember, check valves are like doors that open in only one direction. When water flows in a direction opposite the direction the check valve opens, the water flow forces the check valve more firmly shut.

Remember: the pump is divided into two sections:

· The wet end where the work of pumping water is actually done
· The dry end, where the crankshaft rotates
Seals keep water in the cylinders and oil in the crankcase from mixing.

The Pump Must Have Power

The pump is usually driven by an electric motor or gas or diesel engine. The motor or engine drives a shaft, which rotates, at a speed expressed in revolutions per minute (rpm). The speed in rpm is the number of times the crankshaft rotates in one minute. Each rpm marks the completion of one full pump cycle. Most pumps used in high-pressure cleaners are designed around a crankshaft. The rotation of this shaft is used to rotate the pump’s crankshaft. The crankshaft translates the circular motion into the in and out motion of the plungers moving through the cylinders. This motion is imparted to the pump’s plungers or pistons through connecting rods. This is the reverse of an automobile engine where the pistons move in and out to drive a crankshaft in a circular motion.

The Crankcase
Openings in the side of the crankcase allow the drive rods to connect to the pistons or plungers moving in the cylinders. The crankshaft rotates on bearings, which must be lubricated. In an oil bath pump, the crankshaft and bearings are half-submerged in oil, much like an automotive crankcase. As the crankshaft rotates, the crankshaft dips into the oil and slings it up and so on as each crank journal dips into the oil reservoir in the crankcase. Other pump types may require regular bearing lubrication. These are called dry crankcase pumps.

Other Pump Types
In addition to the three standard types of pumps for which service procedures are discussed here, there are a number of other pump types used in the industry. Two of the most common types of pumps which do not fall into the major categories, combine characteristics of both piston and plunger pumps.

The Hypro 5300 Series And Its Cousins

A small, two-cylinder design built by Hypro and a number of other manufacturers is generally called a piston pump but has both inlet and discharge check valves like a plunger pump rather than using the piston itself as the inlet check valve. Somewhat larger pumps of similar design are available with four cylinders. The duplex Hypros are generally found on small, older cleaners. This Hypro duplex design is also available as a plunger pump.

The two-cylinder pumps have a single connecting rod for the two, vertically opposed pistons. The rotation of the cam-like crankshaft causes the top piston to move up in the cylinder on the discharge stroke while the bottom piston moves up in its cylinder on the inlet stroke. When the shaft turns one-half rotation the opposite action occurs. These pumps are most common on small, inexpensive cleaning systems.

The Superflow

The Superflow series of pumps produced by Cat and plunger pumps but are made in a flow-through design with the inlet check valves integrated into the plungers and operating much like the inlet check valves of a standard piston cup pump. The Superflow pumps also have a split manifold, like most piston cup pumps. The Superflow series is designed for direct-drive mounting and was originally used mostly on gas engine-powered, cold water washers. More manufacturers are incorporating this pump into hot water units, however.

The Diaphragm Pump
Some stream cleaners are equipped with diaphragm pumps. These pumps are more resistant to corrosives than other pump types. Generally these pumps are not capable of producing pressures in excess of 1000 psi.

The Hydraulically Assisted Pump
The hydraulically assisted pump uses hydraulic force to enhance the amount of pressure a plunger or piston type pump can produce. This two-stage design first acts on a larger cylinder filled with hydraulic fluid. This fluid then moves a smaller plunger in the pump cylinder to produce water flow. This pump design was used in some of the first truly high pressure pumps used in high pressure cleaning and some hydraulically assisted pumps are still in service.