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The Pressure Washer

October 10, 2015

Probably the easiest way to understand how a pressure washer functions is to diagnose a unit and see how it is constructed. We have selected a cold water portable unit for this purpose.

  1. First you have your power source (engine or motor) this is what created the energy to drive the pump. It and the nozzle are what determines the cleaning effect.
  2. The heart of any pressure washer system is the high pressure pump. They are a positive displacement reciprocating type pump - either plunger or piston version. Plunger is the most common as they are more efficient and have longer life. Plungers are usually made of ceramic which is very hard and less prone to wear. Plunger pump can be described as a plunger that moves back and forth against a stationary seal or packing. Whereas a piston pump is where the packing is attached to the piston rod and moves against a smooth cylinder wall. Piston pumps generally have better priming characteristics but cannot operate at as high a pressure and have shorter life.

An important feature of the piston pump is that the volume capacity of pump depends on pump speed. So long as pump is rotating the same volume of liquid that goes in must come out. Unlike a centrifugal pump where restriction or increased head creates reduced flow a piston pump has a very flat flow performance curve no matter how high the pressure goes. This is why blockage can be dangerous and why safety devices such as unloader valves must be incorporated into system.

As the plunger moves back, water is drawn into pump cylinder cavity through inlet check valves. When plunger moves forward, the inlet valves close, forcing water to travel through a series of outlet valves and to the discharge port of pump.

Most plunger pumps are either duplex or triplex. Two-piston pumps have higher pulsations and are harder on system components and the operator. Triplex are the most common used for pressure washers as they result in smoother flow.

Newer higher speed pumps also result in less pressure fluctuations and require less starting torque; however, they result in faster wear, have less suction lift capability and sometime noise is high pitched and can be irritating.

Unfortunately since pump is the core of the pressure washer it is generally blamed for any system failing. However, generally 90% of pressure washer problems are not the cause of the pump. Most can be attributed to system restrictions or component wear such as nozzle, etc. Pump starvation of liquid is probably the leading cause of premature pump failure. Inlet line restrictions does not allow adequate water to enter pump resulting in cavitation. Cavitation is when air mixes with water causing small explosions to occur when pressurized which will tear away at packing or piston surface.

The Pressure Washer Industry

Briefly let's talk about proper selection of a pressure washer for your customer's application and types that are available to meet his needs. To better understand this we'll compare high pressure cleaning to washing your hands. Depending upon the degree of dirt and sanitation required you need: water, rubbing action, soap and heat. Relating this to a pressure washer and in tented application you should consider:

  1. Flow - measured in gallons per minute (GPM)
  2. Pressure - measured in Pounds per square inch (PSI)
  3. Chemical - amount and type needed to break down dirt
  4. Heat - speeds up cleaning action, particularly for oil and grease applications.

And I'm adding three more considerations:

  1. Time - how much time do you have or can afford to pay to get a proper cleaning job done.
  2. Cost - What is budgeted for equipment to accomplish above factors.
  3. Power - Available AMPS - H.P.

Pressure Washers are generally broken into two (2) Classes: Hot or Cold Water

Cold Water are the most popular, probably primarily due to cost and ease in manufacturing and use. They are generally limited to 140?F - temperature of your hot water tap. They are broken into two types: electric or gas engine.

  1. Electric are more compact and lower performance when electric current is limited to 115V. They are more economical and sold to Accessories market or for semi-professional use.
  2. Gas engine units are more portable and used primarily outside when noise and gas fumes do not present a problem.

Performance is limited by price you are prepared to pay to get the job done.

Hot Water units are primarily used for commercial or Pumps applications where high temperature are required to break down dirt more effectively. Because they consist of an oil burner and heating coil they tend to be more expensive and complicated to operate. They have however enjoyed probably the largest percent sales growth in the industry due to more efficient manufacturing and promotion.

Most pumps have a dual packing design with a water recycling system which allows water to lubricate packings and bleed back minor leakage from slightly worn packings. When excessive wear occurs and pressure drops significantly packings should be changed. Remember a new pump will sometimes weep slightly until packing take a good set against plungers.

Pump crankcase is very trouble free - like an automobile if properly maintained. Oil should be changed after initial 50 hour break in period and every 500 hours or annually thereafter or when oil gets milky. This could be caused by condensation from temperature changes or oil seal leakage which may occur if vented crankcase plug is not installed and seals are sucked in from vacuum created when oil heats up.

Filters - a good inlet filter is a cheap price to pay to protect your pump from impurities that are in water. Rust, scale and sand can easily clog valves or scratch plungers. Use a 60-100 mesh screen of adequate size for flow and be sure it is checked and cleaned periodically.

Pressure Reducing Valves - are only required if pump is sensitive to high inlet water pressure when city water pressure is in excess of 60 psi. Sometimes chemical injector will not function properly if inlet pressure is too high.

Pressure Gauges - are generally not used by most manufacturers as they tend to malfunction easily unless more expensive glycerin filled ones are used and they add to machine cost. They do however, serve an important role as a ready warning to nozzle wear or pump seal damage which reducing cleaning effect and potentially more expensive repair later.

Unloader Valves - this is the main safety component of a pressure washer. It along with the trigger gun literally controls the traffic flow of water in the system. Without an unloader valve when gun is shut off the pressure will continue to rise until either the motor - engine stalls or pump, hose or gun ruptures. Unloaders divert all or part of the flow to a supply tank or the inlet of pump to prevent pressure from building up. When gun is opened water moves from outlet of pump through hose to gun and nozzle. When trigger is released the valve closes and the unloader is activated either by an increase in pressure (pressure actuated type) or a reduction in flow (flow actuated type). Water flows into channel and pushes piston down which opens bypass valve. The unloader diverts flow of water from outlet side of pump back to inlet side causing water to flow back to pump virtually under no pressure.

Advantages of flow actuated unloaders is that hose and pump pressure is reduced in unload mode. This is safer for hot water coil and when operator does not want fast kick back when gun trigger is depressed. It however, cannot be used on multi gun systems and is generally more expensive and sensitive to adjust.

Pressure actuated type is the common. It traps pressure in hose during unload mode so immediate pressure is available when gun is opened.

Pressure of system can be controlled to some extend by adjusting the tension on the spring holding the piston in the valve in place. Changing nozzle is preferred method of controlling pressure and unloader should only be adjusted to fine tune the system.

If unloader valve goes on/off when gun is shut off either the spring is not properly adjusted or there is a leak in hose, gun or connections. Too tight a spring tension can create a safety problem due to high pressure spikes before it unloads. Sleight bypass of liquid prevents valve erosion.

Other safety devices used on pressure washers are:

  1. Safety or pop-off valve which releases water to atmosphere when pressure exceeds pressure it is set at.
  2. A rupture disk is an artificial weak point designed to burst when pressure highly exceeds system pressure.

Chemical Injectors - Detergents or additives can be easily injected to the flow of water on a pressure washer almost automatically with an injector. This can be done either upstream (apply chemical at full pressure) or downstream (apply chemical at lower pressure) of the pump. Downstream is the most preferred as chemical does not enter pump and do any damage to it. An injectors works like a jet pump. Water enters the injector and accelerates into a jet through the nozzle. The high velocity jet creates a vacuum which causes fluid to be drawn through the suction tube and into the injector. The mixture then flows into a diverging (venturi) passage where suction tube prevents backflow into chemical container when rinsing or at high pressure. Adequate pressure drop based on flow operating pressure must be made to activate venturi principle. This is done by changing to a larger nozzle or diverting flow into other openings which reduces pressure. Injectors are sized by flow and 80% pressure drop must occur to function. Chemical viscosity must also be considered. Too long or too small a diameter of hose or fitting restrictions may cause excessive pressure loss which will not allow venturi to operate.

Trigger Gun - Guns are fairly simple mechanisms designed around a trigger-operated ball valve. The valve ball is held in a closed or forward position by flow of water and blocks flow of water through gun to the nozzle. When trigger is depressed it pushes a pin against ball, forcing the ball off its seat and opening a path for water to flow to the nozzle. When trigger is released gain a spring returns the ball to its seating and flow is again blocked. Gun should be comfortable for operator to use based on the performance

Generally front entry type guns are used on lower pressure units and are less expensive.

Rear entry guns are more comfortable as they have less kick-back and the hose does not get in the operators way.

The gun is probably the most abused component of the pressure washer and thereby the most frequently replaced. It pays to offer a quality one thereby avoiding expensive warranty cost.

Lances - Generally 1/8" or 1/4" diameter. Should be long enough to avoid operator from putting hand in front of nozzle when opening gun or from penetrating foot on higher pressure units. Bent ends generally enable you to spray at an angle, however, back pressure can be fatiguing unless length is sized to operating pressure. Length determines how far away from object being cleaned without getting splash back. Cleaning efficiency reduces dramatically as you increase distance from object being cleaned. For example, at 12" pressure will drop in half from 6" away.

The Nozzle - Next to the pump this is the most important component that determines pressure and cleaning efficiency. The smaller the spray pattern the higher the impact. For example a #5 nozzle changes from an impact of 5.6 lbs. At 15? to 2.6 lbs. With a 40? spray angle. This is why turbo nozzles have become so popular recently. Actually they don't increase pressure but merely use a Zero degree pattern with an oscillating action that covers a larger area faster than you could move a Zero nozzle.

Hose - Hose is generally rated by hose manufacturer for operating pressure with a 4 times burst factor. They are braided and have crimped ends and hose guards at ends to prevent kinking at connections. Diameter and length of hose should be sized to flow and pressure operating at. Pressure or friction loss can increase significantly as flow increases. For example at 3 GPM on a 50 foot length of 1/4" hose the pressure loss is 200 PSI, whereas a 3/8" hose has only a 25 PSI pressure loss.


Rotary Brush - Mounted on end of lance and spins with water pressure providing brushing action required for certain application such as removing road haze from cars where excessive pressure or chemicals could damage paint surface or pin stripping.

Foamers - A foaming nozzle permits the flow of liquid to suck in atmospheric or compressed air into the flow causing it to form tiny air bubbles. This enables the detergent to cling to the surface longer and penetrate the dirt.

Sand Blast Heads - Small amount of sand in high pressure water stream can significantly increase its ability to strip paint. Must have adequate pressure to work effectively, i.e. 2 GPM at 1000 PSI or 3 at 3000 PSI.

Be certain that sand inlet tube is always at top to prevent water from running back into the sand when gun is shut off which will cause it to clog.

Drain Cleaning Nozzle - Excellent for cleaning clogged drains or pipes. Nozzles have a forward jet and several jets at a backward angle which causes nozzle and hose to propel forward knocking off dirt as it travels through pipe.

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