Pressure Washing Pump

To understand the relationship between pressure and flow in a pressure washing system, it is essential to first distinguish between a Positive Displacement Pump (PDP) and a Centrifugal Pump. Pressure washers typically employ the former class, the PDP, which maintains a constant flow rate through the system and serve as the mechanical workhorse of a power washing system. This flow rate, usually advertised as gallons per minute (GPM) and ranging between one and five GPM, is a key specification of most pressure washers.

In fact, the flow rate is the most important characteristic of a pressure washing system, with pressure taking a secondary role. While PDPs can easily produce high system pressures, creating a high-flow system under pressure requires advanced design and robust materials. For this reason, high-flow pressure washers are uncommon in retail stores and even less so in residential garages. Simply put, a high flow rate significantly enhances the cleaning capabilities of a pressure washing system, far beyond what a high-pressure, low-flow system can achieve.

Volumetric Flow Rate of a Pressure Washing Pump

Positive displacement pumps generate flow rates directly determined by the pump's cylinder volumes, the number of cylinders, and the pump's cycling rate. Each stroke of the pump physically displaces a fixed volume of water from the pump's cylinder to the discharge outlet. Consequently, as long as the cycling rate remains constant, the flow rate does not fluctuate. Unlike centrifugal pumps, where flow rate decreases with increasing downstream resistance, PDPs maintain a constant flow rate. Downstream restrictions instead result in an increase in system pressure.

PDPs: The Inverse of a Combustion Engine

A helpful analogy is to think of a PDP system as similar to a car engine—except, rather than combustion gases driving the pistons, the pistons drive fluid through the pump's channels. In a pressure washing system, the flow rate remains constant up to the pump's maximum design pressure. However, if the system pressure is not relieved, catastrophic failure will occur, components will break, and water will spray out of the breach—still maintaining the system’s unchanging flow rate.

Critical Failure: Pressure and Thermal Relief Valves

To prevent such failures, pressure washers incorporate an unloader valve, a specialized type of pressure relief valve. When system pressure exceeds the unloader valve's setpoint, the valve opens a bypass channel, diverting water back to the pump inlet or, in some systems, to a separate water tank. This allows the pump to maintain flow without over pressurizing the system. However, continuous water recirculation through the pump gradually increases the water temperature.

Most pressure washer pumps are constructed from materials like brass, which are temperature sensitive. Overheated water can damage these components, which is why pressure washers are also equipped with a thermal relief valve. The thermal relief valve releases overheated water, protecting the pump from thermal damage.

Additionally, because failure of the unloader valve can result in catastrophic system damage, a secondary pressure relief valve is critical. This secondary valve should be set to open at a pressure slightly higher than the unloader valve's setpoint—typically about 10% higher. For instance, in a system designed to operate at a maximum of 4,000 PSI, the unloader valve might be set at 3,600 PSI, while the secondary relief valve is set just under 4,000 PSI. This configuration ensures that "spike pressure" (brief pressure surges when the unloader valve changes states) does not inadvertently activate the secondary valve.

By incorporating these fail-safe mechanisms, pressure washers ensure consistent operation while minimizing the risk of system failure or damage to pump components.

Controlling Pressure is the Key to Unlocking New Functions

While it is critical to ensure that system pressure is prevented from ever exceeding the design maximum pressure, strategic use of nozzles and downstream injectors can unlock a huge functionality upgrade for a pressure washing system. This new, modern soft washing process relies on carefully crafted cleaning solutions specifically designed to target organic growth on exterior surfaces.

No Replacement for Professional Service

While the concepts here are straightforward, their practical application can be more nuanced than it seems. If you found this information helpful but prefer to leave the hands-on work to the experts, Surface Work is here for you!

We take the guesswork out of power washing by mastering these details through years of experience serving Virginia Beach and surrounding areas. Whether you’re in the Ocean Lakes neighborhood near the beach or the Alexandria neighborhood on the edge of Chesapeake, Surface Work delivers the best power washing services in Virginia Beach. Surface Work makes it easy to get a quote for house washing services by posting the prices online! Contact Surface Work today to experience the difference!