Pressure relief valves are used in most (but not all) pressurized water blasters to prevent the over-pressurization of a blaster's internal parts. Too much pressure can lead to rupture of sealed, failure of parts, and even personal injury. However, in terms of build, they are more-or-less the same as spring-based check valves except that the spring used for pressure relief valves are calibrated to a specific pressure.
Parts of the Valve:
A pressure release valve, like a spring-based check valve, is comprised of a carefully shaped housing that surrounds a small, sculpted plunger (or plug) that is held against one side of the housing by a small spring. The key behind the design is the shape of the housing and plunger combined with its pressure calibrated spring that allows water (or air) to flow around the plunger when a specific amount of pressure is felt, thus preventing over-pressurization of the internals. Of course, for this to work, the pressure relief valve is placed in an opposite orientation compared to how spring-based check valves are placed, otherwise the increase in internal pressure would not open the pressure relief valve since they are, after all, also unidirectional valves.
Water Blasters that Use This Valve:
Some blasters that use spring-based check valves include:
- Super Soaker SC 400 - releases out the rear of the blaster
- Super Soaker CPS 1000 - part of pump valve assembly
- Super Soaker Monster XL - part of pump valve assembly
- Super Soaker Flash Flood - part of pump valve assembly
- Super Soaker Defender - part of pump valve assembly
- Water Warriors Blazer - part of pump valve assembly
- Water Warriors Vindicator - part of pump valve assembly
When there is no flow of water (or air) in any direction,the plunger is pushed snugly against the housing to prevent undesired loss of pressure through the valve. Since it is spring-based, this valve remains properly sealed regardless of orientation and/or standard gravity. As the front-side of the valve faces the pressurized water (or air) side of the air blaster, the valve possess a pressure-calibrated spring which ensures the plunger seals the opening even as there is enough internal pressure to actuate regular spring-based check valves.
When water (or air) is pushing in the direction denoted as forward, the valve will open once enough pressure to push against the pressure-calibrate spring is reached. The term, forward, is used strictly relative to the direction of flow through the valve since the actual position of the valve may have it facing upwards or even towards the back of the full water blaster. Due to the type of spring used, much higher pressures are required to open it by moving water or air. In a practical sense, the optimal pressure setting for the spring should be higher than that for good operational pressure for the pressurized water blaster, but still much less than the expected pressure required that would lead to failure of the system. For air-pressure-based water blasters, their operational pressure seems to be roughly 40-60 PSI; for rubber-bladder-based water blasters, their operation pressure seems to be around 35-45PSI.
Preventing Reverse Flow:
As noted above, thanks to the spring, this valve seals closed when there is not enough force to push the plunger open. As such, when water or air attempts to flow backwards through the valve, pushing in the reverse direction serves only to push the plunger harder against the housing, tightening the seal further. As such, this actually allows a neat bit of engineering to be done as is seen on most modern water blaster that feature a separate pressure chamber. With the correct tubing, check valve, and pressure relief valve configuration, the pressure relief valve can be position such that it remains closed for regular pumping of water and building pressure in the separate pressure chamber. However, one enough pressure is reached in the pressure chamber which would activate the valve, opening it, the excess water would be pushed back into the non-pressurized reservoir, conserving water within the blaster as opposed to letting it dribble out as was what occurred in older water blaster models.
As can be seen illustrated in the diagram above, there is no direct path for water (or air) to flow through the valve in the open state. Even when activated, water (or air) may enter straight into the valve, it must then deflect to go around the plunger, pass the spring, then exit through the out port. As the plunger's head must be larger than the hole it seals, little can be done to improve flow lamination through this type of valve. However, since this valve should primarily be in its closed position, the importance of laminar flow is negligible.
Strengths and Limitations:
The pressure relief valve is a simple, effective way to prevent over-pressurization of a pressurized water blaster. Operating similarly to spring-based check valves, the pressure relief valve works thanks to its pressure-regulated spring and it opposite orientation compared to a typical flow control valve. With few moving parts, pressure relief valves usually have a good active lifetime.
Of course, being spring-based, in time, a pressure relief valve's spring may get stiffer or softer, depending on the material it is made of. As such, the amount of pressure required to open the valve may end up increasing or decreasing over long periods of time (usually a matter of years, not weeks or months). There is no way to replace a faulty spring, thus a faulty valve would need to be completely replaced. If pressure is allowed to get too high, another part of the water blaster's internals may fail; on the other hand, if the pressure relief valve operates too easily, the water blaster's performance will suffer.