How Relief Valves Can Cause Pressure Loss

Various sprayer manufacturers have different ideas as to how to handle the return lines from the spray pump. Depending on the type of pump, there is a need to return the spray material that does not go to the nozzles back into the tank.

Pressure Relief Valve by Spraying Systems

All well designed sprayers have a pressure relief valve in the line coming out of the pump (pressure side). The pump produces a certain volume of liquid: pressure builds up when the outlet of that volume is restricted (the nozzle).

Pressure will continue to build up because there is less going out the nozzle than is being produced by the pump. In order to balance this, a relief valve will divert the extra volume so as to keep the pressure constant.

When the pressure in our sprayer drops, we usually blame the pump, but the first thing we have to look at is the wear in the pressure relief valve. The abrasives in our spray materials wear the seat and plunger in the valve and, when these do not seal properly, they will allow liquid to escape back into the return line.

Consequently, pressure cannot be built up because the relief valve is not restricting the flow because it is leaking. By checking out the relief valve first, we can usually save ourselves a costly and unnecessary pump repair as well as shorten down time, as relief valves are usually very simple to rebuild.

The relief valve is basically a plunger that sits in a seat and is held against that seat by a spring. The pressure in the line presses against that plunger and, when the pressure exceeds the pressure of the spring holding the plunger against the seat, the plunger is pushed away from the seat, allowing liquid to pass through.

The passing liquid then goes out through a line that takes it back to the sprayer tank. When the pressure drops, the spring pushes the plunger back against the seat. This system adjusts continually to maintain a steady pressure as set by the pressure on the spring, which is controlled generally by a screw/handle on the top of the pressure regulator.

A practical example is the pump that is producing 6 gpm for a spray gun using 2gpm, then the extra 4gpms are diverted through the seat of the relief valve, back into the tank. And this could be at any pressure, as long as the spring is strong enough to control it.

That is why we have pressure relief valves rated for 0-100psi, 100-300psi, 100-700psi, etc. Each one of these models has a stronger or weaker spring which generally can be adjusted within those pressure ratings.

Speaking of springs, this could be another cause for losing pressure. Some of these springs are corroded by the materials being by-passed in the relief valve and can weaken or even break and then yes, there will be no pressure at the nozzle.

So when troubleshooting loss of pressure in your rig, first look at the relief valve, (also known as control valve or unit). If the spring tensioning screw or adjustment is all the way down, chances are your failure is in this unit and not the pump.

Another item to look for is a hydraulic agitator (eductor) connected to the relief valve discharge line: This is not recommended for the following two reasons:

The volume coming out of the relief port may not be constant, as it would change with variations in pump speed and number of outlet nozzles open. This could make the the operation of the agitator erratic, as there could be times when nothing is coming out of the relief discharge, therefore no agitation in the tank.
The effect of the agitator (eductor) is similar to a nozzle, producing back pressure in the relief line. Some pumps, especially diaphragm pumps are affected by this back pressure, which could cause premature pump failure. Consequently, make sure that the relief circuit from the valve to the tank is not restricted in any way.

Certain centrifugal pumps, Hypro, for example, do not require pressure relief valves in the discharge circuits. These pumps are designed to allow certain slippage inside the volute (impeller housing) and pressure as well as volume can be safely controlled with a "throttling valve", usually a ball valve in the discharge line that either restricts the output flow or diverts part of that flow back to the tank. A hydraulic agitator (eductor) can safely be used in this system.