Pressure spikes and relief valves on air-assist sprayers

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About Jason Deveau (Spray Guy)

Dr. Jason Deveau has been the OMAFRA Application Technology Specialist since '08. He researches and teaches methods to improve the safe, effective and efficient application of agricultural sprays in specialty crops, field crops and controlled environments. He is the co-administrator of Sprayers101, co-author of the Airblast101 Textbook, a slow cyclist and an even slower runner.

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A properly-sized pump should produce more flow than is needed and work in conjunction with the atomizers to regulate that flow. Typical to high pressure pumps, a piston relief valve (aka regulator) should maintain the desired system pressure through the normal speed range of the sprayer, regardless of the number of booms (or boom-sections) that are on or off. This is achieved by balancing the sprayer pressure against the relief valve spring, which must move freely across a range of flows.

But what does it mean when the pressure gauge briefly spikes off-scale when boom are turned on or off? This is bad for the gauge and will eventually cause it to fail. Quite often, pressure spikes are an indication of one of two things:

  • A dirty or stuck valve
  • An inappropriate spring size
A pressure gauge spiking beyond its range.
A pressure gauge spiking beyond its range.

Relief valve maintenance

Sometimes, pressure spikes indicate a need for valve cleaning and maintenance.

  • The regulator spring cavity may be packed with dirt, which limits valve travel. Clean the housing and spring, and then lubricate and adjust.
  • The regulator may be partially seized or sticky. If the regulator piston and cylinder bores are caked with spray they will ‘hold’ the valve until the pressure/spring balance overcomes the friction.
  • Sometimes valve, and/or the valve guide pin are seized. Disassemble them, clean all sliding surfaces, then lubricate and adjust.
  • Valve/seat wear may have created a leak. You may have already tightened the spring to compensate, but this loads the spring past the pressure balance point you want to spray at. This means that when the booms are shut off, the pressure increases until it reaches the ‘new’ spring balance point. Repair (or replace) the regulator, then lubricate and adjust. Be aware that any leak (external or internal) can contribute to this condition and tightening the spring isn’t the solution.
  • The spring may be damaged (e.g. bent, corroded, etc.). Replace the spring, lubricate and adjust.

Be sure to read the operator’s manual before you do anything. You should understand your sprayer’s design before you perform any maintenance, adjustments or calibration.

Spring size

Sometimes, the relief valve may be mechanically sound, but the spring may not be sized to match a reduced operating pressure. Relief valve springs match the maximum pressure range of the pump. Sprayers operated at lower pressure may be unable to compress the spring. This is common when people switch from disc-core nozzles operated at higher pressure to molded nozzles operated at lower pressure.

This would manifest when one boom is shut off for single-boom operation; there may not be enough pressure to open the bypass. As a result, flow increases over the remaining boom.

Recognizing this problem, some operators have teed-in a second relief valve capable of finer adjustments at lower pressures. Make sure you know what you’re doing if you’re considering this option.

Technically, a spring can either be too weak, or too heavy:

  • The spring may be too weak for the pressure being used (i.e. any adjustment bottoms out). In order to obtain sufficient pressure the operator tightens the spring until it is virtually collapsed, essentially creating a fixed orifice. When the booms are closed the ‘fixed orifice’ doesn’t compensate and pressure rises to force the increased flow through that small orifice.
  • If the spring is too heavy for the pressure being used (any adjustment barely touches the spring when pump is turned off). In this case, the pressure being used will not deflect the spring, so the operator closes the regulator until the ‘fixed orifice’ creates sufficient restriction to flow to achieve the desired pressure. When the booms are closed the ‘fixed orifice’ doesn’t compensate and pressure rises to force the increased flow through, or until the spring begins to deflect.
  • In either situation the spring must be sized so it is in the centre-third of its flex range (i.e. rest state > fully collapsed) at the desired pressure. You can buy springs from the sprayer dealer or hardware supply. Try to maintain original length and diameter of the coil, while varying the diameter of the wire.


In some cases, it is not a matter of valve maintenance, or spring size, but poor engineering. Consider the following:

  • The valve supply and return may be too small for the pump flow. Consult hose and fitting catalogs for flow capacities and lengths. Re-size the hoses and fittings appropriately, and then adjust the regulator.
  • There may be kinks or sharp bends in in the supply and return lines. Re-route the hoses and/or fittings to avoid kinks and sharp bends, and then adjust the regulator.
  • The relief valve may be too small for the pump flow. Consult a regulator catalog for flow capacities and replace the regulator with an appropriate size. Calibrate the regulator spring and adjust.
  • Relief valves have a ‘cracking’ pressure (that’s when the valve just starts to open). Well-designed regulators have small pressure changes from ‘cracking’ to full flow. That information is in their catalogs. Poorly designed regulators have large pressure changes between these two ratings and these regulators should be avoided.
  • The pump may be too big for system. This often happens when sprayers are upgraded and pumps are replaced. Consult the catalogs and reduce pump size or speed, or increase the sizes of the hoses, fittings and regulator.
  • There may be a hydraulic agitator jet on the regulator ‘tank’ line. An agitator jet applies considerable back pressure to a system, and when booms are closed the increased flow causes more than a linear increase in pressure.
  • Broadly, the sprayer system as a whole may be poorly engineered. Inspect and draw a flow path of the sprayer system. Examine where everything is going (or not going). Is it possible someone made changes that the manufacturer did not intend? Consult the manufacturer if you are uncertain. Sometimes, it will have to be re-engineered, which may require expert consultation.

Your pressure gauge can tell you a lot more than your operating pressure – it can indicate a problem with your regulator, pump, lines or overall sprayer engineering. Don’t ignore it – address it.

Thanks to Murray Thiessen, Consulting Agricultural Mechanic, for his contribution to this article.