Why does the pressure gauge spike?

A pressure gauge spiking beyond its range.

A pressure gauge spiking beyond its range.

This article was co-written with Murray Thiessen, Consulting Agricultural Mechanic.

Have you ever see this happen? You finish spraying, you turn off the booms and the pressure gauge briefly spikes off-scale. This is very bad for the gauge and will eventually cause it to fail. The pressure gauge plays a very important part in maintaining spray quality and rate, so it needs to be addressed. What is going on?

The cause is insufficient or restricted regulator capacity. The regulator must 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 regulator spring, which must move freely across a range of flows.

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.

An FMC pressure regulator.

An FMC pressure regulator.

Calibration:

The regulator spring may be improperly sized for the desired spraying pressure. Many sprayers come equipped with regulator springs matched to the maximum pressure range of the pump, not the desired operating pressure. These springs are unable to control the pressure; they are so stiff that the liquid pressure is unable to act on the spring and the valve essentially acts as a flow control (throttling) valve rather than a pressure control valve. Liquid pressure is difficult to control using a throttling valve: it is unable to compensate for changes in flow (like engine speed dropping when driving uphill or closing booms or nozzles). This can cause pressure gauges (which should be matched to your normal operating pressure) to spike. An improperly sized 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.
An FMC pressure regulator - exploded view.

An FMC pressure regulator – exploded view.

Engineering:

  • The regulator supply and return may be too small for the pump flow. Consult hose and fitting catalogues 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 regulator may be too small for the pump flow. Consult a regulator catalogue for flow capacities and replace the regulator with an appropriate size. Calibrate the regulator spring and adjust.
  • Regulator 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 catalogues. 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 catalogues 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.