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Minimizing Paint Line Hour Glassing - by Jim Steiner While there are a variety of causes, hour glassing is simply a momentary absence of paint pressure at the gun. When the paint pressure drops, line width is affected. Think of it like an oscillating lawn sprinkler. To cover a wider area, you must increase water pressure to the sprinkler. If you are covering too wide an area, you must turn the water pressure down to reduce the coverage area.
With high pressure paint machines, most often the hour glassing occurs when the reciprocating pump reverses direction between its up and down strokes. The temporary absence of flow from the pump causes a reduction in system pressure. The result is a momentary low pressure at the gun and the hour glass shape on the road. Does Paint Viscosity Affect Hour Glassing? Yes, it can, but paint viscosity is a variable you may not want to change. Higher viscosity paints generally produce better quality lines with less overspray. For example, water based paints have high solids content and, therefore, tend to have high viscosity. The high viscosity paint increases resistance to movement through the paint system, and can intensify hour glassing. Will Boost Pressure Stop Hour Glassing? Boost pressure can reduce hour glassing to some degree, primarily when caused by high viscosity paint, because it helps keep the suction side of the high pressure pump flooded. A boost system is required when heat exchangers are installed on the suction side of the high pressure pump. The pump cannot draw sufficient paint through the restriction created by the heat exchanger without pump cavitation. Boost pressure in the range of 20-60 psi has been found to be the most effective. However, boost pressure alone will not stop hour glassing caused by worn check balls, seats, or other mechanical problems. How are Mechanical Causes Identified? It can be difficult. Conduct the following brief check when the high pressure pump is on the upstroke. Pressurize the system and briefly open a gun, then close it again quickly. When the gun is closed, observe the high pressure pump carefully. If the surge tank has pressurized and the pump continues to stroke during the upstroke, it most likely indicates that the rod check valve or packing is leaking. Repeat the test again when the pump is on the downstroke. If the surge tank has pressurized and the pump continues to stroke while testing on the downstroke, the foot valve is leaking. Can a Surge Tank or Accumulator Help? Yes, they do. Both devices store paint by compressing trapped gas to compensate for the pressure drop when the piston pump changes direction. Expanding gas in the surge tank forces paint into the plumbing between the pump and gun to help maintain constant pressure. A surge tank is low cost and works well in most applications, but its performance will fade as paint viscosity and paint pressure increase. If the surge tank fills with paint and loses the trapped air inside it, the compensating effect of the surge tank will be lost, and hour glassing can become severe. For high viscosity or high pressure applications, a nitrogen precharged accumulator is the best choice. Unlike a surge tank, pressurized nitrogen in an accumulator is stored inside a flexible bladder. The bladder is charged with 1/2 to 2/3 of the minimum spray pressure required at the guns. This keeps the paint pressure and flow at the guns nearly constant. We recommend a 1 quart accumulator for most systems. What About Size and Routing of Lines? Yes, this can contribute to hour glassing. If the line diameter is too small, or the hard plumbing contains multiple 90-degree turns, the paint can have difficulty passing through the system. The route should be as short as possible with as few turns as possible. We recommend 1/4" ID paint lines as a minimum for flow rates up to 1 gpm, 3/8" ID lines as a minimum for systems with a flow rate of up to 2.5 gpm, and 1/2" ID or larger for systems with a greater flow rate. Any Other Factors to be Concerned With? Good maintenance always helps. Worn tips, clogged paint strainers and similar issues can affect proper pressure and flow and ultimately cause hour glassing. Good maintenance is the place to start. How Can Someone Identify the Cause of Their Hour Glassing? If you can’t determine what the problem is, call us for trouble shooting
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The difference with
paint lines is that we are trying to maintain a particular line width. All
other things being equal, reduced pressure at the paint gun will decrease
the width of the line, while increased pressure increases the line width.
Forward movement of the paint machine during the period of reduced pressure
results in the common hour glass shape. Depending on the amount the
pressure is reduced, hour glassing can be relatively mild with just a gentle
curve, or very severe as if the gun was shut down.