NEW PAINT C I R C U U T I O N PUMP TECHNOLOGY
Peter J. Bankert
Market Development Manager
Graco Inc.
Farmington Hills, Michigan
Presented at:
"Finishing '93 Conference and Exposition"
October 25-28, 1993
Dr. Albert B. Sabin Convention Center
Cincinnati, Ohio
Introduction
Paint circulation is one of the most demanding pumping applications in the
industry. Many new high solids paints require constant flow to eliminate
settling. In fact, automotive paints generally require a fluid velocity of 60
Wmin. to eliminate settling.
The paint circulation pump must be capable of operating continuously - 24
hours per day, seven days a week, 52 weeks a year. This pump literally
must operate 8760 hours per year to keep paint from settling in pipes and
hoses.
Continuous movement of paint through the circulation system can cause
paint shear degradation. This paper will review laboratory results of this
shear degradation and suggest ways to minimize it.
Paint Shear Degradation
Continuous pumping of paint can cause shear degradation.
degradation can result in:
This
I) Increase in paint temperature with a resultant reduction in viscosity,
evaporation of solvents, and possible thermal degradation.
2)
Breakdown in paint rheology which can result in lower paint viscosity.
3) Degradation of metallic flakes which can cause a darkening of the
color of the paint.
Metallic flake degradation has been recognized by the automotive industry
as a cause of color match problems. Additional shear degradation
problems are now associated with some of the newer paint technologies,
such as waterborne paint. Many times these problems do not arise until
new paint formulations are field tested - and even then they may not
become readily apparent due to the different shear levels of different
systems.
There is no single parameter to measure shear degradation. Every paint is
different and it will have a different reaction to the shear from the handling
equipment. The purpose of this paper is not to evaluate different paints, or
to describe what happens to the paint under high shear. The purpose of
this paper is to evaluate equipment and system designs which may reduce
shear.
-
Laboratory tests were performed in order to evaluate various equipment
configurations. The paint chosen for all equipment tests is an automotive
topcoat with large metallic flakes. A single paint was used in all tests to
simplify the test design and have a common baseline for all test results.
The key parameter of evaluation in the tests is the colorimeter readings - in
particular the near specular delta L parameter which is a measurement of
lightness of the coating. This parameter is chosen because previous lab
tests and field reports show this to be a consistent measurement of metallic
flake damage due to shear degradation.
Sources of Shear Dearadation
Previous lab tests have isolated three primary causes of shear
degradation:
1) Back Pressure Requlators (BPR): These devices have small orifices
which are used to create large pressure drops. These regulators are
also in the circulation loop. It is not uncommon for paint to go through
the BPR 100-200 times before it is sprayed. Fluid regulators which are
located at the spray device also have small orifices and high shear, but
the paint only goes through this regulator once. Other fluid regulators
in the circulation system (wall regulators in 3-pipe systems) are also
primary causes of degradation because they are "multiple pass"
reguI ators.
2) Pumps: This is an obvious source of paint degradation due to its
mechanical operation, energy input, and objective of increasing paint
pressure. It is expected that different pump types have different shea;
degradation I eveIs.
3) System Desian: The circulation system design translates the system
needs (flow, velocity, paint demand at spray guns) into equipment
specifications (pump pressure, BPR pressure, pipe diameters). The
system designer can reduce paint degradation by reducing paint
pressures and choosing low shear equipment.
~~
Desians to Minimize Shear Degradation
Equipment: The selection of low shear fluid regulators and low shear
pumps can reduce shear degradation.
Low shear regulators create pressure drops through special designed
orifices which reduce fluid velocity and fluid shear. Most fluid regulators
achieve pressure drop with a ball and seat orifice. In this regulator the
paint is squeezed through the tiny opening between the ball and seat. In
this type of regulator, the fluid velocities can reach 10,000 ft/min. A low
shear regulator has a more laminar fluid path and achieves similar
pressure drops at a greatly reduced fluid velocity. These low shear
regulators are used by many automotive manufacturers to reduce paint
degradation.
System Desian: New system designs can reduce paint degradation. One
system in particular Smart Injection - has been shown to reduce paint
degradation. This concept reduces degradation by eliminating the BPR,
and by reducing pump pressure requirements.
-
In order to understand Smart Injection, it is necessary to understand how a
traditional pumping system works.
Diagram I shows a traditional
circulation system with a pump taking paint from a mix tank and
pressurizing the paint to 200 PSI. The paint flows clockwise around the
loop and retums to the BPR. The BPR pressure in this example is 100 PSI
on the upstream side. Downstream of the BPR is the mix tank which is at 0
PSI (atmospheric). Every cycle through the system, the paint will go from
100 PSI to 0 to 200 PSI as it goes through the BPR to the mix tank and
then through the pump. Paint in a circulation system would make this cycle
on the average of 3 times an hour or 72 times per day.
Diagram II is a schematic of a Smart Injection System. Notice that the
BPR is removed, and the pressurized paint is fed directly into a circulation
pump. The circulation loop is always pressurized, it never returns to the
atmospheric mix tank.
New paint is supplied to the pressurized loop with an injector pump. This
pump must be a positive displacement, variable speed pump which will
stall at a set pressure. In the example shown, this pump injects paint into
the circulation loop at the outlet of the circ pump. The injector pump sets
the system pressure, and is stalled until paint leaves the loop. As soon as
paint leaves the loop through a spray gun, the injector pump senses a
reduced pressure in the loop and cycles on to make up for the lost paint.
The advantage of smart injection includes:
Reduced Paint Deqradation: This is directly due to the elimination of
the BPR and reducing the circulating pumps pressure requirement from
200 PSI (i.e. 0-200) to 100 PSI (i.e. 100-200).
Reduced Pump Enerw: Again this is a direct benefit of not 'Yhrowing
away" the energy in the paint at the BPR. In the example shown, the
BPR pressure "saved" is 100 PSI. This is 50% of the pump pressure of
200 PSI, and the energy savings is about 50%.
Paint Usaqe Reporting: The injector pump only supplies new paint to
the loop when paint is used. A flow meter on this line, or a "smart"
pump can measure flow at this point and report paint usage.
Back-up Pump: With two pumps in a system it is possible to provide
alternate piping and valves to enable the system to continue operating
if one pump were to fail.
Reduced Maintenance:
Pump wear and maintenance is directly
related to pump output. The pump energy savings is a direct indicator
of reduced output, and should relate directly to reduced maintenance.
Reduced Solvent Loss/Temperature Loss: The mix tank is often the
place where circulated paint can lose temperature, lose solvents, and
become aerated. Eliminating the mix tank from the circulation loop
reduces these risks.
Summary:
Lab experiments have been performed by Graco to document the shear
degradation of regulators, pumps and the benefits of the Smart Injections
System. The results were not available for inclusion in this written report,
but will be presented in the October 1993 presentation.
~~
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NOTES
NOTES
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