Paint Mix Room, Pump Supply
& Circulation System Basics
by Todd Anderson,
Technical Application Specialist, Graco Inc.
(4/20/07)
Introduction
This document is intended to explain the basics of pump supply selection and
circulation system design. Specific circulation pipe sizing are not fully addressed in this
document, as in depth line sizing requires further analysis to properly design a complete
paint circulation system. The intention is to provide enough detail to understand the
basics of paint mix room pump selection, pump commissioning and the various types of
circulation systems and their differences. Waterborne materials are also addressed, as
there are specific differences when dealing with waterborne paints vs. solventborne.
Paint Pump Selection
A major component of a paint circulation supply system is the paint supply pump. This
pump is required to supply paint 24 hrs/day, 365 days/year. The most important factor
is to ensure the pump supplying each of the circulation lines is sized appropriately for
the continuous circulation volume required to maintain proper paint shear, viscosity,
particle paint suspension and to minimize paint degradation. The pump must also
provide enough additional volume requirements for intermittent paint supply as gun
drops are in use. The pump must provide the proper fluid pressures for the continuous
and the additional intermittent volumes. It is important to note that the additional pump
flow required when guns lines are opened may be very small depending on the
responsiveness of the Back Pressure Regulator (BPR) under the production conditions.
The most popular pump designs for paint circulation supply are large volume 4 ball
piston and centrifugal pumps. Graco Midrange and High Flo Plus 4 ball piston pumps
(pneumatic and hydraulic) are the most widely used throughout the industry. The Graco
Imperial centrifugal pump is also widely used and proven to be extremely reliable. An
important factor regarding pump selection is paint degradation of metallic materials
caused by the continuous circulation of the material through the pump, piping system
and back pressure regulator. Piston pumps cause the least amount of degradation to
both the brightness and color two-tone effects exhibited by a metallic finish in
comparison to rotary lobe style pumps. Centrifugal and rotary lobe pumps will cause
some initial degradation to the brightness flop of the metallic paint but the rotary lobe
pump will dramatically increase this degradation with extended use. 1
When selecting a piston pump for each circulation loop, Graco recommends a pump
with a continuous duty cycle of 12 cpm (cycles per minute) or less and a maximum
intermittent duty cycle of 20 to 30 cpm. Pump packing and piston wear is directly
related to pump cycle rates. Therefore, pump selection is key to ensure limited wear for
continuous duty operation. Pay close attention to the pump performance curves for fluid
pressure and flow rates at the recommended cycle rates. Maximum piston pump flow
Weiss, K; Handzel, J; Lewis, R; Heilig, W; Korzenowski, J; Adams, J. “WATERBORNE METALLIC PAINTS: A
Comparison of the Degradation Caused by Various Pumps During Circulation.”, SAE, Detroit, MI (1996)
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rates are typically rated at 60 cpm and are poor indicators of circulation pump size.
Pump cycle rates are directly related to pump wear. It is not uncommon to find
installations with pumps running near 4 cpm to minimize wear, especially with
aggressive materials like Xirallic flake materials or heavily pigmented materials.
Graco Imperial pumps should also be sized for the proper flow rates and pressures by
referencing the proper pump curves paying close attention to the viscosity and the
specific gravity of each material. Graco Imperial pumps are sized in stages and
oversizing or undersizing an Imperial pump can produce excess pump strain, limited or
excess pressures and flows with excess energy concerns, typically in the form of heat
and excessive motor current draws.
When selecting supply pumps for waterborne materials, ensure the wetted parts of the
pump are stainless steel. Graco Midrange High Flo, High Flo Plus and Imperial pumps
are well accepted for waterborne materials for high flow circulation applications. This
includes waterborne clear coats as well as waterborne metallics and primers. The
larger High Flo pumps (High Flo Plus), utilize a Plasma (Tuff Coat) pump rod. Caution
must be taken when using Plasma rods with waterborne and other corrosive materials.
Corrosion can occur if the rod is left in a stationary position for a period of time with
corrosive material exposure. To eliminate corrosion concerns with the large High Flo
pumps, hard chrome pumps rods (and pump assemblies) are available.
There is currently a trend in the industry with the use of Xirallic flake metallic paints. The
Xirallic flakes used in these metallic paints are typically 20 to 35 times harder than the
typical metallic (mica) flakes used in the past and will produces different wear results
because of this quality. When selecting a Graco Imperial pump for Xirallic materials,
use the Graco Imperial Severe Duty pump. This Severe Duty Imperial pump has
carbide coated wear parts to combat the abrasive nature of these Xirallic materials and
have worked well in those applications where Xirallic is heavily used. The Graco 4 ball
Midrange pumps offer Chromex or Maxlife rods and Maxlife cylinders as choices for
applications where abrasive materials are used. (i.e. heavily filled Xirallics) The Graco
High Flo plus pump can be supplied with Plasma (Tuff Coat) rods for long life
characteristics (only for continuous duty applications only.)
Part of the pump selection process is the upfront costs of the pump selected for the
circulation application vs. the operational efficiency. Typically air operated (pneumatic)
pumps are the least energy efficient but are typically lowest in installation costs vs.
hydraulic. Hydraulic operated pumps are very energy efficient but require hydraulic
power packs to drive the pump motors, which may push the installation costs higher
than pneumatic and electric. An electrically operated pump is relatively easy to
implement and usually has a minor installation cost impact. Because of the ease of
installation for an electric operated pump, like the Graco E-Flo Plus, and the need to
minimize energy production costs, current industry trends are towards electric energy
efficient paint supply solutions.
Pump Commissioning
Before a paint supply pump is commissioned for service, some circulation pipe system
cleaning may be required. Typically, caustic cleaners are used to clean the paint piping
system before the paint is loaded through the paint supply pumps. These cleaning
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materials are typically corrosive and not recommended at any time for use with Graco
paint supply pumps. If caustics are required to clean the paint circ lines, use an
alternative pump specifically designed for that process. (i.e. a diaphragm pump with the
proper wetted components, compatible with the cleaning materials.) A pump large
enough to easily supply high flow rates/velocities is typically required during this
cleaning process. High flow rates and velocities are required to clean the pipes, these
cannot be produced properly with the paint supply pump. The paint supply pump should
not be used to supply the caustic cleaning materials. The risk of over cycling the piston
pump is likely. There is a definite risk of compromising the packings as well as the hard
components within the Graco piston pumps.
For waterborne applications, the paint supply lines may be loaded with DI water and
circulated before loading the paint in the system. DI water is corrosive, therefore a
dedicated flush pump should be used that is compatible with DI water. If the paint
supply pump is required to cycle the DI water through the circulation lines, this
circulation time should be limited to the shortest time possible. The Graco Midrange
and High Flo Plus pumps require TSL (Throat Seal Liquid) in the wet cup at all times to
keep the rod and packings wet and avoid dried paint on the rod surface.
When
pumping DI water or waterborne materials, the wet cup should never be filled with DI
water. The DI water is corrosive and should never be used in place of TSL. During the
flushing stage of cycling DI water through the Graco Midrange or High Flo Plus 4 ball
pumps, siphoning of wet cup liquid into the pump is normal. Care should be taken to
ensure there is liquid in the wet cup at all time during this DI water flush cycle. Once
paints are loaded through the pumps the siphoning of the wet cup materials will stop.
Graco has found TSL to be compatible with both solventborne and waterborne
materials. (See below for TSL details.) Also listed below are alternative wet cup fluids
for waterborne applications that have been used successfully as alternatives to TSL.
Graco TSL (contact Graco for part numbers)
 Blue Label (This is Graco’s new standard formula)
 Red Label Classic (proven to be a good choice for years, some customers prefer to
continue the use of the red label TSL for paint circ applications.)
Alternative Wet Cup Fluids (Waterborne applications):
Monopropylene Glycol
Butyl Glycol diluted with 10% Water
Butyl Glycol (undiluted)
Circulation System Design Basics
A circulation system is a properly designed piping network used to deliver paint to the
applicator stations as well as maintain and control the material characteristics that are
critical to the finishing process. As discussed early in this document, it is important to
remember most of the systems are circulating paint 24 hr/day, 365 days per year. The
first concern in the design of a paint circulation system is to assure that the proper
pressure and volumes are being delivered to the application stations throughout the
production facility. Included with the design concerns are the ability of the circulation
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system to control and maintain paint shear, viscosity, particle suspension and minimize
paint degradation.
Particle and Pigment Suspension
The material velocities in a properly designed paint circulation system keeps particles
and pigment settling to a minimum and produces a consistent and uniform finish once
applied to the production component. Proper paint mixing and agitation in the supply
tanks are not enough to ensure proper paint suspension. Without the proper circulation
of the paint in the pipe supply and return lines, separation and settling of the paint
pigments and solvents will occur in the piping. This settling is typically considered “dirt”
in the piping with possibility of this settled material ending up on the production part.
Viscosity and Shear Control
Fluctuations in the paint material temperature during the production process can result
in materials that will not atomize properly and adversely affect the quality of the finish.
Some waterborne materials rely on the shear produced by the piping system and the
circulating pump to reduce the material viscosity. Conversely, these same materials
with their unique blends of pigments, micas, metallics, solvents and water, will
breakdown with excessive shear. Care needs to be taken to ensure the piping system
has smooth interior surfaces and transitions. Properly designed paint circulation
systems are an important component used in the control of the paint material
characteristics.
Circulation Rates
Typically circulation velocity requirements for proper material suspension for
solventborne materials are 60 fpm (feet per minute)(.3 m/sec) through the main
circulation loops with 30 to 60 fpm (.15 to .3 m/sec) velocities through the system
branch lines, low pressure returns and gun drops. The same rates are also used with
waterborne materials but care should be take to ensure the minimum velocity rates are
achieved throughout the circulation system. Waterborne materials are typically more
sensitive to viscosity changes and are likely to increase in viscosity when not under a
minimum shear rate. Waterborne materials may not need the 60 fpm (.3 m/sec)
minimum velocity for the main circulation rate and may only require 30 fpm (.15 m/sec).
Ensure the material supplier is in agreement with this before designing the system for
this lower circulation rate. Many paint suppliers are reluctant to agree to the 30 fpm (.15
m/sec) main circulation loop velocity, until they have achieved some history with the
material and found the lower circulation rates are adequate.
Circulation System Design Types
There are typically 3 types of circulation system designs for high flow paint supply
facilities. (Reference attached system layouts.)



Graduated 2 Pipe System (Direct Return, Reverse Return)
Dual Regulated 3 Pipe System
Branch Line System
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In smaller industrial applications, a single circulation pipe “Dead End” system is also
commonly used. This system is typically used by industrial customers with limited
production requirements and will not be discussed in detail here.
Described below are the basics of each of the 3 main types of circulation systems. The
focus is on the basics without the details of specific paint line sizes. Sizing requires a
full analysis of the requirements of the paint system. Ensure the proper material
compatibility of the paint pipe system is used for the current materials used as well as
any future potential materials changes.
2 Pipe Paint Circulation System
A 2 Pipe paint system uses the balancing of pipe diameters to ensure the “balancing” of
pressures and flows for every flow path. Essentially, there are no regulators to balance
the system pressures at each individual paint station. A graduated piping network takes
the place of mechanical pressure control devices for the control of pressure and flow at
each paint drop. The only regulator in the system is the back pressure regulator located
near the paint system supply station to control the overall paint system pressure. Gun
mounted regulators or “Y” restrictors are normally used to control individual spray gun
delivery.

The Direct Return version of a 2 Pipe Circulation System uses a sequential
parallel paint supply and return. The highest pressure paint is supplied to the last
branch and returns from the first paint branch. This type of system may require
higher pump pressures and flows to support multiple branches.

The Reverse Return version of a 2 Pipe Circulation System uses an inverted
parallel paint supply and return. The highest pressure paint is supplied to the
first paint branch but returns from the last return branch. This type of circulation
will require additional paint circulation piping for either the supply or the return.
A properly designed two pipe paint circulation system can be very cost effective and
generally has the lowest pressure, volume and the lowest fill volume of most of the
circulation system choices. The complication of the balancing of pipe sizes to ensure
proper volumes and pressures at the application stations makes this system difficult to
expand.
3 Pipe Paint Circulation System
Commonly called a Third Line System, the 3 Pipe Paint Circulation System is a popular
circulation system because of it’s flexibility. A 3 Pipe system consists of a high pressure
paint supply pipe with pressure regulators at every paint station and a low pressure
paint return line, to allow circulation to every station. In addition to paint station
regulators, the high pressure supply lines and the low pressure return line are equipped
with back pressure regulators located at the paint supply stations. Each paint station in
a 3 Pipe system generally has a spray gun mounted regulator or “Y” restrictors.
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The major advantage of the 3 Pipe system is it’s ability to be easily expanded. This
system generally has the highest pressure, volume and highest fill volume of most of
the circulation choices. The 3 Pipe system has the advantage of ensuring proper
material velocities at each application station by the station fluid pressure regulator.
The 3 Pipe system can be a critical advantage for sensitive waterborne materials that
require minimum circulation velocities at all locations.
Branch Line Paint Circulation System
The branch line system uses a fixed diameter supply pipe in combination with a
graduated diameter return pipe to allow circulation to a group of application stations. At
a give spray booth, the branch line piping will join the supply piping after the last
application station and down stream of a back pressure regulator installed in the supply
piping. The point where the supply piping and the return line branch piping join is
referred to as the branch line termination (BLT). The back pressure regulator installed
in the supply piping at the BLT is referred to as the branch line termination back
pressure regulator (BLT/BPR). Each paint station in a 3 Pipe system generally has a
spray gun mounted regulator or “Y” restrictors.
A Branch Line system requires the least amount of material to create a circulation to the
spray station. This reduced amount may lead to a smaller supply pump than would be
required for a similar sized 3 Pipe system.
A major disadvantage of a Branch Line system is the limitation of the amount of
application stations that can be supplied. As the number of paint stations increase, or
the paint viscosity rises, the pressure drops across the branch line termination reaches
levels outside of the operation capabilities of the circulation pump, the BLT/BPR or the
mix room BPR.
Paint Mix Tank Requirements Specific to Waterborne Materials
Working with waterborne materials requires stainless steel wetted components. The
quality of the stainless for all wetted parts of the system should be of 300 series or
better. 400 series stainless should be avoided as this grade offers minimal corrosion
characteristics with waterborne materials. The material requirements for the mix tank
are no exception. Because of the drying nature of many waterborne materials, care
must be taken to ensure volume levels within the tank are not fluctuating dramically. In
typical solventborne applications, the drying of material on agitator shafts and paint mix
tank walls are usually not much of an issue. Many waterborne materials may not set
correctly and can coagulate one these surfaces. With waterborne materials, the
stainless steel surface evenness require a much higher quality that required with many
solventborne materials. A standard method is to electropolish all the surfaces of the mix
tank to help in reducing the attachment of waterborne materials on the surface. Many
systems incorporate an automatic fill function from the main supply tote tanks to the
paint mix tank to ensure a consistant volume level at all times. These methods are
effective in preventing the drying of paint on the interior walls of the mix tank.
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Filters are an important component of the paint supply module. Large ported bag filters
are the most effective and present minimal issues with shear, especially with
waterborne materials.
Back Pressure Regulator as also a key component of the circulation system supply
module. Traditional ball and seat type regulators can shear some waterbornes as the
coating passes through the ball and seat. Low Shear style Back Pressure Regulators
are specifically designed to minimize the shear of the material. Graco’s air powered
BPR’s offer remote/automatic control of system pressure.
Agitation is a requirement for most systems but may be a concern for many waterborne
materials. Care should be taken with agitators with regard to speed and duration. A
low shear, adjustable speed agitator is essential. Waterborne materials tend to form
foam due to higher surface tension when agitated aggressively.
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PUMP
RETURN
BPR
2 PIPE (DIRECT RETURN)
CIRCULATION
EXAMPLE
Mix Room
1M
2RE
3RE
4RO
5M
LEGEND
BASE COAT 1
6M
1M
2RE
7RE
8RO
9M
3RE
4RO
5M
8RO
9M
BASE COAT 2
6M
7RE
8
M = Manual
RE = Recip
RO = Robot
RETURN
BPR
PUMP
2 PIPE (REVERSE RETURN)
CIRCULATION
EXAMPLE
Mix Room
1M
2RE
3RE
4RO
5M
LEGEND
BASE COAT 1
6M
1M
2RE
7RE
8RO
9M
3RE
4RO
5M
8RO
9M
BASE COAT 2
7RE
6M
9
M = Manual
RE = Recip
RO = Robot
LP RETURN
BPR
HP SUPPLY
BPR
PUMP
3 PIPE CIRCULATION
EXAMPLE
Mix Room
LEGEND
1M
2RE
3RE
4RO
5M
8RO
9M
BASE COAT 1
6M
7RE
M = Manual
RE = Recip
RO = Robot
STATION
FLUID
REGULATOR
1M
2RE
3RE
4RO
5M
8RO
9M
BASE COAT 2
7RE
6M
10
RETURN
BPR
PUMP
BRANCH LINE
CIRCULATION
EXAMPLE
Mix Room
1M
2RE
3RE
4RO
5M
LEGEND
BASE COAT 1
6M
7RE
8RO
9M
3RE
4RO
5M
8RO
9M
BLT
1M
2RE
BASE COAT 2
6M
7RE
BLT
11
M = Manual
RE = Recip
RO = Robot
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