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February 2010

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F o r m o r e t

e c h n i c a l a r t i c l e s a b o u t t h e

p i p e l i n e i n d u s t r y , v i s i t w w

w . p

i p e d a t a . n e t .

PLURAL COMPONENT COATING systems such asPolyurethane and Epoxy coatings consist of two

components that are mixed together at a specific ratio andapplied using airless hot-spray equipment. Often thesecoatings are solvent free, consisting of 100 percent solids.The two-components, referred to as component A andcomponent B are stored in separate containers and have anextended shelf life.

Once component A and component B are mixed togetherin the prescribed ratio, a chemical reaction begins that willultimately cause the material to adhere to the substrate,harden and become the desired corrosion barrier. This is a“curing” process as opposed to a “drying” process brought

about by the evaporation of solvents or water contained inthe coating. This curing is not instantaneous but occurs overtime and passes through several stages.

The curing of the coating material creates problems for airlesshot-spray application.

Airless hot-spray application system

An airless hot-spray application system is shown in Fig.1.There are many manufacturers of this equipment andnumerous variations but the principals of operation are allthe same.

Automation of field application of

plural component coatings to improve

accuracy, productivity, and providedocumentation of the coating

operationby Wayne Duncan

Altus Engineering, Nisku, Alberta, Canada

APPLICATION OF PLURAL component coatings for field rehabilitation projects is normally done manually

using fixed ratio or variable ratio metering units. Even the most experienced coating applicators have a

great deal of difficulty maintaining a uniform coating thickness over the entire surface to be coated. Subsequent

inspection may find high and low thickness readings resulting in rework to obtain the desire result.

Traditional fixed ratio metering units frequently have no metering ratio control for automatic shutdown if the

system goes off ratio. Even the newer variable ratio metering equipment only verifies ratio by counting pump

strokes which does not monitor the deterioration of pump packings or valve seats.

Automation of the application process by using a line travel applicator has greatly improved the accuracy of the

coating operation. Continuous monitoring of the flow rates of both components provides immediate feedbackon metering ratio of the pump unit. Flow rate is also used to determine application speed insuring consistent

coating application thicknesses.

A patented dual pressure purge system uses Argon to purge the mixed coating material from the coatings lines,

eliminating the need for solvent cleaning of the lines every time the unit stops. This also permits the unit to stop

and restart at any point on the line without affecting the quality of coating application.

This paper goes into the details of how the systems works and the data than can be accumulated to document

all the aspects of the coating operation.



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Feed pumps and supply system

A drum of Component A, which is typically pasty at ambienttemperature, is heated by an electric heater (bottom plate ordrum belt) to about 50°C. At this temperature the viscosity of component A significantly decreases and becomes similarto that of component B.

Component B, typically liquid at ambient temperature,usually requires no extra heating prior to pumping into thedevice. Both components are transported from the drum tothe high pressure dosing pumps by individual material feedpumps and material hoses. In general the feed pumps arepneumatically operated piston pumps.

Preparation of components

The high pressure dosing pumps are the core element of thespraying device. There are different systems which are mainly based on combinations of pumps with different volumes orpumps with different stroke length and/or stroke speed. Thedosing pumps pressurize the material to the operationalspraying pressure of at least 3000 psi. In separate lines andin exact proportions of mixing ratio, the two componentsare pushed through the high pressure fluid heaters and gaugeto the mixing block.

Mixing and processing

The mixing block includes valves to open and close or tobypass the fluid lines. In case the mixing block is placed in acertain distance from the machine, the hose package is heated

(electrically, by hot water, etc.). In the mixing block, the twocomponents are mixed by a static mixer. The chemically reacting two component blend then passes through a highpressure hose. Various types of manually or mechanically controlled spray guns can be connected to the dispensinghose.

The mixing block also has a flushing system to pump outthe mixed material when spraying has to be temporarily interrupted or stopped. A powerful solvent flushing orpurging pump is activated when the system is set to ‘f lushingmode’ and the spray gun trigger is pressed. The mixedmaterial is forced out by the solvent, which also flushes andrinses all system components, which were in contact withthe processed material, like the static mixer, the high-pressurehose and the spray gun. This f lushing process is intended toprevent blocking of the device by cured material.

The line-travel coating applicator

The coating applicator, shown in Fig.2, provides rapid linetravel coating application of these plural component coatingsystems. The applicator uses two oscillating airless spray gunsthat apply the coating at a fixed stand-off from the pipe.

The applicator control system is a highly sophisticated, PCbased, electronic control system that monitors and controlsthe operation of the unit. The program runs on a SiemensPC677 computer with a 12-inch touch screen. The programcan also be run on a laptop computer connected to the systemcomputer using an internal WIFI router.



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Flow rate considerations

Obtaining accurate flow rates of components A and B isfundamental to monitoring and controlling any hot spray operation. If you know the flow rate of component A andComponent B you can obtain the actual metering ration of the two components. When you sum the two flow rates, youhave the combined flow rate of the coating material.

All of the commonly used hot spray systems on the markettoday (Graco, Wiwa, etc.) simply count piston strokes toobtain the flow rates. This method ignores the possibility of damaged or worn out packing and the condition of valvesand seats. At best, these methods only give you an indicationof the total amount of material sprayed.

The only accurate way to measure actual flow is by usingflow meters on both the component A and B sides. Eachflow meter totalizes the flow for a period of time. The timeinterval can be adjusted in the software. We use the timeinterval of 1/10 second. This raw data is then smoothed outthrough an averaging routine. All commonly used hot spray

Fig.1. Airless hot-spray system.

Fig.2. Line travel coating applicator unit.




4

systems use positive displacement pumps to meter thematerial. When the pump reaches the end of the stroke, calledthe crossover, all flow stops momentarily as the pump reversesdirection. To avoid erratic readings the software provides arunning average of 50 readings.

Knowing the actual flow rate of both the A and Bcomponents allows you to calculate the metering ratio. Thisis then compared against the coating manufactures specifiedratio and tolerance. For example Protegol UR 32-45R has ametering ration of 3.0:1 +/- 5% . If the calculated ratio isbetween 2.85: 1 and 3.15:1 the coating ratio is acceptable.Our software allows us to raise an alarm when the actualratio varies by more than 3%. In addition, we can tell theoperator which component’s pump is going bad.

Knowing the flow rate, along with some entered parametersallows us to automatically control the coating application.The technician enters the following information on thesettings screen shown in Fig.3:

• Spray pattern width• Spray loss (overspray)• Pipe diameter• Coating thickness• Number of passes to be used to apply the coating

Fig.3. Control panel screen shot.

The Oscillating Speed is determined by:

OsKoRdFP L

SwTD=

-( )1(1)

The line travel speed can then be determined by this equation

Ts KtF L

TD=

-( )1(2)

The computer then converts these values into currentfrequencies that become input to the variable frequency drives controlling oscillation and line travel drive motors.The oscillation drive assembly is shown in Fig.4 and the linetravel drive assembly is shown in Fig.5.

The flip mechanism

The spray guns are designed to rotate away from the pipe atabout a 45 degree from the perpendicular spraying position.The rotation of each gun is created by two air cylinders. One

cylinder causes the gun to rotate in a clockwise direction,the other cylinder causes the gun to rotate in a counter-clockwise direction.

Symbol Description Source Units (Metric) Units (US)

F flow rate flow meter l/min gal/minSw spray pattern width operator input m ftL spray loss operator input % %D pipe diameter operator input m ftT coating thickness operator input mm milsP number of passes required operator input none none

Os oscillation speed (chain speed) calculated m/min ft/minKo constant oscillation speed 9.549 50.546Ts travel speed calculated m/min ft/minKt constant travel speed 0.318 510.57Rd rotating degrees (actual rotating degrees) operator input degrees degrees



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There are two purposes for the flip motion. The first is todirect the gun away from the pipe while the oscillation driveis ramping down and then ramping up when changingdirections. This will prevent excess material from buildingup at these points and is intended to mimic the hand motion

of an applicator when he reaches the end of a stroke.

The unit has two spray guns. Gun 1 coats the side of thepipe closest to metering equipment. Gun 2 coats the side of the pipe farthest from the metering equipment. When Gun1 approaches the 12:00 position a signal causes the solenoidto operate the cylinder causing the gun to rotate clockwise.The signal coincides with the signal to ramp down theoscillating motor prior to changing direction. The signal isbroken once the motor direction is reversed and the motorstarts to ramp back up. Once the signal is broken, the doubleacting cylinder reverses and the spray tip returns to theperpendicular spraying position. Similarly when gun 1

reaches the 6:00 reversal, the second solenoid operatedcylinder causes Gun 1 to rotate counter-clockwise in the samemanner as described above. See Fig.6.

The flip mechanism is also employed at the start of thespraying operation and at the stopping of the spraying. Priorto the start of the spraying operation, Gun 1 is at the 3:00position and is rotated counter-clockwise. Gun 2 is at the9:00 position and is rotated counter-clockwise. This resultsin both guns pointing in a downward direction. When thespraying operation starts (component A and B open), anelectric timer is started. The duration of the time delay is anoperator input to the system. The purpose of this delay is to

Fig.4. Coating applicator oscillation

drive.

allow the material to reach the spray gun tip and wouldessentially be the volume of the hoses divided by the flow rate of the metering pump. Once the delay is over, the gunsare rotated to the spraying position (i.e. the signal to thesolenoid controlling the rotation is broken). At the same

time, oscillation begins at the rate indicated by the pumpflow rate. See Fig.7.

The guns are shown in their flipped position in Fig.8.

Purging problems

Automated airless hot-spray systems usually require frequentstarting and stopping of the coating application. Each timethe application must be stopped, the system is flushed withsolvent as described above to prevent the coating materialfrom curing up in the lines containing the mixed materials.

During the flushing operation you now have a combinationof mixed material and solvent coming out of the spray tip.This combination of coating material and solvent cannot beallowed to go on to the properly coated surface. If this wereto happen all contaminated coating would have to beremoved from the surface.

The lines are full of solvent after f lushing. When the spray isrestarted, the first material out of the lines is a mixture of solvent and coating material. This must not be allowed tocome in contact with the substrate to be coated as it wouldcause a defect in the coating.

Fig.5. Coating applicator line travel

drive.




6

Finally, the use of solvents in field spraying operations canbe very difficult for environmental reasons. Properprecautions must be taken to contain any solvent used inthe flushing operation. Any solvent that escapes into the

environment must be cleaned up.

Patented device

The patented device is a system for using compressed gas forflushing mixed plural component coatings from hoses, andother system components in contact with the mixed material.It is intended for use in automated plural component coatingapplication systems. The patented device replaces the mixingblock, flush pump, spray gun and associated hoses shown inFig.1.

The patented device is shown schematically in Fig.9. A description of the items shown iin the table ( right).

Any inert compressed gas can be used. We selected Argon asit is used in automated welding and is normally readily available on the right of way. A cylinder of Argon normally costs around $14.00. We will use about ½ of the bottle eachday (from 4000 psi down to 2000 psi). The contractor canuse the heel in the automated welding process.

A solvent f lush system is available to clean out the systemduring extended down times such as coffee and lunch breaks.

The solvent does not contact the pipe and can be collectedto prevent environmental contamination.

Data collection

The real benefit of this automation is in the collection andevaluation of data. We are able to record all of the operatingparameters such as actual metering ratio, flow rate, oscillationspeed, line travel speed, operating pressures and GPScoordinates that establish location.

The data can be analyzed to find areas that should be checkedby QC. For example, if the data collected indicated an area

where the coating ratio was close to the maximum tolerance,an inspector can target that area for a more thoroughexamination to insure the coating cured properly.

From the contractor’s standpoint, the data establishes thatthe coating was properly applied. From the pipeline owner’sstandpoint, the data becomes a permanent part of thepipeline record and can be used to enforce coatingperformance warranty provisions.

Conclusions

Automating the coating application process has providedmany advantages:

Fig.6 ( left). Gun flip mechanism during coating.

Fig.7 ( above). Gun flip mechanism at start and stop.

Fig.8. Gun flip mechanism - guns in flipped position.



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Fig.9. Control system flow diagram.

Description Comments

Compressed Gas Cylinder Pressure when full should be at least 4000 psiRegulator 1 Reduces pressure to desired high pressure flush (as much as 2000 psi)Pressure Transducer 1 Pressure Transducer records pressure of gas. Reading monitored by

Programmable Logic Controller (PLC). Error condition when pressuredropsbelow desired level.

Regulator 2 Reduces pressure of gas to desired low pressure flush (as little as 150 psi)Pressure Transducer 2 Pressure Transducer records pressure of low pressure flush gas. Reading

monitored by Programmable Logic Controller (PLC). Error conditionwhen pressure exceeds desired level.

Valve 1 2 position ball valve for high pressure flushOperator 1 Operator, 90 degrees, operates V1 (open-closed)

Valve 2 2 position ball valve for low pressure flushOperator 2 Operator, 90 degrees, operates V2 (open-closed)

• Monitoring the flow rate of the two components allowsreal time determination of the metering ratio and anautomatic shutdown when out of tolerance.

• Using the flow rate we can precisely apply the coating atthe desired thickness minimizing rework for out of tolerance thicknesses and reducing coating material costs.

• Flip mechanism prevents build-up at the 6:00 and 12:00position and allows precise stopping and starting of thecoating operation.

❖ ❖ ❖

• Patented purge system greatly reduces the need forsolvents and clean-up of discharged solvents.

• Data collection aids inspection and provides

confirmation that the coating was applied according tospecification.

• Reduces the number of people required.

• Dumbs-down the application process, allowing less skilledpeople to perform the work.

automation of field application of plural component coatings to improve accuracy, productivity,...

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