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Introduction
Do not perform any procedure in this Special Instruction until you read this information and you understand this information.Failure to properly install, operate, or maintain the SCR equipment may lead to problematic operation, and void the warranty.
Selective Catalyst Reduction (SCR) refers to a method of treating engine exhaust in order to reduce the undesired oxides of nitrogen compounds (NOx). The SCR uses a catalyst which promotes the desired chemical reactions over other possible chemical reactions, while remaining unchanged. Ammonia (NH3) is mixed into the exhaust, and reacts with the NOx in the presence of the catalyst to form harmless compounds (water and nitrogen). In this system, urea ((NH2)2CO) is injected into the exhaust, which quickly decomposes into ammonia under the heated conditions. Urea is used because urea is relatively inexpensive and does not need the special handling that ammonia requires.This Special Instruction will cover the following information for the Caterpillar SCR system: the components, components function, proper installation, system operation, commissioning, maintenance and troubleshooting.
Only use the SCR system with a properly applied and maintained engine. Fouling of the catalyst and a reduced service life can result from the following conditions: turbocharger slobber (oil in the exhaust), extended low load operation and poorly maintained piston rings.
Component Description and Functions
Urea
Due to the toxicity and handling problems with ammonia, aqueous urea (urea salt dissolved in water) is injected into the exhaust steam. The urea quickly decomposes into ammonia. Two concentrations of aqueous urea are typically used, 32.5 percent and 40 percent by weight.32.5 percent urea solution is only slightly alkaline, having a pH of 9.5, like baking soda or soap. This concentration has the lowest freezing point possible for this solution at −11 °C (11 °F), and keeps a constant concentration through freezing.A 40 percent urea solution has more urea per unit of volume than the 32.5 percent solution. However, the 40 percent solution but freezes at a slightly higher temperature of 0.5 °C (33 °F). The urea will also come out of solution (salt out), which could plug the SCR system.Urea is corrosive. Do not store urea in a tank or use supply line that are made of the following materials: aluminum, brass and steel. Use only corrosion resistant materials such as PVC or stainless steel. Any O-rings must be Ethylene Propylene Diene Monomer (EPDM).Urea must be kept below 50 °C (122 °F) to delay decomposition.Use only industrial grade urea. Fertilizer grade urea may contain other elements that can clog the injector nozzle.Compressed Air
Compressed air provides the following functions:
Contributes to the atomization of the liquid urea sprayed into the exhaust stream
Shields the liquid urea in the injector from the exhaust heat until spraying, so that no crystallization occurs which could plug the injection nozzle
Purges the urea line during shutdown in order to prevent crystallizationTypically, compressed air will carry some oil from the compressor and some sediment from the piping. These contaminants can collect in the nozzle. The contaminants can clog the nozzle. Use a coalescing filter/separator that is 90 percent effective that is rated for 1034 kPa (150 psi) and 1699 liter/min (60 CFM).The SCR system will use 226 to 283 L/min (8 to 10 CFM) depending on the nozzle size, at 414 kPa (60 psi) pressure. The system contains an air regulator with a maximum rating of 827 kPa (120 psi) inlet pressure.Dosing Control Cabinet
Illustration 1 g01433875
SCR system overview (1) Catalytic converter (2) Differential pressure sensor (3) Temperature sensor (4) Urea supply from dosing control cabinet to injector (5) Air out to injector from dosing control cabinet (6) Injector (7) Injection duct (8) Mixing vanes (9) Air and urea mixture (10) Engine emissions inlet (11) Exhaust sample line (12) Urea vent (13) Urea return from dosing control cabinet (14) Dosing control cabinet (15) Air compressor (16) Urea fill pipe (17) Ball valve (18) Urea tank (19) Isolation valve (20) Urea supply from tank to dosing control cabinet (21) Air supply to dosing control cabinet (22) Coalescing filter (23) Ball valveControls and Operator Interface
The dosing control cabinet (DC cabinet) controls the dosing of urea, the use of compressed air, and provides for a customer interface.The DC cabinet houses an electronic control module (ECM) that provides the basic logic. The ECM takes input from the various sensors, and provides output to the dosing pump, a solenoid for urea supply, and a solenoid for air supply. The ECM can be communicated with by a standard Caterpillar communication adapter ( 171-4400 Communication Adapter Gp ). A communication adapter plug is provided on the side of the DC cabinet for easy connection.
Illustration 2 g01264279
Inside the dosing control cabinet (1) ECM for the dosing control cabinetConnections for 24V DC power and sensor inputs are provided for through an 18-pin Deutsch style connector, also on the side of the DC cabinet. 110 VAC power (or 220 VAC power for 50 Hz applications) is provided through a separate plug connection.On the front of the DC cabinet there are the following components: main power switch, 24V power supply button, indicator lights and Messenger display
Illustration 3 g01427830
Control cabinet (2) Messenger