Fuel system fouling

Fuel system fouling is related to the amount of water and sediment in the fuel. A by-product of fuel washing is the desludging of the fuel. Washing rids the fuel of those undesirable constituents that cause clogging, deposition, and corrosion in the fuel system. The last part of treatment is filtration just prior to entering the turbine. Washed fuel should have less than. 025% bottom sediment and water. 

Cleanliness of the fuel must be monitored if the fuel is naturally dirty or can pick up contaminants during transportation. The nature of the contaminants depends on the particular fuel. The definition of cleanliness here concerns particulates that can be strained out and is not concerned with soluble contaminants. These contaminants can cause damage or fouling in the fuel system and result in poor combustion. 

Deposition and fouling can occur in the fuel system and in the hot section of the turbine. Deposition rates depend on the amounts of certain compounds contained in the fuel. Some compounds that cause deposits can be removed by fuel treating. 

Carbon residue, pour point, and viseosity are important properties in relation to deposition and fouling. Carbon residue is found by burning a fuel sample and weighing the amount of earbon left. The earbon residue property shows the tendeney of a fuel to deposit earbon on the fuel nozzles and eombustion liner. Pour point is the lowest temperature at whieh a fuel ean be poured by gravitational aetion. Viseosity is related to the pressure loss in pipe flow. Both pour point and viseosity measure the tendeney of a fuel to foul the fuel system. Sometimes, heating of the fuel system and piping is neeessary to assure a proper flow. 

Table 12-4 is a summary of liquid fuel speeifieations set by manufaeturers for effieient maehine operations. The water and sediment limit is set at 1% by maximum volume to prevent fouling of the fuel system and obstruetion of the fuel filters. Viseosity is limited to 20 eentistokes at the fuel nozzles to prevent elogging of the fuel lines. Also, it is advisable that the pour point be 20 °F (11 °C) below the minimum ambient temperature. Failure to meet this speeifieation ean be eorreeted by heating the fuel lines. Carbon residue should be less than 1% by weight based on 100% of the sample. The hydrogen eontent is related to the smoking tendeney of a fuel. Lower... 

The pour point is an indication of the lowest temperature at which a fuel oil can be stored and still be capable of flowing under gravitational forces. Fuels with higher pour points are permissible where the piping has been heated. Water and sediment in the fuel lead to fouling of the fuel system and obstruction in fuel filters. 

In addition to appreciable amounts of water (ASTM D-4176, ASTM D-4860), sediment can also occur and will cause fouling of the fuel handling facilities and the fuel system. An accumulation of sediment in storage tanks and on filter screens can obstruct the flow of oil from the tank to the combustor. A test method is available to determine the water and sediment in fuels (ASTM D-2709). In this test, a sample of fuel is centrifuged at a ref of 800 for 10 min at 21-32°C in a centrifuge tube readable to 0.005 ml and measurable to 0.01 ml. After centrifugation, the volume of water and sediment that has settled into the tip of the centrifuge tube is read to the nearest 0.005 ml. 

Another method of avoiding a heat-transfer surface that may become fouled is to use a temperature-stable liquid in a bath on to which solvent is fed and from which vapour flashes leaving its residue behind. Such a technique is attractive for unstable solvents which decompose or polymerize if heated to their boiling point over long periods. The liquid being heated in the bath should ideally not dissolve the residue although if the concentration of residue in the feed is small and the liquid is a hydrocarbon fuel, it maybe possible to purge contaminated liquid to the fuel system. 

An extensive evaluation of microorganisms recovered from fouled diesel fuel systems conducted by Levy and Hegarty (1991), involving 400 samples from more than 100 distribution and service station tanks, demonstrated that viable aerobic, mesophilic bacteria were present in 75% of the water phase samples tested. The bacterial counts in these water samples were 10 to 10 colony-forming units per milliliter of sample (CFU/ml). Bacteria were recovered from 60% of the fuel phase samples, with maximum counts of 10" to 10 CFU/ml. Viable yeasts were present, at lO CFU/ml, in 30 to 45% of the water phase samples and in 30% of the fuel phase samples. Viable molds were detected in 20 to 25% of both the water and fuel samples, with counts up to 10 to 10 CFU/ml. SRB were detected only in the water phase in approximately 10% of the samples. 

The use of organic coolant (terphenyl mixtures) in a D20-moderated system has been investigated in the Canadian WR-1 research reactor at Whiteshell, Manitoba. Experience has confirmed that zirconium-based alloys are suitable for extended operation in organic coolant provided there is careful control of the coolant chemistry to prevent excessive hydrogen uptake and fuel tube fouling. 

Gasoline engine equipment such as carburetors, injectors, intake manifolds, valve systems and combustion chambers, are subject to fouling by the fuel itself, the gases recycled from the crankcase, or even dust and particulates arriving with poorly filtered air. Three types of problems then result ... 

Physica.1 Properties. Carbonyl sulfide [463-58-1] (carbon oxysulfide), COS, is a colorless gas that is odorless when pure however, it has been described as having a foul odor. Physical constants and thermodynamic properties are Hsted ia Table 1 (17,18). The vapor pressure has been fitted to an equation, and a detailed study has been made of the phase equiUbria of the carbonyl sulfide—propane system, which is important ia the purification of propane fuel (19,20). Carbonyl sulfide can be adsorbed on molecular sieves (qv) as a means for removal from propane (21). This approach has been compared to the use of various solvents and reagents (22). 

Fouling organisms attach themselves to the underwater portions of ships and have a severe impact on operating costs. They can increase fuel consumption and decrease ship speed by more than 20%. Warships are particularly concerned about the loss of speed and maneuverabiHty caused by fouling. Because fouling is controUed best by use of antifouHng paints, it is important that these paints be compatible with the system used for corrosion control and become a part of the total corrosion control strategy. 

Water and particulates are the source of fuel quality problems such as filter plugging, corrosion, and system component fouling. Water can be removed through salt drying, coalescence, filtration, and good housekeeping. 

Eventually, the fouling deposits on the rotor will become so thick that they start to break off, especially if you shut the compressor down for a few hours for minor repairs to the lube-oil system. When the compressor is put back on line, bits and pieces of grayish salt break off, and unbalance the rotor. At 8000 rpm, the high-vibration trip cuts off the fuel to the gas turbine, and the machine is taken off line for repair. 

Another fouling mechanism that can occur is corrosion of boiler tubing and erosion of refractories due to formation of acids and their buildup in the combustion units from conversion of sulfur and chlorine present in the fuel. Fortunately, the amounts of these elements in most biomass are nil to small. The addition of small amounts of limestone to the media in fluidized-bed units or the blending of limestone with the fuel in the case of moving-bed systems are effective methods of eliminating this problem. Other sorbents such as dolomite, kaolin, and custom blends of aluminum and magnesium compounds are also effective (Coe, 1993). 

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