INTRODUCTION
Fuel oil service systems used on steam ships are all basically the same. The pumps are rotary screw, positive displacement, either electric or steam driven. The system pressure is usually 350-400 psig, minimum pressure is 35-50 psig. Electric systems pressure is controlled by a regulating/unloading valve and steam systems have constant pressure regulators governors.
REFERENCES
(a) Propulsion Operating Guide S9LHD-AA-POG-010-LHD1
(b) Ships Fuel and Fuel Systems NSTM Chapter 541
(c) Ships Information Book S9LHD-AA-SIB-020/LHD1-P2
(d) Piping Systems NSTM Chapter 505
INFORMATION
A. The fuel oil service system delivers fuel oil (FO) to the boiler at the proper pressure using vertically mounted, positive displacement, screw type pumps, either motor driven or turbine driven. Suction is drawn from one of two fuel oil service tanks for each fireroom. The function and location of the components of the FO service system are explained below (refer to figure 62B-205-1).
B. There are two main fuel oil service pumps for each fireroom or main machinery space, located on lower level in proper relation to the FO service tanks to obtain required suction. Some ships also have an electric port use pump for initial boiler light off. Fuel oil service pumps are designed to supply FO at the required pressure to the FO burners. During normal operation, both main fuel oil service pumps should be aligned for operation. Normally, one pump is operated and the other is aligned and ready for immediate operation, taking a suction from the same service tank. One fuel oil service pump (FOSP) is usually capable of supplying both boilers at full power.
C. To prevent over pressurization of the pump casing and piping systems a relief valve is installed on each pump's discharge side prior to the discharge valves that relieves excessive pressure back to the suction side of the pump.
D. Electric FOSP's are two speed (high or low) constant speed, so they require regulating/unloading valves which maintain system pressure between the FOSP and fuel oil control valve (FOCV), by allowing excess FO to return to the tank on suction. This pressure is typically 350-400 psig. Steam driven pumps regulate discharge pressure by regulating the speed of the pump, which in turn maintains a constant discharge pressure (350-400psig). At low load conditions, ensure that FO temperature at pump discharge does not exceed 125°F. Usually there is a thermometer installed in the FO manifold on the boiler front. If temperature rise occurs, shift pump to low speed (if electric) or place another pump in operation.
E. Suction and return valves allow excess FO to be returned to the tank on suction. Return must go to the tank on suction. If it went to the other tank, an overflow could result. Some systems will have mechanical interlocks to prevent the overflow of the tank not on suction. The valves may be motor operated (air or electric) or manually operated valves. Other systems have quick closing valves in the suction lines that can be operated locally and at remote stations in emergencies.
F. There is a duplex fire safe fuel oil strainer for each boiler located in the FOSP discharge piping in the fireroom to strain solid particulate matter from the fuel oil prior to entering the FOCV and burner assembly. (Refer to Figure 62B-205-1). They are virtually leak proof in design and require no strainer box enclosure to prevent oil spray in the event of a leak. This is why they are termed firesafe. The only time the strainer is opened is for preventive or corrective maintenance. This strainer is designed with wiper assemblies and purge valves for on line shifting and cleaning. Operation and cleaning of the strainers is covered in the Engineering Operational Sequencing System (EOSS).
G. A fuel oil quick closing valve (FOQCV) is located on each supply line to the boiler front to provide for immediate securing and isolation of the FO supply to each boiler when casualties occur. (Refer to Figure 62B-205-2).
H. The FOCV is located downstream of each quick closing valve to provide individual control of FO supply pressure to the burners in response to the oil demand signal from the automatic combustion control (ACC) system.
I. A bypass around the FOCV is used in the event of mechanical failure of the FOCV. It may be an exact duplicate of the FOCV or a micrometer valve.
J. The fuel oil supply header distributes FO to individual burner root valves. The burner root valve is fully open when the associated burner is operating, and closed when the associated burner is secured.
K. Fuel oil burners are located at the front of the boiler and extend into the furnace and deliver FO and air to the boiler furnace for combustion. Fuel oil burners consist of the air register, control block or safety shut off device (SSD), and the atomizer assembly.
L. The recirculation valve is located downstream of the FO supply header to recirculate FO back to the service tank on suction at times when no fuel oil flow is assured, such as before fires are lighted under the boiler and with the FOSP operating. The positive displacement FOSP must have a complete flow path for fuel and the FOCV can only establish a differential in pressure if there is flow through it. It can also route FO to the contaminated settling tank for purging the system of water contamination through a three way valve or separate piping system.
M. Fuel oil flows by gravity from the service tanks to provide the required suction pressure at the FOSP. The FO service tank level must be maintained between 50% and 95% full capacity. This level is monitored directly by taking tank sounding or indirectly by use of tank level indicators (TLI). Do not allow the FO service tank level to drop below 50% or there may be a risk of losing suction due to the movement caused by sea conditions. This also allows sufficient time to remove contamination from the other service tank if it is found to be contaminated with the sample taken prior to being placed on suction.
N. To stop the pumps in an emergency, a securing switch or trip is typically located in the fireroom operating station (FOS), pump control station and DC deck.
O. Fuel oil from the service pumps flows through the duplex strainers where solid particulate matter its removed from the FO. Strainers should be shifted whenever a 3% or 10 psi pressure drop across the strainers is experienced . Because of the duplex design, shifting fuel oil strainers (FOS) can be accomplished during normal operation in accordance with Engineering Operational Sequencing System (EOSS).
NOTE: If strainers are not of the firesafe type, the boilers will be secured and the FO system depressurized to open and clean FOS, except in an emergency.
P. The FOCV is a diaphragm operated valve that regulates the supply pressure to the burners and has a minimum setting of usually 35-40 psig, to prevent a loss of boiler fires during steam demand changes or failure of the valve.
Q. Fuel oil is directed to the individual fuel oil burner atomizer through the FO supply header and burner root valves (all FO supplied is burned).
R. If the FO burners are the steam atomization type, the steam is taken from the 150 psi desuperheated steam system through the steam supply header (min 135 psi 400°F+/- 10°F). Steam used for atomizing must be at least 135 psi 370°F for initial boiler light off. Shore steam SHALL NOT be used for steam atomization. Pressures and temperatures less than this can cause improper combustion, sputtering fires or the worst case scenario of a boiler explosion. Fuel oil and steam are mixed at the sprayer plate/nozzle in the end of the atomizer before entering the furnace.
S. During first boiler light-off, low pressure air is substituted for shore steam because shore steam is UNRELIABLE and in most cases will not meet steam atomization criteria of 135 psi and 370°F. Straight mechanical atomization can also be used when there has been a casualty to the steam system.
T. The FO flow path begins at the fuel oil service tank by gravity to the FOSP which discharges to the FOS, regulating/unloading valves maintain system pressure (electric systems). Flow continues through the FOQCV to the FOCV which controls FO pressure to the FO burners. If the system was recirculating flow would continue back to the FO service tank on suction.
U. Flange shields are installed on all flanges, unions or mechanical fittings to prevent fuel oil from spraying onto hot surfaces of piping or equipment in the event of a leak at a flange connection or mechanical fitting. Flange shields are fabricated from 3 layers of aluminized cloth, two aluminized layers down or flange side and one facing the outside. Flange shields must be installed on pressurized flammable liquid system flanges and fittings in accordance with Piping Systems NSTM 505.
Figure 1
Figure-2