Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGconditions of load. The discharge pressure of themain feed pump is considerably higher than thepressure carried in theboiler steam drum. Provisionis made for the recirculation of waterfrom the main feed pump discharge back to thedeaerating feed tank, in order to protect the pumpfrom overheating at very low capacity.SAFETY AND CASUALTY CONTROLIn the event of a casualty to a component partof the propulsion plant, the principal doctrine tobe impressed upon operating personnel is theprevention of additional or major casualties.Under normal operating conditions, the safety ofpersonnel and machinery should be given firstconsideration. Where practicable, the propulsionplant should be kept in operation by meansof standby pumps, auxiliary machinery, andpiping systems. The important thing is to preventminor casualties from becoming major casualties,even if it means suspending the operationof the propulsion plant. It is better to stopthe main engines for a few minutes than to putthem completely out of commission, so thatmajor repairs are required to place them backinto operation. In case a casualty occurs, the officeror CPO in charge of the watch should benotified as soon as possible; he in turn mustnotify the OOD if there will be any effect on theship's speed or on the ability to answer bells.LOSS OF VACUUMThe major causes of a loss of vacuum are:excessive air leakage into the vacuum system,improper functioning of the air- removal equipment,improper drainage of condensate from thecondenser, insufficient flow of circulating water,and high injection temperature.1. EXCESSIVE AIR LEAKAGE INTO THEVACUUM SYSTEM may be caused by:a. Insufficient gland sealing steam.b. Vent valve on idle condensate pumpopen.c. Loop- seal filling valve open.d. Bypass valve on drain tank open.e. Drain tank float valve stuck open.f. Taking make-up feed from empty feedbottom.g. Leakage of flanges, fittings, or valvestem packings under vacuum.2. IMPROPER FUNCTIONING OF THE AIR-REMOVAL EQUIPMENT may be due to:a. Insufficient steam to the air ejectors.b. Foreign matter lodged in the air ejectornozzle(s).c. Erosion of the air ejector nozzle, overa period of time.3. IMPROPER DRAINAGE OF CONDEN-SATE FROM THE CONDENSER may becaused by:a. Low speed of condensate pump, indicatingmalfunctioning of the pump'sspeed-limiting governor.b. Condensate pump air-bound becauseof the vent connection from the firststage being closed or not opened wide,4. INSUFFICIENT FLOW OF CIRCULATINGWATER may be caused by:a. Improper adjustment of the overboarddischarge valve (the main injectionvalve being wide open whenever thecondenser is under vacuum).b. Inadequate speed of the main circulatingpump.c. Plugged tubes, resulting from mud,shells, small fish, or kelp beingtrapped against the injection strainerbars or in the inlet water chest.d. Air trapped in condenser.5. HIGH INJECTION TEMPERATUREBasically, the injection temperature limitsthe maximum vacuum (minimum absolute pressure)obtainable in a specific plant, assuming thecondenser, associated equipment, and pipingunder vacuum to be clean and properly operated.Whenever there is a loss of vaccum, the firststep in correcting the trouble is to locate thecause of the casualty. The major possible causesare so numerous that no attempt will be made tolist the proper action required for each one. Therequired action may be very simple, suchas closing the loop seal filling valve, or it maybe much more complicated, such as leaning themain condenser or replacing air ejector nozzles.SALT WATER LEAKAGEINTO CONDENSERIf a condenser salinity indicator shows a risein the chloride content, the source of the contaminationmust be determined immediately. To locatethese sources, test the fresh water fromdifferent units in the system by checking theproper salinity indicators (if installed) and by356I