design considerations for aluminum hull structures - Ship Structure ...

-76-(e)The steel piping must be of Schedule 80 thickness to providereasonably long life in the sea water environment.A galvanized steel pipe system with galvanized steel or galvanizednodular iron valves has the advantage of being more compatiblewith thealuminum structure and reduces the corrosion effect of galvanic couple.However, this protection may last only a year since the zinc will wasteaway. Therefore, the protection of the aluminum structure suggested forthe black steel system is also necessary for the galvanized steel system.The installed cost of this system will be somewhat higher than for theblack steel system.A fiberglass reinforcedplastic pipe system has the advantage of thematerial being inert. IhiLkheadpenetrationswould be with flanged aluminumspools. Valves could be either aluminum or 304 or 316 stainless steel. Thefiberglass pipe can not be bent.and only a few shipboard installationswithU. S. Coast Guard approval have been made. U. S. Coast Guard approvalwould have to be obtained.A PVC coated steel pipe system requires both the inside and outside ofthe pipe to be coated in order to provide full protection to the aluminumstructure. Steel pipe systems with inner PVC lining only are used quiteextensively on shore installationswhere contaminationof the productibeinghandled must be prevented, or where the product is quite active in attackingmetals. It has the disadvantageof being costly, must be purchased in fixedlengths, can not be bent, and the coating is subject to mechanical damage.AqY break in the coating will cause rapid corrosion of the steel.An 18-8 stainless steel pipe system has the advantage of beingcompatiblewith the aluminum structure, can be readily bent and welded andis acceptable to the U. S. Coast Guard. It has the disadvantage of h~ghinitial cost.A 90-10 copper nickel alloy is excellent for use in sea water applicationsbut has the basic disadvantage that it is not compatiblewithaluminum and must be insulated and protected similar to the steel systemsand, in addition, maw heavy wall waster pieces are required.An estimate of piping materials, quantities and costs has been madeof the ballast system within the ballast tanks, Table 14. Certain itemswhich are common to all systems such as bulkhead penetration spools, pipehangers and valve reach rods have not been included. Installationcostsare not included.The welding, fabrication and assembly costs of the aluminum, carbonsteel, 90-10 copper nickel alloy and 18-8 stainless steel piping systemsare assumed to be approximatelyequal and the assembly costs of the plasticand PVC lined steel systems should be somewhat less.As indicated in Table lb it can be seen that the fiberglass reinforcedplastic pipe system is the lowest in material costs as well as having theadvantage of being compatiblewith the aluminum hull structure. Maintenanceof this plastic material for the life of the ship should be very low in cost.It is recommended that this plastic material be used for ballast service inthe ballast tank provided it is acceptable to the American Bureau of Shippingand U. S. Coast Guard. Nodular iron or bronze valves insulated from thehull can be used. If this plastic material is not acceptable to the AmericanFiureauof Shipping and U. S. Coast Guard then considerationshould begiven to the use of an all aluminum ballast system.