High pressure gas (air) cylinders are gen- 

 erally hydrostatically tested at 1.5 or 1.66 

 times their rated pressure and may have a 

 burst pressure of 2 to 4 times the rated. A 

 safety release device is usually required in 

 these tanks. 



Another safety precaution in the handling 

 of compressed air, or any gas, is color coding 

 the tank. Although submersible owners are 

 not required to abide by any particular cod- 

 ing system, and few do, both the American 

 Bureau of Shipping and the U.S. Naval Ma- 

 terial Command recommend that a color code 

 be followed as presented in Table 6.3. 



The amount and service pressure of air 

 carried aboard submersibles varies consider- 

 ably and depends upon the depth from which 

 the water ballast is to be operated and the 

 volume of the ballast tanks. 



Virtually all vehicles diving to 2,000 feet or 

 less use compressed air to blow main and 

 variable ballast. Quite frequently both a 

 high and low pressure system are employed: 

 The low pressure to blow main ballast at the 

 surface, and the high pressure to blow main 

 ballast in an emergency or the variable bal- 

 last tanks when submerged. For example, 

 BEN FRANKLIN uses a 1,422-psi low pres- 

 sure system to blow main ballast on the 

 surface and a 2,874-psi system to blow main 

 ballast at 2,000 feet in an emergency. 



As mentioned, the service pressure of bal- 

 last blow tanks varies widely from vehicle-to- 

 vehicle. The 150-ft depth vehicle of All Ocean 

 Industries carries 40 ft^ of compressed air in 

 two diver-type scuba tanks at 2,250 psi; the 

 15,000-ft ALUMINAUT carries a 4,500-psi 

 supply of air which is used to blow water 

 ballast to 4,000 feet in the event of an emer- 

 gency. 



When air is used to force water ballast out 

 of a tank in open communication with am- 

 bient seawater, its effectiveness as both a 

 deballasting and buoyant force is a function 

 of depth (pressure) and temperature. It is in 

 this context that air shall be considered in 

 the following. From Chapter 2 it was seen 

 that pressure increases at a rate of 14.7 psi 

 with every 33 feet of depth. Temperature, on 

 the other hand, decreased with depth at a 

 rate dependent upon geographic location and 

 time of year. To force water out of a ballast 

 tank into the sea with compressed air, the 

 air must exert a pressure exceeding that of 

 the surrounding water. Having removed this 

 water, the weight of the vehicle is now much 

 less and the vehicle attains a buoyant up- 

 ward force, but air under seawater pressure 

 is of greater density than air at atmospheric 

 pressure and this density must be taken into 

 account when employing an air deb allasting 

 scheme. 



TABLE 6.3 RECOMMENDED COLOR CODING IN PIPING AND COMPRESSED GAS CYLINDERS 



283 



