CHAPTER 8 — Surface-Supplied Air Diving Operations
Change A 8-19
Banks of flasks and groups of valves require only one central label at the main
stop valve.
A volume tank must be part of the air supply system and be located between the
supply source and the diver’s manifold hose connection. This tank maintains the
air supply should the primary supply source fail, providing time to actuate the
secondary air supply, and to attenuate the peak air flow demand.
8-7.2.2
Air Compressors. Many air supply systems used in Navy diving operations
include at least one air compressor as a source of air. To properly select such a
compressor, it is essential that the diver have a basic understanding of the princi-
ples of gas compression. The NAVSEA/00C ANU list contains guidance for
Navy-approved compressors for divers’ air systems. See Figure 8-10.
8-7.2.2.1
Reciprocating Air Compressors. Reciprocating air compressors are the only
compressors authorized for use in Navy air diving operations. low pressure (LP)
models can provide rates of flow sufficient to support surface-supplied air diving
or recompression chamber operations. High-pressure models can charge high-
pressure air banks and scuba cylinders.
8-7.2.2.2
Compressor Capacity Requirements. Air compressors must meet the flow and
pressure requirements outlined in paragraph 8-7.1.2 and 8-7.1.3. Normally, recip-
rocating compressors have their rating (capacity in cubic feet per minute and
delivery pressure in psig) stamped on the manufacturer’s identification plate. This
rating is usually based on inlet conditions of 70°F (21.1°C), 14.7 psia barometric
pressure, and 36 percent relative humidity (an air density of 0.075 pound per cubic
foot). If inlet conditions vary, the actual capacity either increases or decreases
from rated values. If not provided directly, capacity will be provided by
conducting a compressor output test (see Topside Tech Notes, Volume II
Compressors/Process Instruction NAVSEA-00C4-PI-004, Compressor Capacity
Testing). Since the capacity is the volume of air at defined atmospheric conditions,
compressed per unit of time, it is affected only by the first stage, as all other stages
only increase the pressure and reduce temperature. All industrial compressors are
stamped with a code, consisting of at least two, but usually four to five, numbers
that specify the bore and stroke.
The actual capacity of the compressor will always be less than the displacement
because of the clearance volume of the cylinders. This is the volume above the
piston that does not get displaced by the piston during compression. Compressors
having a first stage piston diameter of four inches or larger normally have an
actual capacity of about 85 percent of their displacement. The smaller the first
stage piston, the lower the percentage capacity, because the clearance volume
represents a greater percentage of the cylinder volume.
8-7.2.2.3
Lubrication. Reciprocating piston compressors are either oil lubricated or water
lubricated. The majority of the Navy’s diving compressors are lubricated by petro-
leum or synthetic oil. In these compressors, the lubricant:
Prevents wear between friction surfaces
