The diver’s air supply may originate from an air compressor, a bank of high-pressure
air flasks, or a combination of both.
Regardless of the source, the air must meet certain
established standards of purity, must be supplied in an adequate volume for
breathing, and must have a rate of flow that properly ventilates the helmet or
mask. The air must also be provided at sufficient pressure to overcome the bottom
water pressure and the pressure losses due to flow through the diving hose,
fittings, and valves. The air supply requirements depend upon specific factors of
each dive such as depth, duration, level of work, number of divers being
supported, and type of diving system being used.
Air taken directly from the atmosphere and pumped to the
diver may not meet established purity standards. It may be contaminated by engine
exhaust or chemical smog. Initially pure air may become contaminated while
passing through a faulty air compressor system. For this reason, all divers’ air must be periodically sampled and analyzed to ensure the air meets purity standards.
Refer to Table 4-1 for compressed air purity requirements.
To meet these standards, specially designed compressors must be used with the air
supplied passed through a highly efficient filtration system. The compressed air
found in a shipboard service system usually contains excessive amounts of oil and
is not suitable for diving unless filtered. Air taken from any machinery space, or
downwind from the exhaust of an engine or boiler, must be considered to be
contaminated. For this reason, care must be exercised in the placement and operation
of diving air compressors to avoid such conditions. Intake piping or ducting
must be provided to bring uncontaminated air to the compressor. The outboard end
of this piping must be positioned to eliminate sources of contamination. To ensure
that the source of diver’s breathing air satisfactorily meets the standards established
above, it must be checked at intervals not to exceed 8 months, in accordance
with the PMS.
The required flow from an air supply depends
upon the type of diving apparatus being used. The open-circuit air supply system
must have a flow capacity (in acfm) that provides sufficient ventilation at depth to
maintain acceptable carbon dioxide levels in the mask or helmet. Carbon dioxide
levels must be kept within safe limits during normal work, heavy work, and
emergencies.
If demand breathing equipment is used, such as the MK 21 MOD 1 or the MK 20
MOD 0, the supply system must meet the diver’s flow requirements. The flow
requirements for respiration in a demand system are based upon the average rate
of air flow demanded by the divers under normal working conditions. The
maximum instantaneous (peak) rate of flow under severe work conditions is not a
continuous requirement, but rather the highest rate of airflow attained during the
inhalation part of the breathing cycle. The diver’s requirement varies with the
respiratory demands of the diver’s work level.
In order to supply the diver with an adequate
flow of air, the air source must deliver air at sufficient pressure to overcome the
bottom seawater pressure and the pressure drop that is introduced as the air flows
through the hoses and valves of the system. Table 8-1 shows the values for air
consumption and minimum over-bottom pressures required for each of the
surface-supplied air diving systems.
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Table 8.1. Primary Air System Requirements.
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A properly operated air supply system should never permit
the air supplied to the diver to reach its dewpoint. Controlling the amount of water
vapor (humidity) in the supplied air is normally accomplished by one or both of
the following methods:
-
Compression/Expansion. As high-pressure air expands across a pressure
reducing valve, the partial pressure of the water vapor in the air is decreased.
Since the expansion takes place at essentially a constant temperature (isothermal),
the partial pressure of water vapor required to saturate the air remains
unchanged. Therefore, the relative humidity of the air is reduced.
-
Cooling. Cooling the air prior to expanding it raises its relative humidity, permitting
some of the water to condense. The condensed liquid may then be
drained from the system.
Air supply requirements cannot be based solely
on the calculated continuing needs of the divers who are initially engaged in the
operation. There must be an adequate reserve to support a standby diver should
one be needed.
All surface-supplied diving systems must
include a primary and a secondary air supply in accordance with the U.S. Navy
Diving and Manned Hyperbaric Systems Safety Certification Manual, SS521-AAMAN-
010. The primary supply must be able to support the air flow and pressure
requirements for the diving equipment designated (Table 8-1). The capacity of the
primary supply must meet the consumption rate of the designated number of
divers for the full duration of the dive (bottom time plus decompression time). The
maximum depth of the dive, the number of divers, and the equipment to be used
must be taken into account when sizing the supply. The secondary supply must be
sized to be able to support recovery of all divers using the equipment and dive
profile of the primary supply if the primary supply sustains a casualty at the worstcase
time (for example, immediately prior to completion of planned bottom time
of maximum dive depth, when decompression obligation is greatest). Primary and
secondary supplies may be either high-pressure (HP) bank-supplied or
compressor-supplied.
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 principles
of gas compression. The NAVSEA/00C ANU list contains guidance for
Navy-approved compressors for divers’ air systems. See Figure 8-10.
Figure 8.10. HP Compressor Assembly (top); MP Compressor Assembly (bottom).
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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 highpressure
air banks and scuba cylinders.
Air compressors must meet the flow and
pressure requirements outlined in paragraph 8-6.1.2 and 8-6.1.3. Normally, reciprocating
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. 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 firststage
piston, the lower the percentage capacity, because the clearance volume
represents a greater percentage of the cylinder volume.
Reciprocating piston compressors are either oil lubricated or water
lubricated. The majority of the Navy’s diving compressors are lubricated by petroleum
or synthetic oil. In these compressors, the lubricant:
-
Prevents wear between friction surfaces
-
Seals close clearances
-
Protects against corrosion
-
Transfers heat away from heat-producing surfaces
-
Transfers minute particles generated from normal system wear to the oil sump
or oil filter if so equipped
Unfortunately, the lubricant vaporizes into the air
supply and, if not condensed or filtered out, will reach the diver. Lubricants used
in air diving compressors must conform to military specifications MIL-L-17331
(2190 TEP) for normal operations, or MIL-H-17672 (2135 TH) for cold weather
operations. Where the compressor manufacturer specifically recommends using a
synthetic base oil, the recommended oil may be used in lieu of MIL-L-17331 or
MIL-H-17672 oil.
Using an oil-lubricated compressor
for diving is contingent upon proper maintenance to limit the amount of oil introduced
into the diver’s air (see Topside Tech Notes, March 1997). When using any
lubricated compressor for diving, the air must be checked for oil contamination.
Diving operations shall be aborted at the first indication that oil is in the air being
delivered to the diver. An immediate air analysis must be conducted to determine
whether the amount of oil present exceeds the maximum permissible level in
accordance with table Table 4-1.
It should be noted that air in the higher stages of a compressor has a greater
amount of lubricant injected into it than in the lower stages. It is recommended
that the compressor selected for a diving operation provide as close to the required
pressure for that operation as possible. A system that provides excessive pressure
contributes to the buildup of lubricant in the air supply..
Intercoolers are heat exchangers that are placed between the stages
of a compressor to control the air temperature. Water, flowing through the heat
exchanger counter to the air flow, serves both to remove heat from the air and to
cool the cylinder walls. Intercoolers are frequently air cooled. During the cooling
process, water vapor is condensed out of the air into condensate collectors. The
condensate must be drained periodically during operation of the compressor,
either manually or automatically.
As the air is discharged from the compressor, it passes through a moisture
separator and an approved filter to remove lubricant, aerosols, and particulate
contamination before it enters the system. Approved filters are listed in the
NAVSEA/00C ANU list.
A back-pressure regulator will be installed downstream of
the compressor discharge. A compressor only compresses air to meet the supply
pressure demand. If no demand exists, air is simply pumped through the
compressor at atmospheric pressure. Systems within the compressor, such as the intercoolers, are designed to perform with maximum efficiency at the rated pressure
of the compressor. Operating at any pressure below this rating reduces the
efficiency of the unit. Additionally, compression reduces water vapor from the air.
Reducing the amount of compression increases the amount of water vapor in the
air supplied to the diver.
The air supplied from the compressor expands across the pressure regulator and
enters the air banks or volume tank. As the pressure builds up in the air banks or
volume tank, it eventually reaches the relief pressure of the compressor, at which
time the excess air is simply discharged to the atmosphere. Some electricallydriven
compressors are controlled by pressure switches installed in the volume
tank or HP flask. When the pressure reaches the upper limit, the electric motor is
shut off. When sufficient air has been drawn from the volume tank or HP flask to
lower its pressure to some lower limit, the electric motor is restarted.
All piping in the system must be designed to minimize pressure drops. Intake
ducting, especially, must be of sufficient diameter so that the rated capacity of the
compressor can be fully utilized. All joints and fittings must be checked for leaks
using soapy water. Leaks must be repaired. All filters, strainers, and separators
must be kept clean. Lubricant, fuel, and coolant levels must be periodically
checked.
Any diving air compressor, if not permanently installed, must be firmly secured in
place. Most portable compressors are provided with lashing rings for this purpose.
HP air cylinders and flasks are vessels
designed to hold air at pressures over 600 psi. Convenient and satisfactory diving
air supply systems can be provided by using a number of these HP air cylinders or
flasks. Any HP vessel to be used as a diving air supply unit must bear appropriate
Department of Transportation (DOT) or military symbols certifying that the cylinders
or flasks meet high-pressure requirements
A complete air supply system includes the necessary piping and manifolds, HP
filter, pressure reducing valve, and a volume tank. An HP gauge must be located
ahead of the reducing valve and an LP gauge must be connected to the volume
tank.
In using this type of system, one section must be kept in reserve. The divers take
air from the volume tank in which the pressure is regulated to conform to the air
supply requirements of the dive. The duration of the dive is limited to the length of
time the banks can provide air before being depleted to 200 psi over minimum
manifold pressure. This minimum pressure of 200 psi must remain in each flask or
cylinder.
As in scuba operations, the quantity of air that can be supplied by a system using
cylinders or flasks is determined by the initial capacity of the cylinders or flasks
and the depth of the dive. The duration of the air supply must be calculated in
advance and must include a provision for decompression.
Sample calculations for dive duration, based on bank air supply, are presented in
Sample Problem 1 in paragraph 8-2.2.3 for the MK 21 MOD 1. The sample problems
in this chapter do not take the secondary air system requirements into
account. The secondary air system must be able to provide air in the event of
failure of the primary system per U.S. Navy Diving and Manned Hyperbaric
Systems Safety Certification Manual, SS521-AA-MAN-010. In the MK 21 sample
problem (Sample Problem 2), this would mean decompressing three divers with a
30-minute bottom time using 1.4 acfm per diver. An additional requirement must
be considered if the same air system is to support a recompression chamber. Refer
to Chapter 22 for information on the additional capacity required to support a
recompression chamber.
Many Navy ships have permanently installed shipboard
air supply systems that provide either LP or HP air. These systems are used in
support of diving operations provided they meet the fundamental requirements of
purity, capacity, and pressure.
In operation, a volume source (such as a diesel or electrically driven compressor)
pumps air into a volume tank. The compressor automatically keeps the tank full as
long as the amount of air being used by the diver does not exceed the capacity of
the compressor. The ability of a given unit to support a diving operation may be
determined from the capacity of the system.
