8-2 MK 21 MOD 1

MK 21 MOD 1

The MK 21 MOD 1 is an open-circuit, demand, diving helmet (Figure 8-1). The maximum working depth for air diving operations using the MK 21 MOD 1 system is 190 fsw. The MK 21 MOD 1 system may be used up to 60 fsw without an Emergency Gas Supply (EGS). An EGS is mandatory at depths deeper than 60 fsw and when diving inside a wreck or enclosed space. The Diving Supervisor may elect to use an EGS that can be man-carried or located outside the wreck or enclosed space and connected to the diver with a 50 to 150 foot whip. Planned air dives below 190 fsw require CNO approval.

Figure 8.1. MK 21 MOD 1 SSDS.

Operation and Maintenance

The technical manual for the MK 21 MOD 1 is NAVSEA S6560-AG-OMP-010, Technical Manual, Operation and Maintenance Instructions, Underwater Breathing Apparatus MK 21 MOD 1 Surface-Supported Diving System. To ensure safe and reliable service, the MK 21 MOD 1 system must be maintained and repaired in accordance with PMS procedures and the MK 21 MOD 1 operation and maintenance manual.

Air Supply

Air for the MK 21 MOD 1 system is supplied from the surface by either an air compressor or a bank of high-pressure air flasks as described in paragraph 8-6.2.3.

Emergency Gas Supply Requirements

The emergency breathing supply valve provides an air supply path parallel to the nonreturn valve and permits attachment of the EGS whip. The EGS system consists of a steel 72 (64.7 cubic-foot [minimum]) scuba bottle with either a K- or J- valve and a first-stage regulator set at 135 ± 5 psi over bottom pressure. A relief valve set at 180 ± 5 psi over bottom pressure must be installed on the first-stage regulator to prevent rupture of the low-pressure hose should the first-stage regulator fail. The flexible low-pressure hose from the first-stage regulator attaches to the emergency supply valve on the helmet sideblock. A submersible pressure gauge is also required on the first-stage regulator.

When using an EGS whip 50 to 100 feet in length, set at manufacturer’s recommended pressure, but not lower than 135 psi. If the diving scenario dictates leaving the EGS topside, adjust the first-stage regulator to 150 psig.

Flow Requirements

When the MK 21 MOD 1 system is used, the air supply system must be able to provide an average sustained flow of 1.4 acfm to the diver. The air consumption of divers using the MK 21 MOD 1 varies between 0.75 and 1.5 acfm when used in a demand mode, with occasional faceplate and mask clearing. When used in a free-flow mode, greater than eight acfm is consumed.

To satisfactorily support the MK 21 MOD 1 system, the air supply must:

• Replenish the air consumed from the system (average rate of flow)
• Replenish the air at a rate sufficient to maintain the required pressure
• Provide the maximum rate of flow required by the diver

Pressure Requirements

Because the MK 21 MOD 1 helmet is a demand-type system, the regulator has an optimum overbottom pressure that ensures the lowest possible breathing resistance and reduces the possibility of overbreathing the regulator (demanding more air than is available). The optimum overbottom pressure for all dives shallower than 130 fsw is 135 psi. For those systems which cannot maintain 135 psig when diving shallower than 60 fsw, 90 psi is permissible. The manifold supply pressure requirement for dives 130-190 fsw is 165 psi. For those systems not capable of sustaining 165 psi overbottom due to design limitations, 135 psi overbottom is acceptable.

This ensures that the air supply will deliver air at a pressure sufficient to overcome bottom seawater pressure and the pressure drop that occurs as the air flows through the hoses and valves of the mask.

Sample Problem 1. Determine the air supply manifold pressure required to dive the MK 21 MOD 1 system to 175 fsw.

1. Determine the bottom pressure at 175 fsw:

Bottom pressure at 175 fsw = 175 ´ .445 psi = 77.87 psig (round to 78)

2. Determine the overbottom pressure for the MK 21 MOD 1 system (see paragraph 8-2.2.3). Because the operating depth is 175 fsw, the overbottom pressure is 165 psig.

3. Calculate the minimum manifold pressure (MMP) by adding the bottom pressure to the overbottom pressure:

MMP 78 psig 165 psi= + g = 243 psig

The minimum manifold pressure for a 175-fsw dive must be 243 psig.

Sample Problem 2.Determine if air from a bank of high-pressure flasks is capable of supporting two MK 21 MOD 1 divers and one standby diver at a depth of 130 fsw for 30 minutes. There are 5 flasks in the bank; only 4 are on line. Each flask has a floodable volume of 8 cubic feet and is charged to 3,000 psig.

1. Calculate minimum manifold pressure (MMP).

MMP(psig)=(0.445D)+ 165 psig =(0.455 ´ 130)+ 165 psig = 222.85 psig

Round up to 223 psig

2. Calculate standard cubic feet (scf) of air available. The formula for calculating the scf of air available is: scf available Pf –(Pmf + MMP) 14.7 = --------------------------------------------- ´ V ´ N

Where: Pf = Flask pressure = 3,000 psig Pmf = Minimum flask pressure = 220 psig MMP = 223 psig V = Capacity of flasks = 8 cffv N = Number of flasks = 4

scf available 3000 –(220 + 223) 14.7 = ----------------------------------------------- ´ 8 ´ 4 = 5566.26 scf (round down to 5566)

3. Calculate scf of air required to make the dive. You will need to calculate the air required for the bottom time, the air required for each decompression stop, and the air required for the ascent. The formula for calculating the air required is:

scf required D+33 33 = ---------------- ´V´N´ T

Where: D = Depth (feet) V = acfm needed per diver N = Number of divers T = Time at depth (minutes)

Bottom time: 30 minutes

scf required 130 + 33 33 = --------------------- ´ 1.4 ´ 3 ´ 30

Decompression stops:

A dive to 130 fsw for 30 minutes requires the following decompression stops:

• 3 minutes at 20 fsw
scf required 20 + 33 33 = ------------------ ´ 1.4 ´ 3 ´ 3 = 20.24
• 18 minutes at 10 fsw
• scf required 10 + 33 33 = ------------------ ´ 1.4 ´ 3 ´ 18 = 98.51 scf

• Ascent time:5 minutes (rounded up from 4 minutes 20 seconds) from 130 fsw to the surface at 30 feet per minute. Ascent time: average depth 130 2 =--------=65 feet Ascent time: scf required 65 + 33 33 = ------------------ ´ 1.4 ´ 3 ´ 5 = 62.36 scf

  Total air required 622.36 20.24 = + +98.51+62.36 = 803.48 scf (round to 804 scf)

4. Calculate the air remaining at the completion of the dive to see if there is sufficient air in the air supply flasks to make the dive.

scf remaining = scf available – scf required

= 5609 scf – 804 scf = 4805 scf

More than sufficient air is available in the air supply flasks to make this dive.