How does the “40 percent rule” affect the gas blender?
The 40 percent rule is just part of the oxygen safety equation
Pressures, operating temperatures, equipment design, flow rate, contamination, oxygen compatible components, and operating procedures all play an important role in blender safety
Develop an understanding of what is required to create and sustain a fire in a pressure system, then insure that it doesn’t happen
It is a combination of oxygen concentration, pressure, temperature, contamination, ignition sources, gas velocity, material compatibility and system design that is required for a fire
System design is the cornerstone of a safe oxygen enriched system
Without proper design considerations, a system would be either unsafe to use from the onset or would eventually become unsafe due to contamination build up or material compatibility degradation
Material compatibility is where a material can co-exist with elevated oxygen concentrations and a potential source of ignition, without flashing, based on the systems’ maximum operating pressure and temperature
Why is oxygen compatible a concern in oxygen systems?
Every application differs due to pressures, system design, maximum operating temperature, oxygen concentration, etc.
These variables, combined with the inherent ambiguity of material compatibility, make it important to consult with industry experts for your application
The oxygen clean standard is required to ensure that there is no contamination that could become an ignition source or a fuel source for a fire or deflagration within an oxygen enriched environment under the systems normal operating pressures, temperatures and gas velocities
Why is it important to quantify cleaning results by testing?
To ensure that the system is clean enough to be used in an oxygen enriched environment with the systems working pressure, temperature and gas velocities
Any contamination could become an ignition source or fuel for a fire or deflagration
A build up of condensed hydrocarbons in a pressure system could provide a fuel source in an oxygen enriched environment (above 40 percent oxygen) at the systems working pressure, temperature and gas velocity when exposed to an ignition source
Everything with this system must be oxygen serviced and remain that way
Oxygen service is not as much of a problem with large systems where the gas is mixed into cascade cylinders as the blender has control over the introduction of contaminates
Therefore, 38.6% of the 1800 psi added to the tank must be O2, so this divers tank would be topped with 1800psig of EANx38.6
Appendix -19
172.
Calculate a Partial Pressure Fill Starting with a Partially Full Tank
How would we do the previous scenario with partial pressure blending?
Use continuous blending formula to find the percentage of O2 required (FO2)
Simply plug the FO2 required (in this case 38.6%) into the partial pressure filling formula, that starts with an empty tank
In this case: .386 - .21 x 120 = 26.7
.79
.386 :O2 % in additional mix
.21 :O2 % in air
.79 :nitrogen % in air
120 :difference in bar required
26.7 :amount of additional O2 required
Appendix -20
173.
Calculate a Partial Pressure Fill Starting with a Partially Full Tank
How would we do the previous scenario with partial pressure blending?
Use continuous blending formula to find the percentage of O2 required (FO2)
Simply plug the FO2 required (in this case 38.6%) into the partial pressure filling formula, that starts with an empty tank
In this case: .386 - .21 x 1800 = 401
.79
.386 :O2 % in additional mix
.21 :O2 % in air
.79 :nitrogen % in air
1800 :difference in psi required
401 :amount of additional O2 required
Appendix -21
174.
Fill a Partially Full Tank, Using a Banked Mix, and Topping with Air
Example (Metric): A diver has 80 bar of EANx 32 and wants EANx 36 in a cylinder that has a working pressure of 200 bar. You have a banked mix of 40%
200 x .36 = 72 (bar of oxygen in the final mix)
80 x .32 = 25.6 (bar of oxygen in the present mix)
120 46.4 (bar of oxygen to be added)
200 is the final bar in the tank
.36 is the desired O2 percentage in the final mix
80 is the starting bar in the tank
.32 is the starting O2 percentage in t he tank
46.4 is the difference in the O2 required (in bar)
120 is the difference in the total tank pressure required
Appendix -22
175.
Fill a Partially Full Tank, Using a Banked Mix, and Topping with Air
Then , to calculate the amount of banked mix to be added:
46.4 - (.21 x 120) = 111.6
.19
.19 is the O2 percentage difference between the bank mix (in this case, a 40% mix) and air (at .21)
46.4 is the bar of O2 needed
120 is the total bar to be added to the tank
111.6 is the bar of bank mix to be added (in this case, a 40% bank mix)
Therefore , to properly fill this tank, you would add 111.6 bar of your 40% bank mix, and top with 8.4 bar of air (80 starting bar + 111.6 bank mix + 8.4 air = 200 bar total)
Appendix -23
176.
Fill a Partially Full Tank, Using a Banked Mix, and Topping with Air
Example (Imperial): A diver has 1200 psig of EANx 32 and wants EANx 36 in a cylinder that has a working pressure of 3000 psig. You have a banked mix of 40%
3000 x .36 = 1080 (psig of oxygen in the final mix)
1200 x .32 = 384 (psig of oxygen in the present mix)
1800 696 (psig of oxygen to be added)
3000 is the final psig in the tank
.36 is the desired O2 percentage in the final mix
1200 is the starting psig in the tank
.32 is the starting O2 percentage in t he tank
696 is the difference in the O2 required (in psig)
1800 is the difference in the total tank pressure required
Appendix -24
177.
Fill a Partially Full Tank, Using a Banked Mix, and Topping with Air
Then , to calculate the amount of banked mix to be added:
696 - (.21 x 1800) = 1673
.19
.19 is the O2 percentage difference between the bank mix (in this case, a 40% mix) and air (at .21)
696 is the psig of O2 needed
1800 is the total psig to be added to the tank
1673 is the psig of bank mix to be added (in this case, a 40% bank mix)
Therefore , to properly fill this tank, you would add 1673 psig of your 40% bank mix, and top with 127 psig of air (1200 starting psig + 1673 bank mix + 127 air = 3000 psig total)
If calculations show that the amount of bank mix to be added is greater than the total psig to be added, then some of the gas must be drained from the tank before starting
All calculations must be redone, based on the new starting pressure
The .19 in the previous calculation, will change, based on the bank mix being used
For example, if the bank mix being used is a 32% mix, then the .19 will change to .11 (which is calculated as .32 - .21, bank mix minus air)
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