GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rsterne on September 22, 2022, 05:00:15 PM
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The CGA 320 connector is intended for use with CO2.... It is also the thread used on the common HPA regulators, such as those made by Ninja and others.... In a discussion with Hajimoto, he told me that the limiting factor for pressure was not the regulator itself, but the CGA 320 connector.... I thought that sounded reasonable, so I decided to use Lloyd's Pressure Vessel Calculator to do the math.... Now remember, I am not an Engineer, and I won't guarantee these numbers, but here they are....
The CGA 320 is also known as 0.825"-14 TPI.... The depth of engagement differs from reg. to reg. but from the ones I had available to measure, it seems there are at least 3 complete threads of engagement, providing that the female fitting has that many or more, positioned so that all 3 threads on the reg. bonnet are 100% engaged, and assuming that they are within tolerance....
The O-rings on the regulator are inboard of the threads, so the threads are not under pressure.... They only need to provide sufficient shear strength to provide the threads on the reg. or the female fitting they thread into with at least a 3:1 safety margin at the maximum operating pressure in the tank (not the output pressure, in case the regulator fails and bypasses the tank pressure to the output).... It can be argued that if the regulator is fitted with a functioning burst disc on the output side, that is the maximum pressure the CGA 320 could see, but for the purposes of this thread, I am going to use 4500 psi (310 bar) as the working pressure....
The O-rings seal inside a diameter of 0.700", so the end force on the CGA 320 threads at 4500 psi is (0.700^2 x PI/4 x 4500) = 1731 lbs.... That represents a Shear Stress of 8,260 psi on the thread cross section of each part.... Since the ASME require pressure vessels to have a 3.5:1 safety margin to failure, that means that to be safe for 4500 psi, those threads must not fail at a force of 6060 lbs.... which is a Shear Stress of 28,910 psi.... Since Shear Strength is generally taken as 60% of the Tensile Strength, that requires a Tensile Strength of at least 48 Ksi for both the bonnet and the female part....
On a Ninja regulator, bonnets have been made from aluminum, brass and steel, but it is my understanding brass is no longer used.... We don't know the grade of aluminum or steel used, nor do we have any idea what might be used in "clones" or regulators made elsewhere, or by other companies.... I will look at a list of different materials which might be used for the bonnet or the female CGA 320 it is threaded into.... If the Tensile Strength listed below is less than 48 Ksi, I would not use it for a 4500 psi MSWP.... The tensile strengths I have quoted below are the generally accepted values for the material in question, your material may be more or less....
6061-T6 Aluminum.... 45 Ksi
2024-T3 Aluminum.... 70 Ksi
7075-T6 Aluminum.... 83 Ksi
1010,20 Mild Steel..... 63 Ksi
12L14 Leaded Steel... 78 Ksi
4130 CrMoly (norm)... 97 Ksi
304 Stainless Steel.... 73 Ksi
360 Brass................. 58 Ksi
It is obvious from the above list, that all materials exceed the required 48 Ksi Tensile Strength except that 6061-T6 Aluminum is close (in theory it should give a safety margin of 3.3:1 at 4500 psi).... Again, this all assumes threads within proper tolerances, and 3 full threads of engagement....
While I am looking at this question, I need to address the practice of sealing the bonnet to the female part using a larger O-ring between the bonnet and the face of the part, instead of using the O-rings inboard of the threads, which seal against a 0.700" ID.... While this is a lot easier to do (it does not require accurate boring of the hole inboard of the threads), it increases the diameter of the sealing surface to about 1.0".... The exact diameter will depend on the O-ring used and the shape of the faces compressing it, but I will use a 1" diameter for now.... In most installations, it is probably less (but still larger than 0.700")....
Using a 1.0" diameter increases the force on the CGA 320 thread by more than double, to 3,534 lbf. at 4500 psi.... This increases the Tensile Strength required for a 3.5:1 safety margin to 98 Ksi.... Note that there is not a single material in that list strong enough to meet that criteria.... :o .... If you are using this sealing arrangement, please have a look at the materials used for the bonnet and receptacle, the working pressure, the sealing diameter, and any burst discs fitted.... You MAY be OK with this setup.... or you may be pushing the envelope.... I have one such PCP that uses such an arrangement, and it is regulated at 2,000 psi, with a 3K burst disc fitted to the output side of the regulator, and it uses a 3000 psi bottle.... At 2000 psi, the force on the CGA 320 threads is 1,571 lbf., which means at that pressure I would be OK with 6061-T6, but if the regulator failed and the pressure rose to 3,000 before the burst disc popped, my safety margin would be down to 2.3:1.... but my sealing diameter (less than 1") will increase this.... I feel OK with that, personally, but now that I have done the numbers, you can rest assured I will never seal a regulator bonnet outboard of the threads again.... ::)
Bob
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Thanks for posting Bob. I'll definitely be looking at my calculations again on a build I would like to try. It also has me thinking about the very small brass threads on my BT65 where the valve screws into the air cylinder. Didn't realize brass tensile strength was that low. ;D
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I apologize for the error on the Tensile Strength of 360 Brass.... The most common value is 58 Ksi, and I have corrected that in the OP.... It does, however, vary between 48Ksi and 70 Ksi, depending on the amount of cold working.... Thanks to Ron Burnett for the correction....
Bob