PCP's and Operating Pressures

[Hajimoto]

This topic can quickly get into the weeds and it is not my intent to try and navigate my way though the very complicated sets of standards, codes and accepted practices throughout the world but they need to be generally understood because applying the wrong standards can ruin someone's day.


The players:
 

• DOT- typically shown as DOT on part - Department of Transportation (previously ICC – Interstate Commerce Commission), which is the regulatory body that governs the use of cylinders.
• UN/ISO - typically shown as ISO on part - International Organization for Standardization, is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
• PED - typically shown as CE on part - The Pressure Equipment Directive (PED) is a European set of standards for the design and fabrication of pressure equipment and vessels. It provides the administrative requirement to allow for free placing on the European market without legislative barriers. It has been mandatory in Europe since 2002.
• SELO - typically shown as SELO on part - The SELO China Manufacturer License is an approval procedure that qualifies the manufacturer to produce pressure-bearing products, tanks and vessels.

That said, In the United States the DOT ( Department of Transportation ) enforce the standards set by CGA ( Compressed Gas Association ). In Europe the UN ( United Nations ) and EU ( European Union ) enforce the standards set by ISO ( International Standards Organization ). UN/ISO service pressures are at or above the common fill pressure of a DOT cylinder with a + 10% overfill designation. Also, UN/ISO cylinders contain friendly markings with service pressures rated in PSI and BAR.


ISO service pressures are at or above the common fill pressure of a DOT cylinder with a + 10% overfill designation. Also, UN/ISO cylinders contain friendly markings with service pressures rated in PSI and BAR. Keep in mind that the DOT test pressure are 5/3 times service pressure. Which means if the service pressure is 3000, divide that by 3 you get 1000, multiply that times 5. So a 3000 PSI vessel must endure a hydrostatic test of no less than 5000 PSI. So you can quickly see how someone could misapply the pressure vessel test results to the entire assembly and get into trouble very quickly.


All this said, one must know exactly what standards the pressure vessel(s) were manufactured to meet in order to know what their test limits are.


Remember that we are talking three different sets of numbers here, there are the manufacturers suggested pressure limits, operational limits and limit before failure, which brings us to the term "Maximum".There are two that must be understood in order to venture down the path of testing the limits.

• Maximum Allowable Operating Pressure -  is a pressure limit set, usually by a government body, which applies to compressed gas pressure vessels, pipelines, and storage tanks.
• Maximum Allowable Working Pressure - Is defined as the maximum pressure based on the design codes that the weakest component of a pressure vessel can handle.

When I make suggestions of what I consider a safe operation pressure I use an engineering judgement based on general assessment of the information given by, stamps, spec sheets, material data and know industry standards.


The Gauntlet in .177 and .22 has a brass valve body with one fastener in the bottom that secures the valve body within the pressure tube. They do have a secondary safety should that single screw fail and the valve is forced rearward and that is part #19 which is held in place by the bottom stock screw and the magazine well screw from the top. The .177 and .22 are shipped with a operating pressure of 1100 PSI and I think a safe maximum operating pressure of this arrangement is 1400 PSI. That does not mean that it cannot operate at higher pressures, but higher rated safeties would need to be employed (burst discs, hardened fasteners) to comfortably go there.


The Gauntlet in .25 has a Stainless Steel valve body and 3 fasteners that secures the valve body within the pressure tube. They also have the secondary safety that uses part #19 which is held in place by the bottom stock screw and the magazine well screw from the top. The .25 are shipped with a operating pressure of 1900 PSI and I think a safe maximum operating pressure of this arrangement is 2100 PSI. That does not mean that it cannot operate at higher pressures as the .25 has a more robust overall build and I have seen the .25 run as high as 2600 PSI, but I cannot endorse this as an operating pressure as there is not clear data available on the materials and fasteners used in construction of this rifle.


The long, longer, if you don't know, take the conservative approach and be safe! So yes, manufacturers offer a safe maximum operating pressure which has some wiggle room, to press a mechanical system to its structural limits costs something and it has been my experience that that cost is service life. This service life reduction can be the fasteners that are holding the bottle drop block in place which have no safety other than the screws themselves which if sheared, sends block and bottle skipping across a shop, range or home.


Do you home work and investigate and request as much data as you can from the manufacturer before pushing the limits of pressure vessels you have no idea what their assembly pressure limits are.


Thanks,
Hajimoto

[CraigH]

Thanks for an introduction to the "players"    Some previously known, some not.

[bear air]

Thank you for the write up Haj, HPA is not to be taken for granted.

[scion19801]

nice write up hajimoto. Always good things coming from you and others who are knowledgeable.

[archellas]

Haj,

Thank you for providing info about this! HPA is no JOKE!

If you have ever seen a tire blowout (from about 35 psi pressure) you SHOULD understand what 3000+ psi means (I HOPE!)

Cheers

[scion19801]

Try being next to an airbag blowing out on a fully loaded double axle dump truck and being next to it in a car with driver window down as it makes a low angle declining sharp turn into a work area.. Big BOOM and pressure wave. Scared the kids in the car, and had them crying.

[sharpend]

To weigh in with some personal experience. Pressure vessels are regulated by DOT because these vessels are (can be/ may be) transported on the roads, and require routine re-cert, every 5-10 yrs. Re-cert of a cylinder involves filling the vessel with water completely so little airspace remains, placed inside of a steel water jacketed test apparatus, and the pressure brought up to the 5/3 testing pressure with regard to 3000psi (edit: sorry, mixing units here...nee 200 bar) aluminum cylinders, that being 5000 psi. The water jacket of the test apparatus has a sight glass by which is measured the expansion of the tank, and expansion beyond a certain point will fail a tank, typical cause being metal fatigue. I had a 80cf Luxfer fail and it was a loud pop, rather than an explosion that would have destroyed our building, because there there is little gaseous air in the apparatus, water being incompressible. That being said, I had an instructor at Pearl Harbor who was doing some home mixing of decompression gas, and suffered catastrophic failure of cylinder that totally destroyed the back side of his officer's billet. One more story. I replaced our LPG tanks (edit: 2 LPG tanks) at our house last year. Typically LPGs are certified for twelve years, after which need re-cert, or destroy the tank (by stamping out the cert date and serial #). I have two 9 gal LPGs and when one is empty I drive it into town and get filled. Down the road, just before twelve year service life expires, I will fill both and dope one of them permanently in place at the house. I will buy one new LPG tank, which is transported over the road to get filled, while old fixed tank supplies needs for a day or two. Switch back to newer tank after getting filled. By my estimates, current average gas usage, the old fixed tank will last about another 8 years before empty, that cylinder never transported over the road and presented to the LPG station for refill, which will refuse service because out of cert hydro. Hydro from the Greek, because tested water-filled in water jacket to DOT specs. I have come across oxy tanks collared with "Curtis-Wright" and mfg dates from WWII, with a long list of hydro dates stamped around the shoulder. Anyway, I shoot a Gauntlet 25 and stock 13ci cylinder, and routinely overfill to 3300psi and allow cooling from the adiabatic heating, while loading mags, resetting targets, &c. Just saying, if you are nice to your cylinders and never intend to have someone else fill your cylinders, nor transport on public roads, then a hydro down the road, well....

Mark

[scion19801]

iirc tanks 2" in diameter and under never need hydro or recertification. All tanks above this diameter need it every 5 years.