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  • steel steam pressure vessel, will it rust, pressure questions...

    I have come into need of an autoclave to occasionally sterilize some instruments. Right now I'm considering using 8" schedule 40 steel pipe and having a friend of mine (certified pressure vessel welder) weld the plug and flange on.

    The cycle of the autoclave is as follows:

    Fill bottom of tank with water. Heat to at least 121 deg C, water will pressurize vessel to a pressure of around 30 PSI. Hold at 30 PSI for 30 minutes or so, vent off steam. Continue to cook vessel until interior is entirely dry (no more escaping steam).

    My concern is for rust. I know that it will rust to some extent, but my expectation is that it won't rust toooo badly because of the immediate drying cycle. Total wet time will be under 1 hour.

    As a bonus question, how would one calculate the amount of water required to produce steam of a specific pressure at a given temperature?

  • #2
    It may start to rust lightly after numerous cycles, the steam will have a cleansing effect and cook out any inhibitors or foreign material out of the steel. Normally theres a bit of water treatment in steam production but being an autoclave I'm sure you'll want none????? Almost every autoclave I've ever encountered the innards where SST, any chance of finding some?????

    30 psi will just get you to 121C, what is your steam production system, electric element? You will have to maintain 121 for a guaranteed kill.......If it is electric is the element to also occupy space in the pipe? If so you will loose some level initially for steam production but after it's at pressure it should be minimal being a closed system.......Is this thing going to be supervised.....LOL
    Last edited by hardtail; 04-19-2009, 03:55 PM.
    Opportunity knocks once, temptation leans on the doorbell.....

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    • #3
      Well, according to some hot metal and water is an instant recipe for rust, even if it appears completely dry.

      You will have to maintain 121 for a guaranteed kill.......
      Keep in mind that not everything pathogenic is killed at that temperature. In particular, Radiodurans isn't and neither are prions, the active agent in Mad Cow Disease and Creutzfeldt-Jacob in humans.
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      • #4
        Technically, prions are not alive. It's a protein that has insidious capabilities. I've often wondered if the end time of various life forms comes when a prion emerges that is a good fit. Radiodurans are hardy bacterium. Perhaps an understatement

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        • #5
          it will be supervised. Even if it wasn't, I will use PID controllers for the temp, and install dual relief valves for safety. The elements will heat the entire chamber, not heat from the inside. I will monitor the temperature with a thermocouple inside the chamber, and likely set it just above 121, likely 125.

          I am not overly concerned with prions, as the instruments are not going to see anything important, but sterility is important. 121 w/steam will kill all bacteria. The steam will be produced by the heating of the actual vessel. A small amount of water will be placed in bottom of the vessel prior to heating it, the door will then be closed and sealed. Turn on heater, and wallah, steam.

          I expect it to rust, just not sure how much. I know that pressurized steam is more corrosive than just water, but I really have no clue how much more. I guess we'll see.

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          • #6
            Radiodurans was discovered as a result of the US Army using gamma radiation treatment to preserve food in rations. Even after a high enough dose to partially cook the food some would still spoil. That of course was because Deinococcus Radiodurans will probably inherit the Earth. It is nearly indestructible and has an amazing ability to repair damage to it's DNA. It is thought that D. Radiodurans can withstand high vacuum and the rest of the conditions found in space. It is a candidate for the exogenesis theory of the spread of life through space to Earth.
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            • #7
              So anyway....

              If I want to fill a 1 liter container with steam at 30PSI, and have no water left over...how much water would I need?

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              • #8
                I think 1 liter of steam at 30 psi will condense to 0.01 oz of water at room temperature.

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                • #9
                  Originally posted by snowman

                  The cycle of the autoclave is as follows:

                  Fill bottom of tank with water. Heat to at least 121 deg C, water will pressurize vessel to a pressure of around 30 PSI. Hold at 30 PSI for 30 minutes or so, vent off steam. Continue to cook vessel until interior is entirely dry (no more escaping steam).

                  My concern is for rust. I know that it will rust to some extent, but my expectation is that it won't rust toooo badly because of the immediate drying cycle. Total wet time will be under 1 hour.

                  As a bonus question, how would one calculate the amount of water required to produce steam of a specific pressure at a given temperature?


                  You have just described a retort used to cook canned food. An autoclave and a retort are the same thing, just a fancy word for pressure cooker. You should get years of service out of it. I run a retort at work to cook canned fish. Here is a basic rundown of the retort process:

                  1 Turn on the steam with the vent open until temp reaches 220F, this drives the air out of the retort so you get good heat distribution and a direct correlation between temperature and pressure. ie. 121C = 250F = 15psi

                  2 At 220F (for us it's x number of minutes, but the goal is 220F) close the vent. The temp will continue to rise until the temp controller levels it off at around 252F for us. This is when the cook time starts.

                  3 After about an hour depending on can size, turn steam off and add cooling water through a internal cooling manifold, at the same time we add air into the retort since there is virtually no air in there and you want a little pressure on the cans while they cool.

                  There are different ways to controll the temperature. Since it is a steam only cook, a steam pressure regulator would work and be the least expensive. We have a temp recorder/controller that will work strictly off of temp so we can do overpressure cooks with air added. You can google a steam pressure/temp chart to give you an idea of what you are dealing with.

                  ME

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                  • #10
                    An autoclave is a fired pressure vessel and, depending on location, may need proper certification to operate. The local codes should be checked. In any case properly sized relief devices and controls must be used.

                    If properly designed, operated and maintained, corrosion will be no more of a problem than it is with any steam boiler which, essentially, it is.

                    Michael's description of the operation is the basic procedure to follow, heat and vent to remove all air as it comes up to temperature. Close vents and hold at pressure/temperature for the requisite length of time and cool. You need not water cool if sterilizing instruments, but can allow it to cool naturally. It will have to be vented to prevent a vacuum from forming when cooling.

                    Not all water needs to be converted to steam, a tray can be used to hold the instruments above the water level. Depending on the method of heating, allowing all water to vaporize can lead to some very nasty consequences as the water will prevent localised hearing of the shell and damage that could result.
                    Jim H.

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                    • #11
                      From steam tables - saturated steam at 250؛F (121؛C), will be at 29.825 psia (15 psig). Specific volume of the liquid is 0.017 ft3/lb and of the vapour is 13.821 ft3/lb. So the steam takes up 813 times as much volume as the water. You won't need much water to fill the vessel several times over with steam.

                      Some questions/ thoughts to consider:

                      Put the water in a SS tray. That way you can remove the remaining water without worrying about having to boil the vessel dry. The remaining water will alos have all the remaining dissolved solids that would otherwise be scale on the side of your vessel.
                      As already pointed out, and worth reiterating: Building a heated pressure vessel has legal ramifications. Choose how you want to deal with that fact.
                      AFAIK, a steam autoclave is called that because steam is the source of heat. Steam is a great source of heat, and it is far easier and cheaper to control pressure than temperature (without electronics), and the pressure/ temperature corelation of steam happens to land in a very useful range for an autoclave. But... Is sterilization a result of the heat/ temperature or does the steam affect how sterilization (or cleaning) works? If temperature is the only factor then why bother with the steam? Clean the instruments and build a drying/ sterilizing oven instead and avoid the rust and legal issues. But most of what I know about sterilizing autclaves is based on watching M*A*S*H.

                      Cheers.

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