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Stainless Steel

  1. Sep 16, 2005 #1


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    For a project I am working on, I need a container that is both highly corrosion resistant and strong (able to withstand pressure). I was thinking about stainless steel.

    Basically, the container needs to be able to hold up to a concentrated solution of Sodium Hydroxide (NaOH), if you look at this web page,
    Leave, “Materials” set to “All”, and Select the “Chemical” to be Sodium Hydroxide (80%)”, you can see how various substances will hold up to that concentrated of a solution of NaOH. Now I will not be using a 33.3 Molar Solution of NaOH in real life (mostly because I would like to maintain the use of my eyes for a few more years at least), but why not test the material to the extreme in theory at least (also you will notice that 316 Stainless steel’s rating doesn’t change between 80% and 20% solutions).

    The second requirement, strength, pretty much rules out plastic as a possible container material, since some of the best plastic to hold up to NaOH do not cope with pressure very well.

    Ideally, I should also be able to get the material easily and cheaply.

    So it looks like Stainless Steel is my best bet.
    However, there are so many types/grades of stainless steel; I don’t know which is right for me. 316 stainless steel supposedly hold up against NaOH well (so says the list on that web page), but I wouldn’t know where to get it, and it is probably expensive.

    Then I had a great idea, a pressure cooker! It is already made into a container, it of course can hold pressure, it is made from stainless steel, and it is pretty easy to get one (just go on down to the store). But the type of stainless steel concerns me, all the pressure cookers I can find say they are 18/10 stainless steel, I have no idea what this means (it is probably a ratio of certain components to the steel). How does this compare to 316?

    Wikipedia has a page on stainless steel,
    and it includes a section on the different types, it says that type 316 is used “for food and surgical stainless steel uses”. But I don’t see anything about 18/10

    So does anyone have any advice on my stainless steel dilemma?
  2. jcsd
  3. Sep 16, 2005 #2


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    At what pressure and temperature will the NaOH be stored?

    SS304 is the most common (highest production tonnage) alloy in the 300 series of austenitic stainless steels. The 18-8 refers to nominal 18% Cr - 8% Ni, but in reality it can have 18-20% Cr and 8-10.5% Ni.

    SS316 is second or third most common of the 300 series and is basically SS304 with an addition of 2-3% Mo (for pitting resistance), with a slight reduction in Cr (16-18%) and slight increase in Ni (10-14%).

    The mechanical properties for 304 and 316 are roughly the same assuming the same final thermo-mechanical treatment, i.e. annealed or cold-worked.

    Here is one article on the storage of at least 50% caustic - with alternative alloys. Temperature is a consideration.
    Last edited: Sep 16, 2005
  4. Sep 16, 2005 #3


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    In practice, I cant imagine that I will ever exceed a temperature of about 100 degrees Celsius (at least not at this point). Pressure is a little harder to estimate, epecially since I usually dont do anything with pressure, or dont go to the trouble to measure it. A HIGH estimate would probably be 50 psi, but that is very high compared to what I have now (the pressure gauge doesnt even move, either it is broken, or it just isnt pressurized very much). I also will probably will use about a 3 Molar solution of NaOH (about 11%).

    The more I think about it, the more I like the pressure cooker idea. It has a bunch of build in saftey checks, it is a good type of stainless steel I suppose (every one I see is 18/10), and it is designed for pressure. Not being the type of person to cook very often, or anything passed a microwave, I was reading up on pressure cookers and it seems that most are designed for a max pressure of 15 psi before the valve opens and starts to hissss to let out steam. 15 psi is just over 1 atmosphere of pressure, which I suppose is pretty decent and safer compared to 50 psi.
    Also, I am not too sure how the NaOH would effect the seal on the pressure cooker, if it is rubber/plastic, it should be fine, since according to that compatability chart I had, NaOH was pretty easy on that type of stuff.
    What about the valve on the pressure cooker, can we safely assume it is made of stainless steel too, or perhaps I need to be concerned with the NaOH corroding it too.

    So it loos like, at my opperating conditions, assuming 100 degrees C, 15 psi, and an 11% solution, according to this graph,
    A normal, carbon steel type of steel should work, so if I go into Stainless Steel, I should definately be OK as far as corrosion goes it would seem.
  5. Sep 17, 2005 #4


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    Don'tcha hate it when you write out a response and then - blammo! The whole thing gets blown away! LOL. Ok, time to rewrite, so if this is brief, that's why.

    If this were an industrial application, the cost of the stainless material is generally far outweighed by the cost of machining and manufacturing. So if you decide to make this from scratch, I'd suggest the 316 stainless or even something better like Monel.

    If this is for home/test use, the pressure cooker is good. You can tell if its austenitic stainless or not with a magnet, it shouldn't be magnetic. Except that if cold worked, such as the pressure cooker will be during forming, austenitic SS will become very slightly magnetic. So if you go to the store to buy one, bring a magnet.

    Regarding the valve, my first guess would be a nickel plated brass. The brass is no good, but since its nickel plated, the nickel should protect it just fine.

    Regarding the elastomers, pressure cooker parts are probably food grade silicone, which is good. I don't know what other elastomers are used for food, but in general, nitrile (Buna-N) and neoprene are most common. I see nitrile is attacked by NaOH. Hope that helps.
  6. Sep 17, 2005 #5


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    this is for "home" use, not anything close to industrial.

    I was thinking more about it, and was pricing out pressure cookers, and now I am starting have second thoughts.
    Maybe I could just stick with PVC pipe?

    Advantages of PVC:
    -Cheaper than Stainless Steel
    -Design is more customizable to me than having to use premade pots
    -better insulator, not that I care too much about that
    -I already have it build, rather than re-building it
    -Excellent NaOH resistance

    Dissadvantages of PVC:
    -Cheaper, in the sense of quality
    -Not designed, or even recommended for pressure
    -not as durable

    Advantages for Stainless Steel Pressure Cooker:
    -premade, ready to use, just need to make a few modifications
    -Designed for pressure
    -Several built in safety systems
    -good NaOH resistance
    -more durable, will last for a long time if I take care of it

    Disadvantages of Stainless Steel Pressure Cooker:
    -Onlycan go up to 15 psi before it starts to vent
    -more expensive, and will have to rebuild/redesign again
    -I would prefer a smaller diameter base than what is common is pressure cookers
    -pressure relief valve might evenetually corrode
    -No way to vent pressure immediatly if need be, unlike PVC where I can simply pull out the hose and the pressure is almost instantly gone.

    I like the idea of PVC because it has more of a "built from scratch" feel to it, it is cheaper, and I already have it build. But the pressure cooker is a more professional, heavy duty approch to my project.

    Does anyone know what the standard/normal PVC pipe can hold safely as far as pressure goes? I know that with temperature, the pressure holding capacity decreases rapidly (by about a 5th at 140 degrees F compared to 70 degrees F).
  7. Sep 17, 2005 #6


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    Hey Mr. Jeff,
    Actually, PVC pipe is designed and built for pressure. It's usually used for water, so look for a proper pipe size in a hardware store and make sure its intended for pressurized use.

    Relief valves should be very cheap. For something like this, they only cost a few dollars because they're mass produced. Make sure you have a method of preventing overpressure, properly sized and suited for the system you're installing it in.

    Think about what valves and pressure gauges you need so you can do this safely. Valves and gauges can be purchased very inexpensively so use some thought and make it safe!

    I had a look around and found a graph giving tensile strength of PVC versus temperature to 100 C in MPa. Checking against other data, it seems the data given is ultimate tensile strength, so you have to divide by 2.5 to get allowable stress.

    Allowable stress is the maximum stress you should expose the pipe to. Calculate stress as follows:

    S = Pr/t
    Where S = allowable stress
    P = pressure
    r = radius of pipe
    t = wall thickness of pipe

    The graph of ultimate tensile stress is here at the top of page 10: http://www.georgefischer.co.uk/instaflex2003.PDF

    Don't forget to divide by 2.5 before using the values given on this page.

    And please don't do something stupid and blow your ass off! If you have any questions about making something like this, post them here or shoot me a PM direct.
  8. Sep 17, 2005 #7


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    Really? I was under the impression that it was not, especially since on the side of the pipe, "For non pressure use only" is printed.

    Are there not two types of PVC, a "normal" type and a "tougher" type. perhaps I have the normal stuff and the tougher PVC is what you are refering to.

    I think that the formula might be refering to the "INSTAFLEX" type of plastic pipe, not PVC. I dont see a graph on the top of page 10, or were you refering to the forulas give there?
    On page 8, there is a graph of various plastic pipe tyes as a function of temperature and pressure, and it confirms my suspicions of PVC's ability to hold pressure is significantly lower at higher temperatures.

    I try very hard not to blow myself up, but sometimes accidents happen. That is why I want to be extra, extra careful this time and make sure I do everything right
  9. Sep 18, 2005 #8


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    There is PVC conduit which is not intended for pressure service, and it should be marked as the piece you have is. Conversely there is PVC pipe which is manufactured for pressurized use. In the US it is built per ASTM standards. Here's a web page that gives some background on the different uses.

    Generally, you don't want to use PVC above 140 F. The graph in this reference: http://www.georgefischer.co.uk/instaflex2003.PDF
    shows PVC-C strength all the way up to the boiling point of water (100 C). Note that at 100 C the strength is only 10% of what it is at 20 C. Regarding the page number, I'm going by the page number printed at the bottom of each page. Sorry for the confusion there.

    For pipe dimensions, there's a chart here: http://www.engineeringtoolbox.com/pvc-cpvc-pipes-dimensions-16_795.html
    If you use the value of 3 MPa for PVC-C pipe at 100 C, you should put a 2.5 safety factor (minimum) on that and not expose the pipe to a stress of more than 1.2 MPa. Applying the equation above (note: ASME B31.3 suggests using OD/2 for radius, not ID/2) for 1" schedule 80 pipe, the maximum pressure you should expose the pipe to is 47 psi. So you can see from that example that PVC pipe has some significant ability to retain pressure, even with boiling water. Because you're going above the rated temperature of the material though, I'd suggest not even going to that high a pressure. You really shouldn't use it above 140 F. You might want to take a piece and put it in boiling water and just see for yourself how soft it gets before using it.

    I wonder how you're going to heat something up inside a PVC pipe though, and also how you're going to ensure the system won't be over pressurized. You need at least a pressure gage and some valve to relieve pressure.
  10. Sep 18, 2005 #9


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    So it would seem the PVC I have now is the wrong type. I believe it is a sewer drainage pipe. What type do I need to look for, PVC-C? Is that the kind that can handle pressure?

    I dont seem to be able to recreaste your calculation. The unit for the thickness of the pipe and radius is mm correct? and the answer should give the pressure in bars or MPa?
    The pipe I am using now, the bad type of pipe, has a diameter of 4 inches, so that will dramitically reduce the isuble pressure.

    I will not be intentially heating the contents of the PVC pipe, it will just happen, via a chemical reaction and due to the increasing pressure. The chemical reaction taking place inside will be releasing a gas aswell as a lot of heat. I do have a venting system, a hose built into the side of the pipe to take the case to another PVC pipe. On the second pipe, there is a pressure gauge/valve. So far, I have never seen the little needly on the valve move, perhaps I am just not getting anywhere near the pressure needed to move it, or perhaps it is not working correcltly.
    If I were to blow through a tube into the pressure gauge as hard are my lungs could, should I be see any measureable pressure?
  11. Sep 18, 2005 #10


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    If I'm not mistaken, PVC-C is chlorinated PVC, also called CPVC. It doesn't matter if it's PVC or CPVC/PVC-C, what matters is it must be manufactured to meet ASTM standard D 1785 (assuming you're purchasing the pipe in the US and not Europe or somewhere else). It should be marked as such right on the pipe. In addition, you'll want to know the diameter and schedule rating of the pipe which should also be marked on the pipe.

    Regarding the equation, just rewrite to provide pressure as follows:

    P = 2St/D

    Where D = OD of pipe (inches)
    t = thickness of pipe wall (inches)
    S = Allowable stress (psi)

    For a 1" schedule 80 pipe, the OD is 1.315, wall thickness is .179. If we use as an allowable stress 1.2 MPa (which is 3MPa / 2.5) then in English units that's 174 psi.

    P = 2*174*.179/1.315 = 47 psi

    Regarding how hard a person can blow, it's less than 1 psi. You could use a manometer made from clear tygon tube if you're trying to measure a very slight pressure like that.
  12. Sep 18, 2005 #11


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    OK, I understand you now.
    I need to see if the store has any of the pipe made to meet the ASTM standard, which they probably wont, but I'll check.

    So if I work out the equation for my 4: pipe,
    P = 2(1.2)(.237) / (4.5) = 126.4 kPa = 18.3 psi

    So in the end, it is just slightly higher than the 15 or so I could get with a pressure cooker, but that does assume the temperature is 100 degrees Celsius, if I can cool it, it will be stronger.
    So if I used it as 140 degrees F (60 C),
    P = 2(5.6)(.237)/(4.5) = 589.9 kPa = 85.6 psi

    I was just wondering about how hard one could blow to see if I should have been able to move the pressur gauge, which I couldnt, and I shouldnt have been able to, good, all is well in that respect.
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