Small pressure chamber made from rated NPT components... Safety questions.
Close
Login to Your Account
Page 1 of 5 123 ... LastLast
Results 1 to 20 of 89
  1. #1
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Question Small pressure chamber made from rated NPT components... Safety questions.

    Hey all!
    I want to build a small pressure casting system, the principle is you pour epoxy into a small mold then the whole thing goes inside a pressure chamber and air pressure is applied to crush any entrapped bubbles down to nothing.

    This is a common thing to do, but pretty much all the 'pressure casting chambers' that you can buy online are not ASME coded, and I have heard some horror stories as they seem to be built 'just enough' which is obviously not ideal with pressure vessels...

    Because the parts I need to cast are narrow and long they are a good fit inside 2-3" NPT pipe. I want to build a small chamber from pipe and fittings, with one end cap permanently attached and the other end screwed on and sealed with teflon tape so it can be opened and parts put inside...

    My question: how do I build a system like this safely??? Here's a screenshot of the assembly as it stands in Fusion -



    Currently the components are:
    16" long 3" NPT Schedule 40 welded pipe - McMaster-Carr
    Class 150 Iron 3" NPT Cap - McMaster-Carr
    Class 150 Iron 3" NPT to 2" NPT reducer - McMaster-Carr
    Class 150 Iron 2" NPT to 1/4" NPT bushing - McMaster-Carr
    1/4" NPT Nipple - https://www.mcmaster.com/7727K131/
    High pressure brass cross fitting - https://www.mcmaster.com/50785K233/
    125 PSI Safety relief valve - https://www.mcmaster.com/5784T12-5784T124/
    High pressure 1/4" NPT needle valve - https://www.mcmaster.com/7833K72/
    1/4 NPT Industrial quick connect - https://www.mcmaster.com/6050T15/

    The lowest rated component is rated to 150PSI... Working pressure for the vessel would be 120PSI with the safety relief valve set for 125PSI. The idea behind the needle valve is to allow filling and exhausting the chamber slowly to avoiding the flow of air disturbing the parts inside.

    This is an area where I know enough to build something potentially dangerous, but not enough to be 100% sure that I'm doing everything right. So:

    - Is the design as it stands unsafe for a 120PSI service pressure?
    - What additional measures, if any, should I be taking?
    - Would hydro testing to 150% of service pressure be a good idea? If so does the grease-gun hydro test work fine for this kind of thing?

    Should I ditch the current design and upgrade to class 1000 or class 3000 components for the extra safety factor?? Should I just ditch the whole idea?

    Any and all input appreciated. I bought all the components before realizing that I need to think/research deeper about this, but I'm fine with writing them off as useless if need be. I value my limbs & life more highly than the cost of a pipe fitting or two.

    Thanks!
    -Aaron

  2. #2
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    22,351
    Post Thanks / Like

    Default

    How many times will you screw/un-screw the fittings ?

  3. Likes aarongough liked this post
  4. #3
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Default

    Quote Originally Posted by digger doug View Post
    How many times will you screw/un-screw the fittings ?
    The front cap would get taken off the 3" NPT pipe maybe 2-3 times a week or so. New teflon tape and then re-fitted with parts inside...

  5. #4
    Join Date
    Jan 2007
    Location
    wales.uk
    Posts
    1,844
    Post Thanks / Like
    Likes (Given)
    412
    Likes (Received)
    432

    Default

    It is fairly easy, you hydraulically test it, you can get a bucket pump from hire shops ( I bought mine, not expensive, rothberger is the make, cost £120)
    But I’m expecting to test a lot of pipe soon ( once again god willing) you can then test to your hearts desire, make sure no air in, bottom fill to get the bubbles out,
    Mark

  6. #5
    Join Date
    Mar 2011
    Location
    Denver, CO USA
    Posts
    10,223
    Post Thanks / Like
    Likes (Given)
    58
    Likes (Received)
    5703

    Default

    Quote Originally Posted by boslab View Post
    It is fairly easy, you hydraulically test it, you can get a bucket pump from hire shops ( I bought mine, not expensive, rothberger is the make, cost £120)
    But I’m expecting to test a lot of pipe soon ( once again god willing) you can then test to your hearts desire, make sure no air in, bottom fill to get the bubbles out,
    Mark
    Or fill it with water and apply shop air.
    Test it to 300 or so?

    The thread/unthread will be a problem.

    You can pull a vacuum to degas the resin in the same chamber.

  7. Likes boslab, Screwmachine, BT Fabrication liked this post
  8. #6
    Join Date
    Jun 2002
    Country
    CANADA
    State/Province
    British Columbia
    Posts
    3,202
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    3135

    Default

    Hi Aaron:
    Do you need such high pressures for your application?
    My experience is with acrylic used for making dentures and the pressure pots we used were ordinary kitchen type pressure cookers which go to maybe 20 PSI.
    Many denture acrylics are cured in pressurized hot water, and that low pressure was enough to eliminate bubbles and porosity.

    Others have used something like this from Harbour Fright:
    Paint Pressure Tank - 2-1/2 Gallon.

    These are good to 60 PSI.

    It's probably faster to just modify something like this unless you need the higher pressure.

    If you do, I'd be careful about a couple of things:
    1) make sure the caps have the female thread, so the ends of the pipe are unable to expand and blow the caps off.
    You've done that so that's good.
    2) DOM tubing is better than welded tubing.but at 120 PSi you're in the range where rubberized pneumatic or hydraulic hose is good enough for common service so I am unconvinced it's going to matter.
    3) A quick Google search yields the information that a hydrostatic test of 150% of the working pressure is an industry standard, so you're considered to be good with that.
    If you are really scared and want to build two; you can test one to failure and see what your safety factor is (If you can even pressurize it that high).
    Then you can hydro your keeper to 150% and call it happy.
    A couple of pipe caps and a bit of tubing are cheap!

    If you're still uncomfortable, make a blast casing for it out of a bigger piece of tubing you can slip your pressure vessel into.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

    Oh yeah, buy thick enough tubing that you can face groove it and put in a skinny Oring.
    That will eliminate the need for teflon tape, but you have to make a parallel thread on the removable end so you can wind the cap down enough to compress the oring.
    If you don't have a lathe to do this, you can do it on the Fadal by threadmilling it.
    If you have to make a new cap make it out of 954 aluminum bronze and put a couple of handles on it.
    It'll never seize on the threads that way.

    MC

  9. Likes Pathogen liked this post
  10. #7
    Join Date
    Oct 2005
    Country
    UNITED STATES
    State/Province
    Pennsylvania
    Posts
    22,351
    Post Thanks / Like

    Default

    You could buy some pipe, weld o lets (a cap) a flange for the other end, and a blind flange.

  11. Likes aarongough, dkmc liked this post
  12. #8
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Default

    Thanks guys!

    Couple of things I'm taking away from this:
    1) Hydro testing is a necessity! I don't know why but I figured that 'rated' components were factory tested, knowing this is not the case I will approach the situation differently!

    2) Drop the service pressure. If 120PSI is not needed then I will go down to 60PSI as suggested... This means I can fill the vessel with water and then do a 200% hydro test with shop air as the pressure source, no special equipment needed which is nice. After that I will change out the safety relief valve to 65PSI and charge with a regulator to 60PSI.

    3) I will build a safety cage with provision for charging/discharging the system with the cage closed. Seems like a sensible 'belt and braces' thing to do, no downside and the extra safety is nice.

    Does this all sounds reasonable? I want this thing to be 10000% percent safe. I'm not interested in having my grave read 'died while doing a menial task'... Any extra suggestions welcomed!

    Marcus: those harbor freight pressure pots were exactly the ones I've heard horror stories about! They are not ASME coded and a couple of people have had them fail... I have heard stories of similar ones bulging at their rated pressure which doesn't sound good. Princess Auto sell a similar unit but after hearing the stories I didn't feel safe buying one. Guess I should have felt just as unsafe when thinking about building one

    Doug: I like the idea of flanged fittings, however I'm not sure I trust my welding to the 'welding on a pressure vessel' level. I am comfortable with low level structural stuff but I would stay away from anything bearing weight over people and pressure vessels... I looked on McMaster and it seems like the don't stock any pre-welded flanged pipe? If there's another source for premade stuff then that might be a good option!

    -Aaron

  13. #9
    Join Date
    Dec 2008
    Location
    Vt USA
    Posts
    9,478
    Post Thanks / Like
    Likes (Given)
    1666
    Likes (Received)
    3453

    Default

    rework a small hydraulic cylinder.

    All tested and good to go.

  14. #10
    Join Date
    Jun 2002
    Country
    CANADA
    State/Province
    British Columbia
    Posts
    3,202
    Post Thanks / Like
    Likes (Given)
    0
    Likes (Received)
    3135

    Default

    Hi again Aaron:
    If I calculate correctly, the pipe will see 18,000 lb of force trying to split it at 120 PSI and each end cap will see 850 pounds of force trying to blow the cap off at 120 PSI.
    The hydro test is to give you confidence the whole as-built assembly can take the punishment.
    That's all you care about...the individual components aren't relevant so long as they're all rated over the service pressure, since you can blow a rated fitting right out of the vessel if you install it wrong.

    So yeah...do the hydro test and give yourself assurance your whole assembly is good.
    The individual parts are tested, but...

    With regard to the Harbour Fright pressure pots; if that's their history I wouldn't have one in my shop either!
    I spoke of them only because I know of dental labs who have used them successfully for pressure curing denture acrylic.
    Remember also, that the bomb potential of these devices is volume dependent...this thing is 2 1/2 GALLONS of volume whereas yours is a piddler by comparison.

    I believe your design is very likely more than safe, but I'd hate to have a sphincter clenching cringer moment every time I opened the air valve, so yeah, build it like a brick shithouse, test the snot out of it, and then use it with confidence.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

  15. Likes aarongough liked this post
  16. #11
    Join Date
    Nov 2018
    Country
    UNITED STATES
    State/Province
    Michigan
    Posts
    787
    Post Thanks / Like
    Likes (Given)
    429
    Likes (Received)
    530

    Default

    I agree that a gasketed or better yet, an o-ring flange is a good idea, and you could get a threaded one. That way, the threaded connection will be "permanent" rather than re-assembled every time.

    As to working pressure, start with a modest level and work up to where you're happy. Bubbles formed at atmospheric pressure will be half sized at 15, half that at 60, and half that at 120 psi.

  17. Likes dkmc liked this post
  18. #12
    Join Date
    May 2021
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    166
    Post Thanks / Like
    Likes (Given)
    197
    Likes (Received)
    48

    Default

    Quote Originally Posted by aarongough View Post
    Thanks guys!

    Couple of things I'm taking away from this:
    1) Hydro testing is a necessity! I don't know why but I figured that 'rated' components were factory tested, knowing this is not the case I will approach the situation differently!

    2) Drop the service pressure. If 120PSI is not needed then I will go down to 60PSI as suggested... This means I can fill the vessel with water and then do a 200% hydro test with shop air as the pressure source, no special equipment needed which is nice. After that I will change out the safety relief valve to 65PSI and charge with a regulator to 60PSI.

    3) I will build a safety cage with provision for charging/discharging the system with the cage closed. Seems like a sensible 'belt and braces' thing to do, no downside and the extra safety is nice.

    Does this all sounds reasonable? I want this thing to be 10000% percent safe. I'm not interested in having my grave read 'died while doing a menial task'... Any extra suggestions welcomed!

    Marcus: those harbor freight pressure pots were exactly the ones I've heard horror stories about! They are not ASME coded and a couple of people have had them fail... I have heard stories of similar ones bulging at their rated pressure which doesn't sound good. Princess Auto sell a similar unit but after hearing the stories I didn't feel safe buying one. Guess I should have felt just as unsafe when thinking about building one

    Doug: I like the idea of flanged fittings, however I'm not sure I trust my welding to the 'welding on a pressure vessel' level. I am comfortable with low level structural stuff but I would stay away from anything bearing weight over people and pressure vessels... I looked on McMaster and it seems like the don't stock any pre-welded flanged pipe? If there's another source for premade stuff then that might be a good option!

    -Aaron
    As far as I know, McMaster doesn't sell flanged pipe, but they do sell threaded pipe flanges.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  19. Likes aarongough liked this post
  20. #13
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Default

    Quote Originally Posted by 52 Ford View Post
    As far as I know, McMaster doesn't sell flanged pipe, but they do sell threaded pipe flanges.
    Ah! That's a eureka moment! Great idea!

  21. #14
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Default

    Quote Originally Posted by BoxcarPete View Post
    I agree that a gasketed or better yet, an o-ring flange is a good idea, and you could get a threaded one. That way, the threaded connection will be "permanent" rather than re-assembled every time.

    As to working pressure, start with a modest level and work up to where you're happy. Bubbles formed at atmospheric pressure will be half sized at 15, half that at 60, and half that at 120 psi.
    Thank-you, yes the threaded flange is a great idea! Having never used one before I simply didn't realize that they existed!

    I am definitely prone to the idea of 'more is more better' so I will walk that back and use more most pressures as suggested... As Marcus said most people are casting at 60PSI, so I'm sure that will work for my needs as well!

  22. #15
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Default

    Quote Originally Posted by implmex View Post
    Hi again Aaron:
    If I calculate correctly, the pipe will see 18,000 lb of force trying to split it at 120 PSI and each end cap will see 850 pounds of force trying to blow the cap off at 120 PSI.
    The hydro test is to give you confidence the whole as-built assembly can take the punishment.
    That's all you care about...the individual components aren't relevant so long as they're all rated over the service pressure, since you can blow a rated fitting right out of the vessel if you install it wrong.

    So yeah...do the hydro test and give yourself assurance your whole assembly is good.
    The individual parts are tested, but...

    With regard to the Harbour Fright pressure pots; if that's their history I wouldn't have one in my shop either!
    I spoke of them only because I know of dental labs who have used them successfully for pressure curing denture acrylic.
    Remember also, that the bomb potential of these devices is volume dependent...this thing is 2 1/2 GALLONS of volume whereas yours is a piddler by comparison.

    I believe your design is very likely more than safe, but I'd hate to have a sphincter clenching cringer moment every time I opened the air valve, so yeah, build it like a brick shithouse, test the snot out of it, and then use it with confidence.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Yeah with things like this where I'm at all worried about their general safety I just end up hating to have them in the shop... As a rule I try not to keep or use anything that I'm at all worried about, and so far I have kept all my fingers and don't have any foreign objects lodged in my body... I'd like to keep it that way!

    As you said I will just layer up the safety precautions and sleep soundly!

    -A

  23. #16
    Join Date
    May 2021
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    166
    Post Thanks / Like
    Likes (Given)
    197
    Likes (Received)
    48

    Default

    Quote Originally Posted by aarongough View Post
    Yeah with things like this where I'm at all worried about their general safety I just end up hating to have them in the shop... As a rule I try not to keep or use anything that I'm at all worried about, and so far I have kept all my fingers and don't have any foreign objects lodged in my body... I'd like to keep it that way!

    As you said I will just layer up the safety precautions and sleep soundly!

    -A
    Just out of curiosity, why use pressure versus pulling a vacuum?

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  24. Likes awander, sfriedberg, SAG 180 liked this post
  25. #17
    Join Date
    Jan 2005
    Country
    CANADA
    State/Province
    Saskatchewan
    Posts
    10,394
    Post Thanks / Like
    Likes (Given)
    1420
    Likes (Received)
    3814

    Default

    I'd also suggest looking at a tie-rod style hydraulic cylinder style construction.1/4" wall DOM is normally used for 3500 psi. You get excellent sealing and you can quickly take the thing apart with an impact wrench with a socket. Large pipe threads might require sealing goop to seal, and this can be difficult to unscrew later on. Messy shit to use, too.

    From an Ag supply store, you might be able to buy an extra tailpiece for a standard cylinder in case you don't want to plug off the gland end. However, you could just shorten the shaft up and use it to plug the gland. Chances are good that you can buy the cylinder for less than you can buy the components.

    Or you could apply a thrust force to the shaft/piston and use it to pressure up your chamber, but you will sacrifice working length to do this. With a precharge applied, the sacrificial length would be less. Hell, just think how small you could make those bubbles

  26. #18
    Join Date
    Oct 2014
    Country
    CANADA
    State/Province
    Ontario
    Posts
    1,644
    Post Thanks / Like
    Likes (Given)
    1292
    Likes (Received)
    1405

    Default

    Quote Originally Posted by 52 Ford View Post
    Just out of curiosity, why use pressure versus pulling a vacuum?

    Sent from my SAMSUNG-SM-G891A using Tapatalk
    Seems like you really have to baby-sit the task while using vacuum... The epoxy expands a bunch and have to make sure it's not expanding too much and overflowing etc... Then you run the risk of accidentally introducing bubbles during the pour into the mould as well. In my case I am pouring a powder mixed with an epoxy so I have to keep stirring it to ensure it stays suspended during the pour which means I'll just be introducing bubbles again...

    Pressure casting seems very simple in comparison, just mix the stuff, chuck it in, pressurize it and forget about it! That's the theory at least!

  27. Likes 52 Ford liked this post
  28. #19
    Join Date
    Aug 2019
    Country
    UNITED STATES
    State/Province
    Texas
    Posts
    108
    Post Thanks / Like
    Likes (Given)
    2
    Likes (Received)
    109

    Default

    This is not a "pressure vessel" you are fabricating, it's a piece of standard rated pipe and rated pipe fittings, no different than piping shop air and should be treated as such. One added suggestion is ditch the threaded cap or flanged cap and use a full port ball valve as your access hatch and save a lot of time and hassle.

  29. #20
    Join Date
    May 2021
    Country
    UNITED STATES
    State/Province
    Virginia
    Posts
    166
    Post Thanks / Like
    Likes (Given)
    197
    Likes (Received)
    48

    Default

    Quote Originally Posted by aarongough View Post
    Seems like you really have to baby-sit the task while using vacuum... The epoxy expands a bunch and have to make sure it's not expanding too much and overflowing etc... Then you run the risk of accidentally introducing bubbles during the pour into the mould as well. In my case I am pouring a powder mixed with an epoxy so I have to keep stirring it to ensure it stays suspended during the pour which means I'll just be introducing bubbles again...

    Pressure casting seems very simple in comparison, just mix the stuff, chuck it in, pressurize it and forget about it! That's the theory at least!
    That makes sense. Just a thought, McMaster sells high pressure NPT sight glasses. You could drill and tap the end plate for one. I'm not saying it's a good or bad idea, I'm just saying it's an option if you needed to see into the chamber.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  30. Likes digger doug liked this post

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •