3 replies to this topic

[jakjak]

Hi all,
I have a general question regarding an experiment apparatus that we will be working on. But I need to find out what kind of technical aspect should I be aware of when considering building or even purchasing one.
We have a cylindrical container that is about 0.01 cu.ft (with diam. about 2.3"). It will be partially holding water and will be sealed up. The container will then be heated up to ~ 150degC. The operating pressure is required to be around 75 psi.
My question is what category/code does this container fall under? ie. Pressure vessel or fittings? What is the safety precaution or design factors that we need to be aware of?
Feel free to ask any questions. Any help/advice is appreciated!

Thanks

[Art Montemayor]

Jak:

I presume you are a high school student, and not a Chemical Engineering student in college. I say this because I need to know who I am addressing and what are your abilities or training.

What you are proposing is very dangerous and I would caution you not to proceed to implement what you are describing. I have to presume that you are going to use a 2-1/2” screwed pipe nipple that is 5” long and with screwed pipe caps on each end after you partially fill it with water. Then you are going to heat it (probably with a Bunsen burner or something similar. You will also probably try to have a pressure gage and thermometer on the sealed pipe in order to watch the pressure rise and the temperature climb.

This is not smart experimentation. You can easily maim or kill yourself or someone else with this type of apparatus, so I’m going to give you a short seminar on basic thermodynamics:

You are definitely wrong in thinking that your pressure gage will register 75 psi (you probably mean gage pressure, psig, but are not familiar with the difference).

Absolute pressure (psia) = Gage pressure (psig) + atmospheric pressure (14.696 psi @ sea level)

I can guarantee you that if you heat contained water to a temperature of 150 ^oC, your pressure gage will register more than 69 psia (absolute pressure) and this is equivalent to a pressure gage reading of 54.3 psig (if you are close to sea level) – not close to 75. My main point here is that you should clearly state what specific pressure you are going to have inside that apparatus in order to further study the proposed mechanical strength and determine if the apparatus can safely withstand the true expected pressure. You certainly should know and identify the type of pressure you are talking about – absolute or gage.

Go to the following website: http://webbook.nist....hemistry/fluid/

And you will be able to find out the saturated properties of water at the temperature and pressure that you are proposing. There, you will find out that indeed, water at 150 ^oC will produce an absolute pressure of 69 psia.

Another thing you have to bear in mind and take into consideration before you conduct your experiment is that when you seal the apparatus you will have trapped a quantity of atmospheric air together with the water. Unless you suck an absolute vacuum in the apparatus before introducing the water, you will have a system that is composed of water + air. This will produce a pressure that is higher than that of pure water, so that the pressure reading will not correspond to the temperature read-out of the heated water. You simply do not have a PURE water system.

Unless you can prove that the apparatus will safely withstand the expected pressure, you should not proceed with the experiment. This would be a pretty dumb thing to do – especially if you are not a trained or professional engineer. The apparatus you describe is already impractically too small to have the expected temperature probe, pressure gage, and safety relief device. You have not mentioned any details over these items so I must deduce that you are not prepared nor experienced in what you are proposing. My best advice is: Don’t do it.

[jakjak]

Hi Art,
No, I am not a high school student. But, yes, I do not have much experience in container that will hold such a high pressure. I am aware of the consequences if there was improper handling of such container which might result in overpressure.
To clarify thing, let's assume that the content inside the container will be a two phase fluid. And the container inner volume is approximately 0.01 cu.ft, which translate to a diameter of 2.3" and a height of 3.9". Suppose that the expected pressure inside the container, which will be read from a pressure gage, is 75 psig. The surrounding temperature will be around 150degC.
What I would like to know is what kind of guideline would one follow to ensure safe operation of such container? You have mentioned necessary mechanical strength and also the inclusion of a pressure relief device. But is there a guideline for dimension and operating pressure/temperature for such?

thanks.

[Art Montemayor]

Jak:

I get more concerned when you repeat your obvious Thermodynamic mistakes.

Water heated to a temperature of 150 ^oC will generate saturated steam at a vapor pressure of approximately 54 psig --- NOT 75 psig as you keep insisting. Are you familiar with what I mean by Vapor Pressure? Because you are unable to predict the accurate resultant pressure in your heated vessel, I am concerned that you don't have the ability to control your experiment.

Please open the attached Excel Workbook and look at the sketch and calculation I did to check your work. Note that I have only taken credit for the pure cylindrical section of a vertical oriented vessel with the approximate dimensions you cite. Your volume calculations check out, but once again you fail to state the amount of vapor space that you will dedicate to the generation of steam within the vessel. This is very important. If you don't allow for vapor space you will have a hydraulic bomb that will shatter instantly when the hydraulic pressure exceeds the safe working stress of the vessel you are using. That's why you must state clearly what the physical characteristics and strength of your vessel are. If you are unable to make those calculations or obtain the service of someone who can do that, you are putting yourself in harm's way.

You won't tell me who you are, your training, your age, your resons for doing this, or your technical credentials in qualifying to do what you are describing so I can't tell you more than what I know about your plans. You are, of course free to do whatever you please. But please don't harm or hurt any innocent or naive persons around you. They can't be held liable for your decisions and actions.

Have a safe day.