The Building Blocks

Wouldn’t it be cool if you could just make a supercritical dryer out of Legos®?

Well that’s actually the idea behind the manuclave designs on Aerogel.org, except for instead of Legos we’ll use standard high pressure pipe fittings, valves, and gauges.

Pipe Fittings

Pipe fittings are all around you. They deliver water and steam in houses, liquid carbon dioxide into soda at restaurants, and chemicals and high pressure gases in factories. Pipe fittings can be threaded, meaning they are designed to screw together, or meant to be welded together. We’ll use threaded pipe fittings on Aerogel.org. Threaded pipe fittings are usually defined by the following properties:

  • Thread type
  • Gender
  • Form factor (shape)
  • Diameter or diameter class
  • Material
  • Pressure and temperature ratings

Thread type is the class of fittings that the fitting can screw together with. For comparison, K’nex®, Legos, and generic building blocks only fit with blocks from the same set and don’t really fit together with each other. And if they do fit together they usually don’t fit together well. Same thing with pipe fittings. Different thread types don’t screw together, and if they do, they won’t seal.

Gender refers to whether or not the fitting screws into something or is screwed into. Male fittings insert, female fittings are inserted into. Go ahead. Laugh. Get it out of your system.

Form factor or shape refers to what kind of building block the fitting is. Common form factors are:

  • Pipe plugs, which screw into a female thread to close it
  • Pipe caps, which screw onto a male thread to close it
  • Nipples (go ahead, laugh again), segments of pipe with male threads on both ends
  • Unions, segments of pipe with female threads on both ends
  • Nineties, curved union (elbow)
  • Reduction bushings, parts that allow you to screw a male fitting into a larger female fitting
  • Tees, which have three threaded arms in a T shape
  • Wyes, which have three threaded arms in a Y shape
  • Crosses, which have four threaded arms in a + shape

Tees, wyes, and crosses can have either male or female threads on each arm, but usually are either all male threads or all female threads.

Diameter or diameter class defines the size of the inside of the pipe fitting. Only fittings of the same diameter class will fit together. To join different diameter class fittings you would use a reduction bushing as described above. We use the term diameter class because sometimes the “diameter” of the pipe fitting isn’t really the actual diameter of the inside of the fitting but rather just a label. For example, 1.5″ NPT pipe fittings have an inner diameter of 1.9″.

Material is the stuff the fitting is made out of. Common household pipe fittings are made of polyvinyl chloride (PVC), which is usually a white plastic. Brass and steel or iron fittings are also common in houses. The material depends on the pressure, temperature, and chemical environment the pipe fitting will be exposed to. For example, a manuclave, which sees high pressures, needs stainless steel 304 or stainless steel 316 fittings, where a household steam pipe which sees much lower pressures could be made of common steel or iron.

Pressure and temperature rating determines how thick the pipe fitting will be and the type of service conditions the fitting is safe for. Higher pressure pipe fittings of a given form factor have larger outer diameters relative to lower pressure pipe fittings. Nipples are usually thinner than tees and crosses rated for the same pressure because of the nature of the stresses that result in cylindrical shapes versus sharp corners. The safe operating pressure of a fitting DECREASES with increasing temperature, so keep this in mind! Sometimes fittings are rated by pressure class (for example, Class 1500), but the class number is not the pressure rating-you have to look up the pressure rating for that fitting in that class!

For the manuclaves you’ll build on Aerogel.org, the fittings will have the following parameters:

  • Thread Type: NPT (National Pipe Thread) pipe fittings (note: this is different from NPTF, which don’t fit with standard NPT fittings)
  • Material: Stainless steel 304 or 316
  • Pressure Rating: Class 3000 or Class 6000

Valves

Valves are devices that let you control the flow of fluids (gases or liquids) into a fitting. A sink faucet is a valve. A gas stove knob is a valve. A propane tank valve is a valve. Common valve types include:

  • Ball valves
  • Needle valves
  • Solenoid valves
  • Pressure relief valves

Ball valves are generally high-flow valves. There is a little ball with a hole in it that allows fluid to flow through when turned to align with the direction of the flow. Ball valves are really meant to give all-or-nothing flow control, although if you crack them a little bit you can control the flow somewhat. Ball valves usually open with a quarter-turn of the handle.

Needle valves are generally low-flow valves for when you need to control flow carefully. They have a thin cylindrical shaft inside that blocks flow through the valve until it is screwed up. Needle valves are usually multi-turn valves which gives you precise control over the flow going through them.

Solenoid valves are magnetically-actuated valves which are either open or closed. They contain a metal shaft that blocks flow until it is sucked up by an electromagnet (normally closed type) or contain a metal shaft that is inserted into the way of the flow by an electromagnet (normally open type).

Pressure relief valves are devices that automatically release pressure if a certain preset pressure is reached. We’ve included of these on the manuclave designs on Aerogel.org to make sure your manuclave doesn’t ever get above safe operating conditions.

Gauges

Gauges are devices that measure a property of the fluid inside the pressure vessel. A tire gauge is a guage that tells you pressure inside of a tire. A thermometer is gauge that tells you temperature.

Any pressure vessel should be equipped with both a pressure gauge and thermometer, since the vessel can only withstand a certain pressure and temperature and, in the case of aerogels, you need to know both of these values to do supercritical drying!

Threaded pressure gauges and bimetal thermometers designed to screw into pipe fittings are easy to find. Again you will need to know the thread type, diameter class, and rating for your pressure vessel.

The Safety Factor

At risk of belaboring the point, only use fittings, valves, and gauges rated for the proper pressure and temperature ratings! We have included a safety factor in our designs, that is, we have multiplied the maximum operating pressure and temperature the manuclave needs to withstand by a factor of at least 133% to account for cycle fatigue from repeated use and unknown unknows (unk-unks) that may arise during operation. Furthermore, the fittings themselves have been overdesigned with a safety factor as well to ensure they can withstand at least the pressure they are rated for.

If you’re interested in how engineers determine the thickness of a pipe fitting, do a search for “mechanics of materials and pressure vessels” and “Lame problem” (say “le may”).

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