How does union joint with two male threaded ends work? Does one male end rotate 360 degrees around it axis? How is the inside of the union constructed? Can I connect two pipes with female threads using a union joint and assemble/disassemble it without too much pain? Specifically, I am looking at a stainless steel union joint.
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Typically, most threads on pipes (in the USA at least) are male NPT and all the couplings or fittings are female NPT.
You can connect two pipes with a coupling as long as the last pipe added is free to rotate about its axis.
A union is used for easy removal of pumps and other serviceable equipment that would otherwise not be easily unthreaded. It is also used to attach pipes in which neither pipe can not be rotated about its axis (perhaps because it is assembled to other plumbing, etc). The following image is the best section view of a union I could find. Most of the ones I have used have a flat steel face that mates to a flat steel face with an oring (with no 45 degree bevel like shown in the image). The flat faces allow the equipment to easily be removed sideways out of the plumbing when the union is detached.
Stainless Steel Unions with datasheet links
Most hardware/plumbing stores sell pipe unions and will let you take them apart to see if they fit your needs. Seeing them first hand is the best way to get a feel for how they work.
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$\begingroup$ For what is that 45 degree beval. Do I need O-ring if there is 45 degree beval inside ? $\endgroup$– Sysrq147Commented Apr 4, 2016 at 14:03
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2$\begingroup$ The bevel is properly called a flare in the pressure piping world. It is generally meant to form a metal-to-metal seal with not O-ring in it. $\endgroup$– Ethan48Commented Apr 4, 2016 at 19:08
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$\begingroup$ Thank you ericnutsch and @Ethan48 for Your help and time :) $\endgroup$– Sysrq147Commented Apr 4, 2016 at 20:27