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Old 11-29-2013, 06:26 AM   #75 (permalink)
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Originally Posted by P-hack View Post
If you are bent on 3600 psi then why not use oil as a piston like I mentioned? You can use it with a direct injection pump off an old diesel or any of a myriad of inexpensive hydraulic pumps or combination of high flow/low flow (high psi) pumps. To pump gas at high pressure, the demands on machining precision go up significantly and maintenance goes up as well (and price)., plus "stages" become necessary. A hydraulic pump can run for years, and solves lots of other "sealing" and separation issues.
3gpm@1800rpm, $100
Hydraulic Gear Pump GP F10 6 1 P C 3625 PSI 4000rpm New MP Filtri JP | eBay

Even that little pump will squish 127 cu feet of gas to 3600psi in 5 hours at modest rpm. Just need a high pressure accumulator tank (a tank with a top and bottom outlets would do) and some valves and a controller.
No, I don't want 3600 PSI... This is too much for DIY and will require more precise equipment.

Let's see how "second stage" will work:

Let's assume you compressed gas to 200 Psi with compressor. Easy enough, right? You only need single stage compressor. In my case York 210.

Now I need 1200 Psi. Is it too much? not really. 1200/200=6 times compression not really a rocket science... but

Here it gets tricky. To make compression efficient you have to squeeze gas and leave as less room as possible when you retract piston or diaphragm or whatever you use.

Let's put it this way: You have cylinder/piston. when you squeeze it, gas will overcome pressure at discharge valve (check valve) and go somewhere (your fuel tank etc). All gas that is left in cylinder will re-expand and your 1 stage will have to fight with it's pressure. This is your efficiency loss. Your goal is to leave as less gas as possible after each stroke.

I am thinking about using oil as "filler" Oil will lubricate cylinder and "fill" voids, so gas will have no room left for re-expansion Excess oil will get to oil separator and I will drain it once in a while.

What to use diaphragm or piston/cylinder is a personal choice. Hydraulic cylinder is good for 10 million cycles (this is like 10-20 years of every day use). This is ideal but even if I need to change seals every 3-4 years ($30-$40 set and 1 hours of labor) this is really nothing compare to $2000 every 2-3 years bill for rebuilding 4 stage CNG compressor like Phill.

I was thinking about having hydraulic cylinder with gas on one side and hydraulic oil on the other side but I don't know what will happen in case if gas leaks to oil side?

I will have 2 cylinders one is gas (both sides) and the other is hydraulic ram(both sides). Hydraulic ram will push/pull gas cylinder. When gas is compressing on one side the other side will expand and will be filled with gas by "first stage".

Now potential leaks:
If oil leaks from hydraulic ram I will see it. If gas leaks at piston this will extent compression time. There is only one place where gas can leak out. This is failed rod seal. If gas start leaking at rod seal It will pull oil with it and I will be able to see this during inspection.

Also as an additional safety I want to put gas detector which will shut down everything.

Last edited by vlad; 11-29-2013 at 06:40 AM..
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