Needle valve on the cheap?

I need a needle valve or similar to adjust some pressure with more or less precision. Problems I am facing with commercial solutions at the moment: Needs to withstand about 300psi max pressure, must survive contionous exposure to concentrated ammonia vapour. I already wasted too much money on stainless steel valves claiming to be chemical proof and capable of 6000psi only to find out they use seals that turn to goo in the ammonia vapour and fail. And since the stuff is not really healthy, not easy to get, I would prefer something safer. Real problem is also that the thing should be able to allow for good flow rates when opened fully. Modes of operation: During the active cycle ammonia vapour needs to pass through an opening as big as possible while after that the flow must be shut off. The reverse cycle needs to be adjustable from closed to fully open with great detail to control the flow rate. Current ways to compensate: With a ball valves and extra piping I could seperate the two cycles so I won't have to bother with tiny openings but I would prefer to use only a single connection. With a bypass around the needle valve this could be accomplished better but still looks ugly. Materials out of the question: Copper, brass, normal seals based on rubber, nitrile or similar. Questions: I know fully sealed in.line valves exist for LPG system and similar. Are any of these magnetic valves available for the use with ammonia? And if so: Can they be operated with a neodymium magnet instead of the coil? Even stainless steel valves rated for chemical use and high pressure fail quickly on the seals. Is there anything available that does not use soft seals but for example aluminium instead? Thinking spray pump here... One way ball valve systems like used in simple spray pumps are a great thing. With some aluminium tubing I was able to make a simple valve that with some more attention to detail might even work. But of course there no other way of moving the ball then to use a magnet and with sufficient back pressure even neodymium ones fail to get the ball in the open position and keep it there. Now getting this combine with a needle... If, instead of the ball, a magnet in a teflon shell is used on a spindle I could spin a magnet around the outside to turn the valve with great precision. Has anyone already designed such a thing or would I have to start from scratch here?

Question by Downunder35m   |  last reply