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The relief valve should be a spring loaded "pop safety" valve that reseats automatically after some reduction in the tank pressure from the valve set point - technically referred to as "blowdown". Big safety valves typically have a blowdown of 5% or less. I have never seen typical blowdown figures for the little ones commonly provided for the size tanks we are talking about here (say 80 gal or less). I would guess that they would reseat automatically at some tank pressure 90% or greater than the valve set pressure (no more than 10% blowdown). Say if your single stage tank (150 MAWP) is protected by a valve set at 150 psig, then I would expect the valve to automatically reseat (shut) when the tank pressure drops to no less than about 135 psi. It shouldn't blow at all though with the check valve installed in the tank outlet - that's the reason for the check valve. Safety valves are always considered a last resort in safety of a pressure vessel. Other means should be provided (in this case, the check valve) to prevent intentional overpressure. Could you be safe and overpressurize the tank by 20%? Probably, but no responsible person would recommend doing it intentionally. There is a MAWP stamped on the tank nameplate for a reason. Not to be exceeded. Exceeding that pressure intentionally will encrouch on the safety margin inherent in rating such equipment. In a commercial setting in most states it also would likely be illegal to do so. If your compressors were in a commercial setting, your state likely requires annual inspection and written certification by a qualified state agent or insurance company representative for their safe operation. An installation that permitted such an overpressure would be condemned and would never get an authority to operate. I do not recommend a deliberate overpressure of any amount. It's sort of like the isocyanate issue and painting - how much is too much? Can I do just one tractor with only a charcoal mask? Huh, huh, huh? Violate written safety directions at your own risk :o). I do recommend that once in a while (say annually) you intentionally trip the safety valve with your finger using the little ring - that's what it's for. Do this when the tank is at a high pressure, say just after compressor shutoff. It is a simple check to verify that the valve is not stuck shut for some reason. Just watch your eyes for flying dust when you do that. You might also want to use some hearing protection also - it'll be loud. I doubt you will have a problem with the 3/4" IPS piping (probably standard sched 40). At 50 scfm and 125 psig line pressure the drop is less than 2 psi per 100 ft, not including fittings. If you haven't already done so, I would suggest installing your main regulator at the far end of your "permanent" piping, as far from your compressor as possible. Ideally it should be located right at the actual end use point but that usually is not practical. Most are on the wall where your hose connects up. This is for two reasons: 1. You are correctly using your steel piping as a "cooling medium" to promote condensation of moisture. The piping will be at tank pressure and not your reduced usage pressure. At the higher pressure, the dewpoint of the air is greater than at a lower pressure so condensation is improved (the temperature doesn't have to drop as much to effect some moisture dropout at the higher pressure) plus your water separator will be downstream where the liquid can be collected and removed prior to entering your service hose. 2. Any filters that you may have should be located upstream of your regulator. The higher pressure in that location will result in a lower pressure drop across the unit (for the same flow rate) than if it were located downstream of your regulator. Decreased pressure drop is always a good goal to maximize system performance. You may have something like the a combination separator, filter and regulator such as that marketed by Sharp. That's fine but the best location for it is at the end of your piping run just prior to your hose connection. I suspect you have seen the suggested piping layout at the Sharp website. It's a pretty good one, although I would prefer to come off the top of the header with the drop(s) rather than the bottom as shown. Didn't mean for this to be so long. Just got carried away, I guess.
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