# How do I limit the pressure of a tank to a fixed PSI?

I previously asked about the safety of connecting my 150 PSI air compressor to my auxiliary 120 PSI tank. I was warned of possible compromise of the tank which can lead to injuries and even death. It was strongly recommended that I use a pressure regulator between the two tanks.

• Maximum pressure (PSI) 150PSI
• Air Delivery 3.5 SCFM @ 40PSI
• Air Delivery 2.6 SCFM @ 90PSI
• Compressor Tank Capacity 6 Gallons
• Decibel Rating (Outdoor) 78dBA (one of the reasons for this setup)

• Maximum pressure (PSI) 125 PSI
• Tank Capacity 11 Gallons

My main problem with using an adjustable pressure regulator is that the user can inadvertently exceed 120 PSI using the knob. The innocent user shouldn't be pelted with shrapnel for going past 120PSI. Most regulators available well exceed this maximum.

I was unsuccessful in finding a fixed pressure regulator, at least not for a small air compressor. Perhaps it has a different name? Would a double stage regulator serve the same purpose?

• Does this answer your question? Is it safe to use a 125PSI rated auxiliary air tank with a 150PSI air compressor? – Solar Mike Aug 10 '20 at 20:17
• That was my original question, but since then, I've been looking for something more idiot proof. The answers I got read like an OSHA approved horror novel. – user148298 Aug 10 '20 at 20:22
• And for good reason. – Solar Mike Aug 10 '20 at 20:24
• One thought is to have a regulator that needs a wrench to adjust the spring instead of having a knob. Couple options when I searched for "preset pressure regulator, 120 psi": mcmaster.com/air-compressor-regulators/… store.epsgroupllc.com/… – J. Ari Aug 10 '20 at 20:41
• @J.Ari Wow! They're more expensive than the more complicated regulated ones. This puts me in the strange predicament of choosing between shrapnel shower or groceries. – user148298 Aug 10 '20 at 21:05

In industrial settings, the correct piece of equipment is referred to as either a Pressure Relief Valve (PRV) or Pressure Safety Valve (PSV). These are spring-loaded valves with a few specific characteristics

1. The valve is designed with a spring load, and is ordered with a relief pressure in mind (in your case, 120 PSI)
2. the valve is sized with a valve coefficient + size appropriate to relieve the amount of flow you have in your worst-case-scenario, minus a safety factor (I'm not a design engineer, but generally 10-20% I believe)

There are numerous companies that make PRV's/PSV's, but to order one you will need to know at a minimum;

1. Your maximum volumetric air flow rate possible in the system
2. the pressure you want to relieve at
3. the +/- % tolerance you have (if your target is 120 PSI, can you use 120 PSI +/- 10PSI, or do you need 120 PSI +/- 1 PSI, or even tighter tolerances

You will also want to have a Pressure Reducing Valve or Pressure Regulator (as mart mentioned above) to intentionally control the pressure in your secondary vessel to 120 PSI, as relief valves are not made or intended to be used as a typical control device; They are intended to be emergency or redundancy devices.

For your second question about the pressure regulator, I would be shocked if you couldn't find a pressure regulator the size you're looking for, but in theory there's no reason you couldn't use a throttling valve followed by a pressure regulator I suppose.

• This is the answer, as far as I'm concerned - add a second PRV on top of the existing safety, so that you don't have a single-point-of-failure. – Jonathan R Swift Aug 11 '20 at 14:45
• A second PRV on top of the existing one would mean that if one fails, no pressure relief happens! – mart Aug 12 '20 at 7:29
• @mart sorry for the confusion - I should clarify what I meant in my answer. I meant that from the primary holding vessel / compressor at 150 PSI, there should be a regulator keeping the pressure going INTO the second vessel at or below 120 PSI, and ON the second vessel there should be a PRV in the event of user error w/ the regulator, over-pressuring the second vessel. – ChemE mang Aug 12 '20 at 12:43

I understand your setup as follows (in flow direction):

• compressor with cutout set to 150 PSI
• tank (T1) rated for compressor, with safety valve
• (planned) pressure regulator (PR)
• additional tank (T2), rated 120 PSI, with safety valve

If this is so the safety valve of the second tank will protect you unless you tinkered with or replaced this safety valve.

Now, for the planned pressure regulator, check the flow rate you need and what the compressor delivers. Find the pressure differential ($$\Delta p$$, I'd expect 0.5 bar / 7.3 PSI) of the PR at this flow rate. 120 PSI + $$\Delta p$$ should be below the cutout point of the compressor, or you wont fill your T2.

At startup, the compressor will fill T1, at ~127 PSI (or whatever depending on the PR) T2 will fill until 120 PSI are reached, then PR will close, pressure in T1 reaches cutout point.

Depending on the cutin point of the compressor and relative sizes of T1 and T2 you may get frequent on/off cycles. PR is an additional pressure loss in your system.

The ultimate problem you want to solve appears to be frequent, noisy runtime of the motor. An additional tank won't change the total runtime for a given air demand much, at best it will make longer uptimes and longer downtimes (a good thing in itself IMO).

If someone tinkers with the PR, the worst I see happening is the safety valve venting provided no one tinkered with the safety valve and inefficient operation of the compressor.

However I'd advise a preset pressure regulator.

Thoroughly check if the setup I assume is indeed what you have. Wait with building the thing until the hive mind here had the time to check my reasoning and find possible flaws.

In the long run, a compressor station sized for your application may be more economical.

ETA: The cutin point for the compressor is stated to be at 120 PSI, with the PR set to 120 PSI it is unlikely the pressure in Tank 1 will fall to 120PSI after the initial filling. See if your tools can operate at 100 PSI: 100 PSI in T2 + loss in PR = approx. 107 PSI + plus some safety to account for inaccuracies should be below the cutin point (if that can't be adjusted).

Let me restate that you should not, in any way, fiddle with the safety valves. Safety aside, for the setup to work you need a lower pressure in the second tank.

• My compressor's cutoff is at 150PSI and the cutin is at 120PSI. – user148298 Aug 11 '20 at 17:17
• I added some more data to my question such as volume and CFM. – user148298 Aug 11 '20 at 18:01

Set the air compressor cutin/cutout switch to 120psi. cutout pressure.
Done.

• Two problems. The compressor is a porter cable and the cutin/cutout can't be set. Even if you could, setting it to 120PSI for both would cause the compressor to run all the time. – user148298 Aug 10 '20 at 20:50
• All air compressors have primary controls. If the output of the compressor is directed to two tanks it will shut down at 120psi. Get some help from a friend on this one. What's "for both?" Both what? – RaSullivan Aug 10 '20 at 21:05
• If you set the cut-in pressure to 120PSI and the cut-out to 120PSI, the motor will turn off as soon as it gets up to 120PSI. It will also turn off as soon as it falls to down to 120PSI. The pressure switch works within a range. The porter cable doesn't allow you to change this range, but some other compressors do. – user148298 Aug 10 '20 at 21:09
• My answer mentions setting the cutout pressure, obviously the cutin pressure has to be lower, afterall, it's a differential switch. Your tank mounted, I assume it's tank mounted, compressor should have a gauge port in it, and the receiver, or tank, will have a pressure relief port. The gauge port can be fitted with a tee, and you can add a differential pressure switch at the gauge port. Without knowing the application, flow rates, etc., the addition of a second tank would decrease the run time of the compressor under most circumstances. – RaSullivan Aug 10 '20 at 22:07
• Interesting. I was under the impression that a pressure switch is a sensor that only turns off the motor and doesn't regulate air pressure on its own. – user148298 Aug 11 '20 at 0:16