Wiring new shop compressor

[rls1160]

Thank you all for the insights!

This morning I got it all figured out. I was not able to get the coil to energize. It turned out I just needed to flip the pressure switch hot from L2 to L1. That gave me 240V to the coil and the coil engaged and the motor ran.

View attachment 20240813_101113.mp4

I had to swap two T2 and T3 to get the motor rotation correct. Once I had all the piping sealed, I was able letbthe compressor run until the pressure switch kicked out at 145 PSI's and shut down the compressor.

I leak checked all the fittings and tightened up a few leaders.

I am still trying to figure out how to wire in that auxiliary on/off switch.

Now I just need to button up.boxes and finish up a bit of conduit and reconnect everything in the junction box.

Thank you all,

Rick

 

[mksj]

I have my on/off switch in series with the pressure switch going to the contactor coil. You should have a separate power disconnect for the compressor and I assume it is hardwired. I also have a lighted power switch (240V) so on a separate switch block to give me a visual indicator that the compressor is ON, i.e. the contactor coil is active. One issue that I could see is if the compressor switch is on, but the pressure switch is off, you then turn off the RPC (assuming it disconnects the pass through legs and generated leg). You then either have some air line leakage or use the tank air and the pressure switch activates. Turning on the RPC now will directly activate the compressor and may stall out the RPC or prevent it from coming up to speed. There are a number of ways to put a delay into the circuit on power up/trigger circuit.

 

[rls1160]

I have my on/off switch in series with the pressure switch going to the contactor coil. You should have a separate power disconnect for the compressor and I assume it is hardwired. I also have a lighted power switch (240V) so on a separate switch block to give me a visual indicator that the compressor is ON, i.e. the contactor coil is active. One issue that I could see is if the compressor switch is on, but the pressure switch is off, you then turn off the RPC (assuming it disconnects the pass through legs and generated leg). You then either have some air line leakage or use the tank air and the pressure switch activates. Turning on the RPC now will directly activate the compressor and may stall out the RPC or prevent it from coming up to speed. There are a number of ways to put a delay into the circuit on power up/trigger circuit.

I am going to try and wire in series the on/off run switch that is already incorporated into the motor starter box.

Having a disable switch between the pressure switch and contactor should eliminate any issues with unintended start of the compressor.


I have a main disconnect between my RPC and the 3-phase panel. My procedure is always to turn off the disconnect then the RPC. The reverse is done to power on a piece of equipment.

I am a one person shop. No one else works in this shop or with me. Currently my machines are connected via SOOW cord. I am still in the process of setting up the shop workflow so things are a bit temporary except for the air compressor.

That is the only machine thus far that is hard wired.


Rick

 

[rwdenney]

Not addressing the wiring question at all, which seems to already be in hand.

But something jumped out at me reading the thread for the first time this morning--a pressure switch with an upper limit of 175 psi in a system with a 175-psi pressure-relief valve and a 200-psi tank. I know these 2-stage 80-gallon-tank compressors routinely run to 175 psi, but there's no way in the world I'd run the pressure anywhere near the rating on the pop-off valve, if I'm understanding what you meant. Maybe 2/3 of that value, but no more. Those valves are not intended to be accurate and show variability from one to the next of up to +/- 10%. If the tank's maximum working pressure is 250 psi, the safety relief valve might be rated at 200 psi, and then an upper limit on the pressure switch of 175 psi isn't that likely to open the safety valve (which should never open in practice). If the maximum working pressure of the tank is 200 psi, and the safety valve is 175 psi, then I'd regulate the working pressure to be no higher than 150 psi. Check with the tag on the tank.

TL;DR: The upper limit of the working pressure switch should be well (>10%) below the rating of the safety relief valve, and that should be at least 10% below the maximum working pressure of the tank. Opinions vary on the last point; that's mine, given that I'm not qualified to do the engineering necessary to push reasonable margins.

Edit: A later post said your working pressure switched shut down the motor at 145 psi. For a 175-psi relief valve on a 200-psi tank, that sounds about right to me.

Rick "whose compressor is set to 125 psi max, and then regulated down to 100 psi max" Denney

 

[rls1160]

Edit: A later post said your working pressure switched shut down the motor at 145 psi. For a 175-psi relief valve on a 200-psi tank, that sounds about right to me.

@rwdenny

Attached is the tank certification tag.



If I understand this correctly the maximum working air pressure on this tank is 200 psi. However; I do not know what every designation on this tag means.

Yes the relief valves on the high pressure side of things are calibrated at 175psi.

The current pressure switch is rated at 125 psi. I wanted to replace it with a higher rated switch (new switch has a max rating of 175 psi).

I was not intending to set the pressure switch to 175 psi. As you guys suggested it would be unsafe and counter productive to have both tank and relief pressure at the same value.


I am looking to get 140 to 145 psi.

I hope that clarifies what I was attempting to do.

I very much appreciate the heads up on the the safety factor pressure targets.

If you see anything with the tank tag information that suggests a lower pressure setting please let me know.

Thank you,

Rick

 

[mksj]

Specifications for the the QT-7.5 is the tank relief valve is 200 PSI (Part #110513-200), if replaced with the correct relief valve no reason not to use an upper pressure shutoff of 175 PSI and decrease cycling, unless the motor is undersized. Factory settings for the pressure switch is on at 135 PSI and off at 175 PSI. Also pressure relief valve in discharge line is set to 215 PSI (Part # 111089-215), the last 3 digits are the pressure rating. Maximum allowable working pressure (MAWP) is an American Society of Mechanical Engineers (ASME) designation that establishes the rating for pressure-relief components on vessels, so there is no reason to not use a 175 PSI upper limit unless there are concerns of tank/component issues.

 

[rls1160]

Specifications for the the QT-7.5 is the tank relief valve is 200 PSI (Part #110513-200), if replaced with the correct relief valve no reason not to use an upper pressure shutoff of 175 PSI and decrease cycling, unless the motor is undersized. Factory settings for the pressure switch is on at 135 PSI and off at 175 PSI. Also pressure relief valve in discharge line is set to 215 PSI (Part # 111089-215), the last 3 digits are the pressure rating. Maximum allowable working pressure (MAWP) is an American Society of Mechanical Engineers (ASME) designation that establishes the rating for pressure-relief components on vessels, so there is no reason to not use a 175 PSI upper limit unless there are concerns of tank/component issues.

@mksj. Thank you so much for detailed specifications and relief valve part numbers. Good to know I can rin up to 175 psi with the proper equipment installed.

Much appreciated!

Rick

 

[rwdenney]

@rwdenny

Attached is the tank certification tag.

View attachment 499800

If I understand this correctly the maximum working air pressure on this tank is 200 psi. However; I do not know what every designation on this tag means.

Yes the relief valves on the high pressure side of things are calibrated at 175psi.

The current pressure switch is rated at 125 psi. I wanted to replace it with a higher rated switch (new switch has a max rating of 175 psi).

I was not intending to set the pressure switch to 175 psi. As you guys suggested it would be unsafe and counter productive to have both tank and relief pressure at the same value.


I am looking to get 140 to 145 psi.

I hope that clarifies what I was attempting to do.

I very much appreciate the heads up on the the safety factor pressure targets.

If you see anything with the tank tag information that suggests a lower pressure setting please let me know.

Thank you,

Rick

It's not uncommon for the safety relief valve to be set at the same level as the maximum working pressure of the tank--that value also includes a safety margin. The problem is not the risk that the tank will explode, the problem is that the safety relief valve might vary 10% one way or the other. If it opens at, say, 90% of 200 psi, or 180 psi, and your upper pressure limit is 175 psi (which is itself subject to some variability), then you can expect at some point the safety relief valve might open unnecessarily.

I think your plan as stated is reasonable.

Found this on Garage Journal:

(from the National Board of Boiler and Pressure Vessel Inspectors):

An air receiver must be protected from over-pressure. This is usually accomplished by means of a spring-loaded pressure relief device. The set pressure of the pressure relief device must not exceed the MAWP marked on the air receiver. The minimum relieving capacity of the pressure relief device must meet the requirements of ASME BPV Code Section VIII, Div. 1, paragraph UG-125. Under most circumstances this would require a pressure relief device with enough relieving capacity to prevent the pressure in the air receiver from rising more than 10% or 3 psi, whichever is greater, above the MAWP marked on the air receiver. For an inspector familiar with boiler nameplates which indicate either the maximum generating capacity of the boiler or the minimum required relief valve capacity, verifying the capacity of a pressure relief device on an air receiver will be more of a challenge. The inspector needs to obtain the output of the compressor(s) supplying the air receiver. This information may be on a label or nameplate on the compressor or it may have to be obtained from the compressor manufacturer's published specifications.

The so-called maximum working pressure is sort-of like a speed limit--going over it is illegal but probably not fatal. The burst strength of the tank will be significantly higher. Note that a lot of what's written about pressure relief valves has to do with flow once they open rather than the pressure at which they open--if they don't move air fast enough, they might not be able to keep up aggressively enough with a runaway compressor. That's what the last half of the above paragraph is referring to--sizing the flow of the valve per the capacity of the compressor.

Yes, I know that lots of 2-stage, 5-HP+ compressors sold in stores have 175-psi ratings with tanks that have a MAWP of 200 psi and that have 200-psi safety relieve valves. But if it's a 175-psi safety relief valve as I thought I read in the OP, there is a non-zero possibility that it will open at, say, 162 psi. Keeping the upper limit of the running pressure switch low enough that the potential variabilities don't overlap prevents unexpected opening of the safety relief valve.

Rick "whose 5-HP compressor is single-stage and runs at a lot lower pressure" Denney

 

[rls1160]

Rick "whose 5-HP compressor is single-stage and runs at a lot lower pressure" Denney

@rwdenney

I am the OP (Rick), my compressor is a QT-7.5 200. It is a 3-phase, 2 stage compressor; however it does not have a 7.5 HP motor, it has a 5.0 hp 3-phase motor.

I discussed the following with the local Quincy dealer:

Would there be an issue running the compressor with 5hp versus 7.5hp?

Ans. The hp relates to the amount of air flow (CFM) the compressor would be able to push. The factor used for calculating estimated airflow based on this compressor is 4 -5 CFM/ every 1hp.

So, taking the midpoint of the range, a 7.5hp motor would produce 33.75 CFM

The 5 hp motor would produce 22.5 CFM

I imagine the two variables of CFM and PSI output could vary based on HP.

Please note that my shop 3-phase power is being generated from a 10HP American Rotary ADX Phase converter. So, a 7.5hp motor on this compressor may not run with my RPC. That is why I purchased this particular unit, and why am making the modifications I listed previously.

I am not looking for exact performance. I am looking to maximize what I can get from the set up I have within acceptable safety factors.

I appreciate everyone's input you have all helped me in a big way get this unit up and running. And also keeping me and my shop safe.

Everyone have a great day!

Thank you!

Rick

 

[tq60]

This may have been answered, my input.

You have a magnetic starter with remote control from the pressure switch.

There is a photo of something with knob and 2 terminals with only one wire, that may be protection, so determine what that is.

For interlocking the compressor to something else, that is easy.

Switching to ours.

We have 7.5 hp champion, similar control except ours is single phase.

First mod was to add a small toggle switch to enclosure for the starter.

The wire from the pressure switch that controls the starter or contactor is cut and routed through the switch.

Can control 7.5 hp with little toggle switch.

Next, keep forgetting to turn off when exiting shop, hose reels leak so pg&e reminds us something is on via the bill, so added a standard 120 VAC outlet next to the 50 Amp 240 VAC outlet and connect this to the lighting circuit.

Lights on hot, lights off, dead.

Get a relay with 120 VAC coil and place it inside the starter.

The control wire that WAS connected to the switch is now objected to the common contact of relay.

A new wire is added from the cormally open contact of relay to where the other wire was removed from the switch.

Wire a standard power cord to the coil of the relay and plug into the outlet controlled by light switch.

Lights on, compressor ENABLED, switch must ALSO BE ON for compressor to operate.


Lights out, compressor disabled.

Next will be to replace the toggle switch with on-off-on to allow for normal on, off, or interlocked with Lights.

Given that we do not plan on selling it or needing air in the dark, it will likely never be done.

Shift to your stuff...

Interlock your RPC to the light switch, if t does not already have a small type of switch to control it, a magnetic starter can be used, just be sure the internal coil voltage is 120 VAC, otherwise you will need an adaptor.

If you already have a magnetic starter with remote switches, then just add the relay connection to light switch.

This will now kill everything 3 phase when Lights go out.

If you want to then add secondary control to the compressor, locate the control or supply wire between pressure switch and contactor coil and add a switch to the local housing as master, then you can add a relay with external control for the relay wherever that is needed.

Multi-point would require a series circuit.

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