Installing a pilot unloader/continuous run valve on an electric compressor?

[Bob_McBob]

I recently set up a DV 7.5hp compressor, which is used primarily to run an abrasive blast cabinet. The "issue" I'm having is that even with the largest nozzle option the manufacturer offers and a high pressure setting, the compressor has no problem whatsoever keeping up with the output, so it stops and starts frequently during use. The motor has a somewhat hard start due to the phase converter being a little undersized (the manufacturer changed the rated recommendations in later revisions), so I'd like to avoid frequent starts if possible.

I've read that electric compressors can be used with a pilot unloader valve like a gas compressor, allowing it to run continuously and bleed off excess pressure. What I'm unclear on is how the valve would be physically installed on an electric compressor, since the existing unloader valve is much smaller, and actuated internally by the pressure switch.

How It Works: Piloted Unloader Valve - Rolair Systems

Learn about piloted unloader valves and how they work though this explanation of their components, and adjustment process.

www.rolair.com

 

[The Tool Tyrant]

Here is a little more info regarding your question Bob...

 

[TdK71]

Better check on the duty cycle of you motor and pump, I'm pretty sure they aren't rated for continuous use.

 

[GeoBruin]

Better check on the duty cycle of you motor and pump, I'm pretty sure they aren't rated for continuous use.

I'm curious why you would say that, and especially how you could be "pretty sure" of it? OP has a big, nice, compressor making so much air he can't even use it all and you think it isn't rated for continuous use?

 

[TdK71]

I'm curious why you would say that, and especially how you could be "pretty sure" of it? OP has a big, nice, compressor making so much air he can't even use it all and you think it isn't rated for continuous use?

if this is what he has it's only rated at a 60% Duty cycle.
https://www.dvsystems.com/en-ca/air-compressors/reciprocating-compressors/5-10-hp/is7

 

[GeoBruin]

if this is what he has it's only rated at a 60% Duty cycle.
https://www.dvsystems.com/en-ca/air-compressors/reciprocating-compressors/5-10-hp/is7

According to his post history, he has the HDI model vs the standard duty (SDI) model so it's rated at 80% duty cycle. But in either case, this is new for me because even the motor on my harbor freight 60 gallon compressor is rated for "continuous" duty so that 80% rating must be for the motor/pump "system" and though I can't find it, I'm sure that rating is at a certain pressure or flow rate.

So regarding the OP's question about a piloted unloader valve, does the valve only start bleeding air to atmosphere at what would normally be the cut out pressure on the pressure switch? Or can it be set for something lower? So if he's making close to 30 cfm and typically the cutout pressure is 175 psi, could he set the unloader to something like 125 psi so the pump can "idle" along at the lower pressure and lower the effective duty?

 

[The Tool Tyrant]

Bob, I have installed continuous run pilot set-ups on small single stage units like the Emglo in the video and also large 2 stage Quincy's. The small single stage units work fine unloading through the pilot valve as they don't make high CFM. The QR -25 series Quincy's I installed them on have hydraulic unloaders, so the pilot valve just redirects air pressure to actuate the unloader pins in the suction valves...basically like putting your car in neutral with the engine running. I have never added dual-run to a high CFM pump that unloads via the pressure switch unloader valve. My concern would be making sure that the pilot valve that you use is capable of discharging enough CFM to match what your pump is producing so as not to create back pressure to the pump.

 

[The Tool Tyrant]

To add to GeoBruin's post, don't forget that the OP stated that the compressor is cycling a lot while he is sandblasting, so it's apparent that it is not working at 100% duty cycle anyway. 'Duty cycle' is when the pump is actually working to make pressure, so if it's free wheeling, it wouldn't be considered part of the duty cycle just because it's spinning. In fact the time that it's free wheeling helps to cool it down as the air flow from the sheave cools the pump.

 

[stonesfan68]

One poor man's option would be to open up the drain valve at the bottom of the tank.

 

[GeoBruin]

To add to GeoBruin's post, don't forget that the OP stated that the compressor is cycling a lot while he is sandblasting, so it's apparent that it is not working at 100% duty cycle anyway. 'Duty cycle' is when the pump is actually working to make pressure, so if it's free wheeling, it wouldn't be considered part of the duty cycle just because it's spinning. In fact the time that it's free wheeling helps to cool it down as the air flow from the sheave cools the pump.

See that's the part I'm trying to figure out. To me, "free wheeling" means the pump is just dumping air to atmosphere against no pressure gradient. So if the pilot unloader works like I'm envisioning, it knows when the tank is at some minimum pressure (maybe the same as the cutout pressure, maybe not?) and opens a valve, allowing air flowing from the pump outlet to vent, thus bypassing thr tank inlet. In that case, it is indeed "freewheeling". Once the tanks drains below the set pressure, the valve closes and air is directed into the tank.

Pretty cool.

 

[The Tool Tyrant]

See that's the part I'm trying to figure out. To me, "free wheeling" means the pump is just dumping air to atmosphere against no pressure gradient. So if the pilot unloader works like I'm envisioning, it knows when the tank is at some minimum pressure (maybe the same as the cutout pressure, maybe not?) and opens a valve, allowing air flowing from the pump outlet to vent, thus bypassing thr tank inlet. In that case, it is indeed "freewheeling". Once the tanks drains below the set pressure, the valve closes and air is directed into the tank.

Pretty cool.

That is correct. Most (but not all) pilot valves are adjustable for cut-in / cut-out pressures (within the parameters of the valve spring tension) and you would adjust the cut-out pressure @ approx. 7 PSI LOWER than the cut-out pressure of the pressure switch...otherwise the pressure switch would kill power to the motor before the pilot valve reached it's cut-out pressure.

 

[jayshoppa]

Hi guys this is my first post, please advise if it should be a new thread etc.
Based on the knowledge evident here I thought I'd post about this old IR i'm working on . . .

So it's a simple (old) ((heavy duty)) 5" bore single stage I'm really looking forward to running-
Problems have been a complete failure to identify it enough to find some data about rpm, hp, cfm, and also to even begin to check for any parts availability. Image search hasn't even turned up anything similar.
Based on other compressors I might guess to try a good 5hp motor and 700rpm to start, and I guess to shoot for about 130psi.
Overall condition is great and tight, and until I have more info I will just give cylinder a light hone and fab a new felt crank seal.
____________
To the point TLDR
This pump came with a type of head unloader valve that appears in good operational condition but was disconnected.
It seems like when the unloader piston is pushed down to overcome the spring between it and the valve, it seats a 'hat' ontop the intake valve, effectively closing the valve intake.

From what I read that is opposite quincy and wabco type unloaders that hold intake valve(s) open?

It would seem the suction created wouldn't be as effective at unloading for continuous as just opening the intake valve. Was this ever a "thing" ?

The pump also didn't come with a diaphram/valve for load/unload adjustment but I assume I can add any of those in that can adjust for single stage pressures.

Any input is greatly appreciated!

Since I've found nothing on this machine, am uploading a few vids of it torn down and valve/piston/crank internals etc.

 

[jayshoppa]

To follow up Ken @ Airflo found a manual for it straight away!
The model is simply '5' or '5E' for electric. 1938. There isn't any guidelines on HP or rpm but that's ok.
It is the precursor to more well known 2 stage 'model 41' that was equipped on the rear of 1942 Cletrac M1 crawlers (air force).


About unloaders, the parts diagram confirms this is the original SV unloader, all parts are accounted for (Only 2 springs, hat, and piston), and the design actually does unload by closing air intake of SV (opposite all others I'm aware of).

Looking forward to getting to testing.
Will post up over in 'vintage compressors' then!

/edit - oh and it weighs 220lb dry . . . has 22k1 disc valves