Sanborn compressor pump question

[zKen]

I bought this Sanborn air compressor model G500BPL60H at an auction.

There is a fitting on the compressor next to the airline that charges the tank. This fitting shoots air with every stroke of the piston.

I am at a loss to determine the purpose of the fitting. I downloaded a user manual for the model *500BPL60V (V for vertical tank, mine is H for horizontal tank), which identifies the fitting as connecting to the following very well described parts:
• 145-0084 Tube B-B (this is a ¼ inch airline)
• 058-0017 Nut
• 068-0002 Connector.
The destination of the 145-0084 Tube B-B airline is not at all clear from the schematics in the user manual.

There is an airline from the tank to the pressure switch that I assume serves the purpose of triggering the auto-off when the requisite tank pressure is reached.

I have not yet charged the tank to capacity.

I am inclined to just cap the fitting, as the tank would pressurize more efficiently when the air that shoots out the fitting is forced into the tank.
But I seek the counsel and guidance of the forum, lest I err and destroy my auction prize.

 

[MacMcMacmac]

That is the unloader port. it should be connected to a needle valve on the side of the pressure switch to blow down the line from the pump to the check valve when the compressor cycles off. If someone replaced the pressure switch with one that doesn't have the needle valve, he would have had to remove the line or the compressor would trip out the next time it tried to start against tank pressure. This is a classic case of half-assery, as there is absolutely no way this setup is correct. If you plug that port, and there is no unloader line on your current setup, you will keep tripping out the machine on re-start once there is pressure in the tank. If your current pressure switch has a working needle valve, just run a 1/4" copper line to it with some compression fittings and call it good.

 

[redmondjp]

^ What he said.

Use copper line from a hardware store to replace the missing pipe to the unloader. Don't use plastic tubing - it gets too hot at the compressor head.

 

[zKen]

Understood. Thanks MacMcMacmac and redmondjp. The pressure switch on the compressor is not the model listed in the manual. No idea if the existing switch has a needle valve.

Ordered a replacement 034-0094 SANBORN PRESSURE SWITCH 95 PSI ON 125 PSI OFF FOUR PORT UNLOADER VALVE. Then I will attach a copper line from the unloader port to one of the ports on the pressure switch.

Again, thanks.

 

[The Cobbler]

Then I will attach a copper line from the unloader port to one of the ports on the pressure switch.

Again, thanks.

you attach the unloader to the unloader valve on the switch, not to one of the 4 ports

 

[redmondjp]

Understood. Thanks MacMcMacmac and redmondjp. The pressure switch on the compressor is not the model listed in the manual. No idea if the existing switch has a needle valve.

Ordered a replacement 034-0094 SANBORN PRESSURE SWITCH 95 PSI ON 125 PSI OFF FOUR PORT UNLOADER VALVE. Then I will attach a copper line from the unloader port to one of the ports on the pressure switch.

Again, thanks.

Take a close look at the back and bottom of your existing pressure switch. If it has the built-in unloader (and it should), it will have a 1/4" diameter air line fitting somewhere on it which looks similar to the open port on the compressor head.

 

[zKen]

I found only one 1/4 inch air line connector on the switch. An existing line is attached to it, running from the tank (see first and second photos). Third photo is the backside of the switch, showing a close-up of the connector. Is this connection the unloader?

If that is the unloader that should connect to the port on the compressor, then what is the purpose of the 1/4 inch plastic air line running out of the tank?

I really do appreciate the knowledge that is shared here.

 

[Provincial]

Perhaps someone replaced the original switch with one from a water pump. Those don't have an unloader.

Go to someplace that sells compressors and compressor parts. They will fix you up with a switch that has an unloader.

Here is what one looks like. The unloader is the shiny part on the side of the black plastic case.

 

[MacMcMacmac]

I found only one 1/4 inch air line connector on the switch. An existing line is attached to it, running from the tank (see first and second photos). Third photo is the backside of the switch, showing a close-up of the connector. Is this connection the unloader?

If that is the unloader that should connect to the port on the compressor, then what is the purpose of the 1/4 inch plastic air line running out of the tank?

I really do appreciate the knowledge that is shared here.

It looks like it's plumbed correctly. Drain the tank to zero, plug the open port on the head and let the compressor run to cut out. If you hear a short blast of air from the valve on the pressure switch, you are good to go. If not, you might need to replace the switch with a new one.

 

[gungatim]

definitely looks like someone replaced the pump and motor on that tank. perhaps the original unloader went to a combination check valve, while the replacement pump unloads through an integral port.

hard to tell from the pics, is that inlet pipe soldered to the tank or what?

ultimately you want the pressure in the tank to stay there and not bleed back to the pump (check valve).

then you want the pressure from the pump through the line to the check valve to be relieved before the pump starts up. you can verify if that is happening before you tear into it, maybe it is workable as is with just a pipe plug in that extra port...

 

[The Cobbler]

I agree with the replacement pump and motor, especially with the writing on the motor & the clean bolts on the pump make it even more likely.
given that, the pump on there now doesn't need the unloader port that comes from the head , it can be plugged, just be sure the unloader circuit there now is working properly .

 

[zKen]

I capped the unloader port and charged the tank. Seems to work just fine. It shuts off at 125ish psi and kicks in at 95.

Now I can attach the belt shroud and level the thing off on the dirt floor. Concrete is getting higher on the priority list.

My new switch has arrived. I guess I'll save it for later use.

Thanks all for the help!