Air Compressor control

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daves1459
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Air Compressor control

Post by daves1459 » Sun Jul 07, 2019 8:06 pm

I'm building a Winton Brown 1 1/2" scale Westinghouse 9 1/2" air compressor for my NYC & HR 999 project. I plan to install straight air brakes to as close as practical emulate the original air brake system. One of the items to that end is a governor to regulate the air compressor. I have acquired a couple of designs that I think can be made to work adequately. However, in discussing the project with a few modelers they told me that they omit the governor and simply let the air compressor stall against main reservoir pressure. Then, when the air pressure drops after some brake applications the air compressor simply starts and runs until it again stalls. Anybody out there have experience with such an arrangement? Is there a particular steam to air piston surface area ratio that is used? For example if the boiler steam pressure varies from 80 to 100 psi with a 100 psi average and the desired main line air pressure is 60 psi is the steam piston surface area made to 6/10 that of the air piston?

Dave

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Bill Shields
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Re: Air Compressor control

Post by Bill Shields » Sun Jul 07, 2019 10:06 pm

letting the compressor stall is the easiest way to operate this system.

You will want a regulator on the discharge of the accumulator to the air brakes so that you have a consistent reaction of brakes to actuator lever.

What the set (outlet pressure to brake cylinder) pressure will be is going to have to be determined by trial and error.
Too many things going on to bother listing them.

RET
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Re: Air Compressor control

Post by RET » Mon Jul 08, 2019 7:37 am

Hi Dave,

I am assuming that what you are building is not a "simple" two cylinder compressor, but a working cross compound one. If that is the case, there are a couple of things to consider.

First, apparently people who have built successful ones in 1/8th scale have made the steam side "simple" not compound, but made the air side compound. This allows you more freedom in determining the discharge air pressure against system losses which can be high (the "simple" version can give a discharge pressure over 100 psi. if desired) but it also requires a more complex control system.

If you simply allow the compressor to "stall," you can get into the situation where the steam fully condenses in the compressor side and it fills with water. A controlled leak would mean that the compressor would run slowly all the time and potentially avoid this situation. The best solution would be to have a settable control valve on the air side that would shut off the steam to the compressor and dump the compressor air discharge to atmosphere, thus allowing the compressor to run down completely. This would eliminate the condensation problem.

The reason I'm adding my two cents to this is because I'm in the process of taking over the finishing of a 1/8th scale compound compressor which I am assembling from left over parts and it is made with the steam side in a "simple" configuration. I am assembling the steam side now. Once I get that working, I have to build the air side which will be done as a "compound" setup.

I am very impressed with the quality of workmanship in the parts that I am working on. The valving, porting, passages etc. are also complex and I'm learning a lot from it. I fully expect to have it work when I'm finished. Building the air side will be relatively simple by comparison. Judging by the quality of the parts I'm working on, even in 1/8th scale this is not a viable project for the average builder.

The ultimate goal is to finally build the same thing in 1/16th scale, but that is another project for another day. I am beginning to see how it can be done, but the air output will be reduced by a significant amount.

Richard Trounce.

daves1459
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Re: Air Compressor control

Post by daves1459 » Wed Jul 10, 2019 10:07 pm

RET,

My air compressor is actually is a single stage single cylinder design. The prototype application is 1893. I don't believe the cross compound designs arrived until 20 or more years later.

It crossed my mind that stalling the compressor would cause pipe, steam chest, and cylinder to fill up with condensate. I like the idea of some sort of relieve valve on the main air tank. I suppose a safety valve with a small blow down reset pressure would be an easy way to go.

Thanks, Dave

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Re: Air Compressor control

Post by Soot n' Cinders » Wed Jul 10, 2019 10:15 pm

One idea I've had is a safety valve on the tank plumbed to a pneumatic valve on the steam line. The safety would be plumbed to the pneumatic valve so the valve would close when the safety opens. A small leak between the safety and valve would allow the valve to open again when the safety resets.
This is how a prototype governor works, just all combined into one small appliance.
-Tristan

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Fred_V
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Re: Air Compressor control

Post by Fred_V » Thu Jul 11, 2019 7:04 am

I have a CAD drawing for an 1-1/2 scale pump governor. Send me a PM if you would like a copy.
Fred V
Pensacola, Fl.

Berkman
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Re: Air Compressor control

Post by Berkman » Thu Jul 11, 2019 8:48 am

With Tristan's idea, you could use non scale off the shelf components and hide them somewhere, perhaps in the cab or under the cab. Anyone built something like this?

RET
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Re: Air Compressor control

Post by RET » Thu Jul 11, 2019 9:58 am

Hi Dave,

Clippard should have most of what you need and you can make what they can't supply. The control valve you will likely have to make, although you should be able to modify a Clippard spool valve. Since the Clippard valves are made to work on air, you should have no problems. They may even sell a spring biased spool valve. This valve is described as follows.

Think of a spool valve with "O" rings on the spool for port sealing (like the Clippard spool valves). Now if you apply a bias spring to one end of the valve with a screw to the other end of the spring for pressure adjustment, you will have the control valve you need. Feed the non-spring end with the discharge air pressure and have a check valve in the main air line after the compressor discharge but before the control valve. When the control valve gets up to the desired pressure, the spool will shift against the spring allowing air to close the steam supply to the pump and open the dump valve on the pump air discharge in the line before the check valve. The dump valve is connected between the compressor discharge and discharge check valve. If you sketch this all out, you can see how the system would work.

As the air is used, the pressure falls and the control valve spool will move back to its initial position. This can be used to dump the air pressure on both valves, the steam supply valve and the air dump valve so the compressor will easily start up again because there will be no condensate to get rid of.

Most of the components don't have to be large and so can be tucked away out of sight or made to look like all the other piping that runs around a locomotive.

Let us know how this works out.

Richard Trounce.

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Fred_V
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Re: Air Compressor control

Post by Fred_V » Sat Jul 13, 2019 12:23 pm

Fred_V wrote:
Thu Jul 11, 2019 7:04 am
I have a CAD drawing for an 1-1/2 scale pump governor. Send me a PM if you would like a copy.
Correction, the drawing is for a 2.5" scale governor.
Fred
Fred V
Pensacola, Fl.

RET
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Re: Air Compressor control

Post by RET » Tue Jul 16, 2019 12:24 pm

Hi Dave,

In case there is any confusion, here is a sketch of the circuit I described before. The basic concept can be tailored to any compressor of any size.
Compressor schematic.jpg
This is the simple control schematic I was describing.
As you can see, there is a normally open valve in the compressor steam supply line. There is also a normally closed valve teed off the compressor discharge before the check valve. Both valves are air operated. When both valves aren't pressurized, the compressor runs. Clippard will likely have these air piloted valves. A small dump valve is all that is necessary and will probably work better.

When the control valve hits its set pressure, it applies air to both valves, causing the steam to be shut off to the compressor and the compressor discharge to be dumped to atmosphere. This way, the compressor will run until there is no more steam and then stop. Thus there will be no steam in the compressor and lines to condense and cause problems when the compressor restarts.

When the system pressure falls, the control valve will shift the other way and the control lines will be vented to atmosphere causing both valves to shift back and the compressor will run again.

The control valve will likely have to be made. What you need to make is a simple "spool" valve with sealing "O" rings on the spool. The spool needs to be biased to one side with an adjustable spring so you can set the desired pressure. When the compressor discharge pressure is applied to the the non=spring end of the spool and the pressure becomes high enough to push the spool against the spring, the "O" rings on the spool will connect the supply port to the control piping causing the steam and dump valves to shift. When the air pressure drops, the spool will shift back causing the pressure on both valves to be vented to atmosphere.

One small note. When you are using "O" rings in a sliding seal like this, make the groove so the "O" ring just slides in with no lateral play in the groove. Also cut the groove deep enough so there is only 4 to 7 thousandths radial "pinch" on the "O" ring itself. If you use the Parker recommendations, there will be way too much drag and the spool won't move easily.

Hope this helps.

Richard Trounce.

daves1459
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Re: Air Compressor control

Post by daves1459 » Wed Jul 17, 2019 9:58 pm

Hello Richard,
Thank you for taking the time to draw your schematic and it's description. I now understand your intent. I see possibilities. Do you think the control piping for the "Normally Closed Valve" could come directly from the "Main Control Valve"? If so I could make the "Normally Open Valve" look like a prototype governor and place it near the compressor as the prototype. Clippard makes a lot of valves. it seems that a pilot operated adjustable on-off valve would likely be available.

Dave

RET
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Re: Air Compressor control

Post by RET » Thu Jul 18, 2019 8:13 pm

Hi Dave,

It doesn't really matter how you route the control piping from the main control valve as long as each valve (steam supply and dump valve) responds properly to the main control valve signal. Each valve must actuate when the main control valve shifts and return to its unactuated position when the control valve shifts back and dumps the pressure. The control valve and the dump valve don't have to be very large, but the steam supply valve should be large enough so it can pass all the steam required by the pump. 1/4" NPT should be more than enough; 1/8" NPT. would likely work.

You can probably make most of the components look like something that belongs on the locomotive to the average observer, but you likely won't fool most of those in the hobby if they look closely.

Richard Trounce.

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