Poppet valve sizing for tender axle pump

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DaveD
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Poppet valve sizing for tender axle pump

Post by DaveD »

Hello all,

I'm redesigning my 1.6" P7 tender axle pump after spending too much time with the original, made from a bronze casting that turned out to have a bunch of porosity defects; it cannot be repaired. I used poppet valves and these worked really well. I got the idea from Jack Bodemann's thread here: viewtopic.php?f=8&t=80839. I will use poppet valves in this redesign. It will be a two cylinder pump with each cylinder 3/8" diameter.

My question is about proper sizing of these poppet valves. I fabricated four for the discarded pump to fit in 1/4" passages, and each of them has three ball-milled cuts, with each cut providing about 0.010 sq. in. area, for a total of 0.030 sq. in. I plan to feed the boiler and return water to the tender tank through 1/4" ID tubing, which has a cross sectional area of around 0.050 sq. in. It's returning water to the tank when the pump is idling that I'm concerned about. It would be nice to have as little back pressure in this pump as possible when it isn't doing anything.

It appears that if there is a flow restriction to be concerned about it will be with the poppet valves themselves, as currently designed. Poppet clearance (upward movement and cross-sectional area to the side and above) is great enough that there will be no problem there. If this is a potential problem, and I don't know whether it is, should I make larger valves, e.g., 5/16" diameter, with more cross-sectional area to better fit into a 1/4" delivery system overall? I haven't been able to find any engineering data on this, so I suspect the answer will be in empirical experiences.

Thanks for any assistance.
Dave Dalton
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Fender
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Re: Poppet valve sizing for tender axle pump

Post by Fender »

Dave,
You said that the pistons will be 3/8” diameter but I didn’t see what the stroke will be. That will give a better idea of the flow rate through these valves.
Dan Watson
Chattanooga, TN
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LVRR2095
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Re: Poppet valve sizing for tender axle pump

Post by LVRR2095 »

Couldn’t you sleeve the porous bronze casting? It would save making a whole new pump.

Keith
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DaveD
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Re: Poppet valve sizing for tender axle pump

Post by DaveD »

Dan, Half-inch stroke, 3/8" bore.

Keith, No. There are porosities throughout, including in the feed side and where the O-rings seal the caps. Plus, it appears that these porosities were caused by foreign material, like stones and such; silver-soldering wouldn't help either. I wish that I had noticed this earlier, but...
Dave Dalton
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Re: Poppet valve sizing for tender axle pump

Post by LVRR2095 »

Dave….sorry to hear that, it would have made things a bit easier.
Keith
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DaveD
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Re: Poppet valve sizing for tender axle pump

Post by DaveD »

Keith, It's not so bad. This is the first large assembly disaster in many years building this locomotive. How many years I won't say, as I would be embarrassed if I did. Dan knows and might tell you, but I hope he doesn't! Huh, Dan? :mrgreen: I'm very careful but extremely slow, and that is what it's about for me. I'm kind of in a hurry now because I would like to finish this locomotive before I'm finished!
Dave Dalton
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Re: Poppet valve sizing for tender axle pump

Post by Berkman »

I would think about it from the long-term maintenance and servicing standpoint as well.

I'd absolutely want some grease fittings that are accessible from the bottom, and hopefully a way to change O rings etc without taking the boiler off. I'm sure others would love to see the drawings of the new design you come up with.
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SPSteam2491
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Re: Poppet valve sizing for tender axle pump

Post by SPSteam2491 »

Very good idea placing the pump on the tender instead of the engine. Far better for serviceability and it's easier to drop a tender truck than lift off the boiler.

Are you planning on having the bypass valve on the tender or the engine? With it on the tender, you could keep the return line much shorter to reduce backpressure from the piping. If you are able to keep the cross sectional area of the flow around the poppet valve larger than the pipe ID, and the over length of the return line short, your limiting factor for the system will be the cross sectional area of the piping (in your case 1/4" ID).
Thanks
John LaFavor
Pacific Design Shops
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Re: Poppet valve sizing for tender axle pump

Post by Berkman »

Is it though? A pump on the driver axles is "engine driven" A pump on the tender truck axle seems much more likely to potentially lock up, albeit though easier to remove in case of work.
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DaveD
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Re: Poppet valve sizing for tender axle pump

Post by DaveD »

I'm happy to see all the interest in this thread. However, no one has come close to getting around to answering the question I have about poppet valve size.

I would like to take credit for some of this, but aside from redesigning the pump as a fabrication instead of using a casting, and using poppet instead of ball valves, I can't take any credit. Many of you recall that Bruce Hamilton, Baltimore, made the first two of these President series B&O locos and I found the third set of castings. So far so good, until I learned that the axle pump casting must have been made from the bottom of the bronze pouring, chunks of rock and all. And, of course, credit for the poppet valves goes to Jack B.

If you missed it, this is a tender axle pump. Ten or fifteen years ago I rebuilt a 3/4 scale loco for an aging friend in Richmond, VA, as he could no longer see to work on it and wanted to sell it. The pump on that loco was on a main driver axle and had to be rebuilt; I swore I would never build one like that. A tender axle pump is far easier to get to, as it takes about a half hour or less to drop the truck.

Mike S., who owns the President Adams, the second of Bruce's creations, finds it difficult to get to the pump, which is located in front of the front tender axle with little clearance between it and the front frame casting. I've improved on that by clearing out a bit of the front coupling casting in that area. To oil the eccentrics, Mike added some oiler tubes that go in from the side below the frame and stop above, dripping on top of the eccentric area. He adds a few drops of oil there at ever run. The eccentric rods are thick enough (3/8") that I could put zerk fittings on the bottom side, but I believe an oil drip like Mike installed would be ultimately easier to get to and as effective. Comments?

Mike also put the bypass valve in the tender as part of the valve, operated from above, that supplies water to this pump. The return tube is located where he can see it inside the tender tank right in front of the hand pump; that way he can see if the pump is hitting on both cylinders.

So all I need to continue my re-design is some good comments about just how much flow restriction I can deal with here without causing back pressure when the pump is simply bypassing water back to the tank.
Dave Dalton
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SPSteam2491
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Re: Poppet valve sizing for tender axle pump

Post by SPSteam2491 »

Personally, I have taken off the axle pump on all the steam engines I own but they are a good option for getting water into the boiler.

A pump on the tender will not be near any heat source keeping it cooler. The RPM of the pump will also be faster due to smaller wheels on the tender so a smaller piston can be used to pump the same amount of water with respect to distance traveled. Thus decreasing the amount of force needed to pump the water.

If you are going to have a condition that would lock up the axle pump, would you really want that failure to be tied to the drive system of the engine? The engine will continue to plow through the failure until something breaks. Where a failure on a tender axle pump would cause the wheels to lock up and slide. It's far easier to replace a tender wheel than to replace a drive system component for a worst case scenario.

Dave, you may have missed the second part of my earlier comment. I was asking about the cross section of the available space around the popper valve as compared to the cross section of the pipe. If you are able to keep that cross section larger than the pipe ID, you will not be adding any noticeable additional back pressure to the system outside of the length of pipe.
Thanks
John LaFavor
Pacific Design Shops
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DaveD
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Re: Poppet valve sizing for tender axle pump

Post by DaveD »

John,
Yes, I missed that part. But I stated in my first post that "Poppet clearance (upward movement and cross-sectional area to the side and above) is great enough that there will be no problem there.". The potential problem is in the cross-sectional area of the three flutes within the poppet itself. I am attaching photo clips. The first is of the poppet valves and red arrows point to the area of the minimum cross-sectional area, the flutes placed there with a ball-end mill. Each of these three flutes has a cross-sectional area to the wall of the 1/4" hole that they fit into of 0.010 sq. in., for a total of 0.030 sq. in.

The second photo clip is of one of the caps and the diameter of the opening is where the poppet valve top resides, which is somewhere between 7/16"and 1/2", whatever it is (I forget) that will provide at least 0.050 cross-sectional area between the 3/8" valve top diameter and the ID of this opening. Inside this cap you see a slotted support that prevents the top of the poppet valve from going any higher (upside down here) into the cap. The slot cross-section totals of these four slots are at least 0.050 sq. in. This is the topside, or outlet side of the pump. the bottom side is made in a similar fashion but with a 4-legged spider inserted into the 1/4"casting bore to prevent the valve from rising too much.
image_2022-08-26_154236365.png
poppet valve cap with clearances.JPG
Dave Dalton
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