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Author Topic: Pratt Institute Power Plant  (Read 661 times)

admin

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Pratt Institute Power Plant
« on: May 21, 2017, 06:59:49 AM »

I have seen a few videos about the power plant at Pratt Insitute, and about the guy who has operated it for years (Conrad Milster), and was glad that the school preserved such a nice example of how things use to be.

Today I ran across a pdf document that gives quite a bit of information about the Pratt plant, and it is a interesting read (so far, I have not yet finished it yet).

https://www.asme.org/getmedia/745d7e61-1b79-40cc-847e-e2c0afaeebf6/25-Pratt-Institute-Power-Plant.aspx
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cae2100

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Re: Pratt Institute Power Plant
« Reply #1 on: May 21, 2017, 02:03:40 PM »

Interesting read. Here's the video I was talking about in the email, it's a automatic engine like in the document and really interesting to just watch the exaust when the engine is running.
https://www.youtube.com/watch?v=oY3ur4sPwlU
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Re: Pratt Institute Power Plant
« Reply #2 on: May 21, 2017, 09:41:14 PM »

That is a nice video of a vertical engine.

I remember reading up on the pros and cons of a variable cutoff governor vs a throttling type, but unfortunately I forgot all the details.
I will need to refresh on that topic.
Seems like a lot of the governors were fail-safe, in that if the governor belt broke, the governor would stop the engine.

That is the first I had heard about the uneven valve and valve face wear using the variable governor, but it makes sense now that he mentions it, since the valve would not override the seat on each stroke.

A balanced D-valve would also help with valve wear, but they can also cause steam leakage if they relieve too much pressure off the valve.

The Speedy Twin valves are quite interesting in that they are semi-balanced, but use a totally different method for balancing than a typical balanced D-valve.
« Last Edit: May 21, 2017, 10:02:38 PM by admin »
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cae2100

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Re: Pratt Institute Power Plant
« Reply #3 on: May 22, 2017, 01:53:15 AM »

How does it produce uneven wear with a variable cutoff governor if it's just going over the same area as a normal slide valve path? I was going to try to design a variable cutoff governor into my engine eventually, so Im really interested in how it would cause more wear vs a normal slide valve.
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admin

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Re: Pratt Institute Power Plant
« Reply #4 on: May 22, 2017, 03:18:22 AM »

I have not studied the flywheel governors much, but for the Stephenson link, which provides variable cutoff, when the link is in "full gear" (I think that is the right term), then the valve travels to the full extend allowed by the eccentric motion, and maximum power is produced by the engine since the cutoff is late, and thus the full boiler pressure is acting on the piston until late in the stroke, as late as perhaps 80% or more.

Efficiency in the full gear position is not very good since the expansion of the steam is not being used much or at all.

When the Stephenson's link is changed to a position that provides early cutoff, the travel of the valve is limited, and cutoff can be at perhaps 15-20%.
Early cutoff allows maximum use of the expansive power of steam, and thus is much more efficient if the engine still produces enough torque to pull the required load.

Locomotives used the Stephenson link quite a bit, and used the full gear position when starting from a standstill, and then raising the link up (I think the term is "linking up") once the train was underway and maximum torque was no longer required.

So it has to be assumed that many locomotives operated for the majority of their operating time with the link in the up position, and thus the valve was not traveling the full extent of the valve seat.

I assume that the valve face was refaced every so often, on a regular maintenance schedule, and the valve probably ground off flat at the same time.

I think the valve seat wear is something that would accumulate over a long period of time, but I can't say exactly how long "long" would be.

The Dake steam engine had uneven wear on its valve face (the inner piston doubles as the valve face), and yet it was a very viable and commercially successful steam engine design, so uneven wear on the valve seat is not something I would worry about.

The Soule Speedy Twin does not have variable geometry valve gear, and so that engine is not as efficient as an engine with variable cutoff, but the Speedy Twin was designed specifically as a compact and powerful sawmill carriage engine, and the idea was to get the carriage from one end of its travel to the other while cutting a very large log, as fast as possible, and no other steam engine was as well suited to the task.
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admin

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Re: Pratt Institute Power Plant
« Reply #5 on: May 22, 2017, 03:40:18 AM »

You can see the valve travel as the link position is varied in Charles Dockstader's Stephenson's link valve gear program.

I think I can post it here since this is a non-commercial, non-profit steam engine site.
All copyrights for this program belong to Charles Dockstader.

I started to try and convert his programs to something more modern (he has programs for all sorts of valve gear, and they may still be available online), but then I got into 3D modeling, and I figured out that I could run simulations of the actual valve gear on engines I was designing, and unlike Dockstader's program, the exact shape and dimensions of the valve and valve seat are used in the 3D model, so it is super accurate.

« Last Edit: May 22, 2017, 04:10:53 AM by admin »
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admin

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Re: Pratt Institute Power Plant
« Reply #6 on: May 22, 2017, 04:11:06 AM »

I am trying to recall if a typical D-valve still has valve seat overtravel while the valve gear in the the full linked-up position.

Valve gear designs tended to vary, and some designs limited the maximum valve travel such that only 80% or so of the steam port was opened at maximum valve travel.
Other designs had valve over-travel, where the leading edge of the D-valve actually traveled beyond the edge of the port, thus giving 100% port opening.

Edit:
I looked at the 2D drawings I have for the bottle engine, and they use the same valve and valve seat layout as a Stanley 20 hp engine design (scaled).
When the D-valve is at 0% cutoff, ie: the valve is at the point where admission begins, then the far side of the valve is at the far edge of the valve seat, so in other words, when the Stephenson's link is fully linked up, the valve travel is at a minimum, and the valve at the very least travels to the edge of the seat, even if it does not over-travel, so there would not be any abnormal valve seat wear if this type engine was run for extended periods of time with the valve gear in the early cutoff position.

So that makes me wonder about the configuration of the valve gear that used a flywheel-mounted governor.
I will have to do some digging, but I would think that a flywheel-mounted governor could be configured just like a Stephenson's, and at a minimum, the valve could travel up to the edge of the valve seat.

But even if the old flywheel-mounted governor systems did not allow the valve to travel to the edge of the valve seat, there is no reason why your design could not allow the valve to travel to the edge of the seat at maximum cutoff.
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admin

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Re: Pratt Institute Power Plant
« Reply #7 on: May 22, 2017, 04:28:29 AM »

You can see from the photo below of the sectioned locomotive steam chest that even at minimum valve travel, the opposite side of the D-valve still over-rides the end of the valve seat.

This locomotive valve appears to be a balanced type.


The photo comes from this post:
http://www.classicsteamengineering.com/index.php?topic=1247.msg9321#msg9321
which I moved recently into its own topic.
« Last Edit: May 22, 2017, 04:33:33 AM by admin »
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cae2100

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Re: Pratt Institute Power Plant
« Reply #8 on: May 24, 2017, 12:59:21 AM »

I saw all of that stuff before, I still cant wrap my head around why it would cause premature wear or more wear than a standard setup. I was going to go with something almost identical to a reversing type stevenson valve, but have the other side locked rather than using two eccentrics, and have the governor attached to the link to move it up or down and control the speed that way. I wasnt going to start on it till way down the road, but it was mainly just something to tinker with really. Also, I cant use exe files, I run a linux system and usually I just compile the stuff from code anyhow.
« Last Edit: May 24, 2017, 01:01:22 AM by cae2100 »
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admin

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Re: Pratt Institute Power Plant
« Reply #9 on: May 24, 2017, 01:13:46 PM »

I guess the bottom line it that unbalanced D-valves are known for being high-wear items in general.

You can calculate the amount of force pressing a D-valve against its face and see why the wear would be high.

And many locomotives used higher pressures, I think up to 250 psi, and superheated steam on top of that, which I guess is why you see a lot of balanced D-valves used on locomotives.
I think eventually many locomotives used piston valves.

It would be interesting as an experiment to detach a D-valve from its eccentric and try to move it by hand when there is pressure on the steam chest.
I am guessing you would not be able to move by hand most D-valves that are under pressure.

And the forces also create wear on the eccentrics and associated linkage.

I have considered using piston valves in the past, but I like the simplicity of the D-valve, and I think I could cast a steam chest arrangement for a D-valve easier than for a piston valve.

I know that piston valves if properly designed and machined will work well, but I am not sure I could machine one and be assured that the parts of the valve would not catch on the ports in the cage.

I am planning on using a balanced D-valve design, and I consider that a good way to get many of the benefits of a piston valve without the complexity of a piston valve.
The popularity of the balanced D-valve in the steam locomotive world tells me that it is a viable design.

And one of the good things about a D-valve is that they will self adjust with wear of the valve and seat, assuming that the wear is somewhat even, and that is something that a piston valve cannot do.


Steam engines went through a long period of development before they became obsolete, and I have had to find a particular steam engine era that I want to concentrate on.
I really like the designs of the late 1800's because they were the culmination of many years of research and development, they were very hardy and reliable working engines, and they were still simple enough to be able allow most of the moving parts to be seen while the engine is running.
For me, there is something fascinating about watching all the parts move on a steam engine, and the slow speeds of many steam engines enhances the visualization.

I have been offered more modern steam engines (built in the early 1900's) for a very reasonable price, and declined these offers, because the engines had totally enclosed crankcases, pressurized oil systems, etc. and lacked the visual charm and grace of the older engines.

It is entirely possible that a D-valve traveling on a shorter stroke will create a different wear pattern that if it were traveling full stroke.
I don't have the hands-on experience to say one way or the other, so can only speculate about how it could be from a theoretical standpoint.

I try to talk to the operators at these shows to get first-hand knowledge of steam engines, and I find that information to be very informative and interesting.
I talked to one guy at Soule who rebuilt the twin marine engine, and I learned quite a bit just from that conversation.
« Last Edit: May 24, 2017, 01:17:58 PM by admin »
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cae2100

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Re: Pratt Institute Power Plant
« Reply #10 on: May 24, 2017, 03:24:58 PM »

my engine would be running at 20-30psi usually, maximum 40 so the pressures shouldnt be too awefully high. I went with a bit of a softer bronze so it will tend to wear with the engine rather than wearing into the cylinder. That cylinder is brutal on cutters, the aluminum alloy that I went with is so hard that it dulled out 3 HSS cutters before I could even finish machining the port face on the thing.

I was thinking about the stevenson's link that is used on a marine engine to reverse them, adding the governor setup to adjust that. I think for as much use as mine would be used, I dont think I really have any worries about wear and such in the engine. If it does, I can just face it off again in the shaper and it'll be good.

I think Ive talked to that guy also, I think I caught him off guard when I started asking questions that most people wouldnt know unless they were into steam engine building and such, lol.
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