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Plumbing Part VI - Blowdown
Valve
Nelson Riedel Nelson@NelsonsLocomotive.com
Initial: 9/28/03 Last
Revised: 04/24/2006
When I started
to think about the blowdown I realized that I'd never seen the actual valves
on a Shay --- and none of my photos showed the valve. On a
subsequent trip to Cass I found and photographed the valve on several different shays.
As I understand normal operation, the valve is occasionally opened when the boiler is under
pressure to expel debris that has collected in the mud ring. The
valve can also be used to drain the boiler. Some of the local folks
attach a hose and use the hot water under pressure to hose down their
locomotives after use. Maybe the most important use is to initially
fill the boiler ---- just connect a garden hose to the
discharge pipe.
The blowdown valve I decided to make is like one that Jim Buchanan ( Buchanan
Machine Works ) uses on his Climax. Jim sent me
a copy of the design from May/June 1997 Live
Steam. Jim later told me that he
would not use this design again because scale eventually builds in the
boiler and the scale then cuts the O Ring necessitating frequent
replacement. He suggests a
design with bronze body and stainless type wiper. Since
I already had the valve constructed I decided to try it and see if I had the
same problem.
| The photos above and on the right show the three main
pieces. The outer disks are turned from bronze --- left over
from making crankshaft bearings. Brass or stainless would also
work great. The middle disk is stainless
steel. The next step was to drill and tap six 4-40 holes on a
1.25" bolt circle to hold the two bronze pieces together.
The holes in the thinner piece were threaded.
Next, the three pieces were assembled and a 1/4" hole
drilled through all three pieces at a radius of
0.302" |
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| The 1/4" hole in the thicker bronze disk was enlarged with a Letter F drill and then taped 5/16 MTP
The 1/4" hole in the thinner disk was drilled 7/16" and the input pipe on the right
machined from brass
stock. The length is not critical. The inside is
drilled 1/4" The critical measurements are the 5/16"
diameter 0.55" long stub. The sealing O-Ring (7/16"
OD - 5/16" ID) rides on the stub.
The pipe was then silver soldered into the thinner bronze disk with
the tip of the stub flush with the inside surface
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Note that the center disk is 0.142" and the recess it rests in is
0.156" deep so there is a 0.014" end float. The O-Ring recess is 0.055" deep which
with the 0.014" end float gives
a total height of 0.069" for the O-Ring. The O-Ring nominal
diameter is 0.070" with a range of 0.067" to 0.073".
The width of the slot is 0.062" so the O ring is squeezed a bit which should push
it out a few thousands to give a good
seal. I've tested the valve at 125 psi air and no leaks.
If it starts to leak I'll turn a few thousands off the thicker disk to
reduce the depth of the recess for the rotating disk.
The author of the note in Live Steam (I didn't get the
part with the author identified) suggests a Viton O-Ring. Viton
has a high temperature rating but is not recommended for
steam. I'm going to try the NSF-Approved Ethylene
Propylene (EPDM) that is rated up to 300 degrees --- with 100 psi steam I'll
be
stretching it a few degrees, but I figure it should be OK.
The next photo shows the finished parts. The input pipe was
silver soldered to the disk on the right which cause the darker
color. The end of the operating
handle will be cut to length and drilled after the valve is mounted and the best position of
the release handle determined.
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| The photo on right shows the back of the assembled valve with
1/8" NPT stub that will screw into the boiler. |
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| The front of the finished valve. The operating lever is
retained with a 2-56 screw. Two orthogonal holes were drilled
in the shaft so the the handle can be mounted on either
side.
The discharge pipe is 5/16 tube. The tube will be replaced
by a nipple, elbow and long tube that will pass through a hole in
the side of the frame similar to discharge pipe on Cass No 11 shown
earlier. |
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Update (12/07/03): The blowdown valve was
installed in the back of the boiler as shown in the next photo.
A short piece of
5/16" stainless drilled to 15/64" was silver soldered between the
blowdown and the elbow in the discharge path. This was after a
5/16" MTP brass nipple broke. This stainless tube has an ID
very close to 1/8" pipe so there will not be much flow
restriction. A 3/8" OD soft copper tube threaded 1/8" NPT on each
end is used for the discharge pipe. The soft tube together with a bracket under the frame to support the tube
and the use of a stainless tube between the elbow and valve should prevent
the 5/16" tube breaking in the future.
While taking apart the valve to do the above modifications I noticed a
cut in the O-Ring just like Jim Buchanan predicted. However,
that was before it was connected to to the boiler, it had just be tested
with compressed air. I removed the O-Ring for closer inspection and
found it cut all the way through ----- then I remembered not having the
correct size O-Ring and cutting down a larger diameter one. Best order
the correct size.
| Update (12/10/03): Yep, here's another update.
Shortly after I did the update above Dan Staron suggested that it
would have been better to use a tee rather than an elbow in
the discharge. The straight end of the tee could be closed
with a plug but opened if the blowdown became plugged and a rod run
though to unplug it. Good ideal!. I'd also
decided to hook up the operating link. I had been
postponing the link figuring that I could reach underneath and open
the valve with my hand. However, then realized that there are
no seals in the output side of the valve and it will likely leak
---- don't want boiling water over my hand. Also,
had obtained the correct O-Ring so went back at it.
The photo on right above shows the tee ----- or more accurately, a
1/4" MTP cleanout plug in the elbow.
The operating link is
1/8" stainless with a cotter pin in the bottom. The top
of the link is threaded 5-40 and doesn't quite reach the underside
of the cab floor
when the lever is pushed all the way down (valve
closed).
The handle shown on the right is a loop of 3/32" stainless
rod silver soldered to a length of 3/16" brass
rod. The bottom of the rod is tapped 5-40 and
screws over the link. The handle can be unscrewed when it
is necessary to remove the floor. |
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I'll update this again after some experience with scale ----- not long
now. LocoParts has reduced the price of their blowdown valve ---
probably a good alternative to changing O-Rings.
Update 6/6/04: The valve had been in use for a number of steaming
hours and no problem yet with the O-Ring. I also noted that Bob Reedy
used this type of blowdown on his Three Truck Climax (Live Steam
May/June 2004). The drawing is such that I'm pretty sure he's the
author of the original design I got from Jim Buchanan
| Update 2/10/06: The shay is in the workshop for
annual maintenance and a few improvements. I had the cab floor
off so thought it was a good idea to open the blowdown valve and check
the condition of the O-Ring. The photo on right shows the
inside of the valve. Everything has a rusty discoloration but
the surfaces are smooth and there is no scale. (Maybe the
LSB-8000 is keeping the scale away.) The O-ring was removed and
found to be undamaged after two years service. However, I have
some Viton O-Rings of the correct size (#011) so will replace the
O-Ring. Unlike EPDM, Viton is compatible with oil so I'll
lubricate the valve with steam oil before reassembling it.
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| Update 4/24/06: While doing a final test before
the operating season the shaft in the blowdown valve broke.
Maybe the new Viton O-Ring was too hard for the clearance (one
shouldn't try to fix it if it ain't broke.) The
blowdown is hidden so function is more important than appearance.
McMaster-Carr Miniature Ball
Valve # 4112T13 fit perfectly in the tight space as shown in photo on right. I had
to make new handle for the valve with the on and off positions 45
degrees from the horizontal. (Why didn't I think of this before?) The valve has PTFE seats and Viton seals and is
rated for 300 degrees F. The WOG pressure rating is 1000
psi at 70 degrees but is not rated for steam. |
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