Superheaters and Throttle Linkage

[NW Mailing List]

The reason that superheater tubes would burn out is that the heat of the
hot, burning gasses from the firebox must be carried away from the steel of
the tubes by the movement of a fluid through them, in this case the steam
itself.  Without some sort of mass flow through the superheater tubes, the
tubes will quickly heat up from the transfer of heat to them from the
exhaust gases.  This will be in the order of 2500-3000 degrees F exhaust gas
temperature.  And the tubes will heat up to that temperature.

 

Just having the superheater full of steam is not sufficient as the heat
energy must be moved out of the tubes so that the tube does not overheat and
burn/melt.

 

The boiler is different as the flues are covered with liquid water and any
heat is going into the water to turn it into vapor or a gas (steam).
Excessive heat shows up as popping the pressure relief valves and venting
steam to atmosphere.  

 

So, a locomotive drifting down hill for a long period of time with any type
of throttle that is essentially closed will cause the superheater flues to
heat up perhaps to a serious temperature.   But, the crew should have
reduced the fire and left the throttle cracked to keep the steam moving and
to keep the cylinder condensation minimized or eliminated via the blow offs.

 

So, yes, the front end throttle can help 'some' but the problem doesn't go
away that readily.   Theoretically, a system that bled steam through the
superheater tubes and back into the boiler steam dome might do the trick if
one could generate the pressure drop across the superheater flues to get the
steam to flow.  This is not actually very easy to so.  Venting the
superheater to atmosphere while the throttle was closed would be quite
wasteful.  

 

Gary Rolih

Cincinnati 

 

 

  _____  

From: nw-mailing-list-bounces at nwhs.org
[mailto:nw-mailing-list-bounces at nwhs.org] On Behalf Of NW Mailing List
Sent: Monday, August 25, 2008 3:24 PM
To: NW Mailing List
Subject: Re: Superheaters and Throttle Linkage

 

I've read somewhere that the front end throttle was developed (at least
partly) to solve the problem of superheater tubes burning out because they
were empty of steam when steam dome located throttles were closed.  (The
steam ran from the throttle valve through the superheater and then into the
cylinders.)  The front end throttles were downstream from the superheaters,
so the heaters were always full of steam. 

 

The tube burning issue probably related to the way a particular class of
locos was operated and the degree of superheat.  I THINK that later locos
had higher degrees of superheat and most likely longer tubes more
susceptible to burning out. 

 

The multiple-valve front end throttles may have been easier to operate,
particularly at higher pressures (although I THINK that most/all
dome-located throttle valves were "balanced" , so engineers didn't have to
work against boiler pressure).

 

pete groom

On Aug 25, 2008, at 6:06 AM, NW Mailing List wrote:





Bill -

 

The As, E2s, E3s, Js (including at least one of the J 4-4-2s) K 4-8-2s,
selected Ms and M-1s, all M-2s, all S-1 and S-1a 0-8-0s, selected W-2s, all
Ys except the Y-1s, and all Zs were equipped with superheaters.

 

Suggest you consult the revised edition of Bud Jeffries' GIANT OF STEAM; Bud
has the numbers of the Ms and Ws that had superheaters.  W-2 778 pictured in
August Thieme's book has a superheater.

 

The presence of a linkage to the front end only denotes the presence of a
front-end throttle; most of those used by the N&W were American Multiples
which were located in the superheater header.  Some Y-5s used a Bradford
front-end throttle which was in a dome ahead of the stack.

 

All the others had the throttle located in the steam dome; the K-1s used an
external throttle rod going from the cab to the side of the steam dome.  The
others had the throttle rod going into the boiler through a packing gland in
the back ot the boiler head, and up to the dome.  Ms and Ws had the throttle
rod going into the back of the steam dome above the boiler top from the
linkage inside the cab, because the engineer didn't sit behind the rear of
the firebox; he sat beside it and the throttle was up on the side/top of the
boiler.

 

Engines with superheaters did NOT usually have a front-end throttle, but
engines with front-end throttles had superheaters.

 

EdKing

----- Original Message -----

From: NW <mailto:nw-mailing-list at nwhs.org>  Mailing List

To: nw-mailing-list at nwhs.org

Sent: Monday, August 25, 2008 1:20 AM

Subject: Superheaters and Throttle Linkage

 

 

I was looking at the picture of Y-3a 2077  on the August page of the NWHS
2009 Classic Calendar and noticed that it didn't have the throttle linkage
on the engineer's side to the smokebox like the Y-6's and and A's.   After
doing a little bit of research I discovered that locomotives with a
superheater usually had a front-end throttle which I hadn't heard of
previously.   The linkage on the Y-6's and A's apparently go to such a
front-end throttle.  I guess this means that the Y-3's, or at least this
one, were not superheated.  I think that the actual throttle valve in a non
front-end throttle situation was located in one of the domes; in the case of
the Y-3a, I would guess that it would be the 2nd dome back from the stack.
The dome just behind the stack appears to be a sand dome.  My questions:
how did the linkage get from the cab to the dome that had the throttle -
nothing is apparent in the photo.  Also, which N&W engines were superheated?
Were the J's? Were the K2a's?  Were the K1's.?

 

Thanks,

Bill Mason

 

 

 


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