[N&W] Re: Slide Valves vs Piston Valves

[nw-mailing-list at nwhs.org]

Mason -

I don't think so.  The cylinders rarely wore out; the pistons and valves
didn't wear on the cylinder casting itself; there was a cylinder liner
pressed into the cavity which was machined out to the proper inside diameter
for the piston to run in, with its packing rings.  The same was true of the
slide valve seats; they weren't part of the cylinder casting.  And piston
valves used a liner machined for the valve and the ports.  When these liners
wore out they could be replaced without messing with the cylinder casting.

If anything happened to the cylinder castings themselves, it was probably
wreck damage.  And the cylinders for those engines were cast in halves (both
the slide valve cylinders and the piston valve cylinders) so that if one
half was damaged a new one could be bolted on without replacing the whole
thing.

The Z-1b program was just that - a program.  How the individual engines were
selected for it has not come to light yet, but it was very unlikely that the
condition of the cylinders had anything to do with it.  The 1331 was the
lowest numbered engine selected; why none of the 1315-1330 bunch made the
cut is a mystery, so far.  Probably none of the Z-1s were selected because
of the Walschaerts valve gear.

There's a good bet that the 1399 was selected for the Z-2 because of its
number, although why they'd go for the 1399 instead of the 1400 is anybody's
guess.

And why they selected 74 instead of 75, or 70, is also anybody's guess; the
only thing might have been the curtailing of work during the depression.  It
could have been that the program was originally intended to do all 175 of
them.

EdK
-----Original Message-----
From: N&W Mailing List <mailing-list at nwhs.org>
To: N&W Mailing List <mailing-list at nwhs.org>
Date: Wednesday, June 30, 1999 10:58 PM
Subject: Re: Slide Valves vs Piston Valves


 >Mason Cooper asks:
 >
 >Ed - I have been told that when the slide valve cylinders became worn out
 >the engine was made available for conversion to superheating. This was
 >because the cylinder saddle represented such a large part of the expense.
 >Is this true?
 >
 >----------------
 >
 >Gary Rolih adds:
 >
 >Regarding the use of slide valves on superheated engines, the superheated
 >steam is MUCH hotter than saturated steam.  The higher temperatures cause
 >lubricating oil failure i.e it starts to break down by the splitting of the
 >long hydrocarbon chains into shorter chains,  These have much less
 >viscosity and diminish the oil films ability to carry load.  Slide valve
 >engines used common lubricating oil; superheated engines had to use oil
 >specifically developed to not break down at the higher steam temperatures
 >
 >A short, simplified lesson in thermodynamics.  Materials can exist in three
 >states solids, liquids and gases.  These states are functions of
 >TEMPERATURE and PRESSURE.  For a material such as water, known as a simple
 >compressible substance, it can exist as a mixture of water- liquid- and
 >steam or vapor- a gas simultaneously.
 >
 >Boiling water on the stove at normal atmospheric temperatures and pressures
 >has steam above it which can be seen because it is part gas and part liquid
 >water suspended in the gas.  This is a multiphase condition where the
 >pressure and temperature become interdependent variables.  The mixture here
 >is roughly 60% gas and 40% water.  If you add more energy as heat, the
 >ratio of gas to liquid changes and gas becomes more dominant.  At the right
 >amount of heat addition- at the same temperature and pressure- no liquid is
 >present and the vapor is all gas.  This is saturated steam.  After this
 >point is reached, the addition of more heat causes the interdependence of
 >the temperature and pressure to CEASE.  The heat addition causes the
 >temperature or pressure or both to change independently of each other.
 >This point of change, if one plotted a chart of Temperature vs. Pressure
 >for water forms a line dividing the states called a "vapor dome".  Inside
 >the dome you have steam and liquid in combination; outside only a gas can
 >exist. This region can be called superheated.
 >
 >The significance of working in the superheated range is that one can get
 >higher pressure drops in the system and much greater thermal efficiency of
 >the expansion process.  But, superheated steam by it definition consists of
 >a gas only.  Which is where the name DRY STEAM comes from.
 >
 >
 >