While discussing the issue of steam locomotive rosters, the following issue came up in the email discussion group.

Q: [A.P. Robinson wrote:] With respect to the Z1a / Z1b conversion issue, I thought (in my extremely limited wisdom) that the obvious spotting difference between the Z's were the cylinders. (One group had slide valves, the other had piston valves.)

A: [Ed King notes:]
Not only your wisdom but all N&W conventional wisdom felt that way. This was something I stumbled upon almost by accident, and then I looked really closely at my N&W photo of the old 1438. Bells went off, because that photo was from a period 8-10 years earlier than the Z-1b program started.

[Mason Cooper opined:]
Generally saturated steam engines had slide valve cylinders because they were more effiencient, but they could have piston valves. Superheated steam pitted the plates on slide valve cylinders as they slid back and forth which is why the conversions always had piston valve cylinders.

[Ed King responds:]
Mason - the use of piston valves predated superheaters by more than ten years. Slide valves had limited port area because the ports were flat and their width was limited. The ports of piston valves were around the entire circumference of the valve, and weren't limited by a flat space.

I like to use the N&W as an example of piston valve use pre-superheater. N&W had (if I remember off the top of my head) 638 or thereabouts locomotives equipped with piston valves before the first superheater hit the property. Count 'em up. The A 4-6-0s, the rebuilt B 2-8-0s, the E, E-1 and E-2 Pacifics, the J Atlantics, the M, M-1 and M-2 4-8-0's (there's 275 right there), the rebuilt U 4-6-0's, the V 4-6-0s, the W-1 and W-2 2-8-0s, the X-1 0-8-8-0s (high pressure cylinders), and the Y-1 2-8-8-2s. The first superheaters were either on the E-2a Pacifics or the M-2 a or b or c 4-8-0s. And, of course, all the Zs came with them.

It was quickly found that slide valves didn't work with superheaters because the lubricants of the day couldn't protect such a great flat sliding valve under pressure (the steam pressure acted upon the valve even with balancing strips) at the superheated temperatures and valve seats were quickly worn and/or scored. In contrast, the steam pressure had no effect on the piston valve; it was completely balanced and it was much more easily lubricated. This factor, plus the port area, doomed the slide valve.

Except on the Virginian.

[Mason Cooper asked a related question:]
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?

[Ed King replies:]
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.

[Gary Rolih writes:]
Slide Valve Wear: With a sliding plate rubbing against a flat surface driven by a reciprocating rod with possible bending loads applied to the rod from the valve mechanism, the slide plate/valve and the mating surfaces would experience a wearing into the top of the saddle at the valve seat and a thinning of the valve. The top of the saddle would be either resurfaced by planing or face milling until the surface dropped down too close to the centerline of the piston to maintain the valve slide-to-valve mechanism dimensions, i. e. too much side load when the two don't line up.

A typical patch probably would be to machine a thin plate of cast iron and bolt it on to the saddle. For a thin, worn valve, a new casting would be used. The top cover plate over the valve would also be relatively cheap to make from a new casting. Chances are that the top cover and valve itself would have to be matched in thickness or shimmed to control the gap in the assembly. These exercises are not difficult and are common in machining and machinery building.

Welding on cast iron is tricky due to the high level of carbon, especially the graphitic forms, in cast iron. Good practice is to weld, then furnace anneal the part to remove the residual stresses put in by the welding process. The melting and resolidification of the iron and carbon still makes bad stuff like big lumps of iron carbides surrounding areas of pure iron. These are stress risers causing fatigue crack starting sites and load concentrators. So, probably, most shops avoided that activity.

Resurfacing the valve seat while the saddle was mounted on the locomotive is possible with a horizontal boring mill or a shaper made to overhang the work piece. The drilling would be done with a radial drill- think big drill press with a swing arm. These kind of machine tools were common in those days.

Steam erosion of the valve passages in the saddle, I cannot comment upon. That would be difficult to repair with patches or building things back up with weld. Serious erosion would likely be a scrapping criteria.

I would suspect that conversions were dictated only when the saddle was cracked or the passages were worn "through" in some manner that was not easily repairable.

Note that a conversion from inside admission to outside admission, like on the W-2's and M's, would require that the saddle be removed or the cylinder castings taken off of the saddle. The machining change requires boring operations on the side of the components facing the boiler. To get access, the parts must be off of the loco. Certainly the smokebox area has some significant work to do on it, too.

A great deal of any modification project is the amount of work necessary to get the components to a state where they can be worked on. This time must be calculated into the job to see if the whole exercise is cost effective. A lot depends on configuration. Some valve changes were bolt-ons which means the loco was not long out of service. A job which demands total tear-down, and 6 months out of service, would probably not be done especially if business was good and the loco was needed.

[Gary continues:]

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 film's 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 its definition consists of a gas only. Which is where the name DRY STEAM comes from.