The Steamboating Forum

Hey Fred

In correspondence with Mike he mentioned something that I had never heard of, he referred to the throttle wire drawing effect. Now I have another odd puzzle to work out because I thought that in order to "super-heat" steam you had to add heat?

How does the steam gain super-heat after a restriction? Wouldn't the pressure "drop", on the other side of the throttle valve thus reducing the temperature of the stem?

Any thoughts or maybe a link to an explanation?

Cheers
Scott

This all makes sense, and I hope I can explain it satisfactorily.

The first step is to read and understand to discussion "Wet, Dry, and Superheated steam" topic in the FAQ section of this forum. After you understand this, then come back here to get the rest of the story.

Scott,

Steam exhausts from a turbine or engine at a lower temp and pressure than the steam supllied. I a "perfect" world the loss of pressure and temp equals the the work done.

In a "perfect" throttle or reducing valve, the steam exhausts from the valve at a lower pressure, but having done little work, it will retain much of the initial energy, and there by having a temperature higher than the saturation point for that pressure. On the ship I was on 1200psi desuperheated steam was reduced to 150psi. The reducing valve was followed by an in line desuperheater that would inject feedwater into the line to reduce the temp to near saturation for 150psi .

A simple explanation in a world where nothings SIMPLE, & nothings PERFECT.

David

To continue: "In correspondence with Mike he mentioned something that I had never heard of, he referred to the throttle wire drawing effect"

ANS: Throttling steam from any higher pressure to a lower pressure is common in all steam systems, and the term "wire drawing" refers to the cutting of valve seats where high pressure steam was severely restricted by having the valve disc very nearly closed, and as the steam flows across the valve seat, it would sometimes cut a groove across the seat. This groove was similar to what might happen if some tough piano wire was placed across the valve seat, the valve nearly closed, and the piano wire was drawn across the valve seat, cutting a groove. Hence the term "wire drawing" in days of old, where valve seats were commonly damaged by this throttling process. In most of today's valves this seat damage in not found, but the old terminology has remained. This really has nothing to do with the thermodynamic processes here, but I just thought a bit of language history might be of interest.

Now for the technical part: When dry steam is throttled from a high pressure to a low pressure across an insulated valve or other restriction, the energy level of the steam does not change. No heat is lost from the steam, and no work is done by the steam, so the same energy level remains in the steam.

The energy level of 150 PSIG dry saturated steam (with a saturation temperature of 366F) is higher than the energy level of, say 50 PSIG dry saturated steam (saturation temperature 298F). Notice that the 50 PSIG steam has a significantly lower saturation temperature.

If we throttle 150 PSIG dry saturated steam down to 50 PSIG, the saturation temperature of the steam changes, but at the outlet condition of 50 PSIG, the energy level remains at the high value associated with the 150 PSIG steam. All of the original 150 PSIG steam energy is still in the 50 PSIG steam. Hence the higher energy level associated with the 150 PSIG dry saturated steam shows up as some superheat (temperature above 298F) in the 50 PSIG steam. In fact the temperature across the throttling restriction does drop somewhat, but not all the way down to 298F, therefore the outlet is superheated steam at 50 PSIG. For this case the 50 PSIG steam would be at 328F, having superheat of 30F above saturation temperature.

I hope this explains how we gain both superheat, and have a temperature drop across a restriction.

Hey Fred. David

Ok been to the FAQ and read that. Yes now I understand better what is going on during the throttle partial open event. Steam seems to have a whole physics chapter all it's own. That has explained why some of the throttle vales I have seen were made the way they were.

As it turns out I know exactly what "wire drawing effect" is, I just did not know they already had a name for it. One of the machines I built at work is used to apply a waterproofing membrane on walls below grade. It uses two 5 to 1 air transfer pumps to move the material out of the barrels and into the pump and then it uses a huge 35 to 1 air motor to move part A and B down separate lines to the gun that has a mixing head. The stuff looks just like bubble gum and is as sticky as hot bubble gum.

Anyway when I built the machine I placed a set of ball valves in various locations so it would be easier to clean out. I expressly labeled the valves as "DO NOT open or operate while main pump is cycling". Well you know how monkeys are, the first day it was on the job I got a call from the head monkey telling me the machine was going nuts. Long story short it finally came out that the he knew more about the machine than I did and was using two of the valves to dump pressure during lunch so he would "not" have to purge the head. (the reason the head is purged is because the material sets very fast and if left in the head the whole thing has to be removed and baked to burn the crap out of the passages.)

After a few cycles he had cut the seats on the ball valves and had part A and part B running together through places it was not supposed to be. When they got my brand new machine back to the shop I was so pissed I told the owner that if the head monkey did not clean the machine out before he went home tonight he may as well pay to have a new one built. Head monkey did not go home that night till somewhere around 3am.

Anyway I thank ya'll very much for the information. This time round I learned two things and came away without any more questions!

Cheers
Scott

All very interesting but not at all what I was talking about in my email to Scott.

There is no way in hell I can do justice this subject in a post here without staying up half of the night, boring you all half to death and revealing my, at best, sketchy grasp of the subject.

Wire drawing, as I was talking to Scott about it, is shown on an indicator card as a place where a port is closing rather slowly and thus the energy considerations in the previous posts shows up on the card. When the same situation occured with a throttle or a governor, it became called Wire Drawing even though it doesn't show on an indicator card.

There was quite a tussle in the latter nineteenth century between the Corliss engines, which controlled the output through varying the cutoff, and conventional engines which used a flyball governor. The Corliss won, by the way. When you do a Google search under wire drawing and indicator cards, almost all of the hits are from over a hundred years ago when this was an issue.

We primitives with our little launches are not about to control our engines with highly variable cutoff valve gear so we just throttle the steam to the engine. The energy loss doesn't amount to much but it does make an engine squeak some of the time when it is first throttled back after a hard run.

The engine has been running on saturated steam. Suddenly it's getting some superheat. The rings were happy. Now they are not. A short shot of cylinder lube and Voila! A quiet engine.

I've seen those gutters on locomotive safety valve seats. Every year on the 90 ton shay, I as the sacrificial fireman and, incidentally, the son of the engineer, got to squat down in the steam dome and lap the seats clear of the grooves. Slave labor in our times!

Time for grandpa to toddle off to bed.

Mike

I love this type of high-level discussion. Checking for new posts while drinking my first cup of coffee has become a good habit...

Now, let's make the next step and consider the consequences of throttling with regard to the overall efficiency of our small steam plants. The Pearl twin engine on my boat (which however has a slightly larger bore than the nominal 2 1/2") reaches maximum speed at a boiler pressure of just 3 bar. I have limited this "maximum speed" to ca. 280 RPM. The max. operating pressure of the fire tube boiler is 10 bar. Therefore, there are two ways of reaching maximum speed (or any other, lower one): a) Throttle valve fully open, thereby controlling the speed by varying the boiler pressure between 1 and 3 bar. b) Maintaining a "reasonable" boiler pressure (6-8 bar) and throttling down to adjust the desired speed.

And the big question...: Which way renders a higher overall efficiency? Way b) is my favourite one, everything feels OK, and not only for tactical reasons. There might be a thermodynamical explanation as well. Any thoughts?

Albert

Engine efficiency with high boiler pressure, throttled down to the desired output, will produce some superheat in the engine inlet, and this can reduce initial condensation somewhat. That is a little better than running the boiler at the low pressure with no throttle valve.

Better yet, run the boiler at maximum pressure, and use variable cutoff of steam in the cylinder, that will give the best economy.

Because of the valve gear in the Margaret S, and the low engine speed, I always use cutoff control for adjusting load. That arrangement has 9 lever notches forward, and 9 notches astern, so there is a wide range of cutoff choices. The boiler stays close to maximum pressure, and the throttle valve is usually wide open. When a passenger is running the helm, I will show them the main steam throttle valve and let them know that turning this ball valve handle stops the engine, just to keep things simple for a novice. I am usually only away from the helm long enough to load the cannon anyway.

Variable cutoff is at the core of the Corliss engine's good efficiency. Each end of the cylinder stroke has separate intake and exhaust rotary valves. A flyball governor controls the closing point of the exhaust valves. They are way cool to watch. There are even shock absorbers to ease the shock of closing. It is possible (and common) to see a Corliss under very light load to have steam admission for only perhaps 10-20 percent or less of the stroke.

With locomotives and their highly variable loads, the valve gear is set at full (or as near to full as the gear will allow) stroke admission and the throttle is used for control until the train is well started and there is good traction. Then the engineer starts backing off on the percentage of admission. It's called 'hooking it up'. You can hear the exhaust note change to a sharper bark as this happens. There are limits to how far you can hook up Stevenson and Walshearts gear without the valve events getting less than optimal. Fred has weird valve gear and can take advantage of it to show us up!

On marine engines with loads that are largely proportional to r.p.m., we get away with a much smaller ranges of valve adjustment. I have only one position in reverse and two in forward. I use the full admission in forward for very low speed when the engine might stall (it's a single) or when reversing at low r.p.m. for the same reason. The rest of the time I back off to the second notch. If I don't the pressure starts to drop which gets my attention.

Mike