The Steamboating Forum

OK, It's been WAY to dead around here! Here's a question to maybe generate some new post.
The engine is 2 1/4" bore with a 3 1/4" stroke, double action, running at 250rpm at 100lbs steam pressure. What will the bore and stroke be for a piston pump driven at 250rpm to make up the water used by the engine?

A 2.25 in X 3.25 in Double Acting Engine at 250 RPM, 100 PSIG steam pressure, produces about 1 to 1.5 Horsepower (for mean effective pressures of 70 to 100 PSI).

Water rates for small single expansion engines is in the vicinity of 60 pounds of feedwater per BHP-hr (Brake Horsepower - Hour).

Thus this engine would require from 60 to 90 Pounds per Hour (PPH) feedwater.

A small single acting plunger pump (with volumetric efficiency of 90%) would have the following dimensions at 250 RPM:
0.5 in bore, 0.75 in stroke = 75 PPH feedwater
0.625 in bore, 0.75 in stroke = 118 PPH feedwater

A short stroke pump is better to reduce hammering.

The smaller pump might be considered minimum, the larger one maximum required, allowing auxiliary steam use, blowdown, etc.

fredrosse wrote: ...The smaller pump might be considered minimum, the larger one maximum required, allowing auxiliary steam use, blowdown, etc.

Don't forget WHISTLE BLOWING That's more than just an "etc." I'm not even sure if that falls into "auxiliary steam use".

Hi Fredrosse

W.B.Hall claims in Funnel no. 125 page 50 that the actual steam consumption could be as high 2.6 times the theoretic calculated, for saturated steam.

This figure is reduced by superheating, and comming close to 1 at 100 degr. C superheating.

As my own boiler is far to small, I have been interested in this cylinder condensation isue ever since.

If the consumption you mention = 60 lbs/h per BHP is about right, the "cylinder condensation factor" could be reduced to approx 2 for saturated steam (depending slightly on the mechanical efficiency)

Would you call 60 lbs conservative?

Should - (for mean effective pressures of 70 to 100 PSI) - be understood only as part of the "rule of thumb"?

In the actual case it seems that 100 PSI mean effective pressure would be very hard to obtain.

Something like 25" vacuum and a 98 % cut off.

Best regards
Jørgen

“W.B.Hall claims in Funnel no. 125 page 50 that the actual steam consumption could be as high 2.6 times the theoretic calculated, for saturated steam.

This figure is reduced by superheating, and coming close to 1 at 100 deg. C superheating.

As my own boiler is far to small, I have been interested in this cylinder condensation issue ever since.

If the consumption you mention = 60 lbs/h per BHP is about right, the "cylinder condensation factor" could be reduced to approx 2 for saturated steam (depending slightly on the mechanical efficiency)

Would you call 60 lbs conservative?”

ANS: The 60 LB/BHP-Hr is a rough estimate, based on my judgment of typical engine consumption. As a comparison, extensive tests were made years ago in “Live Steam” magazine, with a similar, (simple double acting slide valve engine) but smaller engine using various amounts of superheat, engine speeds, valve gears, etc. The test engine was about 1/4 HP, and used in the vicinity of 80 LB/BHP-HR steam consumption. The 2-1/4 x 3-1/4 launch engine would probably do better. Other available test data is usually found for much larger engines, typically in the 100-500 horsepower range. These engines did much better with lower steam consumption per output horsepower, but they are way too large for comparison to our small launch engines.

“Should - (for mean effective pressures of 70 to 100 PSI) - be understood only as part of the "rule of thumb"?

In the actual case it seems that 100 PSI mean effective pressure would be very hard to obtain.

Something like 25" vacuum and a 98 % cut off.”

ANS: The mean effective pressures of 70 to 100 PSI was used to roughly estimate the output horsepower potential only, and yes, getting 100 PSI BMEP would probably be difficult with the stated steam conditions, and represents an upper bound for this engine’s output. Economical cutoff would bring the BMEP down considerably.

fredrosse wrote:“extensive tests were made years ago in “Live Steam” magazine, with a similar, (simple double acting slide valve engine) but smaller engine using various amounts of superheat, engine speeds, valve gears, etc. .

Hi Fredrosse

Do you by any chance have a copy of the mentionen test, or can you point out the relevant isues of Live Steam?

Best regards
Jørgen

There is a whole series of issues (about ten issues, I think) with the engine tests, starting with the October 1989 issue of Live Steam Magazine. Author is Christopher Leggo.

Hi

I checked the Live Steam Web site. There are 14 parts in that article series called October test.

I just ordered a photo copy.

It looks as if there should be reading for a couple of evenings

Jørgen