Feel water pump question

[daysaver1]

I realize there are probably several hundred answers to this question but I'm trying to find out if my proposed pump will be sufficient. I've about had it with my existing Hypro pump and plan to build a simple displacement piston pump that I'll run off the same mechanics as currently drive my vacuum pump at 1/2 shaft speed via a standard Scotch Yoke.

My boiler is a Semple 40 foot fire tube and engine is a Semple 3x4 simple. My condenser vacuum pump pushes out what I think is a small amount of condenced water every stroke, but I've never measured it and now with the boat mostly dismantled, I want to know.

My plan is to have a .625" piston travelling on a 2" stroke, which at 300 RPM engine speed, I calculate to pump about 1.6 US quarts a minute.

So the question is if you've ever calculated amounts of feed water make up, and if so, does my number seem to work?

Bruce Jahn

[steamboatjack]

Bruce,

The amount of feed water put in a boiler is the same as the amount of steam coming out in mass terms i.e. Kg or Pounds. You should be able to get the maximum steam output for the boiler and work out the mass of feed from steam tables. I would suggest the pump capacity needs to be 1.5 to 2 times this for an engine driven pump. Hypro's are used extensively and work well, If yours does not you need to overhaul it (kits available) or speed it up by using a belt drive or similar.

regards
Jack

[artemis]

Determine the amount of water (in the form of steam) that is consumed in one hour operation of the engine:
Bore = 3"
Stroke = 5"
RPM = 300
Steam Pressure = 100 psi gauge
HP=5

Volume of piston (in cubic inches) in one revolution (two strokes) times RPMs times 60 (mnutes in an hour) divided by 1728 (number of cu.in. in a cu.ft.), divided by 3.881 (number of cu.ft. of steam per pound of water at 100 psi) divided by 8.34 (pounds of water per gallon of water):
{[(1.5" x 1.5") x Pi x 5" x 2 x 300 x 60] / 1728} / 3.881 (=189.72lbs/hr) / 8.34 = 22.78 gal/hr

All of the above assumes the admission of steam for the full stroke (90%), usual valve and gland leakage, cylinder condensation, etc. This amounts to about 37.9 lbs. steam/water per HP hour which is a decent figure for a small single cylinder engine with no real variable cutoff (I assume the reverse is by slip eccentric).

Since there are 7.4805 gal/cu.ft. of a liquid, then you need 3.045... cubic feet or 5262.2 cubic inches of water an hour.

If your plunger/ram pump (the best choice by the way for a feedwater pump, particularly if running around 200 strokes per minute or less) is 0.625" punger diameter by 2 inch stroke, then:
(.3125" x .3125") x Pi x 2" = 0.6135... cu.in. per stroke or 8576 strokes per hour or 143 strokes per minute. Generally 1.5 times the amount of feedwater required is considered best for the pumped supply (allows for whistle blowing, providing steam for tea or espresso machine, catch-up if the water level is low, etc) then you pump would need to make 215 strokes per minute. Nicely within all the parameters.

The above calculations and "rules of thumb" follow the best marine steam practice for small marine engines. They are reasonably accurate.