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 Post subject: Testing Boiler Output
PostPosted: Sun Jul 19, 2015 4:03 pm 
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Determining the steam flow output capability of a small steam boiler can be accomplished by several methods. Tests will generally be fairly crude with small wood or coal fired boilers that are hand fired, as the consistency of firing is generally quite variable. None the less a rough approximation of the boiler’s output can be made.

With oil or gas firing, a much more constant load can be maintained on the boiler, so better results are available.

Be aware of safe practices during testing, secure piping such that its dischadge direction is fixed, and keep people and pets away from potential danger and burns. Pets are usually smarter about this than humans.

Orifice Output Method.

In this test, a steam flow orifice, discharging to atmosphere, determines steam flow according to Napier’s Formula for saturated steam critical flow, where the outlet pressure of the orifice is less than 50% of the inlet pressure:

Steam Flow (PPH) = Knap x Ae x P1

Where: Steam Flow id expressed in Pounds per Hour (PPH)

Knap = Napier’s Flow Constant, 51.0 (PPH/PSIA/in2)

Ae = Orifice Effective Area, Square Inches

P1 = Orifice Inlet Absolute Pressure, PSIA

P2 = Orifice Outlet Absolute Pressure, PSIA, and P2 must be less than half of P1

Note that throughout this testing, all pressures are expressed as Absolute Pressures, not Gauge Pressures. Absolute pressures are generally the gauge pressure plus the local atmospheric pressure.


The Orifice Effective Area depends on the shape and length of the orifice, plus the bore diameter of the orifice. The general formula for Effective Area:

Ae = Cd x A

Where: Ae = Orifice Effective Area

Cd = Coefficient of Discharge, or how close the actual orifice or nozzle area achieves flow compared to an ideal perfect theoretical orifice.
Cd typically is 0.6 for a small short orifice with a sharp inlet bore, and ranges into the 0.9-0.98 range for orifices with well rounded and polished entrance.

A = Orifice Actual Area

For small boiler testing, use a sharp edge short orifice, around 1/8 inch diameter. Do not break the sharp inlet edge of the orifice entrance. Use a Cd of 0.6

In this test, the orifice is sized to match the boiler’s expected output at about 75% of the normal steam operating pressure. The boiler is brought up to 75% pressure, full firing commences, and all steam output is directed thru the orifice.

While maintaining water level in the boiler, full firing continues, and steam pressure at the orifice will climb or decrease until steady (constant pressure) is achieved. After holding steady pressure for a period of about 15 minutes, with steady feedwater flow maintaining boiler water level, record the orifice inlet pressure, then calculate the steam flow according to Napier’s Formula.

If pressure climbs to the maximum boiler working pressure, then you need a somewhat larger orifice. If pressure falls too low, then a smaller orifice should be setup.

Holding steady conditions is the key here, so pumping of feedwater should be fairly steady, as well as steady firing. Have a helper record the boiler steam pressure every 30 seconds during the test, as well as the boiler water level elevation.

Condensing Tank Method

In this test, the entire steam output of the boiler is valved into a cold water tank, where the steam condenses, and heats the water. The setup is the same as above, and the procedure is the same, with the following exceptions:

In place of the orifice, a small outlet throttle valve is substituted, so the flow resistance can be adjusted to match the boiler operating pressure while discharging to atmosphere.

An insulated metal water tank holding several gallons of water with an accurate submerged thermometer is provided, about 10 gallons capacity for our typical launch boilers. A typical large thin wall stainless steel cooking pot will do fo this, wrapped with an inch or so of fiberglass insulation or terry cloth layers. The tank is filled to about 90% capacity with ordinary tap water at ordinary ambient temperature, say 50F – 70F. Accurately record the mass amount of water in the tank, plus the tank metal weight.

The output throttle valve is connected to an output chamber (you can usually use a large pipe Tee for this) which has a tube connection leading into a submerged outlet within the water tank, plus a large ball valve for atmospheric steam discharge. For our typical launch boilers, the tube to the water tank can be about 1/2 to 3/4 inch diameter, about 3 to 6 feet long, and the large ball valve 3/4 to 1 inch diameter. All piping and tubing must be metal, thin wall copper or stainless steel or carbon steel is proper.

The boiler is brought up to full pressure, with full firing. Adjustment of the outlet throttle valve holds pressure, and all steam output is directed to atmosphere.

While maintaining water level in the boiler, full firing continues, and steam pressure at the boiler will be steady. After holding steady pressure for a period of about 15 minutes, with steady feedwater flow maintaining boiler water level, Quickly close the large ball valve, and start recording time with a stopwatch.

Steam will instantly flow into the tank and start condensing. When the tank water is heated to 140F to 170F, open the large ball valve and record the time. If you wait too long here, the condensing process will become less and less effective because the water will approach 212F, giving violent bubbling, with steam escaping the water tank and thus energy loss to the ambient. We want all of the steam energy to be captured in the water tank. After opening the large ball valve, immediately stir the water to assure even temperatures and record the final temperature.

Holding steady conditions is the key here, so pumping of feedwater should be fairly steady, as well as steady firing. Have a helper record the boiler steam pressure every 30 seconds during the test, as well as the boiler water level elevation.

Now a simple energy balance here will determine how much steam flow the boiler produced during the stopwatch period.

Condensation Energy = Energy Gain of Tank Water + Energy Gain of Tank Metal
Here we use “British Thermal Units” (BTU) as the typical USA energy unit. Use Metric units if you are more comfortable with them.

The energy gain of the Tank Water = Mass x Specific Heat x Temperature Rise

The energy gain of the Tank Metal = Mass x Specific Heat x Temperature Rise

For the water, the specific heat is 1 BTU per Pound (mass) per F Degree

For the Steel, the specific heat is 0.1 BTU per Pound (mass) per F Degree

The specific heat of other tank metals is usually close to 0.1, and the accuracy of this number is usually not too important, as the water mass is several times the metal mass in typical testing.

Calculate the Condensation Energy.

Steam from a launch boiler generating saturated (or very slightly superheated) steam will give up about 1000 BTU. Thus the steam generation rate for the test is:

Steam Generation (PPH) = (Condensation Energy / 1000 ) x (3600 / Seconds)

Where the time in seconds is the stopwatch reading, the time condensation was occurring.

Combined Testing

You can combine both of these tests, and get a better average output number. If the tests are conducted as described, the two test methods will be prodicing very similar results.

Repeating the tests a few times would be standard practice, to assure consistent results.


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 Post subject: Re: Testing Boiler Output
PostPosted: Tue Jul 21, 2015 12:32 am 
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Attached drawings of a typical test orifice for small launch boilers, made by pressing a brass insert into a small pipe nipple. Orifice size should ideally be reamed, and it is important to NOT break or chamfer the sharp inlet edge in any way. This orifice will have a discharge coefficient close to 0.6

This orifice would be better in stainless steel, but more difficult to machine.


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ORIFICE NIPPLE.jpg
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ORIFICE INSERT.jpg [ 13.89 KiB | Viewed 5623 times ]
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 Post subject: Re: Testing Boiler Output
PostPosted: Tue Jul 21, 2015 12:51 am 
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An example of the Orifice Test, using an orifice the same as detailed above, 1/8 inch diameter sharp edged inlet.

Firing the boiler on test, it is found that steam pressure can be held at 140 PSIG with all steam directed thru this orifice, discharging to atmosphere.

1 Determine orifice effective area:

Area = 1/8 x 1/8 x Pi / 4 = 0.125 * 0.125 x 3.14 / 4 = 0.0123 square inches gross orifice area

Since this is a sharp edge inlet orifice, the discharge coefficient Cd will be 0.6

Effective Area = 0.0123 x 0.6 = 0.00736 square inches effective.

2. Determine upstream absolute pressure

PSIA = PSIG + Atmospheric Pressure = 140 PSIG + 14.7 PSIAtmospheric = 154.7 PSIA

3. Determine steam flow with Napier's Formula:

PPH = Knapier x PSIAinlet x Effective Area = 51 * 154.7 * 0.00736 = 58.1 Pounds Per Hour Steam Flow


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 Post subject: Re: Testing Boiler Output
PostPosted: Tue Jul 21, 2015 8:02 am 
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Location: Lopez Island, Washington State, USA
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Great stuff. There will noisy afternoons happening.

A: I had no idea that the ration of diameter to bore length could be that much. Another words, I thought that for a .125" diameter that the length had to be much less than .025". Good to know.

B: I have no idea how I would set this up without inducing a heart attack from pumping my manual feed water pump. Right now my only two feed water methods are that hand pump and the engine driven pump and it would totally mess up the test to run the engine. All the other feed water pumping schemes I can think of just now also consume steam. I guess I need a second boiler to run the accessories during the test.

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 Post subject: Re: Testing Boiler Output
PostPosted: Tue Jul 21, 2015 8:24 am 
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Orifice bore length could be much less than 20% of the bore, but for ease of machining the 20% ratio (0.025 on a 1/8 inch orifice) is easier, and the discharge coefficient is still close to 0.6

Yes, pumping feedwater is something a teenage helper might be employed for, saving you from a heart attack. Alternately you might run your main engine or feed pump with an air compressor?

Another option might be to run the main engine with the boiler you are testing. Boiler, then orifice, then main engine. As long as the engine inlet pressure (orifice outlet) stays below half the main steam (orifice inlet) pressure, the orifice will be in critical flow, and the testing works OK. With the main engine only working the feed pump (propeller disconnected or out of the water) the low engine inlet pressure should work OK.

If you do it this way you might still be dumping some steam to atmosphere to assure a low orifice outlet pressure, the lower the better. Remember we are talking absolute pressures here.


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 Post subject: Re: Testing Boiler Output
PostPosted: Tue Jul 21, 2015 8:36 am 
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Disconnecting the prop would probably work for me.

My keel condenser wouldn't have any flow across it so my hot well would run hot. There is no lift from the hot well to the pump so pumping boiling hot water would probably work O.K.

The engine would be under very low load so the drop across the orifice would be not that much less that to atmospheric.

I will never bother to do it but it's an interesting thought experiment. I can see myself at the public dock attempting to explain what I'm up to. Kinda like putting your car's bumper against a building and racing the engine!

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 Post subject: Re: Testing Boiler Output
PostPosted: Tue Jul 21, 2015 2:56 pm 
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An example for the condensing test.

Stainless steel water pot, ten gallons US capacity, weight = 12 pounds
Filled with 9 gallons of water at 60F, weight = 75 pounds

Firing the boiler with steady conditions achieved, and steam dump to atmosphere, then dump ball valve is closed and timing initiated.

The tank is getting much too warm to touch, and almost full, at 7 minutes. At 7 minutes 14 seconds the dump valve to atmosphere is opened (no more steam to the tank), the tank stirred for uniform temperature, and the final temperature indicates 152F.

1. Calculate Condensing Energy, Temperature rise = 152 - 60 = 95 F degrees temperature rise.

Water Condensing Energy = 75 poundsmass x 1 BTU/poundmass/F Degree x 92 F Degree = 6900 BTU

Tank Metal Condensing Energy = 12 poundsmass x 0.1 BTU/poundmass/F Degree x 92 F Degrees = 110.4 BTU

Total Condensing Energy = 6900 + 110 = 7010 BTU

Condensing Energy Rate = Energy per unit of time = 7010 BTU / ( 434seconds / 3600 seconds/hour ) = 58,148 BTU per hour

Since the condensation of 1 pound of steam is about 1,000 BTU, the boiler output is about 58 pounds per hour (PPH)


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