The Steamboating Forum

3. Testing Boiler Output with a Discharge Orifice

This method uses the boiler operating normally, with steady conditions. All of the steam output is fed through an orifice of known size, and the pressure at the inlet to the orifice determines the steaming rate.

Again, steady conditions are generally required, the boiler is running with maximum firing rate, normal feedwater flow, and is holding a steady pressure at the inlet to the steam flow orifice.

The orifice is sized so that it will pass full steaming rate at somewhat less than rated steam pressure. While firing the boiler, steam pressure is initially low, but will build up to a steady state pressure, and that steam pressure will be used to determine the boiler's steaming rate.

For example, say you have a boiler that you think produces 150 PPH (pounds per hour) steam at 114.7 PSIA main steam pressure. Note that we must use absolute pressure here, not gauge pressure. Absolute pressure is generally taken at gauge pressure (100 PSIG) plus 14.7 PSIA atmospheric pressure.

The Rankine formula for a steam orifice is:

Steam Flow (PPH) = 51 x Effective Orifice Area (square inches) * inlet PSIA

The effective area of any orifice is defined by a “coefficient of discharge”, Cd, which would be 1.0 for a perfect frictionless orifice. The effective area of an orifice depends on its shape; real orifices always have a Cd less than 1, and for typical sharp edged orifice the Cd is close to 0.6 for our purposes a simple sharp edged hole reamed into a flat plate will have an effective orifice area equal to 60% of the calculated hole area.

Say we have an orifice with 1/4 inch diameter. The hole area is Pi*Dia^2 / 4 = 0.049 square inches. The effective area is 60% of this value, = 0.030 square inches.

At 114.7 PSIA the steam flow is calculated at:

Steam Flow (PPH) = 51 x 0.030 (square inches) * 114.7 PSIA = 172 PPH

So we have an orifice that is somewhat oversized for our expectations, that is OK.

Now the boiler is fired and steam pressure is raised to about 70 PSIG, then, with full fire the steam is directed thru the orifice. Continuously firing the boiler, you are able to push steam pressure up to 80 PSIG, and hold that steam pressure. That is as high as you can get. Now the boiler is at maximum output, and you can calculate the steaming rate.

Steam Flow (PPH) = 51 x 0.030 (square inches) * (80 PSIG +14.7 PSIA) = 142 PPH

Note that if steam pressure cannot be continuously held at the measured pressure, errors would be introduced similar to the previous example. The measured steaming rate by this method is only valid when considering the steam pressure that you can continuously maintain.

Back to the original question by petethepen1 on this thread:

"My thoughts are that one would include 1/4 of the surface area of the mud drum, probably none of the steam drum as it is not in sight of the fire, 1/2 of all of the bottom of the steam generating tubes, but should one include the three layers above that cannot see the fire?"

The ASME Code delineates how to determine heating surface, which is divided into furnace radiation surface and convection surface (all of the tube surface which is not counted as radiant surface, plus other surfaces that see hot gas circulation, and is not counted as radiant surface.

The boiler shown has about 15 square feet of tube outside surface area, of which 1.2 square feet is radiant heating surface. The lower mud drum has about 0.8 square feet radiant surface, for a total radiant surface of about 2 square feet for the entire boiler. Convection surface is about 15 square feet total, counting those portions of the lower and upper drums which experience hot gas circulation.

In terms of required safety valve capacity, which closely relates to steaming capacity, the ASME Code provides steaming rates according to how the water tube boiler is fired: Hand fired, 6PPH per square ft convection surface, and 8PPH for radiant surface, giving 102PPH nominal steaming capacity. If oil fired, the rates are higher, and would also represent hand firing with a forced fire, 10PPH per square ft convection surface, and 16PPH for radiant surface, giving 182PPH nominal steaming capacity.

For metric conversion, this boiler has 1.39 square meters convection surface, and 0.185 radiant surface area, with equivalent steaming capacities of 40 kg/hr hand fired, and 82 kg/hr oil fired.

From a practical point of view I would have no way to deal with the feed water issue. That is, when I normally use the boiler near capacity I am running the engine with its shaft driven feed water pump. I would probably drop in my tracks trying to keep up with a test like this with my hand pump!

My home water system only runs about 50 p.s.i. I suppose if I lived in a city with higher system pressure I could try that. Of course them I would have to deal with city neighbors calling the fire department or police over the noise. Ah, well.

I have an engine cylinder head testing flow bench that side stepped having to measure air density by having the tested device in series with a sharp edged orifice as described. It was surprisingly difficult to get the orifice right. I finally made about ten of them and discarded the few that had significantly higher flow.

"From a practical point of view I would have no way to deal with the feed water issue. That is, when I normally use the boiler near capacity I am running the engine with its shaft driven feed water pump. I would probably drop in my tracks trying to keep up with a test like this with my hand pump!"

I believe these tests can be conducted with an engine driven feed pump, except for the test using the change in water level indication, and that test requires no feed pump at all.

Hmm. Using the engine driven pump means running the engine and that would be consuming some unknown quantity of steam.

I'll give some thought to the water level thing. In my VFT there would be a linear relationship between level and quantity of water. Makes the measurement easy. There is the issue of the heating area changing both in quantity and quality as the level changes.

I'll bet that with say a quarter inch diameter orifice passing steam to atmosphere, the level would be dropping alarmingly fast. I'm not sure of that though. I do know that when I dump open my 1 1/2" blowdown valve, the level is off the bottom of the glass in seconds. Makes the cat run fast too.

So perhaps I might use a relatively large orifice like .25" (easier to make accurately). Have a regulating valve followed by a gauge up stream of the orifice. A flowmeter basically.

I could have a relatively large pipe up stream of the orifice to let the flow stagnate a bit so the orifice would be seeing more laminar flow. With orifices smaller than about .25" I would get into the regime of the hole to thickness ration being too small. I'm not sure I can ream a hole in shim stock that wouldn't have some weird edge shape.