Orifice Discharge Coefficient

[fredrosse]

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 1.0

The Cd for an orifice is strongly influenced by the shape at the fluid flow entry, and a sharp edged orifice typically has a Cd very close to 0.6 if the orifice plate thickness is no more than about 20-25% of the orifice hole size.

If the machinist just touches the inlet hole edge to break the sharp edge, then with the slightest cut the Cd goes up several percent, and reliable results become difficult. If the inlet hole is countersunk on the inlet side, the Cd will go up into the 0.8 - 0.85 range, and reliable results are difficult unless the orifice is calibrated with a known flow rate. If the hole is very well rounded and polished (think of the end of a musical horn), then the Cd will be 0.97 - 0.99.

For our purposes a simple sharp edged hole reamed into a flat plate, countersunk on the DOWNSTREAM side will have an effective orifice area equal to 60% of the calculated hole area. The straight cylindrical portion of the flow path should be about 20-25% of the hole diameter.

It is relatively easy to make a few orifices from ordinary pipe plugs. Chuck up a pipe coupling in a lathe, screw in the pipe plug, machine the orifice, and screw into your steam piping at a conveinent location.

[Old Steamer]

Fred,
This is all good stuff but is it relevant to our engines which typically have HP in single figures?
Regards,
OS

[fredrosse]

This is really associated with the thread on this forum "BOILER POWER", which addresses how to determine the steaming capacity of a boiler (any size). One method outlined measuring the steam flow rate based on the pressure upstream of an orifice, and the relevant details about orifice variations are given here.

[Cyruscosmo]

Hey Fred

Here in the near future I am going to set up the bigger of the two coils I showed you to determine how much steam I can get out of it. From what I have seen of other coil setups this coil compares favorably except for the sheet metal tube wrapped around the outside. I am going to remove this tube and give it a go. When I get some good numbers then I can add a preheat tube wall around the outside run it again and see how much better it works. Last I will add a super heater.

The smaller coil is obviously shot with that big hole and the rust pitting. I have another one that is in working condition and did not sit in a frame full of rain water for a couple months.

What assembly of fittings should I use for setting up a test rig to determine flow rate within a reasonable degree of accuracy? Like pressure gauges, temperature gauges, valves and so forth. I want to get a better idea of what this coil assembly can output in numbers I can work with instead of speculation.

On initial calibration of the orifice, what method would you suggest. Water under pressure, air? In your post you outline a method of making the orifice, what size range should I start with? Do I need to dump the steam into a chamber after the orifice to be measured or can I just vent to atmosphere?

When I fire the coil I will be measuring the oil used in a setup like Bart's fire box that he linked the PDF for.

After I get a good base line I will add a steam atomizer and work out the size of the pump I will need for a LaMont setup. Like Bart said in one of his posts, if I can't get a good reliable pump working there is no point in messing with a LaMont boiler.

Cyruscosmo

[Lopez Mike]

On my old flow bench that I used to test cylinder heads, intake manifolds, carburetors and fuel injection bodies for flow, I used a sheet metal orifice that had the same diameter hole as the tested device. It had a sharp up stream edge and a very slight downstream bevel and was arbitrarily assigned a co-effient of .60.

Since in this bench design the air went through the tested device and then the orifice (after a flow straightener before the orifice) the other variables such as air density and humidity cancelled out and I could document the flow co-effient of the tested device in a non-dimentional way.

I have no idea what the hell might be happening when doing this with steam! The state changes as the steam expands must be really complex. Fred??

[fredrosse]

Just added "Testing Boiler Output" to the FAQ section of the forum. Rather long winded and boring, but it will give reasonable results.

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

The 1/8 inch diameter orifice shown here would pass 43 Pounds per Hour (PPH) steam flow with 100 PSIG inlet pressure (114.7 PSIA)

If all the orifice dimensions were doubled (to 1/4 inch diameter orifice), then it would pass 4x as much, or 172 PPH @ 100 PSIG.

[Cyruscosmo]

Hey guys

Thank you for the information and the pictures Fred.

That seems pretty straight forward. So we start with a coil in an insulated enclosure. The enclosure sits atop a Kast-O-Lite 30 fire box with burner assembly that can be changed out easily between test runs.

The input of the coil will get a temperature sensor, strainer and some form of flow sensor. The strainer at this point is to keep the flow sensor functioning during tests. After the tests it would not be needed in the system.

The output of the coil will have another temperature sensor and a strainer? I know there should be a strainer at some point before the pump but placing it at the pump intake will restrict flow and may cause cavitation. Any thoughts?

The output of the coil is fed tangentially into the separator. The steam safety valve, steam shut off valve, running steam gauge, drain valve, feed water input from pre-heater and so forth are plumbed into the separator.

Heated water swirls around the inside of the separator, enhancing steam separation. The pump intake port at the bottom of the separator is arranged so that the swirling water enters straight on.

A fixture is made with a shut off valve, temperature sensor, pressure sensor, orifice bypass valve, orifice, then vent to atmosphere/water tank. Can I do the water tank temperature rise test at the same time?

Then it is just a matter of making a lot of steam and keeping track of time and numbers.

Does that sound about right?

Cyruscosmo

[fredrosse]

From your post it appears you are going with a Lamont type boiler. Two questions:

How will you regulate feedwater flow?

What type of Lamont circulating pump will you have that can take the saturated water inlet and still function??

"In your post you outline a method of making the orifice, what size range should I start with?" ANS: Use an orifice sized to pass 1 PPH per every thousand BTU oil burner nozzle size, at maximum safety valve set pressure. For example, if the oil burner nozzle is for 1.0 GPH (Gallon per Hour), that is 140,000 BTU per hour, so size the orifice to pass 140 PPH steam flow at the boiler maximum allowable operating pressure. Say you select 200 PSI maximum operating pressure, assuming you can get valves steam rated for this. Then your test orifice would be sized for 140 PPH at 215 PSIA inlet pressure. When you test the pressure achieved upstream of the orifice will be something less than the maximum, in the vicinity of 70%, approx.

"Do I need to dump the steam into a chamber after the orifice to be measured or can I just vent to atmosphere?" ANS: you can just vent to atmosphere, the only criteria is to keep the steam flow measuring orifice outlet pressure less than half the inlet pressure.

"The output of the coil will have another temperature sensor and a strainer? " ANS: I would think these are not really necessary, if you are generating saturated steam in the coils, with a Lamont boiler the temperatures throughout will all be close to the saturation temperature as a function of boiler pressure.

In case you do not have steam saturation tables, this function gives saturation temperature as a function of pressure with fair accuracy: Tsat (F) = 117.17 * Psat (PSIA)^.22322

"I know there should be a strainer at some point before the pump but placing it at the pump intake will restrict flow and may cause cavitation. Any thoughts?" ANS if the strainer restricts flow enough to interfere with pump operation, then it needs to be
bigger, and have very small pressure drop. This would be true for any location between the separator and the pump suction.

"A fixture is made with a shut off valve, temperature sensor, pressure sensor, orifice bypass valve, orifice, then vent to atmosphere/water tank." ANS; Could you make a simple piping diagram to show how this is arranged?

Yes, you can run the orifice and tank heating tests at the same time. Just be sure to keep the steam flow measuring orifice outlet absolute pressure well below 1/ 2 of the inlet pressure.

[Cyruscosmo]

Hey Fred

Yes I am going to give the LaMont a go. At this point in time I have a crap load of raw materials a set of new boat plans and an idea of what I want. So for now I am tinkering with various ideas.

Regulate the feed water flow? I was going to use the method most of you already use. Exhaust from the engine goes into the condenser, and then a pump grabs the condensate and dumps it into the hot well. You know the rest... Will that not work?

The pump? I have two pumps in mind. One is a plunger style and the other is motor/shaft/impeller. I would like to make the plunger style work as I can drive it with a steam driven piston which is easy enough to make.

About the strainer, it occurred to me that if the strainer “were to collect” enough stuff to cause any slowing of the flow there would be a problem of cavitation. That is why I would mount the strainer before the steam separator because if a little cavitation occurred at that point the bubbles would be removed in the separator. If the strainer were mounted at the inlet of the pump then the bubbles would be more likely to cause pump problems. That is just my guess...

I have read that the LaMont style boilers rarely have any scale problems, however I think tempting fate is a foolish idea. I do intend to use distilled water on fill up and a condensing system but no matter how careful you are something always gets into a plumbing system that is the perfect size to bring everything to a halt.

But first of all I need to build a pump. Like Bart pointed out without a pump system that will work under those conditions there is no point in going further. I know there is a way to do it, just gotta figure out how. So we do some tests and figure out what works then add another piece.

Cyruscosmo

[fredrosse]

" So for now I am tinkering with various ideas. " In my view, tinkering with machines is great fun if you have the time for it. Good luck with your endeavors.

"Regulate the feed water flow? I was going to use the method most of you already use." ANS: Most of us use fairly erratic methods, which would not work well on a LaMont boiler. For example, my VFT boiler can be steaming at full output, feed flow can stop, and it will be over four minutes for the boiler water level to drop one inch, plenty of time for coarse control methods. A watertube boiler has less water inventory, and requires faster response, probably around than a minute for a one inch drop in level. With a 3 horsepower LaMont boiler having a 3 inch diameter separator, one inch drop in level would occur in just 9 seconds. The Lamont would need reliable and quick acting automatic control here.
"The pump? I have two pumps in mind. One is a plunger style and the other is motor/shaft/impeller. I would like to make the plunger style work as I can drive it with a steam driven piston which is easy enough to make." ANS: I have been a corporate pump consultant for many years, and have thought about a suitable pump for a LaMont circulator, which has very challenging design problems. The only reliable method I have found is to mount the pump 10 to 15 feet below the separator, however this is no solution on a steam launch. Hope you can find this solution, maybe the steam car group has worked up something?

"About the strainer..." ANS: Yes, strainer upstream of the water level in the separator would be a good place to put it. Note that the two phase steam/water mixture passing thru here will be often somewhat violent, so use a sturdy strainer.

"I have read that the LaMont style boilers rarely have any scale problems, however I think tempting fate is a foolish idea. I do intend to use distilled water on fill up and a condensing system but no matter how careful you are something always gets into a plumbing system that is the perfect size to bring everything to a halt." ANS: Good policy. Note that distilled water is good, however collected rainwater is very nearly as good, and easy to collect from roof drains. After about 15 minutes of rain flushing my roof surface, the collected rainwater is 1 - 2 PPM (Parts per Million) TDS (Total Dissolved Solids), almost deionized water. For comparison, the lake water we all use for our steamboats in PA is 115 PPM, and my city drinking water is 265 PPM.

"But first of all I need to build a pump. Like Bart pointed out without a pump system that will work under those conditions there is no point in going further. I know there is a way to do it, just gotta figure out how. So we do some tests and figure out what works then add another piece." ANS: Good luck with that. This is why I would use a monotube boiler with no LaMont circulating pump. Good reliable automatic control of separator water level throughout the variable load range of the boiler, no steam generating coil burnout, nothing but a common cold water feed pump as the only moving part of the entire boiler & feedwater system.