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Simple boiler control

Posted by steamerandy 
Simple boiler control
August 17, 2013 06:26AM
Jim talked about the Waterman control a while back. I proposed this before. But I think I have a better explanation worked out.

This would be for a mono tube steam generator. I may be way off but it was an inspiration for the White proportional system. The thing is that it's just to simple. Must have already been tried. Why would it not work.

The basic idea is using a donkey feed pump but the water cylinder and steam cylinder are proportioned to pump equal pound of water for equal pounds of steam at the intended operating steam conditions. Water density is fairly stable compared to the steam. For example say we wont the output steam to be at 1000 PSIA and 800F having a specific volume of 0.6878 ft^3/lb If the feed water is between 100F and 200F Using Keenan and Keys steam properties I figured some pump ratios.

       100F    200F  Feed water temp
780   40.746  39.516

800   42.649  41.362

820   43.573   42.259
The above are specific volume ratios of feed water at 100F and 200F to steam temperatures of 780F,800F, and 820F at 1000PSIA
I used compressed feed water. Maybe should be atmospheric. Not a big difference. Feed water temperature makes much more of a difference in the ratios. Pressure makes a difference in the specific volume of 5*10^-6 between 1000PSIA and atmospheric

Using a pump ratio of 42 will put the steam temperature between 790 and 810 F if 1000 PSI is maintained by firing. With that volume displacement ratio the steam cylinder would have a 42 to 1 mechanical advantage needing only 23.81 PSI to match the 1000 PSI boiler pressure. Figure with extra margen would still be less then a 30 PSI drop across the steam side of the pump. So we would have 970 PSIA out of the pump.

The pressure is maintained by the fire. If the temperature is high then steam is less dense (higher specific volume) The more pounds of water are pumped than pounds of steam used in pumping it. That should effect a temperature drop sense we are heating more liquid and vaporizing it. Likewise when the steam temperature is low less then 1 pound of water is pumped per pound of steam. Anyway that is the concept.

Blow some holes in it please.


A fuel pump could be hooked to the water pump and pump proportional fuel to water.

But using an electric fuel pump would probably be best. If not moving the fire would maintain pressure. And be ready to go.

Edited 2 time(s). Last edit at 08/17/2013 06:49AM by steamerandy.
Re: Simple boiler control
August 17, 2013 12:26PM
steamerandy Wrote:

> Blow some holes in it please.


My first impression is, this isn't going to work. The reason is, a monotube boiler is far more dynamic than a fixed ratio FW system would allow.

The first dynamic is, the superheat zone varies wildly depending on the pressure and the amount of water in the coil. Just open the throttle and the superheat zone is suddenly multiplied many times over; this results in wildly varying heat absorption rates from various parts of the coil depending upon whether they contain steam or water (or both).

It's been proven over many efforts that a dynamic feed system works best, with multiple feedbacks into the system.

The Lamont works because it defines the economizer, evaporation and superheat zones--and maintains them through dynamic flows regardless of the amount of steam being drawn off the generator.

Just my opinion.

Re: Simple boiler control
August 17, 2013 07:42PM
Thanks Bill.

I agree that the Lamont is a lot easier to control. A similar boiler was built in England having no recirculation. It simply had the generator output entering low in the stand pipe. Baffles in the stand pipe forced a long path for steam to travel up through the water. The water in the stand pipe would be at saturation temperature. When you have a saturated mixture of steam and water entering the stand pipe the steam simply separated out to the top of the stand pipe. When you had superheated steam entering the stand pipe the super heat was given up to the water in the stand pipe as it traversed through the baffles to the top. The heat given up to the water caused evaporation making more saturated steam and lowering the water level. The stand pipe water level was simply controlled by the firing rate. Producing saturated steam mix raising the water level or superheated steam lowering the water level. Saturated steam was taken off from the top of the stand pipe. The comical boiler was for the dry cleaning industry and built to produce saturated steam. But for superheated steam it would be simple to run through a super heater section like a Lamont.

"It's been proven over many efforts that a dynamic feed system works best, with multiple feedbacks into the system."

What feed backs are there other then pressure and temperature in a mono-tube boiler?

The stand pipe adds water level to the control and provides a separation of water and saturated steam going to the super heater. You do not have control of the superheat temperature. Just assurance that you have superheat. The only control is fixed by the placement and length of the super heater, When industrial processes require very specific super heated steam temperature a separately controlled super heater is used.

I think I still did not explain the control system well enough. Let me try again

The control system has three inputs. Usage, Pressure, and Temperature.


Actually it's not the feed that is dynamic, It' is the usage.

That is where the proportional feed pump comes in. It is in series with the load metering water into the steam generator in proportion to that being used by the engine. So when the throttle is suddenly opened the pump will respond feeding water at a higher rate. The feed proportional is for a specific pressure and temperature. The system does require the pressure to be held. Pressure is the feed back controlling the fire, heat input. This system does relay on pressure being held and may need some volume of steam reserve to dampen pressure fluctuations. The pump is also controlling the temperature. With a stable pressure the pump will pump less water when the temperature is low and more water when temperature is high.

As an argument of all the variables inside the boilers. The changing zones etc. All I can say is that experience has shown that the White flow-motor control system worked astonishing well. It didn't address all that internal goings on and worked quite well.

Some of the problems I am worried about is that specific volume ratio is not a single point but a line. I am trying to control for a specific pressure and temperature state output. But the specific volume that of that steam follow a line of changing pressure and temperature. The is lower pressure and temperatures having the same specific volume as well higher. So it is a line passing through my control point. Shown in the diagram below.

Looking at the constant specific volume ratio line above. Pressure horizontal and temperature vertical, If the steam state isto the left or above the constant ratio line excess water is being pumped. On the right or below the line too little water is being pumped.

A fully mechanical proportional system would have a piston fuel pump driven by the donkey pump. A mechanical linkage that can adjust the fuel pump stroke from the steam pressure.

The Lamont has a good quantity water at the saturation temperature in the stand pipe if it were to burst. That water would flash to steam just the same as a fire or water tube boiler. So there is some explosion hazard with a Lamont. it's not likely to suffer overheating failure as a water or fire tube boiler does in low water saturations. But in an accident the stand pipe could suffer collision damage and burst.
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P_T_PumpRatioChart.JPG 49.5 KB open | download steamerandy 08/17/2013 Read message