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Scope
Scope
Steam Generation covers overall system design and component selection up to steam entering the steam chests, including water and fuel storage, the steam generator itself, the accumulator, main throttle and other valves, and instrumentation.
Generator/Accumulator System
Generator/Accumulator System
Instead of a fire-tube boiler to raise steam, the vehicle will use an intermittently fired monotube steam generator to charge a steam accumulator. The accumulator then provides steam to the cylinders and accessories. The steam generator will need to be oversized to be able to charge the accumulator while it is being discharged, and the steam accumulator big enough that the time between generator firings is reasonable. Ideally, you'd get several minutes of driving in between a minute or two of firing.
Advantages of the intermittently fired accumulator design:
Fire tube boilers are difficult to build, an accumulator can be a standard ASME code VIII Div 1 pressure vessel
Fire tube boilers have many failure points
Fire tube boilers have many nooks and crannies for scale and cruft to accumulate and that are hard to clean, accumulators can be completely emptied and inspected
Steam systems are dangerous due to the energy stored in pressurized water above ambient boiling temperature. In a fire tube boiler, energy is actively being added to the energy storage device, and failures in that energy addition can produce failures (like overheating the crown sheet or flue plate). In the trike system, energy addition (steam generator) and energy storage (accumulator) are separated. The steam generator could rupture but the accumulator would not be affected.
The fuel burner runs at one output; fuel and air settings can be set once and not adjusted during operation.
Starting up a fire tube boiler takes a lot of time because you have to heat the entire boiler. Conceivably, one could use steam output from the generator right after startup, while the accumulator is still cold (some special valving would be required).
Basic Steam Generation System
Basic Steam Generation System
Firing procedure:
Apply power to burner blower and pump. Fuel cutoff solenoid remains unenergized due to flow switch interlock.
Apply power to feed pump. Once flow picks up, burner will turn on automatically.
Monitor pressure and level in accumulator. Injected fluid from generator will tend to either increase or decrease the fluid level in the accumulator. Decrease feedwater flowrate to decrease accumulator level or increase feedwater flowrate to increase accumulator level.
If accumulator is too full, vent some fluid via blowdown valve
If accumulator is too empty, turn off burner power and pump cold water into accumulator
When accumulator is charged, turn off burner and feed pump power
Boiler Feed Pump
Boiler Feed Pump
The boiler feed pump transfers water from the water tank, which is at atmospheric pressure, to the generator coil, which, during generator operation, is at some pressure higher than the accumulator pressure. There is a market for DC agricultural sprayer pumps that get to 150 psi, albeit at relatively low flow rates. These are not constant speed and have a pump curve despite being positive-displacement pumps; the motor gets increasingly bogged down as pressure increases.
Shurflo models 8030-813-239 (Viton) or 8030-947-230 (EPDM) are rated to 0.8 GPM @ 150 psi, using 12 amps at 12VDC
Remco 3323-1L1-63B is rated to about 0.66 GPM @ 150 psi using 9.25 amps at 12VDC, but can go up to 0.45 GPM @ 200 psi using 11.5A at 12VDC
Northstar 26820200 has no published curve but has the highest max flow of 2 GPM and highest max pressure of 200 psi, maybe 1.0 GPM @ 150 psi?
Remco 3323-1K1-63B is rated to 1.35 GPM @ 150 psi using 9.5A at 12VDC
Alternatively, an inverter may be needed to power a constant speed pump to deliver higher flowrates at good pressure.
The pump will be protected by a mesh filter, and the water valve on the outlet of the water tank isolates the system.
Instrumentation and Controls:
Pump on/off switch at panel
Variable speed control or unloading valve, foot pedal or panel dial
Visual flow indicator/meter such as rotary vane or rotameter at panel
Flow switch for burner interlock, with panel indication
Shurflo 8000 series pump, https://www.pentair.com/en-us/products/business-industry/water-supply-pumps/spray-pumps/8000_series_diaphgram_pumps.html
https://dwyer-inst.com/en/products/flow/flow-transmitters/paddlewheel-in-line/seriessfi-800.html
Oil Burner
Oil Burner
DC powered oil burners are available for off-grid buildings and portable hot water/steam pressure washers. These include a motor that powers a blower and fuel pump, an electric arc igniter, as well as the adjustable air vents needed to dial in an efficient flame.
Beckett ADC/SDC series DC powered burners are rated up to 350k/700k BTU/hr, which is equivalent to 10/20 boiler HP (though this is the burner, not the boiler), or 20/40 mechanical HP using the 15% overall efficiency figure from Principles of Locomotive Operation by Arthur Wood.
Instrumentation and Controls:
Panel on/off switch for entire burner system. Burner is only turned on during firings.
Burners come equipped either with continuous duty ignition (igniter is on continuously during firing) or intermittent duty with a CAD sensor (igniter is only on while flame is not detected). If firing periods are expected to be long, could reduce the load by an amp or two by using the intermittent duty version.
Burners come with fuel solenoid, which will be interlocked with water flow switch and accumulator high pressure switch. Each interlock will have its own indicator light on the panel.
Beckett ADC burner, https://www.beckettcorp.com/product/adc-oil-burner-0-75-to-2-50-gph-dc-power/
Generator Coil
Generator Coil
The generator coil consists of stainless steel tubing arranged around the burner such that the exhaust gases from the burner must go around and in between the coils. We see that there are hot water pressure washers are available with either vertical or horizontal coil and burner arrangements, so we have some flexibility.
The burner and coils in hot pressure washers perform the same function as the steam generator, so we can use their data to ballpark how much of the heat input can be expected to transfer to the fluid. Using data from the Hydrotek SC35005VG diesel-fired pressure washer with Beckett SM or SF series burner,
Heat transferred to fluid = (Flow Rate) * (Temp Rise) * (Heat capacity of water)
Heat transferred to fluid = 4.7 [gal/min] * 140 [°F] * 1 [Btu / (°F*lb)] * 8.34 [lb/gal] * 60 [min/hr] = 329 kBTU/hr
Coil efficiency = (Heat transferred to fluid) / (heat input to coil)
Coil efficiency = 329 [kBTU/hr] / 454 [kBTU/hr] = 72%
This figure is a very rough figure because it depends on coil design, and also because the coil contains only liquid water in a pressure washer application vs liquid water and some amount of saturated steam in a monotube boiler. The steam absorbs heat less effectively than the liquid water, reducing the overall effectiveness of an otherwise identical coil. The takeaway is, if we can reach 65-70% transfer efficiency, we are performing comparably against a commercial heat transfer coil and we can feel good about it.
Esteem is a fired heater company who has published a guide to fired heater design. Esteem and other sources indicate that 70% of the heat transfer in a commercial unit comes from just the radiant section, giving us a clear focus for design.
Operation of the monotube steam generator will fall under 755.022(a)(3)(B) in the Texas Boiler Code; this section exempts small (<12" diameter), manually fired boilers used for education and recreation from boiler certificate requirements.
Instrumentation and Controls:
ASME Code I safety valve set to a higher setting than that of the accumulator - The tubing in the generator will be rated to hundreds of psi, so the valve will just be sized for the max possible steam rate and set at the next highest setting above (150 psi + pressure drop across across check valve to accumulator at max flow)
Pressure gauge
Inline sight glass? - In theory, allows operator to check if generator output contains liquid water
"Saturation Gauge"? - Combination pressure and temperature gauge with indications of saturation line, liquid, and superheated regions. This could be made by taking apart a remote reading dial thermometer and standard pressure gauge and mounting them in a single "dial" at right angles to each other. Cool, but really only necessary if the steam generator has that much range and the state of the output is not otherwise obvious.
Steam Accumulator
Steam Accumulator
The pressure vessel stores saturated water ready to boil off and provide steam for the cylinders, thus acting as energy storage between generator firings. Unlike boilers, accumulators can be built to ASME Code VIII Div 1, the standard for normal process pressure vessels. Off the shelf compressed air receivers with compatible MAWP and temp ratings are available from Manchester Tank, Samuel Tanks, and others, although the default NPT fittings are only 3/4" max. A custom unit with larger fittings may be required. They are also available from the factory with an epoxy lining, however after market tank coatings such as KBS may also be feasible.
It unclear to me whether steam accumulators are covered by the Texas Boiler Code, but to be safe we will use vessel(s) of less than 12" diameter to fall under exemption 755.022(a)(3)(B).
Spirax Sarco has a good walkthrough on sizing an accumulator for a stationary plant application.
15 gallon vertical air receiver drawing from Manchester Tank, https://www.mantank.com/products/standard-air-receivers/
Instrumentation and Controls:
ASME Code VIII safety valve sized and set according to the ASME rules from the pressure vessel's nameplate MAWP
Inlet check valve
Vacuum breaker/check valve - Prevents accumulator from developing negative pressure when cooling down
Pressure gauge
Level sight glass
Left/right/(rear?) blowdown valves
Main outlet valve - Isolates valve chests from accumulator
Aux outlet valve - Isolates accessories from accumulator
https://www.mcmaster.com/3885N116/
Regulator/Throttle Valve
Regulator/Throttle Valve
Most of the valves in the system are on/off 2-way valves, but the throttle valve needs to control steam flow linearly. As this valve will be controlled via a foot pedal (the accelerator pedal), the force needed to move the valve is a factor. There are a few options:
V-notch ball valve such as an Inline 304F (V notch option listed in catalog) - Instead of the normal full or reduced bore opening, the ball has a v-notch cut into it which is revealed proportionally over the 90 degrees of the ball valve opening. These can have a reasonably linear Cv curve, however the valve itself will need to be larger than other on/off valves in the system to account for much of the bore of the valve being blocked off even at 100% open. Ball valves take a relatively high amount of torque to turn, but the force is completely uniform from open to close. They are also available with a spring-return handle from the factory.
Globe valve such as the Spence K-series - These are industrial process valves designed to match with a pneumatic actuator, but without the actuator, they operate simply by lifting the stem. They are usually arranged such that the inlet line pressure presses the valve into its seat when closed. This means there is some force needed to get the valve off its seat in addition to whatever friction forces are present due to the valve packing, but once the valve is off the seat, this force may diminish significantly. This behavior may lead to interesting feedback on the foot pedal.
Ideally (from both an efficiency and operational fun standpoint), starting from a stop will involve applying the throttle, then eventually linking up with the reverser/Johnson bar and flooring the throttle until it's time to come to a stop again. If operation actually works this way, we may want an over-center type linkage where the returning force on the pedal suddenly drops off at the 100% position so it's easy to maintain without fatigue.
Design should also include a mechanism to automatically re-open the cylinder drain valves when the pedal is fully released. This way, you can't forget to re-open them starting from a stop. The drains can then be shut off manually at speed.
https://ballvalve.com/pdf/Inline_Digital_Products_Brochure_2023.pdf
Advanced Steam Generation System
Advanced Steam Generation System
One goal of an advanced steam generation system could be to replace the electric burner and pump with more prototypical equivalents running off of steam or compressed air.
Steam Injector
Steam Injector
The prototypical way to pressurize water in a live steam locomotive is to use a steam injector. This also extends into live steam modeling, with a cottage industry of machinists supplying working scale injectors of their own design. While efficient and prototypical to operate, the flow rate may not be sufficiently controllable for the steam generator. An electric or pneumatic pump would also still be needed on a cold start; you cannot operate a steam injector using compressed air.
Full-size Metcalfe injectors are available from 1 to 25 GPM
Eccentric Engineer offers injectors up to 2 GPM
Metcalfe steam injector, https://www.steaminjector.co.uk/steam-injectors
Air Compressor
Air Compressor
Steam powered air compressors are very prototypical, and provide a wonderful breathing beat to an idle engine that makes it seem alive. Compressed air can also be stored for long periods of time. This means that the trike can be started with air stored during the previous run. In theory, the battery could be completely eliminated, with lights and other electric accessories powered directly by alternator/dynamo, just like on full size locomotives.
Steam/Pneumatic Atomizing Burner
Steam/Pneumatic Atomizing Burner
Instead of an electric burner package, a steam-powered atomizer could be used, just like on a full-size oil-burning locomotive. A steam pressure reducing regulator would allow a needle valve to dial in the right steam flow for on/off operation. Electric arc ignition would still be required to set off the flame, and a parallel air system would be needed for startup. Alternatively, if the air usage is not large, it may make sense to always run the atomizer off of air.
Pneumatic Boiler Feed Pump
Pneumatic Boiler Feed Pump
While not prototypical, pneumatic diaphragm water pumps pressurize water up to the incoming air pressure. These are usually rated up to 100-120 psi, so not full boiler pressure, but would be useful to start the system to the point where an injector could take over.
VEVOR pneumatic pump, https://www.vevor.com/diaphragm-pump-c_11112/air-operated-double-diaphragm-pump-1-2-inlet-7gpm-pneumatic-nitrile-qby4-15-p_010483066329
3-way Throttle Valve
3-way Throttle Valve
A standard 2-way valve allows throttling to occur, but there is also the possibility of using a 3-way to allow for motion while the accumulator is still warming up from a cold start: The 3-way valve modulates between sending all generator output to the accumulator (pedal not depressed) and to the valve chests (pedal completely depressed). Any generator output "unused" for running the cylinders right then and there goes towards charging the accumulator for the first time. Either an additional throttle pedal would be needed for this "start mode", or a few on/off valves to set between "start" and "run". There should also be some way to make sure water doesn't make it to the cylinders from an unfired generator. Finally, the pedal may behave quite irregularly as steam pressure would tend to seal the valve against either seat when fully depressed or fully released.
https://www.spencevalve.com/en-us/spence-products/regulators/control-valves/k-series-control-valve
Alternative Steam Generation
Alternative Steam Generation
Off-board Generation
Instead of the steam generator, additional installed accumulator capacity could be used to allow the trike to run through an outing without recharging. This allows for alternative off board sources of steam. For example, a home thermal solar array could charge a stationary accumulator, or directly charge the accumulator(s) mounted to the trike when it is home. The additional accumulators could live on a tender trailer and attach via hose to the trike's normal accumulator, bypassing the generator for carbon-neutral running.
Note that such a stationary solar boiler would not be manually fired and would thus not be exempt from requiring a boiler certificate.
On-board Gasification
One disadvantage of the steam generator/accumulator scheme is that it prevents the direct use of solid fuels like wood; there isn't a way to turn a wood fire on or off for the steam generation cycles. One potential alternative is a gasification setup that generates flammable gas from a bed of wood chips or pellets, possibly mounted on a tender trailer. With soot and particulate exhaust yet a sustainable fuel source, a bumper sticker opportunity presents itself: "Dirtier than your diesel, more renewable than your Tesla".
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