What Are Once Through Steam Generators

This term is new to me too.

Maybe it points at generating steam in one pass, meaning: you start with cold water and make steam, without regeneration, return or bypass.

OTSG boilers do away with internal recirculation and have no steam drum. This allows for lighter construction using a separator instead of a steam drum etc.

They have many advantages and are now often used for heat recovery from things like gas turbines etc. I guess that you do away with drum blowdown and speed up the steam production cycle which is great for heat recovery use.

Try googling Benson boiler - I think by Siemens.

Hi... judging by an article I found regarding increasing the output of a Turbo-generator, it seems like a OTSG is used for steam generation using the hot exhaust of a turbine.

Indeed your guess seems right ... BFW goes in one pass where its heated, then boils and exits as wet steam.

I'm still looking for more details on OTSG.

Hmm, in a typical boiler you have "regeneration, return and bypass" ..? I'm guessing they pertain to the natural convection circulation of the BFW in the boiler tubes and mud drum,,, but I'm guessing here. What are they?

Thanks!

There are all kind of steam generators. Some have a circuit to re- heat the wet steam after use. In the old days part of the steam was also used to preheat the (bituminous) fuels.

Once-through steam generators are heat exchangers that convert water to steam (or other heat-transport liquids to vapor) in a single pass. Liquid water/whatever enters at one end and steam/vapor exits at the other end. Heat is drawn from some other fluid (liquid, gas).

The 'once-through' designation means that the heat exchanger is a single-pass one where water/liquid enters at one end, gets vaporized in the straight pipe through which it travels, and exits at the other as steam/vapor. There's no second pass from doubling back into the heat-exchange zone ... the steam exits the heat exchnager right away.

OTSGs are used in supercritical power generation. Supercritical fluid, that is a fluid whose pressure is beyond the its own critical pressure, enters at on end and exits the other end after having picked up heat. The fluid then goes on to a turbine in which it expands, and in which its pressure and temperature drop. The turbine spinds to generate power.

The fluid, still supercitical then passes into a condenser, which cools it and transforms its state to liquid. The liquid then gets pumped to supercritical pressure and passed through the OTSG again.

Why are OTSGs used for supercritical applications, including power generation? Because supercritical fluids have an important property: they don't boil. Supercritical fluid, when heated, don't change phase ... they remain supercritical fluids ... so there's no phase mixing like in boilers. For example, in sub-supercritical steam boilers, water vapor and liquid water are both present in the boiler's risers, which absorb heat from the boiler's furnace. Liquid water circulates upwards, and as it absorbs heat, part of it vaporizes in the risers. The vapor-liquid from the risers then gets released into the boiler's drum, where the vapor (steam) gets separated from the liquid water.

In supercritical applications, there's only one fluid phase: the supercritical one. Because of this, two phases like vapor and liquid don't need to be separated, so a device such as a steam drum is neither needed nor used.

In non-supercritical applications, if a liquid enters one end of a heat exchanger and nothing but vapor comes out the other (so again, no need to separate vapor and liquid by using a boiler drum), that heat exchanger is a steam generator. It's called a once-through generator if the fluid flows through in a single pass.

Simple as that. Cheers!

DZ

one small correction, you said:

"OTSGs are used in supercritical power generation. Supercritical fluid, that is a fluid whose pressure is beyond the its own critical pressure, enters at on end and exits the other end after having picked up heat. The fluid then goes on to a turbine in which it expands, and in which its pressure and temperature drop. The turbine spinds to generate power. The fluid, still supercritical then passes into a condenser, which cools it and transforms its state to liquid. The liquid then gets pumped to supercritical pressure and passed through the OTSG again."

in expanding as it passes through the turbine, the fluid (typically water) is no longer supercritical. it is typically re-heated again and goes through a medium pressure turbine, and then finally a low pressure turbine.

All previous comments are not accurate. Please look at Struthers-Wells site. OSTG for oil field service are designed to generate about 80% quality steam/water mix to inject into the oil bearing formation, heating the sand/rock/oil so the oil will flow easier to producing wells. The entire system of steam generators, produced fluid treating, water treating for recycle, oil de-watering, oil-water emulsion metering and lease accounting have been extensively researched and developed in the southern end of the California Central Valley. Bakersfield is the center of activity.

The quality of steam produced is limited by TDS and hardness remaining in the treated water and the desired injection pressure. "Packaged' steam generators are reliable, low cost, efficient and available in standardized sizes up to and over 50 MM Btu/Hr @ 3000 psig. They are designed to run 24/7 unattended with only daily check-up, and include all the instrumentation and safety systems needed to run for months(years) with minimal maintenance.

The advantages are numerous, and include:

a)Low cost, b) easy to add or reduce capacity without capital cost penalty, c)high efficiency, d) unattended operation, e)reliable, f) tolerant of low quality feed water g) meet all emissions requirements with many stack gas treatment options, h)no 'blow-down' -all feedwater goes down-hole, i) designed for, proven reliable in oil field service, j) can use produced oil as fuel.

These ARE NOT at all like power or utility steam boilers, but are optimized to take advantage of the 'low quality -i.e. +/- 80%' requirements for the steam, which drastically simplified feed water treating and reduces boiler scaling (and burn-through) potential

OTSGs are also used for supercritical power applications.

DZ

ALL previous comments are not inaccurate.

Please see this site .

Thanks Keith,

It looks like there are different kinds of Once Through Steam Generators.. I am intersted in the application you are talking about -> SAGD oil production.

Thanks for your description, and that from all the other folks. Its been very helpful & interesting!

Cheers,

RM

I went to the Struthers-Wells web site... I only saw OTSG mentioned, but no information about it was given. Where should be look to see info on OTSG?

Thanks!

It seems we have here lots of people who use Once Through Steam Generators laying claim to the definition of the term.

My guess (there is a small OTSG on a project I am working on) is that in a power generation system and where steam is used as a heating medium (conventional power stations, refineries, chemial plant etc) steam is generated goes off does some useful work and the condensate is returned to the boiler. There it is boiled up again goes off does some work as steam (either good qality, saturated, super heated, supercritical or whatever).

In a OTSG unit steam is raised the steam (and I think it could be in any form) goes off does some work and the condensate is 'lost' or rather never returns to the boiler. The water for the boiler is always from a fresh supply and so each water molecule just goes through the boiler once.

In my case the steam is used for regenerating a water purification system - the steam and stripped hydrocarbons are condensed, go to settling tanks the oil is skimmed and the water goes into the produced water system for disposal. In this case the water quality needed is low (in comparison to power system Boiler feed Water) but high compared to drinking water - we actually take the feed from the potable water system and do some extra treatment.

Not correct. In most of the OTSG's the condensate is reused. The term refer to what happens inside the boiler. What happens outside is not relevant.

What extra treatment did you use on your potable water?

Unfortunately I never found out. It was part of a vendor package that i was expecting to review. But the detailed bid and that part got held off. I would expect there would be softening, oxygen scavenging as a minimum.

Pump and dump.

No.

http://plainsjustice.org/files/ElkRun/DNR_App_20080603/Elk%20Run%20References/Supercritical%20Boiler%20Technology%20Matures.pdf

Once through boilers have been around since the sixties and were at one point some of the most efficient units in the world.

One thing that turbine people would find interesting is that some of the cross compound units of this design were 3600 RPM on both the HP and LP/Secondary side.

The technology is as follows

Normally these boilers are vertical in construction,

There is a vertical insulated shell (about 3mm thick and 1 mtr in diameter and 1500mm in height )with monolithic refractory casted at the bottom.

there are two shells concentrically placed having 25 mm gaps.

It is closed from top side by refractory flange having one inlet for fresh air. Air enters in the gap between outer insulated shell and first inner shell from top.At bottom a gap is maintained where air takes upward path and enter as heated primary and secondary air for combustion. this increases the efficiency.

A coil is prepared of using 25 mm dia pipe which is of boiler quality. This is about 500mm internal diameter coil wound on a a drum. The tubes are welded at the contact surface so that air can not go from inside of the coil to outside surface. The internal part acts as heat exchange surface in contact with the flame.A burner is fitted from top which fires from top to bottom.Top end of coils pipe is fed with water using positive displacement piton pump,The water gets heated in the path. The second end of the coil at the bottom is connected to another coil wound in flat spiral

shape is called serpentine or pancake. The flame strikes the same and delivers heat in most efficient way. due to highest velocity of gasses and turbulent flow, coefficient of convection is highest.

The flue gases take turn from outside surface of tube and are exhausted from top of the boiler from top refractory.

As the name suggests the water at ambient temperature enters the tube and steam exits at the other end normally at 14 kg/ sq cm

The capacity of the coil to hold water is designed in such a way that it remains out of the provisions of laws applicable to boilers (30lit)

The boilers available are from 100 kg to capacity of 1000kg of steam per hour maximum.

The higher capacity is built but is covered by the laws meant for boiler authority.

Advantages

Instant on / off is possible with flip of a switch. You can switch off the boiler in tea brakes or lunch brakes. Hence fuel saving is very high.

High thermal efficiency.

very compact

does not explode. Even if by an accident tube punctures the coil is inside hence not dangerous .

Very easy to maintain.Stack temperature is about 225 to 250 C

If pumping speed is increased then it can operate as hot water generator. Hotels use it

this way. Capital cost is low.

Fuels used are LDO, NATURAL GAS / LPG ETC.

I suggest to search " three m boilers India" Thermax India ltd India

Thermax had collaboration with Wanson Belgium for this type of boilers.

kiran nawathe

Different applications have different standards and our plant is no different. Suffice to say that we do have a "Once Through" steam generator configuration. The resultant steam is very high quality supercritical steam.

The following is taken from:

http://en.wikipedia.org/wiki/Critical_point_(thermodynamics)

The term "critical point" is sometimes used to denote specifically the vapor-liquid critical point of a material. The vapor-liquid critical point denotes the conditions above which distinct liquid and gas phases do not exist.

In water, the critical point occurs at around 647 K (374 °C or 705 °F) and 22.064 MPa (3200 PSIA or 218 atm)^[1].

As the critical temperature is approached, the properties of the gas and liquid phases approach one another, resulting in only one phase at the critical point : a homogeneous supercritical fluid. The heat of vaporization is zero at and beyond this critical point, so there is no distinction between the two phases. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed. The critical pressure is the vapor pressure at the critical temperature. On the diagram showing the thermodynamic properties for a given substance, the point at which critical temperature and critical pressure meet is called the critical point of the substance. The critical molar volume is the volume of one mole of material at the critical temperature and pressure.

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Our "Once Through" steam generator operates above the critical point and supplies very high quality supercritical steam to turn our turbine/generator at 3600 RPM.

There are other applications which utilize "Once Through" steam generators which may or may not require this high quality "supercritical fluid".

One respondent mentioned "wet steam" as the output steam but I can assure you, wet steam may be used in some applications, but not in large turbine generators which operate at 3600 RPM. The Turbine Water Induction for using saturated steam "wet steam" would get someone in "hot water" (pun intended) in a hurry. Not to mention the fact that co-workers may be heading down the road "kicking rocks" because the instrument of their livelihood (the turbine) would have suffered irreparable catastrophic failure which would severely hamper the likelyhood of any employer to be able to keep a number employed.

Check also "Clayton Steam generator" website. By my knowledge they were one of the first that produced this system. The website shows you all you need. I visited a Clayton factory in Belgium in the 70's when most of the industries were still using the conventional horizontal ones, as De Naeyer and Wanson. In the beginning Clayton had a prime market for ship steam generators.

Back in the early 1970's when gas became scarce, I built a steam power plant used for a steam car. I used the construction plans created by Richard J. Smith (deceased); a very clever mechanical engineer. He lived in Midway City, CA and I visited his garage many times. Incredibly amused when he had tumblers mixing chemicals for the local 4th of July fireworks display. Made the fuses and rockets. The Smith steam generator was a once-through type, comprised of water going into about 300 feet of spiraled, pancaked layered, 3/8" copper tubing (used for preheating) and about 50 feet of "used for superheating" seamless stainless steel 1/2" tubing at the outlet. The generator was built for producing 1000 degree F at 1000 psi superheated steam. I have a few pictures, if interested. I built an entire steam power plant; closed cycle using a Roots blower turning a fan that cooled a large radiator that received the "wet" exhaust steam from the engine. The engine was a modified six cylinder Kiekhaefer engine; often the basic engine would have been salvaged from a sunken Mercury outboard engine. The water used for steam contained Chevron cylinder oil and a GUNK additive for lubrication. It was a fun project for about two years of my life when I lived in Van Nuys, CA. A lot of work.

The question is OTSG for SAGD operations. I can see there are a lot of comments related to OTSG in other industries, such is power but that is not for SAGD, those OTSGs are more like power boilers or heat recovery convection sections.

OTSG for SAGD basically is a fired heater with the cylindrical radiant section lying flat on the ground with 1 main forced draft burner at front end. The flue gas go through a hog trough and then up to the convection section to recover the heat to bring the efficiency to around 90%.

The coil inlet is high pressure BFW and the outlet typically will have 80% quality steam. Coil size and material used typically : 3" A106 Gr C pipe. Steam then go through a steam separator and then eventually go down to the well pads and under neath the ground to heat up the oil sands for abstracting oil.

Bill C

Dear Mr. RockM,

Once Through Steam Generators - INTERNAL WATER CIRCULATION IS TOTALLY NIL. The water enters at one inlet and comes out as STEAM at a very high pressure and Temperatures, at the outlet.

At HIGH PRESSURE the DENSITY of STEAM is high or INCREASED and the DENSITY of WATER is LOW or REDUCED. In the conventional low or medium pressure boiler - the DIFFERENCE in the DENSITY of STEAM and WATER is the FORCE for NATURAL CIRCULATION OF WATER with in the Boiler.

At a pressure of 1400 psi (or 98.6 Kg/cm^2) and above, the DIFFERENCE IN DENSITY between water and steam is NOT SUFFICIENT ENOUGH to keep the NATURAL CIRCULATION of WATER in the Boiler. Therefore FORCED CIRCULATION is adopted. Pl. refer the STEAM TABLES for precise data on volume, density of water, steam etc.

At a pressure of 3202 psi or 220.8 Kg/cm^2 the DENSITY DIFFERENCE between WATER and STEAM IS ZERO and is same at 19.2 Lbs/Ft^3 or 37.4 Kg/M^3. Therefore water is NOT CIRCULATED and passess through the tubes and comes out as steam. Hence called as ONCE THROUGH BOILERS.

DHAYANANDHAN.S, 22-4-11.

A domestic kettle classes as a OTSG.