Steam Generation 1 OTSG
Technical Profile
Steam is often required for heavy oil or oil sands production. A common piece of equipment in oil fields is the Once Through Steam Generator (OTSG). The OTSG comes in a variety of capacities and pressure ratings - most common from 1500 to 2500 psig (100 to 170 barg).
The OTSG is basically a continuous tube heat exchanger where preheating, evaporation and superheating of feed water take place consecutively. Many tubes are arranged in parallel connected to a common inlet and outlet. Cold water enters one end and steam and water exit the other. Hot flue gas flow counter-current to that of water.
Generally, the OTSG makes 70 to 80% steam. They do not vaporize all the feed water fed to them. If 100% steam is needed from an OTSG, a steam separator, or series of separators, is used to separate hot water from saturated steam. The beauty of an OTSG is that it can handle much higher levels of silica and other dissolved solids than a packaged boiler since these solids stay in solution with the feed water that is not vaporized. The OTSG does not utilize blowdown, which limits its thermal loses. It needs much less pretreatment than a conventional boiler. Typical feed water quality is listed below:
Parameter |
Limit |
Rationale |
Total Hardness |
< 1 ppm |
Ca, Mg, Fe induce scaling in generator tubes leading to reduced thermal efficiency and tube hot spots that can lead to tube failure. Softening is required. |
TDS SAC (IX) WAC (IX) |
< 7,000 ppm < 30,000 ppm |
Wet Steam Generators can handle high TDS because some water always accompanies the produced steam (i.e., less than 100% steam). Therefore, the generator can handle TDS up to the limit of solubility in the liquid phase at the operating temperature and pressure (for instance 20% liquid for an 80% steam generator) beyond which point precipitation occurs and tube fouling follows. However, TDS concentration is limited by cation exchange softening where too high a concentration of TDS leads to hardness leakage through the softeners. Capital and operating expense for WAC is much higher than SAC. |
TSS |
< 5 ppm |
Suspended solids promote sludge formation and fouling in softening equipment. Filtration is required. |
Oil |
< 1 ppm |
Excess oil leads to softener fouling. Oil breakthrough to the generator will promote foaming, tube fouling and coking that reduces thermal efficiency and may lead to tube failure. Efficient oil removal is required. |
Oxygen |
< 0.01 ppm |
Oxygen leads to localized tube corrosion. It can be removed through degassing, or a residual concentration of oxygen scavenger chemical may be maintained. |
Alkalinity (bicarbonate) |
< 2,000 ppm |
Alkalinity control is critical to keep Silica in solution, but too high a concentration will lead to excess hydroxide formation that can lead to caustic embrittlement of generator tubes. Moderate alkalinity helps reduce acid corrosion and maintain Silica solubility. |
Silica |
< 150 ppm |
Silica is controlled by controlling the solubility of silica in the liquid phase with alkalinity. Alkalinity should be maintained at least 3 times Silica concentration. A limit of 150 ppm is generally accepted as the standard without the presence of scaling ions like Ca, Mg, Fe. |
pH |
7 - 12 |
Slightly alkaline pH leads to good operation. Below 7 pH acid corrosion becomes a problem in the generator tubes. Above 12 pH hydroxide concentration may lead to caustic embrittlement of generator tubes. |
It is possible to design an OTSG for 100% superheated steam for the power generation market for instance. In this case, it must be designed for dry running conditions as the feed water vaporizes in the tubes rather than a steam drum. Tubes are generally made of special high nickel alloys that are expensive and create long lead time for fabrication and delivery. They are small diameter and thin wall tubes. Suspended and dissolved solids should be removed prior to the OTSG. Oxygen removal is not as critical as package boiler operations due to the exotic metallurgy of the tubes. For dry run condition, TDS should be below 50 ppb or less than a conductivity of 0.25 µmho/cm.
For power plant applications, almost no steam is lost to the process. A makeup rate of 0.1% or less is common as no blowdown is needed. A packaged boiler system will generally realize a makeup rate of 2.5% or higher due to steam loss and blowdown. Therefore, while the OTSG is more expensive to fabricate, it is more economic to operate.
