Steam Generation Package Boilers or Heat Recovery Steam Generators (HRSG)
Technical Profile
Steam is often required for heavy oil or oil sands production. Particularly for oil Steam Assisted Gravity Drain (SAGD) production, an alternative to the Once Through Steam Generator (OTSG) is a traditional boiler. A package boiler, also known as a Heat Recovery Steam Generator (HRSG), comes in a variety of capacities and pressure ratings. The most common pressure rating for oil field and SAGD operations range from 1500 to 2500 psig (barg).
Unlike an OTSG, a boiler is generally constructed using carbon steel rather than exotic metallurgy. This allows for more economic fabrication and shorter delivery times, but higher operating costs due to steam loss from blowdown and water pretreatment costs offset this advantage to varying degrees. A typical boiler contains a water/mud drum and steam drum connected together with a series of tubes that run through a firebox. Steam is produced off the steam drum. Unlike an OTSG, a boiler is not designed for dry running.
Maintaining high water quality is fundamental to good boiler operation. Calcium and magnesium must be removed to prevent buildup of scale on tube walls. Heavy scaling will reduce heat transfer leading to hot spots and potentially tube rupture. Silica (SIO2) can form scale below 600 psig (41barg). Above 600 psig, silica starts to volatilize and transfer through the steam to deposit on downstream equipment and production formations. Chlorides are very soluble in water and will carryover with steam and corrode downstream equipment creating chloride pits. Oxygen is highly corrosive in boiler systems and must be removed. Carbon dioxide must be removed as well since it will combine to form carbonic acid that corrodes downstream equipment. Total dissolved solids (TDS) must be removed to avoid extremely high blowdown rates. TDS must be controlled in the boiler below saturation limits that would promote precipitation. Typical TDS requirements are shown below:
Drum Pressure, psig |
Maximum Boiler Water Solids, mg/L |
Steam TDS Corresponding to Max. BW TDS |
Maximum Total Alkalinity, mg/L as CaCO3 | |
0-300 |
3500 |
1.0 |
(1) | |
301-450 |
3000 |
1.0 |
(1) | |
451-600 |
2500 |
1.0 |
(1) | |
601-750 |
1000 |
0.5 |
(1) | |
751-900 |
750 |
0.5 |
(1) | |
901-1000 |
625 |
0.5 |
(1) | |
1001-1800 |
100 |
0.1 |
(2) | |
1801-2350 |
50 |
0.1 |
(2) | |
2351-2600 |
25 |
0.05 |
(2) | |
2601-2900 |
15 |
0.05 |
(2) | |
- 20% of Actual Boiler Water Solids
- For TDS < 100, total alkalinity is dictated by boiler water treatment
Boiler operating pressure in the 1500 to 2500 psig range requires removal of TDS to 50 mg/l or less. TDS is typically reduced using Reverse Osmosis or Evaporation.
