|Oxygen control in boiler circuits by Antonio Venturelli, Termochimica Impianti Srl
Water used in high-pressure boilers must be very pure or else severe corrosion can rapidly destroy the materials of construction. Water fed to boilers consists of raw make-up water and condensate returns , most commonly a mixture of the two. Both contain oxygen and carbon dioxide in solution in varying amounts. The make-up water storage vessel is often in contact with the atmosphere, so that dissolved gases can enter the boiler water here. Another source of dissolved gases is leakage into the condensate or through pump seals.
While oxygen and carbon dioxide could be removed by the addition of scavenging chemicals to the feedwater alone, this practice is extremely costly in all but the smallest plants. Generally speaking, plants generating in excess of 10,000 lb/hr of steam can justify the installation of a Deaerator.
If the percentage of make-up to the system is above 50% and/or the CO2 content or alkalinity of the feedwater is high ( CO2 + HCO3 exceeding 100 ppm) the use of Deaerator will result in worthwhile savings even below 10,000 lb/hr boiler capacity.
Although Deaerators are capable of reducing oxygen and carbon dioxide to very low levels, they should not be considered as a substitute for proper internal boiler feedwater treatment. They will, however , greatly reduce the amount of “scavenging chemicals” which must be fed to maintain proper residuals with the boiler.
According to the definition as given by the Heat Exchange Institute Standards , the Deaerator is a package unit capable of removing from the water all dissolved oxygen in excess of 0.005 cc/liter ( 7 parts per billion) at all loads up to and including the rated capacity.
The Deaerator will accomplish the following :
1. Oxygen & Carbon Dioxide removal from boiler feed water
2. Raise the temperature of the feedwater
3. Improve heat transfer in boiler
4. Utilize low pressure exhaust and flash steam
5. Common collection points for high & low temperature condensate returns
6. Economizing of overall operation with strong reduction in the chemical treatment costs.
Termochimica ‘s technology for Deaerators, made by Eng. Antonio Venturelli which is the most experienced designer of Deaerators in Europe, are based on “spray and tray “ action with particular care to heat transfer between steam and water in countercurrent flow.
This design involves three sections for an efficient deaeration :
1st by special spray valves , self - adjusting spring loaded type, providing the breaking down of incoming water into thin films or droplets. By this action, the distance a bubble of gas has to cover to be released , is greatly reduced and a large percentage of the gases are removed in this section.
2nd by perforated special trays through which the water rains in countercurrent with degassing steam. Fresh gas-free steam enters from the bottom of deaerating tower and flows upwards scrubbing out the traces of residual oxygen and carbon dioxide.
3rd by reboiler section in order to maintain an uniform distribution of degassing steam flowrate under the trays .
These internals are mounted directly in workshop, before delivery, in order to simplify all start-up activities in field by Client.
Gases O2 and CO2 mixed with a little steam flowrate are discharged to the external atmosphere through a vent line completed with calibrated orifices and vent condenser direct contact type.
With more than 100 equipments operating in the world , according to the most restrictive specification and guarantees in terms of oxygen content and stripping of other gases from boiler feed water, Termochimica is leader of this particular product named Deaerator for water treatment in energy industry.
More details on this technology are reported on Termochimica web site www.termochimica.com
Termochimica is a slim Company, with a strong basis technology and a large flexibility to understand all requisition by Customers , operating with continuous improvement of our design in degassing systems completed with accessories and instruments.
Our competitors remain limited to heavy companies offering a too much dispersive range of products while Termochimica represent the “Specialist of Deaerator” with the awareness that we offer the best product in short time and with more competitive prices.
The positive results of our mission are confirmed by the long reference list of Deaerators supplied since 1969 to the main world-known EPC companies (such as ABB LG, Agip Gas, Alstom Power, Aramco Overseas Company BV, Enelpower, Foster Wheeler, JGC, Nuovo Pignone, Snamprogetti ,Technip, Tecnimont, and others).
Recently Termochimica awarded an order for the supply of Deaerators for a project in Qatar. This order confirm our strong presence in Middle Est , in addition to other Deaerator’s supplies in Saudi Arabia Emirates and Oman territories.