By Ed Wolfe
Another article "How Much Cash is Going Up in Smoke" give a few examples of how to improve energy efficiency in a plant. Here are several more.
Deaeration is a two step process to remove dissolved oxygen from boiler feedwater. The first step is mechanical deaeration, where live steam heats the boiler makeup water in a pressure vessel to a temperature of 250°F and then strips the oxygen to the atmosphere.
The second treatment step is chemical deaeration of the boiler feedwater by the addition of oxygen scavenging chemicals in the deaerator storage vessel. The chemical treatment assures that there will be no free oxygen in contact with carbon steel. If present, oxygen will form corrosion pits throughout the steam system, causing serious damage.
Several plants have reversed these steps by having the cold boiler makeup water first treated with a vacuum deaerator, then adding an oxygen scavenger chemical, before flowing through heat exchangers where process waste heat is picked up. The potential amount of steam saved is as much as twenty percent of the makeup water flow rate.
The storage vessel is the terminal collecting point for the boiler feedwater. Knowing all the flows of water and steam to the deaerator is the key to calculating the boiler makeup water requirements. Boiler feedwater consists of the following three flows which are combined in the deaerator storage vessel and then pumped as feedwater to the boiler.
The impurities in the makeup water are diluted by the blending of returned plant steam condensate and the deaerator heating steam condensate.
Doing a heat and material balance around the deaerator storage vessel is the key to determining the amount and quality of the boiler makeup water required for a specific steam system to calculate the amount of heating and stripping steam required to operate the deaerator. The steam tables published by ASME are used. Knowing the amount of the steam condensed for heating the cold water in the operation of the deaerator, plus the amount of returned condensate, will provide the dilution factor for the impurities in the boiler makeup water. This is the result of the blending of both the returned condensate and the deaerator condensate with the makeup water. Knowing the limits of each impurity in the boiler makeup water allows plant operations to control the treatment steps in the most cost effective manner.