Boiler feedwater deaerators

Knox has provided the following graphs for estimating the required vent steam from boiler feedwater deaerators. Vent steam rate depends upon the type of deaerator (spray or tray type) and the percentage of makeup water (in contrast to returning condensate). Low makeup water rates require relatively lower steam vent rates, but there is a minimum rate required to remove CO2 from the returning condensate. 

Many industrial separation processes that have been traditionally implemented using the classical mass-transfer operations—such as distillation or stripping—can be replaced by membrane separations that can be more environmentally friendly (such as the boiler feedwater deaerator of Example 2.14) or less energy intensive (such as the distillation-membrane hybrid of Problem 6.26). This replacement can be very challenging since it requires the production of high-mass-transfer-flux, long-life,... 

Figure 21.1 is a sketch of a typical boiler feedwater deaerator. The lower portion of the deaerator acts as a surge vessel for the boiler charge pump. The working part of the deaerator is the smaller vertical... 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

The deaerated treated boiler feedwater then enters the boiler. Evaporation takes place in the boiler and the steam generated is fed to the steam system. Solids not removed by the boiler feedwater treatment build up in the boiler, along with products of corrosion. These are removed from the boiler by taking a blowdown (purge) from the boiler. The steam from the boiler goes to the... 

Thus, in this example, assumption of the deaeration steam allows the steam balance to be closed. However, this is based on an assumed deaerator flow. The actual flow to the deaerator can be calculated from a heat balance around the deaerator. Figure 23.23 shows the flows into and out of the deaerator. If the boiler feedwater flow and condensate flows are known, together with an assumed value of the vent steam, then the flowrate of deaeration steam can be calculated from an energy balance. 

Normally, after being heated, these streams are used in the boiler area (deaerator feedwater, cold return condensate, boiler feedwater, RO feedwater) or in the combustion chamber (air preheat). However, economizers can be used to recover and supply heat elsewhere, such as hot process water or hot utihty water, especially as used in the food processing and pulp/paper industries. Additionally, recovered flue gas waste heat can be used indirectly i.e., remote process streams can be heated locally with hot steam condensate, and then the cooled return steam condensate can be reheated in the flue gas economizer. An... 

For corrosion and safety reasons, the condensate recovered from these sources is best not returned to the deaerator for use as boiler feedwater. However, depending on the contaminant, the condensate may be reused for a number of services. Our favorite reuse of such contaminated condensate is as a replacement for velocity steam in the heater-tube passes of a fired furnace. 

The humble deaerator, operated by the Utility Department, is an interesting and important component of any process facility. Oxygen is a highly corrosive element, and if left in the boiler feedwater, would rapidly oxidize the boiler s tubes. 

The dissolved air left in boiler feedwater (BFW) is stripped out with steam, in the deaerator shown in Fig. 8.9. The cold BFW has been taken first from the Mississippi River and then filtered to remove sand and sediment. Removal of the bulk of the calcium salts that would cause hardness deposits in the boilers is often accomplished by hot-lime softening. If excess C02 gas appears in downstream units consuming the steam, it is the fault of the lime softening, not the deaerator. 

Removing oxygen from boiler feedwater in deaerators... 

Oxygen is a particularly reactive element. Actually, oxygen is a very potent gas and reacts aggressively with exposed metal surfaces to form oxides. Of all the corrosive substances encountered in a process plant, few exceed oxygen in reactivity with steel pipes. If a considerable amount of oxygen is left in boiler feedwater, interior corrosion of the boiler tubes will be rapid. The bulk of the oxygen, or dissolved air, is stripped out of the boiler feedwater in a deaerator. 

Figure 15.1 Deaerator is intended to remove 02 from boiler feedwater.

It is the second of these two areas that is of interest in this chapter. Heat is delivered for steam production in many separate stages. Preheat is first supplied to the low-pressure deionized water prior to deaeration. Higher pressure product (HP boiler feedwater) is further preheated to around 100°C for supply to the waste-heat boiler. The waste-heat boiler is then able to vapourize the high-pressure deionized and deaerated water for final delivery to the steam superheater. 

A clamp-type pipe joint connector failed a short time after it was put into service. This clamp, shown in Figure 7.48, had been used to join two ends of NPS 8 carbon steel steam piping, operating at a pressure of approximately 11 MPa (1600 psi). The boiler feedwater used in this process had been softened by cation exchange (Ca <-> Na) and was fully deaerated. The feedwater was recycled water from an oilfield operation and contained several thousand ppm dissolved chlorides. The steam was 80% quality and at a temperature of approximately 330°C. 

This two-stage deaeration enables the temperature of the steam-boiler feedwater, which comes from the second-stage deaerator, to be held constant. The feedwater boiling point depends on the pressure of the steam fed to the deaerator. 

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