Steam system deaeration

Fig. 31. Steam system of a pulp and paper mill where PRV = pressure reducing valve, DSH = desuperheater, and DA = deaerating. To convert MPa to...

Against the amine feed gain due to recycling, it is necessary to offset amine losses of 5% or more from BD, 5% from a deaerator, 5 to 10% where steam system leaks in older systems may occur, and 5 to 10% loss in large, complex, multipressure systems. 

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... 

A boiler with a capacity of 100,000 kg-tr1 is required to produce steam of 40 bar and 350°C. The feedwater from the deaerator is at 100°C and contains 100 ppm dissolved solids. Maximum dissolved solids allowed in the boiler is 2000 ppm. The steam system operates with 60% condensate return. Enthalpy data for steam are given in Table 23.10. Calculate ... 

Steam systems. An investment cost of 35 per lb/h of total steam generation capacity is recommended for preliminary estimates (costs as of 2001). This represents the total installed costs for gas- or oil-fired, forced-draft boilers operating at 250 to 300 psig, and all appurtenant items such as water treating, deaerating, feed pumps, yard piping for steam, and condensate. 

The available energy flou through five major sections of sulphuric acid plant is given in figure 2. The major inputs to this system are sulphur and pouer, with demineralised (DM) water uet air, process water and cooling water from environment. The useful outputs from the system are sulphuric acid and steam. Losses to environment include heat losses from various equipments blowdown water steam from deaerator vent warm water and stack gas. 

The last example of Chapter 11 dealt with the case where a liquid and a gas were present together, but the gas was inert with respect to the liquid. However, there are a large number of systems on a process plant where a liquid and its own vapour are present together evaporators, condensers, steam drums, deaerators, refrigeration systems, stills and distillation columns. These systems exist in a state of vapour-liquid equilibrium, and their behaviour is significantly different from the gas-liquid system dealt with in Chapter 11, Section 11.6. The liquid and its vapour will have the same temperature, and it will not be possible to decouple the mass and energy balance equations for the liquid from those of the vapour. The way to obtain the necessary time differentials explicitly is to use the Method of Referred Derivatives. 

Table 2.15 Revamp options for deaerators, boilers, steam turbines and steam system. Equipment/system - Objective Options...

The purpose is to develop a steam balance for operational supervision as well as for identification of improvement opportunities in the steam system. Models for boilers, turbines, deaerators (DAs), letdown valves, desuperheaters, and steam flash tanks are discussed in the previous chapter. Historian and distributed control system (DCS) data will be coimected to steam balance so that the steam balance is capable of dynamically balancing the steam and power demands due to process variations, units on or off, and weather change. 

Calculate the steam prices based on enthalpy-based method for the steam system in Figure 17.1, which consists of major components of a complex steam system boilers with deaeration and makeup, back-pressure steam turbine, condensing turbine, process steam demand, steam letdown vale, desuperheater, and so on. The economic data are provided in Table 17.1. 

In another process plant, the steam system operates with a large amount of LP vent. To prevent loss of valuable condensate due to LP vent, a condenser was installed to cool down the LP steam and return LP condensate back to the deaerator. Although this solution saves condensate, it did not resolve the LP long issue. It was later identified that a driver switch could help to reduce the LP vent In the boiler house, there are three forced draft (FD) fans currently run by MP-LP extraction turbines but fans have spare motor drivers. The operation policy acceptable to the plant was to use steam turbines for reliability reasons. The engineer wanted to establish the value of the driver switch to minimize the LP dump. 

In addition to being present throughout the steam system at startup, air enters whenever there is a shutdown that allows steam to condense and pull a vacuum. Also, it is constantly present if the deaerator is malfunctioning and feedwater conditioning-chemical dosage is inadequate. Carbon dioxide is also present in steam produced from water that is not completely demineralized. Besides the adverse effect of these gases on heat transfer, their presence leads to... 

Carbon dioxide is often ignored in steam systems. However, when absorbed in water, it forms carbonic acid, which can be corrosive to all parts of the steam and condensate system. Its potential presence is frequently overlooked in the design of heat exchangers, steam traps, condensate systems, deaerators, and water-treating systems. Most steam systems require continual addition of makeup water to replace losses. Makeup water must be adequately treated, by demineralization or distillation, to remove carbonates and bicarbonates. If these are not removed, they can be thermally decomposed to carbon dioxide gas and carbonate and hydroxide ions. The ions will normally remain in the boiler water, but the caron dioxide will pass off with the steam as a gas. When the steam is condensed, the carbon dioxide will accumulate since is is noncondensable) be passed as a gas by the steam trap or if the condensate and carbon dioxide are not freely passed by the steam trap, become dissolved in the condensate and form carbonic acid. If carbonic acid is formed it can have a pH approaching 4 and be very corrosive to copper and steel. Even if both the gas and condensate are passed freely by the steam trap, the gas will become soluble in the condensate when subcooling occurs. If oxygen is present, the corrosion rate Is accelerated. 

Deaerators. Because the condensers in steam systems operate with internal pressures below one atmosphere, air may leak in. Some of this air dissolves in the water that forms as steam condenses. If this air remained in the water as it reached the hoiler, rapid rusting of hoiler surfaces would result. To prevent this, an open heat exchanger, called a deaerator, is installed. Water is sprayed into the deaerator as a fine mist, and steam is also admitted. As the steam and water mix, the water droplets are heated to their boiling point but not actually boiled. The soluhiUty of air in water goes to zero as the water temperature approaches the boiling point, so nearly all air is forced out of solution in the deaerator. Once the air is in gaseous form, it is removed from the system. 

As methanol emissions continue to be monitored more closely, there are some methods of reducing the methanol in the deaerator vent. The vent stream could be condensed and sent to the reformer or the steam system. Catalytic combustion could be used to reduce the methanol. A scrubber system could be added to remove the methanol. In some instances a condensate stripper is added instead of the deaerator to remove the ammonia, methanol, and other contaminants from the condensate. This system recycles the vent stream to the reformer as process steam and the bottoms are mixed with the incoming boiler feedwater makeup. However, this system adds considerable capital cost to a project. 

Operation with the package boiler had not permitted all the auxiliary and ancillary plant to be loaded to capacity so that only during this phase could the commissioning of such items as the rotary air pumps, condensate extraction and feed water pumps, feed heaters and the associated bled steam system, and deaerator controls be undertaken. 

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. 

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