Steam and Condensate System

Water Hammer and Condensate Backup Steam-Side Reboler Control 

In this chapter we discuss process equipment such as 

A few of the nasty features of this sort of process equipment we discuss are 

When considering the steam side of steam-heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 12.1. The steam is on the tube side. 

The steam enters through the top of the channel head of the reboiler. Any superheat in the steam is quickly lost to the tubes. Superheating steam does very little in increasing heat-transfer rates in a reboiler. Actually, when considering the temperature difference between the steam and the process fluid, it is best to use the saturated steam temperature as the real temperature at which all the heat in the steam is available. For example, assume the following steam flow to a reboiler  

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Consequently, any and all waterside or steamside problems that may hinder or sometimes completely prevent the achievement of these objectives must be addressed. In practice, there are a myriad of potential problems that may develop in the steam and condensate systems, and (as in other areas of the boiler plant) many of these problems are... 

Oxygen, nitrogen, carbon dioxide, ammonia, hydrogen sulfide, sulfur dioxide, and other contaminant gases are generally present to some degree in all steam and condensate systems. 

Thus, oxygen infiltration gives rise to a range of localized steam and condensate system corrosion reactions and products. These reactions may, in turn, lead to further downstream problems of corrosion debris transport when the condensate returns to the FW system. 

In areas where the water has a high natural alkaline hardness, the potential for contamination of the steam and condensate system by C02 may be very significant. And in those plants requiring a high percentage of MU water, the risk of serious condensate system corrosion to the steel pipework, valves, traps, threaded joints, and points where noncondensable gases can collect is very real. 

Where carryover occurs, much of the solids content is deposited in the first parts of the steam and condensate system, such as superheaters, but the balance can be transported all the way back to the pre-boiler system and from there to the boiler itself. Thus, a chain of cause and effect may once again develop in a manner similar to the progression of problems in other areas of the boiler system. 

Where new boilers are married to old steam and condensate systems, similar effects occur because of the transport of debris and the resultant blockages of valves and steam traps. Where the replacement boilers are of a higher rating than the existing boilers, the problems are further exacerbated (often due to surging, faster steaming times, or increased steam velocities). 

Process leaks into the steam and condensate system may take the form of phenols, glycols, and various other organics that produce an... 

In certain direct steam-contact process applications (such as in food and beverage processing or pharmaceutical preparations) the use of amine-based products in steam and condensate systems is subject to legal restrictions. Also, the use of ammonia or amines may be dependent on the materials of construction employed or technical limitations (such as the risk of copper alloy corrosion). 

In systems where the steam and condensate systems are extensive, it is important that any contamination of returned condensate be controlled either by monitoring and dumping or by condensate polishing. 

Steam and condensate systems, corrosive gases in 284 carbon dioxide carryover 288... 

The planning and design of the steam and condensate system is based on the mass and energy balance. Startup, shutdown, and operation under partial load must also be taken into account (Table 2.5-2). 

When condensate returns to a boiler, it may be contaminated by pipeline corrosion or with process materials. Likely sources of the latter include exchanger leakage and poorly installed process tie-ins. Carbon dioxide and oxygen are the chief causes of corrosion in steam and condensate systems. Section 12.4.3.3 discusses the treatment of boiler feed water to remove these contaminants, as well as hardness. 

Steam Leaks from Steam Pipes (Visible) The most common causes of steam leaks come from holes in steam pipes and threaded pipe connections in a steam and condensate system. Steam flow through a leak can be calculated using an orihce equation, which is based on the diameter of the leak, pressure at the inlet of the orifice, and pressure at the outlet (atmosphere). 

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. 

Trace through the cooling water system. Starting at the cooling water pumps, mark the cooling water lines and control valves, as per above. Repeat this procedure for the unit s steam and condensate systems. Treat... 

There is some disagreement as to the desirable feeding point for film-forming inhibitors. All inhibitor suppliers say that the materials can be fed directly to the steam and condensate systems. Some suppliers recommend adding the inhibitor to the feed water or directly to the boiler and say that the inhibitor will evaporate with the steam and condensate in a thin, continuous film. However, most of the commercially available filming inhibitors are formulated produets, each component having a somewhat different volatihty (and solubility) and, therefore, the preferred point of addition should be the steam header. 

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