Steam condensers, malfunctions

The bigger the radiator, the more heat is provided to a room. The bigger the radiator, the faster the steam condenses to water inside the radiator. A larger radiator has more heat-transfer surface area exposed to the condensing steam. Unfortunately, the radiator shown in Fig. 13.1 is suffering from a common malfunction. Water-hardness deposits have partly plugged the condensate drain line. Calcium Carbonate is a typical water-hardness deposit. 

To summarize, the two most common malfunctions of a steam condenser (or radiator), are... 

Instrument and operating air can in most of the cases described here be obtained by way of the air compressor in the air separation unit. The instrument air is dried in switchable gel dryers at pressures between 6 and 8 bar to a water dew point (referred to atmospheric pressure) which is clearly lower than for the lowest local temperatures, in order to prevent steam condensation in piping, instruments and control valves. Sufficient air to shut down the plant in case of malfunction is stored in an air receiver of appropriate capacity. The air compressor of the air separation unit is almost in all cases supplemented by a stand-by instrument air compressor which normally produces only the air rate required to start up the plant. 

Vapor binding, or air lock, is another common cause of household radiator malfunction. Often, the vapor accumulating in the radiator is CO,2, rather than air. The C02 originates from the thermal decomposition of carbonates in the boiler. Regardless, air and C02 form a noncondensable vapor in the radiator. These noncondensables mix with the steam in the radiator. The noncondensables then reduce the concentration of the steam, by dilution. The diluted steam has a lower partial pressure than pure steam. The lower the partial pressure of the steam, the more difficult it is to condense. As the rate of condensation of the steam drops, so does the heat radiated by the radiator. 

Quite clearly, this discussion will raise many points for the designers to consider. Similarly, each of the other hazard prompts can be addressed. They will undoubtedly raise questions about the design of the venting system (toxic and flammable emissions), how to deal with a failure of the cooling water supply to the condenser, how to control and monitor the effluent discharge even under conditions of plant malfunction, instrument failure, loss of other services such as electrical supply and steam, human error, ease of safe maintenance and so on. The prompt internal fire may lead to a debate on the start-up of the system, when acetone vapour and air will be present initially. 

Anticipated operational occurrences are off-normal events, usually plant transients, which can be coped with by the plant protection systems and normal plant systems but which could have the potential to damage the reactor if some additional malfunction should happen. Their typical frequency of occurrence may be more than 10 year Some of the anticipated occurrences (PIEs - postulated initiating events) are due to the increase of reactor heat removal (as might occur for an inadvertent opening of a steam relief valve, malfunctions in control systems, etc.). Some are due to the decrease of reactor heat removal (such as for feed-water pumps tripping, loss of condenser vacuum and control systems malfunctions). Some are due to a decrease in reactor coolant system flow rate, as in the case of a trip of one or more coolant pumps. Some are connected with reactivity and power distribution anomalies, such as for an inadvertent control rod withdrawal or unwanted boron dilution due to a malfunction of the volume control system for a PWR. Events entailing the increase or decrease of the reactor coolant inventory may also happen, due to malfunctions of the volume control system or small leaks. Finally, releases of radioactive substances from components may occur. 

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... 

Increase in reactor heat removal inadvertent opening of steam relief valves secondary pressure control malfunctions leading to an increase in steam flow rate feedwater system malfunctions leading to an increase in the heat removal rate. —Decrease in reactor heat removal feedwater pump trips reduction in the steam flow rate for various reasons (control malfunctions, main steam valve closure, turbine trip, loss of external load, loss of power, loss of condenser vacuum). 

The steam trap neither aids nor retards condensate drainage, unless it is mechanically malfunctioning, meaning it is sticking either open or closed. Also, replacing the steam trap with a condensate drum and LRC also neither retards nor aids drainage. Condensate drainage is a function of system hydraulics, as discussed in subsequent sections. 

And these two malfunctions are also the most common problems we encounter in the design and operation of shell-and-tube heat exchangers used in total condensation service for hydrocarbons, as well as for steam. 

I have written a separate book pertaining to a wide variety of vacuum system problems Troubleshooting Vacuum Systems (Wiley, 2012). And my chapter on steam surface condensers also deals with pertinent vacuum system malfunctions. 

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