Condensate return systems

High in stalled co.sts. The incremental piping required for the steam supply system and the condensate return system must be installed, insulated, and, in the case of the supply system, additional steam traps are often required. The tracer itself is not expensive, but the labor required for installation is relatively high. Studies have shown that steam tracing svstems typically cost from 50 to 150 percent more than a comparable electric tracing system. 

When steam pressures in the chest are near atmospheric, condensate can rise in the shell and drastically reduce avail-ahle surface—if the trap is too small to dump steam into the condensate return system or if the condensate return pressure is greater than the calculated chest pressure required. In these cases, the steam pressure will have to rise in the chest to overcome this error, if steam pressure is available. If not, the rehoiler will not deliver design flux. 

Air vents are most effective when they are fitted at the end of a length of 300 mm or 450 mm of uninsulated pipe that can act as a collecting/cooling leg. Air is an excellent insulating material, having a thermal conductivity about 2200 times less than that of iron. The last place where it can be allowed to collect is in the steam space of heat exchangers. Further, as it contains oxygen or carbon dioxide, which dissolve readily in any subcooled condensate that may be present, the presence of air initiates corrosion of the plant and the condensate return system. 

No single set of recommendations can cover condensate return systems. These divide naturally into at least three sections, each with its own requirements. 

Defunct or poorly operating traps allow heat-containing condensate, insulating air, and corrosive gases to accumulate within the condensate return system, thus reducing efficiency and greatly increasing the risks of system deterioration. 

The condensate return system is a post-boiler section system that includes all steam traps, condensate lines, associated manifolds and valves, condensate receiving tanks, save-all tanks, condensate pumps, and other auxiliaries for condensate recovery. 

Flash steam often may be usefully recovered by fitting a flash vessel either in a common condensate return system or after the steam traps, where large volumes of HP steam are used. Flash steam recovered in this way can be discharged to an LP steam delivery system. 

Carbon dioxide, which damages condensate return systems via the formation of carbonic acid. 

To remove oxygen, carbon dioxide, and other noncondensable gases from the condensate to maintain the necessary vacuum and to minimize the potential corrosion problems that these gases can cause in the condensate return system. 

Condensate system requirements are to check the condensate pumps, steam traps, and overall condensate return system. A loss of condensate return necessitates an increase in MU water to compensate for the shortfall in the FW system. 

In many condensate systems it is not unusual, in practice, to find condensate return systems where the pH is apparently high (> 7.5) and yet very significant corrosion is taking place. There are several reason for this ... 

The pickup, transport, and redeposition of corrosion debris and deposits can happen anywhere in steam distribution and condensate return systems and are not confined to any particular boiler plant size or pressure rating. For example, deposit pickup may occur in a superheater with redeposition taking place perhaps in a pressure reducing station, steam trap, or condensate line. The starting point for transport mechanisms is often a combination of BW carryover and condensate line corrosion. 

Condensate drainage devices, 70 148 Condensate polishing ion exchange in, 74 417 in steam-generating systems, 23 227 water softening method for, 26 122-123 Condensate return, in heat pipes, 73 226 Condensate return systems, 70 147-148 Condensate systems, in industrial water treatment, 26 136-137 Condensation, 9 281-282. See also Polycondensation control of VOCs by, 26 679-680 ketone, 74 570... 

High installed costs. The incremental piping required for the steam supply system and the condensate return system must be installed, insulated, and, in the case of the supply system, additional... 

Condensate Return Systems. In a process plant, steam traps are used to drain and return condensate. Given proper application and continuous maintenance, these can operate with minimal steam leakage. Correct installation is also important (12). 

Steam Rurity. The usual function of steam purity limits is to protect the turbine from deposition and subsequent corrosion. In systems where the steam is used for chemical processes, the specific process may create additional requirements for steam purity. For instance, in food processing (qv), regulations may limit or prohibit hydrazine in the steam, so ascorbic acid or sodium sulfite must be used as an oxygen scavenger. When steam is sent to processes and returned as condensate, it may be necessary to add pH control agents to the steam to control corrosion at various points in the process and condensate return system. Table 1 gives typical steam purity recommendations for steam turbine protection. The recommendations vary, depending on whether the... 

Steam-heated calandrias with process boiling temperature less than 100°C can present control problems, especially at reduced rates and during start-up. In most such cases, low-pressure steam is used for heating. Control is usually achieved by throttling the entering steam in order to reduce the pressure at which it is condensed. At reduced rates this often results in steam pressures less than atmospheric or less than the steam condensate return system pressure. The steam is usually removed through steam traps which require a positive pressure differential to fiinction. In order for the trap to function, steam condensate floods part of the steam chamber imtil the steam pressure is sufficient to operate the trap. This leads to poor control and all the problems associated with condensate flooding. 

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