Question of the Day: Boiler Carryover

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Taxonomy

• Cooling Boiler & Wastewater
• Industrial Water Treatment
• Industrial Water Treatment
• Industrial Water Managment

5 Answers

1. I agree with most of the comments made prior to this one as to definitions.

   Carry over is specifically any solid dissolved or suspended in boiler brine that becomes entrained in steam output as particles of solid material, micro-droplet mist, or generally wet steam.  Physical causes include poor level control in boiler drums, poor upstream treatment of boiler make-up water, especially where high silt/clay surface waters are the source, and failure/misapplication of boiler internals (mist eliminators).  Failure to use the correctly pure water for steam attemperation can also result in "carry-over" although this is actually resulting from direct contamination of the steam.  Anything related to carry-over must be strictly prevented in Universal Pressure Boiler, or once-through boiler as all the water entering is converted to steam.

   The impact depends upon the application, thusly for heat exchange carry-over may result in impingement attack of surfaces, leading to failure, deposit accumulation also will result in loss of heat transfer efficiency.  For rotating equipment (bladed Rankine cycle turbines), deposits on blades will eventually spall off unevenly, and this gives rise to excessive vibration, or even sudden catastrophic failure of blade rows due to sudden release, gross rotor imbalance, if the problem is that severe.

2. Carry over is a process to carry the Impurities and water treatment chemicals from boiler water into steam through mechanical and vaporous carryover. I.e  salts of  oxides, hydroxides, and silica. It can deposit in area of process for production.

   • Deposit on the turbine blade resulting in corrosion and efficiency losses.  
   • Deposits in turbine valves prevent valve closure resulting destructive overspeed.  
   • Deposits in the superheater and reheater can cause reduced heat transfer resulting tube overheating, and exfoliation.  

   Mechanical Carry-over reasons are following,

   

   • poor design, damage, or incorrect installation of steam drum moisture separators
   • poor control of boiler drum level
   • high rate of pressure changes
   • foaming of the boiler water due to high concentrations ofTDS, alkalinity & organics.

   Vaporous Carry-Over (Volatility) -  A distribution ratio concentration of a specific molecular compound in the steam to its concentration in the liquid water.  

    

   I have cover practical problem of boiler water treatment in my book Fundamental Boiler Chemistry & Preventive

3. When it foams in Boiler upper drum the boiler water carries over along with steam. The loss in feed makeup and quality of steam matters. The steam remains wet as superheater also affected.

4. As a generalized category of carryover, I have always related it to be caused by the following but no exclusively to:

   • Operational - If a boiler is not properly brought up to pressure and ready to meet the steam demand, priming can occur. 
     Mismanagement of COC's by over cycling and lead to carry over.
   • Mechanical - If there are malfunctioning high level sensors or ancillary steam/heat exchanger components causing an imbalance in back pressure then vacuum, carryover can occur.
   • Contaminants - F.O.G. ~ Fats, Oils and Greases being returned in the condensate and subsequently entering into the DA will ultimately enter into the boiler. The increased surface tension, similar to the over cycling of the boiler water, will lead to carryover.

   The impact, to name a few, will cause:

   • System failure
   • Unreliable performance
   • Contaminated/wet steam ~ leading to insufficient heat exchange
   • Product contamination in cases of direct steam contact operations.

5. High TDS (exceeding the recommended cycles of concentration) 

   surges in stream demand

   poor design

   contamination from condensate or improper chemical treatment