Determination of Boiler Carry-over

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</td> <td valign="top" height=380 BGCOLOR="#FFFFFF" TEXT="#080000"> <div> <B>Determination of Boiler Carry-Over in Utility and Industrial Steam Systems</B></div> <div> Impurities and water treatment chemicals from boiler water are carried over into steam through mechanical and vaporous carry-over (boiler carry-over).  When steam purity exceeds specific limits (low ppb concentrations of salts, oxides, hydroxides, and silica), these chemicals can deposit on the turbine blade surfaces, resulting in corrosion and MW and efficiency losses.  Deposits in turbine valves prevent valve closure, possibly resulting in destructive overspeed.  Deposits in the superheater and reheater can cause reduced heat transfer, tube overheating, and exfoliation.  Boiler carry-over (mechanical carry-over) is usually guaranteed by the boiler manufacturer.  Jonas, Inc. now offers a service that includes a review of the boiler water and saturated steam sampling systems and determination of the carry-over for utility and industrial steam systems.</div> <div> <B>Background</B></div> <div> Boiler carry-over has two components: mechanical carry-over (M) and vaporous carry-over (V).  Concentration of impurities in steam depends on the total carry-over, T=M+V.  </div> <div> <I><B>Mechanical Carry-over </B></I><B>-</B> is the entrainment of water droplets in steam exiting the boiler drum.  It is a measure of the separation efficiency of the boiler for a specific load, drum level, and boiler water chemistry conditions.  The actual mechanical carry-over should be known for a given plant; based upon validated testing during commissioning or shortly after.  It should be checked against industry accepted limits and the equipment warranty (Figure 1).</div> <div> High mechanical carry-over can be an indication of one or more of the following:</div> <div> <IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">poor design, damage, or incorrect installation of steam drum moisture separators</div> <div> <IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">poor control of boiler drum level</div> <div> <IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">high rate of pressure (or load) changes</div> <div> <IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">foaming of the boiler water due to high concentrations of:</div> <div> <IMG BORDER="0" SRC="Symb075c002d00c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">total suspended solids (metal oxides, colloids)</div> <div> <IMG BORDER="0" SRC="Symb075c002d00c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">alkalinity (OH-) (OH-/TDS > 0.2)</div> <div> <IMG BORDER="0" SRC="Symb075c002d00c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">organics (ingress through makeup system, water treatment chemicals, oily matter)</div> <div> <I><B>Vaporous Carry-Over (Volatility) </B></I><B>- </B>is the evaporation of volatile molecular impurities from the boiler water.  The degree of vaporous carry-over is expressed as a distribution ratio, which is the ratio of the concentration of a specific molecular compound in the steam to its concentration in the liquid water.  The amount of vaporous carry-over depends on the chemical species that are present in the boiler water and the boiler pressure (Figure 1).</div> <div> <TABLE BORDER="0" CELLPADDING="2" CELLSPACING="1" WIDTH="493" BORDERCOLORLIGHT="#C0C0C0" BORDERCOLORDARK="#808080" FRAME="BOX" RULES="ALL" ALIGN="BOTTOM" HSPACE="0" VSPACE="0" > <tR> <tD VALIGN=TOP HEIGHT= 416 ><NOBR><div> <IMG SRC="1a3c29a0.jpg" border=0 width="450" height="410" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> </NOBR></tD> </tR> <tR> <tD VALIGN=TOP HEIGHT= 62 WIDTH="487"><div> <B>Figure 1.  </B></div> <div> <B>Mechanical and Vaporous Carry-Over as a </B></div> <div> <B>Function of Boiler Drum Pressure</B></div> </tD> </tR> </TABLE></div> <div> <B>Carry-over Test Description</B></div> <div> Carry-over measurement is performed by simultaneous continuous or periodic (grab samples) monitoring of the sodium concentration in both the boiler water and the saturated steam.  The first step in measuring carry-over is ensuring that the samples that are being obtained are representative.  Jonas, Inc. will review the boiler water and saturated steam sampling systems and make recommendations for improvements, if necessary, prior to the start of the monitoring.</div> <div> <I><B>Representative Sampling</B></I> - It is recommended to use EPRI Isokinetic Sampling Nozzles and the flow velocity through the sample lines should be ~ 5 ft/s (1.5 m/s).  These are described on the Jonas, Inc. website at <A HREF="nozzle.htm" TARGET="_top" TITLE="EPRI Isokinetic Sampling Nozzles (Probes"><U>www.steamcycle.com/nozzle.htm</U></A>.</div> <div> Once the sampling system is working correctly, Jonas, Inc. will install and calibrate two sodium analyzers; one on the boiler water sample line (either blowdown or downcomer) and one on the saturated steam sample line.  The concentration of sodium in each sample will be monitored for a period of ~2 days.  A data acquisition computer will record the data and the total boiler carry-over of sodium will be determined from the real-time data using the following relationships:</div> <DIV ALIGN="LEFT"></DIV> <div> <IMG SRC="1a4af2c0.gif" border=0 width="431" height="44" ALT="Microsoft Equation 3.0" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> <IMG SRC="1a527150.gif" border=0 width="551" height="21" ALT="Microsoft Equation 3.0" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> During this test period, the boiler drum level, boiler pressure, load, chemistry, etc. can be varied to represent selected operating conditions.  This testing should be performed during commissioning of a new unit and repeated periodically to verify proper operation of boiler drum internals, or if there is a change in boiler water treatment. </div> <div> For drum boilers on all-volatile treatment (AVT) and oxygenated treatment (OT), the concentration of sodium in steam may be less than the analytical detection limit and, for a carry-over test, a sodium compound, such as Na<FONT SIZE="1" COLOR="#000000" FACE="Arial" >2.6</FONT>PO<FONT SIZE="1" COLOR="#000000" FACE="Arial" >4</FONT>, may have to be added to the boiler water. </div> <div> <B>References</B></div> <div> <TABLE BORDER="0" CELLPADDING="4" CELLSPACING="1" WIDTH="565" BORDERCOLORLIGHT="#C0C0C0" BORDERCOLORDARK="#808080" FRAME="BOX" RULES="ALL" ALIGN="BOTTOM" HSPACE="0" VSPACE="0" > <tR> <tD VALIGN=TOP HEIGHT= 19 WIDTH="23"><div> 1.</div> </tD> <tD VALIGN=TOP><NOBR><div> O. Jonas. "Transport of Chemicals in Steam Cycles." <I>Corrosion/85</I>, NACE, 1985.</div> </NOBR></tD> </tR> <tR> <tD VALIGN=TOP HEIGHT= 33 WIDTH="23"><div> 2.</div> </tD> <tD VALIGN=TOP WIDTH="523"><div> <I>Interim Consensus Guidelines on Fossil Plant Chemistry</I>. EPRI, Palo Alto, CA: June 1986. CS-4629 and other EPRI Guidelines.</div> </tD> </tR> <tR> <tD VALIGN=TOP HEIGHT= 33 WIDTH="23"><div> 3.</div> </tD> <tD VALIGN=TOP WIDTH="523"><div> <I>Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers. </I>ASME. New York, 1994.</div> </tD> </tR> <tR> <tD VALIGN=TOP HEIGHT= 33 WIDTH="23"><div> 4.</div> </tD> <tD VALIGN=TOP WIDTH="523"><div> O. Jonas, et. al. "Development of Steam Sampling." Presented at the EPRI International Conference on Fossil Plant Cycle Chemistry.  Baltimore, MD.  June 1991.</div> </tD> </tR> </TABLE></div> <DIV ALIGN="LEFT"></DIV> <div> <B><IMG SRC="1e266430.gif" border=0 width="358" height="67" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></B></div> </td> </tr></table></body>