Sampling and Data Collection Considerations

Calculation of total carryover requires determination of a low volatility parameter (typically sodium though other parameters, such as chloride or sulfate, could be considered as an

alternative or in addition to sodium) in the saturated steam and boiler water of the unit. Ideally, the readings should be collected as close to simultaneously as practical.

When one or more on-line analyzers will be used to collect the data the outputs of the

instruments should be checked and calibration performed if needed. If a single online analyzer will be used for both sample points, sufficient time must be provided to allow for switching of samples and stabilization of readings. In cases where on-line analyzers are not used, grab samples should be collected as close to simultaneously as practical.

The boiler blowdown is the preferred sample point for carryover assessment as it is more representative of the liquid phase which may enter the steam with the saturated vapor. It should have a somewhat higher concentration of the constituent of interest than a boiler downcomer sample, which will tend to be slightly diluted by the feedwater entering the boiler.

For carryover assessment purposes, saturated (drum) steam is the only acceptable sample point for steam. Superheated and reheat steam samples are not representative for carryover assessment since impurities in the saturated steam may form deposits prior to the superheater and reheater.

Also, impurities from previously deposited solids may later dissolve and be present in the superheated and reheat steam. In addition, the composition of superheated and reheat steam is influenced by use of feedwater for attemperation.

Monitoring of the saturated steam provides verification of compliance with the boiler

manufacturer’s performance guarantee for steam purity, which often applies only to the saturated steam. Depending on boiler design the saturated sample source may be from single or multiple steam leads (off-take lines). The boiler operator should ensure that a representative sample is taken. In older boilers with multiple steam lead arrangements, some of the leads may be isolated and no longer operable. In such cases, resultant steam purity data does not provide complete assurance that contamination is not entering the steam path.

With proper sampling and analysis, it is thus possible to effectively monitor the total carryover of impurities into the high pressure turbine. As carryover is influenced by boiler water pH, the operator should note this at the time the sodium or other constituent readings are checked. Also, if a downcomer sample of boiler water must be used, the condition of the feedwater should be noted. These additional details will be helpful in interpretation of results including comparison of findings from carryover assessments made over time. Such an approach greatly improves the chance for identifying and correcting problems before target values for impurities allowed in the reheat steam are exceeded.

The saturated steam chemistry data and trends over time can also be related to the performance of the steam drum moisture separators by measuring the carryover of impurities from the boiler into the steam. Excessive carryover may indicate poor moisture separator performance. When the design includes multiple steam leads and these can be sampled individually, it should be possible to identify areas of greatest concern.

The latest cycle chemistry guidelines are based on findings of a series of EPRI sponsored investigations of volatility [4-10]. In the case of constituents such as silica and copper, volatile carryover is a significant contributor to total carryover over a broad range of operating pressures [8,9]. However assessment results for most impurities including sodium, chloride and sulfate has confirmed that mechanical carryover of has the greatest impact upon total carryover and steam purity, particularly at boiler drum operating pressures below 2500 psi (17.2 MPa). As discussed here and in the individual cycle chemistry guidelines, total carryover from drum boilers is considered now a Core Monitoring Parameter which should be routinely checked at intervals of about every six months so as to minimize the risk of steam contamination and possible damage to the turbine.

3.5 References

1. Cycle Chemistry Guidelines for Fossil Plants: All-volatile Treatment, Revision 1. EPRI, Palo Alto, CA: 2002. 1004187.

2. Cycle Chemistry Guidelines for Fossil Plants: Phosphate Continuum and Caustic Treatment.

EPRI, Palo Alto, CA: 2004. 1004188.

3. Cycle Chemistry Guidelines for Fossil Plants: Oxygenated Treatment. EPRI, Palo Alto, CA:

2005. 1004925.

4. Behavior of Ammonium Salts in Steam Cycles. EPRI, Palo Alto, CA: December 1993. TR-102377.

5. Volatility of Aqueous Sodium Hydroxide, Bisulfate and Sulfate. EPRI, Palo Alto, CA:

February 1999. TR-105801.

6. Vapor-Liquid Partitioning of Sulfuric Acid and Ammonium Sulfate. EPRI, Palo Alto, CA:

February 1999. TR-112359.

7. Volatility of Aqueous Acetic Acid, Formic Acid and Sodium Acetate. EPRI, Palo Alto, CA:

July 2000. TR-113089.

8. Behavior of Aqueous Electrolytes in Steam Cycles: The Solubility and Volatility of Cupric Oxide. EPRI, Palo Alto, CA: November 2000. 1000455.

9. The Volatility of Impurities in Water/Steam Cycles. EPRI, Palo Alto, CA: 2001. 1001042.

10. Vapor-Liquid Partitioning of Phosphoric Acid and Sodium Phosphates. EPRI, Palo Alto, CA: 2003. 1007291.

11. Assessment of the Ray Diagram. EPRI, Palo Alto, CA: 1996. TR-106017.

12. Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs).

EPRI, Palo Alto, CA: 2006. 1010438.