By Brad Buecker, ChemTreat, Inc. and Ken Kuruc, Hach

According to reviews that proceed to look in commerce publications, mixed cycle energy technology will, for a while, stay an vital bridge know-how between declining coal-fired energy and rising renewable sources.  Additionally, with the increasing evolution of inexperienced hydrogen know-how, mixed cycle energy may stay a robust power supply for many years. 

However, as is well-known within the business, many corporations try to function mixed cycle crops “lean and imply” to maintain prices to a minimal.  This can have fairly unfavourable penalties, notably with regard to chemistry management within the plant’s warmth restoration steam mills (HRSGs).  There are many examples of apparatus failures and shutdowns from water/steam chemistry upsets, through which both lack of coaching or inadequate staffing allowed issues to develop.

A crucial side of HRSG chemistry management is monitoring of feedwater conductivity and pH .  Conductivity measurements (notably cation conductivity, as outlined beneath) enable detection of contaminant in-leakage that might in any other case trigger important issues within the steam generator.  Additionally, particular and cation conductivity readings could be utilized to precisely calculate feedwater pH.  Steady pH management is vital to assist mitigate flow-accelerated corrosion (FAC), which has plagued many crops and has even led to a number of failures of feedwater and economizer piping over the past three many years. [1]  Modern know-how permits the entire crucial devices to be positioned on one compact pattern panel part, with readings that may then be distributed to the plant management room or every other location in or outdoors the power.  The current article examines this know-how.

The Importance of Measuring Conductivity

All waters conduct electrical energy, and the conductance (or alternatively the resistance to electrical circulation) is straight associated to the dissolved solids focus.  Specific conductivity (SC) is a basic measure of all dissolved ions in a water pattern.  For instance, fully deionized water has an SC of zero.055 microsiemens (µS) per centimeter (cm), as measured at 25°C. Drinking waters usually have a conductivity of a number of hundred µS/cm particular conductivity. 

Because condensate and feedwater for steam-generating energy crops should be fairly pure to reduce corrosion and scale formation, make-up water programs are usually designed to supply effluent per the next tips:

  • Specific conductivity:  ≤zero.1 µS/cm
  • Sodium:  ≤2 parts-per-billion (ppb)
  • Silica:  ≤10 ppb

This water can have an aggressive affect, together with the aforementioned flow-accelerated corrosion, on the carbon metal, which generally constitutes feedwater piping, warmth exchanger shells, economizer tube bundles, and low-pressure HRSG evaporators.  pH has a big affect on the water’s corrosion potential, as illustrated by the next well-known diagram.

Figure 1.  Influence of temperature and pH on iron dissolution from carbon metal. [2]

For many years, a regular process has been feed of ammonia (NHthree) to condensate/feedwater to ascertain a mildly alkaline pH and decrease corrosion.

            NHthree + H2O Û NHfour+ + OH    Eq. 1

Ammonia dosing is really useful for many HRSGs to keep up pH25C inside a variety of 9.6–10.zero.  But within the realm of high-purity water chemistry monitoring, two difficulties exist with precisely monitoring this chemistry:

  • Measuring pH straight in high-purity water is troublesome and could be liable to errors.
  • Ammonia will increase condensate SC, making SC readings problematic for detection of impurity ingress (e.g., from a condenser tube leak), because the ammonia affect can simply masks small however nonetheless problematic contamination.

Regarding the primary concern, the next desk outlines the…

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