A well-planned and designed steam and water analysis system (SWAS) with the right instrumentation can help provide the critical insights you need to monitor cycle chemistry, protect equipment and ensure safe operation.
Industry organizations such as EPRI and IAPWS recommend “minimum key instrumentation” to ensure proper plant operation. According to IAPWS TGD2-09, “Every plant should have a minimum level of instrumentation which can uniquely identify key parameters and drivers to each/every failure/damage mechanism which can occur at that plant.”
Typical online instrumentation locations
- Condensate (CPD) – Monitoring condensate usually takes place at the condensate pump discharge (CPD). This is essential to warn of contamination due to condenser leakage, regenerant chemicals from the makeup plant.
- Feedwater – In conventional plants, feedwater is monitored at the economizer inlet (EI) downstream from low-pressure feed heaters, the deaerator and high-pressure feed heaters. In combined cycle plants, the appropriate sampling points are usually downstream of the feed pumps.
- Boiler drum – Monitoring boiler drum water varies according to the chemicals being used. Ideally, samples are taken from appropriately designed sampling probes in the boiler downcomers.
- Steam – In drum boilers of conventional plants or heat recovery steam generators (HRSG), steam can be sampled at two locations: upstream of superheaters or downstream of the superheaters.
- Make-up water – Consider monitoring both newly purified water at the make-up water treatment plant outlet and stored condensate, which might contain dosed ammonia or amine and other contaminants.
key swas instrumentation
Every plant should have instrumentation that provides reliable chemistry monitoring of locations and parameters around the cycle. A well-designed sample system helps keep cycle chemistry in control with accurate, timely data for corrective action.
Conductivity – Monitoring the conductivity of feedwater, boiler/evaporator water or condensed steam provides an immediate indication of the amount of ionic solutes present in a sample. Conductivity indicates the onset of a fault condition, such as contamination from a condenser leak, and can quickly determine trends in chemical conditions within a boiler/evaporator.
Conductivity after cation exchange (cation conductivity or CACE) – Measuring conductivity after water has passed through a column of strongly acidic cation exchange resin can indicate the presence of potentially corrosive ionic contaminants. CACE is commonly used to monitor condensate, feedwater, and condensed steam. It’s extremely effective for quickly indicating the onset of cooling water ingress and for detecting contaminated makeup water.
pH – The pH of a solution is a key parameter in controlling circuit chemistry. The online measurement is made potentiometrically using a pH-sensitive glass membrane electrode and a reference electrode. A practicable target is to measure pH to within 0.1 on the pH scale.
Sodium – Measuring sodium ions in a sample provides a very sensitive indication of contaminants to condensate in plants with water-cooled condensers and also of the carryover of dosing solutes from boiler drums into steam. Glass electrodes are used at controlled high pH (achieved by the dosing of Diisopropylamine (DIPA) or a suitable amine into the sample flow) to provide reliable measurements down to concentrations of ~0.1 PPB.
Dissolved oxygen – Targets are set for dissolved oxygen concentrations in feedwater for two main reasons. High concentrations of oxygen can put equipment at risk for acidic corrosion, which can lead to sudden large-scale tube failures. Very low concentrations of oxygen can cause enhanced iron transport and flow-accelerated corrosion in feedwater and boiler water where all-volatile treatment (AVT) is practiced. As such, measuring dissolved oxygen is essential to optimizing the passive layer formation throughout the feedwater system.
Degassed conductivity after cation exchange – This involves instrumentation to measure conductivity after cation exchange when the most volatile weak acids have been removed from solution. It’s most useful for determining the condition of steam from a condensed sample, as it can be related to the purity criterion for operation of a condensing steam turbine.
Phosphate – A colorimetric technique is used in which the sample is mixed with an acidic solution containing ammonium molybdate together with a redox controlling reagent. In the presence of phosphate, a blue compound is generated and its concentration is detected spectro-photometrically.
Silica – Targets may be set for silica in drum boilers providing steam to turbines to decrease the risk of deposition within the turbine. Since the essential requirement is to meet a target in the steam, it’s generally more appropriate to measure the silica content of the steam. The measurement of silica uses a colorimetric technique where the sample is mixed with ammonium molybdate at a controlled low pH to generate a blue compound that can be detected spectro-photometrically.
Total Organic Carbon (TOC) – The presence of organic materials in make-up water can negatively affect water treatment plant operation. Some organic materials can form deposits that impact heat transfer and could generate potentially corrosive degradation products.
Learn more about SWAS instrumentation in our webinar: Maximize Your SWAS Instrumentation, including:
- The ins and outs of steam and water analysis systems
- Key instrumentation and measurements within your SWAS
- pH and conductivity measurements
- Implications and considerations when upgrading instrumentation