New Guidance On Boiler Feedwater Sampling and Monitoring Released by ASME

A new document, “Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers” is now available from ASME - the American Society of Mechanical Engineers. It was prepared by the Water Technology Subcommittee of the ASME Research and Technology Committee on Water and Steam in Thermal Systems and provides recommendations for sampling and monitoring required to maintain the suggested water chemistry limits outlined in:

1) “Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers,” and

2) “Consensus on Operating Practices for Control of Water and Steam Chemistry in Combined Cycle and Cogeneration Power Plants.”

The boilers covered by these recommendations include:

Industrial water tube, high duty, primary fuel fired, drum type with superheaters and/or process restrictions on steam purity

Industrial water tube, high duty, primary fuel fired, drum type without superheaters

Industrial fire tube, high duty, primary fuel fired

Industrial coil type, water tube, high duty, primary fuel fired

Marine propulsion, water tube, oil fired, drum type

Electrode type, high voltage, recirculating jet type

Single-pressure industrial heat recovery steam generators

Importance of Effective MonitoringEffective monitoring of makeup water, condensate, feedwater and boiler water qualities and steam purity is necessary to control deposition and corrosion within the boiler, steam and condensate systems. The absence of adequate monitoring and control can lead to increased costs, major system component failures, operational upsets and/or unscheduled outages and is ill-advised from the viewpoint of safety, economy and reliability.

Click here to enlarge imageA key component of effective monitoring is sampling. The sample must be truly representative of the stream being monitored. This requires a proper sampling system, adequate system flushing, the required sample velocity and appropriate sample handling during the time between sample collection and analysis. If proper procedures are not followed, analytical results may not be representative of the stream being monitored, and the results will be meaningless and potentially harmful if used as the basis of operating decisions.SamplingThe full document includes diagrams of the recommended sample points for makeup, feedwater, blowdown, steam and condensate for both soft water and high-purity makeup systems. Corresponding tables provide additional information on sampling frequency and recommended testing for each sample location.

Figure 1. Sampling Nozzle InstallationClick here to enlarge imageRecommendations for sample system design and conditioning are also provided. Samples at temperatures exceeding 77°F must be cooled prior to collection to maintain a representative sample, prevent the loss of sample volatiles and assure the safety of collection personnel. In addition, several other general requirements for sample conditioning systems exist to minimize the pickup or loss of particulate and metal oxides and prevent contamination during low flow/pressure situations:

Design sample lines and other system components using inert metal, which is resistant to corrosion by steam and high-purity waters (typically 304 or 316 stainless steel).

Extend the water sample nozzle into the pipe a distance 0.23 times the internal radius of the pipe, up to a maximum of 2 inches. The nozzle should be cut at a 45° angle and should face into the direction of flow (see Figure 1). A Pitot tube is an acceptable equivalent.

Keep sample lines as short as possible, ideally less than 30 feet.

Minimize the number of valves, fittings and elbows or bends.

Bend tubing whenever possible, rather than installing a fitting.

Avoid traps and pockets in which fluid or sludge can collect.

Throttle sample flow at the outlet of the cooler only. Multiple sample coolers may be necessary to achieve the recommended sample temperature.

Pitch the sample line downward at least 10° toward the sample outlet.

Limit sample lines to 1/4 or 3/8 inch tubing to facilitate flushing.

Design sample lines for a velocity of 5-6 ft/sec.

Design isokinetic sample probes as per ASTM D 1066, Standard Practice for Sampling Steam and ASME PTC 19.11, Water and Steam in the Power Cycle.

When sampling superheated steam, locate a roughing cooler as close as possible to, but not more than 20 feet from the sample tap in order to “stabilize” contaminant concentrations. Insulate the line between the tap and the cooler.

Recommended purge times relative to sample line size are provided in Table 1. Sufficient time must be allowed for equilibrium to be established when flushing a sample line or following a change in sample flow rate. High-purity water samples typically require longer continuous flushing (24 hours recommended) to achieve representative samples at the low concentrations of particulate contaminants present. Flow rate should not be readjusted within 45 minutes of sampling. Velocities of 5-6 ft/sec. are recommended.Isokinetic sampling probes are required and should be used at their designed flow rate when sampling two-phase systems, such as sodium in steam and iron in water.Samples taken for dissolved oxygen, high-purity conductivity and high-purity pH must be tested immediately at the sample location or must be determined using a continuous in-line analyzer because of the effect of air absorption on final results. At a sample conductivity of <10 μS/cm, the sample is prone to carbon dioxide absorption from air, which will change both conductivity and pH values.Analytical TestingThe consensus document also provides interferences and special considerations for obtaining accurate and meaningful results with specific analytical tests typically used for monitoring. Parameters discussed include but are not limited to: dissolved oxygen, iron, copper, hardness, pH, organic matter, silica, total alkalinity, specific and cation conductivity, total dissolved solids and ORP. About the Author: Debbie Bloom is first vice chair of the ASME Research and Technology Committee on Water and Steam in Thermal Systems. She’s also a principal consultant at the Nalco Company, where she’s worked since 1977. Contact: 630-305-2445 or [email protected]Sampling Procedures
NOTES for Table 1 at top of page:
1. Standard, accepted good sampling practices require a minimum of 3 sample system volumes be purged before a sample is considered “representative” of the system. For example, ten feet of 1⁄4” tubing with a wall thickness of 0.035” would require 18 seconds to flush while 100’ of the same tubing would require 3 minutes to flush.2. High-purity water samples typically require longer continuous flushing (24 hours recommended) to achieve very low concentrations of particulate contaminants present. Flow rate shouldn’t be readjusted within 45 minutes of sampling.Get your copy of the new guidelines at http://catalog.asme.org