Technical Tips

STEAM PURITY STUDIES

WHAT IS A STEAM PURITY STUDY?

A study of the amount of contamination found in the steam from a boiler under various operating conditions.

WHAT INFORMATION DOES A STEAM PURITY STUDY PROVIDE?

The study can quantify amount of boiler water carryover in the steam.

The study can help determine what is causing the boiler water carryover.

WHY IS A STEAM PURITY STUDY IMPORTANT?

The purity of the steam delivered to a turbine, a superheater or a process is often critical to the proper performance of the equipment. Boiler solids accumulating in equipment can cause costly failures due to corrosion or overheating.

HOW IS A STEAM PURITY STUDY PERFORMED?

The study requires an ASTM Steam Sampling Nozzle be installed in the saturated steam header off the boiler. The nozzle is installed in the superheated steam header if superheated steam purity is to be checked.

The steam sample is condensed and supplied to a continuous recording analyzer monitoring conductivity or sodium or silica on a continuous basis.

During the study the amount of contamination in the steam is measured at various boiler loads and under changing load conditions.

If process contamination in boiler feedwater is a factor the purity of the steam will be monitored with contaminated feedwater supplied to the boiler.

The following steps will illustrate how the study is performed:

1. Confirm installation of an appropriate ASTM Steam Sampling Nozzle in the steam header.
2. Install a sample cooler to condense the steam as close to the analyzer as practicable.
3. Install the Continuous Recording Analyzer as close to the sample point as practicable.
4. Initiate steam flow to the cooler and check operation of the analyzer.
5. Run the system with normal stable high load operation on the boiler for at least 24 hours if possible.
6. Repeat step 5 under one or more different boiler load conditions over the next several days if possible.
7. To examine the impact of changing boiler load operate at a stable load for at least four hours and then swing to a higher load as quickly as possible.
8. Repeat step 7 swinging from a higher load to a lower load.
9. Shut down the boiler blowdown to allow the TDS to exceed the recommended limits for at least 6 hours at a normal STABLE operating load and record the steampurity
10. Add caustic soda to increase the boiler hydrate alkalinity to 50% higher than its recommended value and record the steam purity.
11. If feedwater contamination is an issue operate for at least 6 hours at normal STABLE operating load with contaminated feedwater and record the steam purity.
    

The conditions under which each test is performed must be noted on the strip chart at the time that test is being run.

For questions concerning the design and sizing of the sampling nozzle or selection of the analyzer, please email Bain & Associates Consulting, Ltd.

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