TABLE OF CONTENTS The most well-intentioned chemical treatment program may be virtually worthless without representative sampling. However, sample collection and transport to on-line instruments requires great care. Some of the key factors that go into representative sampling include:
Extracting the sample from the bulk solution so that it represents actual conditions in the process fluid
Conditioning the sample to prevent deposition of dissolved constituents in the sample line
Maintaining linear velocity of the sample within a suitable range to prevent deposition or entrainment of suspended solids
Further conditioning the sample to pressures and temperatures that allow for accurate analyses by on-line instruments
While some aspects of sampling have been highlighted in previous chapters, the following discussion provides a more focused look at this aspect of steam generation chemistry.
As many of the previous case histories illustrated, chemistry upsets can occur very suddenly. Except in very low-pressure units, grab sampling by itself is not recommended, as this only allows the chemist to obtain "snapshot" views of chemical conditions. Much can happen during the intervening periods. Online monitoring has become virtually essential for proper chemistry control.
Tables 7-1 through 7-4 list recommended sample points and sampling parameters for electric utility and industrial co-generation steam plants. Some of these analyses may not be absolute requirements, and readers of this book may argue for or against some of the analyses that are or have not been mentioned. However, schemes outlined will allow steam generation personnel to closely monitor boiler conditions.
