Radioactive tracer measurement of silicon transfer in boiler-turbine systems

International

Journal

of Applied

Radiation

and Isotopes,

1970, Vol.

21, pp. 87-91.

Pcrgamon

Press.

Printed

in Northern

Radioactive Tracer Measurement Silicon Transfer in Boiler-Turbine

Ireland

of

Sys terns G. C. SNYMAN Atomic

Energy

and J. K. BASSON Board, Pelindaba and

R. K. DUTKIEWICZ Supply Commission, Johannesburg,

Electricity

South

Africa

(Received12 May 1969)

A method is described for the determination of the amount of silica being entrained in the steam of high-pressure boilers employed in large power stations, using silicon-31 as radioactive tracer. Good correlation was obtained with cases of known turbine blade deposition and the results have been used to decrease silica entrainment.

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MESURE

PAR

SILICIUM

INDICATEUR

DANS

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RADIOACTIF

SYSTEMES

DU

TRANSFERT

DE

CHAUDIERE-TURBINE

On decrit une methode pour doser la quantite de silice qui se trouve entrainte dans la vapeur des chauditrcs a haute pression utilisees dans les grandes centrales Clectriques, en employant le silicium-31 comme indicateur radioactif. On a obtenu un bon rapport avec des cas oh la deposition sur les lames de turbine Ctait connue et les resultats ont servi a l’amelioration de l’entrainement de la silice.

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Es wird ein Verfahren beschrieben fur die Bestimmung der Siliziummenge, die im Dampf von Hochdruckkesseln in grossen Kraftwerken unter Benutzung von Silizium-3 1 als radioaktivem Indikator mitgerissen wird. Gute Ubereinstimmung wurde erzielt mit Fallen bekannter Turbinenschaufelablagerungen und die Ergebnisse wurden dazu verwendet, urn das Siliziummitreissen zu verbessern. 87

C

88

G. C. Snyman, J. K. Basson and R. K. Dutkiewicz INTRODUCTION

generally accepted that in a boiler-turbine system the amount of silica in the steam must be below 0*02 ppm in order to prevent silica deposition on the turbine blades, which results in a decrease in cycle efficiency. The ratio ofsilica-insteam to silica-in-water has been determined by a number of investigators(1-3) and was found to be a function of temperature and pressure. The concentration of silica in the steam is small, and some of it is often in a colloidal form which makes chemical determination difficult. These two factors make it difficult to determine the steam silica, and it has been the practice to determine the boiler-drum water silica content and to use the theoretical silica ratio to determine the amount of silica in the steam. In spite of keeping the boiler-drum water down to a safe concentration, serious turbine blade deposition had occurred at one of the South African Electricity Supply Commission’s power stations and it was decided to determine experimentally whether mechanical entrainment of drum water was occurring in the drum (due to faulty steam purifiers) or whether the silica was entering via some other route such as the boiler attemperator supply. IT

IS

CHOICE

AND

PRODUCTION

OF

TRACER Various chemical and radioactive tracer methods have been used for the determination of water entrainment in boiler drums.f4) However, most of these methods suffer from one or other disadvantage, the most common being a lack of sensitivity. Since the solubility of silica in steam is relatively high and since silica is the compound which caused the turbine blade deposition, it was decided to use radioactive silicon as tracer in these experiments. Silicon-31 is the only radioactive isotope which can be produced in suitable activities for such an experiment but its nuclear characteristics pose three problems : (1) The half-life of only 2.62 hr demanded that the test be carried out very soon after preparation ofthe radioisotope, and also required that the duration of the experiment be limited to a few hours.

(2) Since slSi is virtually a pure beta emitter and the samples were in liquid form, GeigerMtiller liquid counters (Veal1 type) were used to measure the activity. Because of their inherent low efficiency and small sample capacity, rather large activities of 31Si had to be used. (3) The small neutron cross-section (110 m barns) for the production of 31Si required large amounts of silica to be irradiated in the reactor to produce sufhcient activities for the tests and also required a very pure chemical for irradiation to minimize the production of interfering activities. Finely powdered quartz was used as the source material and trial irradiations were performed to establish its purity. Several specimens of quartz were tested and the best material selecGamma spectrum analysis and half-life ted. measurement indicated that the radiation purity of the source, i.e. the 31Si beta radiation expressed as a percentage of total beta radiation from the source, was about 99.5 per cent soon after production and 98.6 per cent after 4 half-lives of the 31Si. The major impurities were 5sMn and s4Na. Typically, the weight of water in a boiler was about 100,000 kg, the counting sample 10 g and the efficiency of the counter 10 per cent (boiler capacities between 1 and 2 million lb of steam per hr). About 250 mc were thus required to obtain a reasonable count-rate. About one curie of 3?Si was produced by irradiating 45-50 g of quartz for 5 hr in the Atomic Energy Board’s reactor, SAFARI- 1, at a flux of about 5 x 1013 n/cm2 sec. This weight of quartz was also regarded as the maximum weight that could safely be put into the boiler water without producing an unduly high silica concentration in the boiler. EXPERIMENTAL

PROCEDURE

The radioactive quartz, sealed inside aluminium hydraulic rabbits and packed in lead containers, was taken to the site of the experiment. In all cases-some sites were 250 miles away from the reactor-the site was reached within 4 hr after irradiation. For some tests a light aircraft was used to limit travelling time from Pelindaba to the power station sites, Using long tongs and appropriate equipment

89

Radioactive tracer measurement

two 0.5 cm dia. holes were pierced into the aluminium rabbit and the quartz powder emptied into a plastic bucket containing water. (To reduce airborne activity, this operation was performed under a plastic cover.) The quartz powder was stirred into suspension and then tipped into a receptacle connected to the suction of the extraction pump of the set under test. Quartz dissolves quickly under the very high temperature and pressure conditions of a boiler. Cooled samples of drum water, saturated steam and turbine condensate were taken every 15-30 min. The samples were taken in 2-l. plastic bottles to which non-radioactive silicon solution had been added as carrier. From these samples 10 ml were put into a Geiger-Miiller liquid counter and the beta radiation measured. Great care was taken in cleaning the counters and regular background between samples, counts were taken during the experiments to ensure that the counters were free of contamination. Because of the short half-life of 31Si, decay corrections had to be made. A crystal-controlled master clock was used to register the time at which each sample was counted. To assist in rapid calculation of the decay, a set of decay corrections was computed beforehand and the counts were immediately corrected for background and decay and then plotted. This helped to determine when equilibrium was achieved, and also checked counting errors. The tests were terminated when the radioactivity had decayed to such an extent that the counting statistics became poor. RESULTS

AND

DISCUSSION

The object of the experiments was to determine the ratio of silica-in-steam to silica-in-water for a number of boilers and, by comparing this ratio with the theoretical ratio, to determine whether any mechanical water entrainment was occurring. This ratio was calculated by dividing the radioactive count-rate of the steam by the count-rate of the water. Both the water and steam count-rates were found to exhibit temporal fluctuations but these were generally in-phase, resulting in a ratio which exhibited only statistical variation. Table 1 shows some of the results obtained from these experiments and Fig. 1 shows a

TABLE 1. Results of silicon-3 1 determinations of the silica-in-steam to silica-in-water ratios for 7 power station boilers

Boiler tested A B C C D E F G c* * After arators.

Drum pressure psig 1305 1375 1330 1330 1710 1685 1290 1330 1330 modification

Observed silica ratio o-0035 0.0165 0.0121 0.0119 0~0110 0.0104 0.005 1 0.0033 0.0039 to boiler

drum

Theoretical silica ratio o-0034 0*0042 0.0037 0.0037 0~0102 0.0098 0.0033 0.0037 0.0037 steam

sep-

typical curve of radioactivity in a boiler against time. The drum-water activity rises slowly due to the time taken for the silica to reach the drum. The condensate (feedpump) activity starts high since it is into the condensate that the silica is injected. This figure exhibits two other points of interest. Firstly, the water activity rises to a maximum and then decreases slowly; this decrease is due to the loss of radioactivity in the steam and also to leakage of drum-water as observed during the test. The second point of interest is the fact that the condensate activity is below the steam activity, pointing to a possibility of silica deposition on the turbine blades. During this test it was found that the silica ratio was higher than the theoretical ratio, and the combination of this high ratio with the high silica content in the drum-water led to a predicted figure of steamsilica higher than the 0.02 ppm recommended, and blade deposition could have been expected. The maximum count-rate was recorded about 24 to 3 hr after injection, or about 2 half-lives after production of the activity. Because of the large decay correction required towards the end of each test, large fluctuations in corrected counts were obtained, as can be seen from Fig. 1. It was therefore important to calculate the ratios of silicon in the steam to silicon in the water from the mean counts over the whole equilibrium period, rather than to attempt to interpret individual ratios. The accuracy of the method, based on counting

90

G. C. Snynan, J. K. Basson and R. K. Dutkiewicz

91

Radioactive tracer measurement

statistics of actual counts recorded, was f6 per cent for one standard deviation in the worst case (Boiler G) and better for the other sets. The repeatability of the measurements on Boiler C (see Table 1) is an indication of the accuracy of this method.

CONCLUSION The radiotracer method developed for the determination of silica ratio was found to be Good easy to implement and very sensitive. correlation was obtained for those cases where the silica ratio was higher than the theoretical value and with known blade deposition. As a result of the findings of these experiments

3

the steam scrubbers of one boilertype were replaced, i.e. the scrubbers of boiler C were replaced with a type identical to those in boiler F. This resulted in the elimination of the water, and hence of silica entrainment, as shown by the third experiment on boiler C in Table 1.

REFERENCES COULTER E. E., PIRSH E. A. and WAGNER E. J. I’ Trans. Am. SOL. mech. Engrs 75, 689 (1956). 2 STRAUB F. G. Steam Turbine Blade Deposits. Illinois * Eng. Exp. Station, Bulletin Series No. 364 (1946). 3. KENNEDY G. C. Econ. Geol. 45, 629 (1950). 4. Handbook of Industrial Water Conditioning, 6th Edn. Betz Laboratories Inc., Philadelphia (1962).