A variety of sensors are used to locate the position of devices or liquid levels in vessels They are described in other chapters in connection with the devices or vessels with which they are usually mechanically integrated.

Techniques for sensing and indicating the positions of control rods are discussed in Chap. 7, Sec. 7-3.7 and in the examples of Sec. 7-4

There are many examples of level sensing in pressur — lzed-water and boiling-water reactors (Vol. 2, Chaps. 15 and 16), for example, sensing the water level in a boihng-water — reactor vessel Usually, the sensors are differential-pressure transducers or a series of pressure-actuated switches. In sodium-cooled reactors, level sensing can also be accom­plished with resistance or induction probes or with acoustic devices. These sensors are described in Vol. 2, Chap 17, Sec. 17-4.3

The steam systems of all nuclear power plants (and of fossil-fueled plants as well) include a variety of level sensors, ranging from simple sight tubes to pressure transducers.

4- 6 STEAM PROPERTIES SENSING

4- 6.1 Quality

(a) Definitions. Steam quality The percentage by weight of dry steam in a mixture of saturated steam and suspended droplets at the same temperature.

Moisture The percentage by weight of suspended droplets of water in a mixture of dry saturated steam and water droplets at the same temperature.

(b) Sample Collection. Sample collection is carried out according to ASTM D1066 or ASME Performance Test Code, Part II. A few salient points are extracted here. It is important to note that the ASME Performance Test Code does not recommend using the electrical conductivity method for determining the moisture content of steam. A recommended form of sampling nozzle is shown in Fig. 4.39.

Fig. 4.39—Recommended sampling nozzle. (From American Society of Mechanical Engineers, Supplement to Power Test Codes, PTC 19.11, Part II, p.6, 1970.)

The pipe or tube in the sampling nozzle extends across the pipe on a diameter to within 0.25 in. of the opposite wall. The drilled holes face upstream in the pipe and are spaced so that each port represents an equal area of pipe section. For a representative steam sample, the hole size in the sample tube must be chosen so the rate of sample flow is equal to the rate of steam flow. The shortest possible connection should be used between the sampling nozzle and the calorimeter or cooling coil.

(c) Moisture Determination. Throttling Calorime­ter. This is a simple device (Fig. 4.40). Its essential details are a throttling orifice admitting steam to an expansion chamber and a thermometer well entirely surrounded by the low-pressure steam from the throttling orifice. The principle of operation is the equality of initial and final enthalpies when steam passes through an orifice from higher to lower pressure, provided there is no heat loss and the difference between initial and final kinetic energies is negligible. Two conditions are necessary in the use of a throttling calorimeter: (1) there must be a significant pressure difference between steam in the sample and steam in the expansion chamber and (2) the quality of the sample must be high enough to produce a measurable degree of superheat (8°F) in the calorimeter.

For example, if the pressure in the expansion chamber is atmospheric, the temperature is 280°F, and the sample pressure is 135 psia, the constant enthalpy line on a Mollier chart shows the initial moisture to be 1%, or 99% quality.

Fig. 4.40—Throttling calorimeter. (From American Society of Mechanical Engineers, Supplement to Power Test Codes, PTC 19.11, Part II, p.14, 1970.)

Quality can be calculated from the following formula:

X = h2,~— X100 (4.16)

hfg

where X = the quality (%)

h2 = the enthalpy of superheated steam at the calo­rimeter pressure and temperature hf = the enthalpy of saturated liquid in the mixture prior to throttling

hfg = the enthalpy of vaporization of steam entering the calorimeter

Separating Calorimeter. The throttling calorimeter cannot be used when the enthalpy in the calorimeter chamber is equal to or less than the enthalpy of saturated steam. Either a separating or universal instrument must be used. In the separating calorimeter (Fig. 4.41), water is separated out from the steam and read in a graduated gage glass. The quality (%) is

X = w +Mxl0° (4Л7)

W + M

where M = the weight of dry steam condensed after passing through the calorimeter

W = the weight of water as read from the gage-glass scale

R = the weight of water corresponding to heat loss by radiation

With an insulated calorimeter, the radiation loss can be neglected.

The accuracy of the separating calorimeter is somewhat less than that of the throttling calorimeter.

Throttling Separating Calorimeter. The throttling sepa­rating calorimeter is made up of two calorimeters, a

throttling calorimeter and a sepaiating calorimeter of low and high range, respectively, in series The steam first passes through a throttling orifice and, if the moisture is not excessive, the quality is determined as m a throttling calorimeter If the moisture is outside the throttling — calorimeter range, the separating calorimeter is at once available and no delay is caused by its use

Separating Throttling Superheating Calorimeter. A throttling calorimeter can be connected to the exhaust of a separating calorimeter if the separation of moisture in the separating calorimeters is not complete. The quality of the original sample can be found by multiplying the qualities for each process.

Radioactive Tracers. Radioactive tracers can be used to determine steam quality from a boiling-water reactor. This method is generally limited to steam pressures under 1000 psi because of the occurrence of volatilized salts m the steam. Concentrations of specific radionuclides in condensed steam and boiler-water samples are determined with a multichannel analyzer Steam quality, in percent, is calculated from the tracer activities as follows

efficiency, or utility. Solids in the steam may be in different proportion to each other than the solids in the water from which the steam is derived.

(b) Gravimetric Determination. The steam is evapo­rated to dryness, and the residue is chemically analyzed in the classical manner. The method is described in ASTM D1069, Tentative Method of Test for Suspended and Dissolved Matter (Suspended and Dissolved Solids) in Industrial Water and Industrial Waste Water.

(c) Electrical Conductivity Method. The conductivity of a condensed steam sample is proportional to the concentration of lomzable constituents dissolved in the sample. The conductivity, expressed in micromhos, is meaningful only if it is compared with data from a gravimetric determination. The gravimetric-determination data are the primary standard. Usually the steam sample is given a preliminary treatment through degassers to remove most of the gaseous impurities that contribute to the measured conductivity. Unfortunately, degassers are not very effective m removing amines, hydrogen sulfide, and sulfur dioxide.

There are four sources of interference with respect to the calibration of electrical conductivity measurements

1. Dissolved volatile substances, such as ammonia, amines, H2S, II2, C02, and S02, increase conduc­tivity. Ammonia and hydrazine are commonly used in once-through boilers for pH adjustment and dissolved-oxygen scavenging.

2. Dissolved solids, such as the oxides of silicon, copper, and iron, ionize very little and therefore have little influence on conductivity.

3. The conductivity of pure water, as small as it is, must be subtracted from the combined conductivity.

4. The current-carrying capability of each ion species is different at any one temperature, and temperature coefficients are different, therefore, calibration de­pends on an assumed composition that may change in any one system and is almost certain to be different in different systems.

(d) Sodium Tracer Method for High-Purity Steam. The

method is described in ASTM D2186. Generally, this method assumes that the ratio of sodium concentration to impurity concentration in the steam is equal to the ratio of sodium concentration to impurity concentration in boiler water

where As is the activity in steam and Aw is the activity m boiler water.

(4 18)

4- 6.2 Purity

(a) Definition. Steam purity refers to the solid matter in steam. Solid matter is defined as materials in steam which are solids at room temperature and which are capable of deposition as solids m superheaters, steam lines, turbines, or other steam-utilizing apparatus so as to reduce capacity,

where St = concentration of impurities in steam Ss = concentration of sodium m steam Wt = concentration of total solids in boiler water Ws = concentration of sodium in boiler water

The values on the right-hand side of this formula are determined by ASTM methods

The principal advantages of this method are its freedom from interferences, its ability to measure extremely small concentrations of impurities, and its rapid response to transient conditions. Sample temperature control is not required in the method.

(e) Determination When Silica and Metal Oxides Are Present. Electrical-conductivity measurements are not always reliable when significant quantities of the oxides of metals or silicon are present. These oxides do not ionize significantly. Eliminating these impurities in the feedwater is the best precautionary measure. Metal oxides carried over into the turbine can plate out on the blades and impair efficiency. If these substances are present in significant quantities, determination should be made from one of the following ASTM methods D857, aluminum, D859, silicon, D1068, iron, D1687, chromium, D1688, copper, and D1886, nickel.