Thermowells are protective devices for the sensors of temperature indicating, recording, and controlling instru­ments As used in out-of core locations in a nuclear power plant, temperature sensors may be exposed to a wide range of pressures and temperatures and to a variety of poten­tially corrosive materials

This section includes a description of the basic types of thermowells and their materials of construction, a summary of methods for ensuring that the thermowell design will survive the mechanical stresses met in service, and a guide to the selection of thermowell materials

(a) Connection to Process Vessel. A thermowell is usually secured to a process vessel by threads, flanges, or welding (Fig 4 14)

III

The threaded connection, normally using standard-taper pipe threads, is most popular owing in large measure to its simplicity and low cost Standard threaded well connec­tions range in size from */2 in to 1 ln NPT, with specials % in to 2 in NPT meeting most requirements

Flanged assemblies of any size and/or pressure rating are available Normal means of well mounting are provided by ASME-approved welding techniques, with follow-up

machining to provide any standard sealing-face configura­tion. Flanges are commonly used to seal long thermowells or those wells which are inserted into large vessels. An alternate flange type well is the nonwelded Van Stone well with integral flange, using a lap-joint flange to hold it in place. Also available is the ground-joint type with a machined ball that mounts in a socket between a pair of mating flanges. These latter two designs have an advantage in that as thermowell replacement becomes necessary, flanges may be reused with the new assembly.

Welded connections are normally used where process pressures are too great for flanged or threaded assemblies or where long-term inexpensive connections are desirable The welded-in type is commonly used in conjunction with high-pressure, high-velocity steam lines. This type well is frequently furnished with close tolerance limits on outside diameters in the area to be welded. These are tapered-stem wells with greater wall thickness in the weld area but with relatively low mass at the end to improve response with tip-sensitive temperature-measuring devices.

(b) Length, Bore, and Wall Thickness. Overall well length is determined not only by desired — insertion length but also by external extension of the connection end. Most threaded connection wells require an additional 2 in. of nonimmersed length to provide threads and wrenching surface. Welded or flanged wells normally require at least 1.25 in. of extra length for instrument-connection thread­ing and welding surface. Where there are layers of thermal insulation, a lagging extension should be added between the process connection and the instrument connection.

Bore size (both length and diameter) depends on the thermal sensing element to be used. The fit between the sensor and the inner wall of the thermowell must be good if accuracy and rapid response are to be achieved [Sec. 4-2.1 (i)]. Care should be taken to prevent heat loss to surroundings and to avoid variations caused by stratifica­tion of process fluids. Where clearances between measuring element and bore are minimal and welding must be performed in the field, a counter bore of 10 to 20 mils greater diameter than the bore should be made. This counter bore should be carried sufficiently far past the welded area to avoid distortion in the bore due to heat of welding.

To withstand mechanical stresses, the thermowell wall should be thick. However, to provide rapid response to process-temperature changes, the wall should be thin (and the immersed well mass should be minimum). These conflicting requirements have been met by using tapered thermowells, in which the tip has a thin wall for optimum heat transfer and a thick mounting for improved strength. The design of these wells is discussed in the next section.

(c) Design of Power Test Code Thermowells. The American Society of Mechanical Engineers recommends a standardized Power Test Code thermometer well, as shown m Fig. 4.15. Wells of this design, with 6 in. minimum wall thickness, are expected to satisfy 95% of the present needs.

Fig. 4 15 —Power Test Code thermometer well (From Sci­entific Apparatus Makers Association Standard RC 21-4-1966.)

The following design procedure enables a user to determine if a well selected for thermometry considerations is strong enough to withstand specific application conditions of temperature, pressure, velocity, and vibration. This design procedure does not allow for effects due to corrosion or erosion. If corrosion or erosion is anticipated, additional wall thickness must be allowed in all exposed sections to prevent premature well failure.

The nominal size of the sensing element is considered here to vary between % in. (6.35 mm) and ?8in. (22.225 mm). For this range the dimensions of the thermo­well are assumed to be those given in Table 4.9.

Table 4.9—Thermowell Dimensions (in.)* Dimension Nominal size of sensing element 7, 7s 7, a "/.a X A (mm.) ‘X,, 1 4 1 6 Iі/, 1’/, 17, a В (min ) 7s 7, ‘У < 6 I’/, a I’/, d (mm.) 0 254 0 379 0 566 0.691 0 879 d (max ) 0.262 0 387 0 575 0 700 0 888 * From Scientific Apparatus Makers Association Standard RC 21-4 1966

A thermometer well must be able to withstand (at the operating temperature) the static stress associated with the maximum operating pressure of the process vessel. The maximum allowable pressure is computed from the formula

P = KjS

where P = maximum allowable static gage pressure (psig)

Kj = a stress constant depending on thermowell geom­etry

S = allowable stress for thermowell material at the operating temperature as given in the ASME Boiler and Pressure Vessel or Piping Codes (psi)

For wells constructed as shown in Fig. 4.15 with dimen­sions as given in Table 4.9, the stress constant has the values listed in Table 4.10. For wells of other dimensions, the stress constant is given by (4.6) where (see Fig. 4.15) В is the minimum outer diameter (inches) at the well tip and Fg is a factor varying between 2 0 and 1.0 as shown in Table 4.11.

Table 4.10—Values of the Stress Constants Kj, K2, and K3 * Nominal size of sensing element Stress —————————— constant % % 9/16 "/l6 X >4 ‘8 ‘ 1 6 ‘1 6 ‘8 K, 0.412 0 334 0 223 0 202 0.155 K2 37 5 42 3 46 8 48 7 50 1 K3 0 116 0 205 0.389 0.548 0.864 *Irom Scientific Apparatus Makers Association Standard RC 21-4-1966.

Thermometer wells rarely fail in service from the effects of temperature and pressure. Since a thermowell is essen­tially a cantilevered beam, vibrational effects are of critical importance If the well is subjected to periodic stresses that have frequency components matching the natural fre­quency of the well, then the well can be vibrated to destruction. In nuclear power plants the temperature of high-velocity fluid streams (steam, water, etc.) must be measured Thermowells immersed in these streams (thermo­well axis transverse to flow direction) are subject to periodic stresses attributable to the cyclic production of
vortices in the wake of the flowing fluid, the “von Karman vortex.” The frequency of these stresses, fw, is

fw = 2.64^ (in Hz) (4.7)

D

where V = fluid velocity (ft/sec)and В = well diameter at tip (in ), see Fig. 4.15. The natural frequency of the thermo­well (cantilever structure) is

fn = Kf (0 (in Hz) (4 8)

where E = elastic modulus of well material at the operating temperature (psi)

7 = specific weight of well material (lb/in.3)

L = length of well (in.) (see Fig 4.15)

Kf = a factor depending on well dimensions (Table 4.12)

The wake frequency fw should not go above 80% of the natural well frequency, fn,

^<08

In

If the ratio r is over 0.8, the well will tend to vibrate to failure.

Table 4.12—Values of Kf* Well length (L), in. Nominal size of sensing element % l У < 6 ‘У.. 7 ‘8 21, 2.06 2 42 2.97 3.32 3.84 2 07 2 45 3 01 3.39 3 96 4 2 08 2 46 3 05 3.44 4 03 io‘/2 2 09 2 47 3 06 3 46 4 06 16 2.09 2 47 3.07 3 47 4 08 24 2 09 2 47 3.07 3.48 4.09 ‘From Scientific Apparatus Makers Association Standard RC 21-4-1966

Table 4.11—Values of F3* (Note t = В — d, D = 2B) t/D FB t/D FB From To From To 0 084 0 091 2 0 0 150 0 169 1 5 0 092 0 099 1 9 0 170 0 199 1 4 0 100 0 114 1 8 0 200 0 219 1 3 0 115 0 129 1 7 0 220 0 239 1 2 0 130 0 149 1 6 0 240 0 249 1 1 0 250 Up 1 0 *From Scientific Apparatus Makers Association Standard RC 21 4 1966

In any practical situation, the fluid velocity, V, is fixed, and the parameters under the instrumentation engineer’s control are the well dimensions. Once the size of the sensing element is decided on (e. g., on the basis of speed of response, ruggedness, etc.), the thermometer-well outer diameter В is fixed (Table 4.9), and the wake frequency (Eq 4.7) is determined. The only well parameter remaining (except materials of construction, see next section) is the well length, L. Since fn decreases with increasing length (Eq. 4.8), the requirement for fw/fn to be less than 0 8 imposes a limitation on the length, L.

The maximum length of a thermometer well for a given service depends not only on the vibratory stresses imposed by the flowing limit but also on the steady-state stresses

(drag) of the flowing fluid. These stresses limit the well length according to the following formula
(c) Frequency ratio (Eq. 4.9) r = 986/1305 = 0.755 <08 (satisfactory) Step 4 Maximum length calculation (a) Magnification factor (Eq. 4.11) 0.755 Fm'(l — 0.7552) J 5
v(S-K3P0) 1 + Fm
к? V
(4.10)
(in in.)
where V = fluid velocity (ft/sec) v = specific volume of fluid (ft3/lb) S = allowable stress for well material at operating temperature per codes (psi) Po = static operating pressure (psig) K2, K3 = stress constants (Table 4 10) The factor Fm is a “magnification factor” dependent on the ratio r of wake frequency to the natural frequency of the well
(b) Maximum length (Eq. 4.10)
0.4134(9725 — 0.389 X 2000)
46.8 350
(1 + 1.325)
5 3 3 in. > 4У2 in. (satisfactory)
Conclusion 1 he well selected is satisfactory for the application. An example of the installation of a typical thermo­couple or lesistance thermometer is shown in Fig 4 16 The thermowell shown is a heavy-duty weld-in well.
w n
r
(d) Materials Used in Thermowells. Because of their ability to withstand chemical attack from process fluids, stainless steels are most frequently used in thermowells Customarily, stainless steels are put in three groups martensitic, ferritic, and austenitic. The martensitic steels contain 11 5 to 18% Cr, <2.57% N1, and 0.06 to 1.20% C They have a ferritic structure when annealed but take on the properties of a martensite when cooled. They can be heat-treated, hardened, and tempered to provide a wide range of mechanical properties for use m abrasive environments or where particularly high strength is required. Martensitic steels are in the 400-senes stainless steels, excepting the ferritic grades, and are strongly magnetic. Examples are the AISI types 403, 410, 414, 416, 420, 431, and the 440 letter series. The ferritic steels contain 11.5 to 28% Cr, no N1, and 0 06 to 0.35% C. They are always magnetic and do not respond to heat treatment. They are strong and ductile when properly annealed and are generally more corrosion — resistant than the martensitics. Examples are the AISI types 405, 430, 430F, 442, and 446. The austenitic grades are chrome—nickel alloy steels with a maximum carbon content of 0.25% and with 7 to 30% Cr and 6 to 36% N1. They are nonmagnetic in a fully annealed condition but become slightly magnetic with cold working They are generally tougher and more ductile than martensitic and ferritic steels and have a much higher corrosion resistance The austenitic steels belong to the 300-series. Selection of a stainless steel requires consideration of which properties are most desirable for the application corrosion resistance, strength at operating temperature, oxidation resistance, particularly at elevated temperatures, availability in a form suitable for fabrication, or ease of fabrication (machinability, weldability)
Example To clarify the use of the above formulas, consider the following example It has been determined that a 4V2-m. well is required to accommodate a % 6-in. sensing element that will measure the temperature of superheated steam at 2000 psia, 1050°F, and flowing at a velocity of 350 ft/sec. If the thermometer well is to be made of type 316 stainless steel dimensioned according to Table 4.9, will the well be safe5 Step 1 Obtain the necessary data as follows
v Specific volume of steam L Modulus of elasticity at 1050° F 7 Specific weight of metal at 70° F S Allowable stress at 1050° F	0 4134 ASME Steam ft3/lb Tables, 1967 22 35 B31 1 0 1967, x 106 psi App C 0 290 Ib/in 3 9725 psi ASME Code, Sec VIII
Step 2 Maximum static pressure (Eq 4.5) P = 0.223 X 9725 = 2170 psig > 2000 psia (satisfactory)
Step 3 Frequency calculations (a) Natural frequency (Eq 4.8)
(Use of dimensions and specific weight values for 70°F instead of for 1050°F is partially compensatory and causes no significant error.) (b) Wake frequency (Eq. 4.7)

RTD CONNECTIONS Зо 4n 5 о Co 5o Зо ЗО 5o I T red’ r ; X X X X CD RTD 5 $ 5 RTD 5 3 TERMINAL 4 TERMINAL

T/C CONNECTIONS + — + — + I-C T/C C-A T/C C-C T/C

Fig. 4.16—Typical thermocouple and resistance element installed in heavy-duty thermowell. (Courtesy Bailey Meter Co.)

The principal properties of commonly used grades of stainless steels are summarized in Table 4.13.

Other materials may be used m thermowells. Tables 4.14 and 4.15 give the recommended allowable stress values and maximum operating temperatures for a number of thermowell materials.