introduction to thermowells
Introduction to Thermowells:
Materials—The Longevity Factor-
In general, the thermowell material to be chosen for an installation is governed mainly by the corrosion conditions the well will face. The high polish given to all stainless and Monel wells provides maximum corrosion resistance. Occasionally, the primary consideration is one of strength rather than corrosion resistance. For example, a stainless steel well may be required for high pressure water service instead of a brass well which would be satisfactory from a corrosion standpoint. Consult the pressure-temperature ratings given for each well type. Wells are also available in special grades of stainless steel, Chrome-molybdenum steel, Silicone bronze, Hastelloy B & C, Nickel, Titanium, and Monel. Tantalum jackets are also available.
Connection—The Installation Factor-
In these pages, you will find standard bore wells of threaded, flanged (A.S.A. and Van Stone), and socket weld types. All threaded wells are made in easily welded or brazed materials. This is important for installations requiring seal welding or brazing. The pipe thread provides mechanical strength, and the weld or braze provides the seal.
Flanged wells (other than Van Stone type) consist of a bar stock well which is solidly welded to a top quality flange. Standard construction uses a primary “J” groove weld and a bevel groove secondary weld. The underside weld is machined to produce a clean fillet. This double welded construction eliminates the possibility of crevice corrosion since no open joints are exposed from either inside or outside the installation. Socket weld wells are simple to install—simply weld them into place. These wells fit A.S.A. standard socket weld couplings or flanges. The resulting installation is clean and tight.
Insertion Length—The Accuracy Factor
The distance from the tip of the well to the underside of the thread or other connection means is the insertion length (designated as “U”). For best accuracy, this length should be great enough to permit the entire temperature-sensitive part of the element to project into the medium being measured. A properly installed element will project into a liquid a distant equal to its sensitive length plus at least one inch. In air or gas, the element should be immersed its sensitive length plus at least three inches.
Thermocouples and thermistors have short sensitive lengths. They can be used with shorter insertion length thermowells. Bimetal thermometers, resistance thermometers and liquid-in-glass thermometers have sensitive portions between one and two inches long. Therefore, the minimum standard insertion length of 2 1/2" (63.5 mm) must be entirely immersed in liquid for proper accuracy. Filled system thermometer bulbs may have sensitive portions from one to several inches in length. Determine the sensitive length of the bulb before choosing an insertion length. Above all, be sure that dead length, i.e., that required to pass through walls, pipe fittings, etc., is taken into account when choosing the necessary well insertion length.
Bore Size—The Interchangeability Factor
Almost any installation uses several types of temperature measuring sensors. The selection of a standard bore diameter can produce extreme flexibility within the plant. The same well can accommodate either thermocouple, resistance thermometer, bimetal thermometer, or test thermometer. The bore sizes of wells shown in this handbook cover the most commonly used temperature-sensing elements as follows.
0.260" (6.6 mm) Bimetal Thermometers 1/4" (6.4 mm) Stem
Liquid-in-glass Test Thermometers (unarmored)
Other elements having 0.250" maximum diameter
0.385" (9.8 mm) Thermocouples (#14 Gage)
Liquid-in-glass Test Thermometers (armored)
Other elements having 0.35" (9.5 mm) maximum diameter
Tapered or Straight Wells— The Velocity Rating Factor
Tapered shank wells provide greater stiffness with the same sensitivity. The higher strength-to-weight ratio gives these wells a higher natural frequency than the equivalent length straight-shank wells, thus permitting operation at higher fluid velocity. Refer to “Velocity Ratings of Wells.”
Velocity Ratings of Wells
Well failures, in many cases, are not due to the effects of pressure and temperature alone. Inadequate strength of well can be due to improper choice of wall thickness or material. NEWPORT assumes no responsibility for the failure of a thermowell except as stated in the NEWPORT warranty found in the front of the Temperature Measurement Handbook.Less familiar, and more dangerous, are the vibrational effects to which wells are subjected. Fluid, flowing by the well, forms a turbulent wake (called the Von Karman Trail) which has a definite frequency based on the diameter of the well and the velocity of the fluid. It is important that the well have sufficient stiffness so that the wake frequency will never equal the natural frequency of the well itself. If the natural frequency of the well were to coincide with the wake frequency, the well would vibrate to destruction and break off in the piping.
Recommended maximum velocity ratings can be found for every standard well length and material cataloged. To reduce the complexity of presenting this information, the ratings given are based on operating temperatures of 1000°F (537°C) for wells made of Carbon Steel (C-1018), A.I.S.I. 304, & A.I.S.I. 316. Values for brass wells are based on 350°F (177°C) operation. Limits for Monel wells are based on 900°F (482°C) service. Slightly higher velocity may be possible at lower temperatures.
Where single values appear in the velocity tables, these may be considered safe for water, steam, air or gas. For shorter insertion lengths, consideration is given to the velocity pressure effect of water flowing at higher velocities. The values in parentheses, therefore, represent safe values for water flow, while the unbracketed values can be used for steam, air, gas and similar density fluids.
It should be pointed out that the values given are intended primarily as a guide. To be safe, check each well with your own calculation. If you have operating conditions requiring special well designs, our Engineering Staff is available to assist you in all cases. NEWPORT assumes no responsibility for the failure of a well except for its repair or replacement.