SPECIFICATIONS

Parker thermowells are available in virtually any material to fit your application. Contact the factory regarding availability of materials not listed in the “Ordering Information” guide on page 5.

The strength of a thermowell depends on several parameters that relate thermowell construction to the installation environment. For most industrial applications, standard Parker thermowells provide the necessary strength if the material, style, and length are correctly specified for the application parameters: fluid type, temperature, pressure, and fluid velocity. It is important to note that most thermowell failures are caused by vibration that is induced by fluid flow.

 In addition to providing this selection guide, Parker offers assistance in correctly selecting thermowells, given the application parameters. This service is available for a nominal charge. Contact the factory for more information.

All 2100 Series thermowell bodies are machined from solid bar stock. Flange mounts are welded to the thermowell body. The material type and traceability code are etched on each thermowell. Additional tagging for specific customer requirements is available.

 

MANUFACTURING STANDARDS

Mill Standard +0.000” / -1/32”

±0.010”

±0.050”

±0.050”

¼” ±0.050” (unless otherwise specified)

Polished to 16 RMS

±0.003”

 

THERMOWELL TERMINOLOGY

Process Connection	External means to connect thermowell to process system. Wells can be threaded, bolted (to matching flange), clamped, or welded in place.
Instrument Connection	Internal threads to connect temperature instrument to thermowell.
“U” Dimension	Length of thermowell immersed into process system. Measured from the base of the process connection to the end tip of the well.
“T” Dimension	Also called “lag length” or “lagging extension”. Extends length between the instrument and process connections to accommodate vessel or piping insulation.
“A” Dimension	Instrument insertion length into thermowell. Equal to bore length.
“D” Dimension	Also called “tip diameter”. Diameter of thermowell shank at the end tip of the thermowell. This dimension may vary with process connection and/or shank design.
“Q” Dimension	Also called “root diameter”. Diameter of thermowell shank below the process connection. This dimension may vary with process connection and/or shank design.
Bore Diameter	Dimension of internal bore to match the diameter of the instrument inserted into the thermowell.
Stepped Shank	Also called “reduced tip”. The shank O.D. is reduced over the last 2½” of the “U” dimension from the standard root diameter to ½” O.D. The stepped shank is available with a 0.260” bore diameter only.
Straight Shank	Shank O.D. is the same from the root diameter (“Q” dimension) to the tip diameter (“D” dimension). The straight shank is generally used with a 0.385” or larger bore diameter, but is also available with a 0.260” bore.
Tapered Shank	Shank O.D. is gradually reduced from the root diameter (“Q” dimension) to the tip diameter (“D” dimension). The tapered shank is recommended for heavy duty applications characterized by high vibration, pressure, temperature, and/or velocity.

 SELECTION CONSIDERATIONS

For best temperature measurement accuracy, the “U” dimension should be long enough to permit the entire temperature-sensitive part of the measuring instrument to project into the medium being measured.

Liquid temperature measurement: A properly designed thermowell will extend into the fluid an amount equal to the length of the temperature-sensitive zone plus one inch or greater.

Gas temperature measurement: A properly designed thermowell will extend into the fluid an amount equal to the length of the temperature-sensitive zone plus three inches or greater.

The temperature-sensitive zone for thermocouples and thermistors is short (right at the tip of the device), enabling measurement accuracy with limited immersion into the process fluid.

Bi-metal thermometers, resistance temperature detectors (RTDs), and liquid-in-glass thermometers have bulbs with temperature-sensitive zones between one and two inches long.

Filled-system thermometer bulbs may have temperature-sensitive zones from one to several inches in length.

While Parker offers thermowells with bore diameters up to 0.718”, the most common are as follows:

0.260” bore:

• Bi-metal Thermometers (¼” stem)

• Thermocouples (¼” sheath)

• RTDs (¼” sheath)

• Liquid-in-glass Test Thermometers (unarmored)

• Other elements having 0.252” maximum diameter

0.385” bore:

• Bi-metal Thermometers (⅜” stem)

• Thermocouples (8 and 14 gage)

• Liquid-in-glass Test Thermometers (armored)

• Other elements having 0.377” maximum diameter

Tapered shank wells provide greater stiffness for the same sensitivity. The higher strength to weight ratio gives these wells higher natural frequency than for equivalent straight shank wells, thus permitting operation at higher fluid velocities.

In most cases, thermowell failures are not due to the effects of pressure and temperature. The calculations necessary to provide adequate strength under given conditions are familiar enough to permit proper choice of wall thickness and material.

Less familiar are the vibrational effects to which thermowells are subjected. Fluid flowing past the well forms a turbulent wake (the Von Karman Trail), which has a definite frequency based on the diameter of the well and the velocity of the fluid. The thermowell must have sufficient stiffness so that the wake frequency will never equal the natural frequency of the thermowell itself. If the natural frequency of the well were to coincide with the wake frequency, the well would vibrate to destruction and break off.

Table 1 provides recommended maximum velocity ratings for common well length and material combinations. To reduce the complexity of presenting this information, the ratings given are based on operating temperatures of 1000°F for carbon steel, 304 SST, and 316 SST wells. Ratings for brass wells are based on 350°F service. Ratings for Monel wells are based on 900°F service. Slightly higher velocity is possible at lower temperatures.

The velocity ratings provided are extremely conservative and intended primarily as a guide. Wells are safe from vibrational destruction if the resonant frequency is well below the wake frequency, or if the fluid velocity is constantly fluctuating through the critical velocity point. Nevertheless, if the installation is not hampered by a sufficiently stiff well, it is recommended that the values given not be exceeded.