The standard material tables listed for each thermowell series are adequate for most requirements.

These standard materials are inventoried in sufficient quantities to insure prompt delivery demands are met.

INSERTION LENGTH

The insertion length is the section of the thermowell from the closed end to the underside of the mounting thread or other connection method. This length is specified as the "'U" dimension. For best response and accuracy, this length shall allow the greatest area of the sensing element's (RTD, thermocouple, thermometer, etc.) sensitive section, to protrude into the medium being measured. Above all-be sure that dead length-i.e., the length required to pass through walls, pipe fittings, etc., is taken into account when choosing the necessary insertion length.

Bi-metal thermometers, resistance temperature detectors and liquid-in-glass thermometers have sensitive sections between one and two inches in length. It is important that the entire standard insertion length of two and a half inches be immersed in the fluid for proper accuracy.

Filled system thermometer bulbs may have sensitive portions from one to several inches in length. Determine this sensitive length of the bulb before choosing an insertion length.

Thermocouples, thermistors and some rtds have short sensitive lengths. Therefore, they can be used in thermowells with shorter insertion lengths.

A properly installed sensor, in the case of liquids, project into the fluid an amount equal to its sensitive length plus at least one inch. In the case of gas or vapor, this projection should be equal to the sensitive length plus at least three inches.

BORE SIZE

The bore sizes of wells shown here cover the most commonly used temperature sensing elements as follows:

.260" Diameter Bore:
     Bi-metal thermometers (114" stem)
     Thermocouples (1/4" sheath)
     RTD's (1/4" sheath)
     Liquid-in-glass test thermometers (unarmored)

.385" Diameter Bore:
    Bi-metal thermometers (3/8" stem)
    Thermocouples (8 and 14 gauge)
    Liquid-in-glass test thermometers (armored)
    Other elements having .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 length straight shank wells, thus permitting operation at higher fluid velocity. Refer to "Velocity Ratings Of Wells."

VELOCITY RATINGS OF WELLS

Well failures, in most cases, are not due to the effect 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 wells are subjected. Fluid flowing by 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 well must 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 following text , a recommended maximum velocity rating can be found for most 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 for wells made of carbon steel (C-1018), A.l.S.l. 304 and A.l.S.l. 316. Values for brass wells are based on 350° F operation. Limits for Monel wells are based on 900° F service. Slightly higher velocity is possible at lower temperatures.

Where single values appear in the velocity tables, these may be considered safe for water, steam, air, or gas. In the shorter insertion lengths, consideration is given to the velocity pressure effect of water flowing at higher velocities. The values in parenthesis represent safe values for water flow, while the unbracketed value may be used for steam, air, gas, and similar density fluids.

The values printed are extremely conservative and are intended primarily as a guide. Wells are also safe 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.

If you have operating conditions requiring special well designs, our engineering staff is available to assist you.

More Information