Home > Applications

THERMOWELLS (DRY WELLS)

A. Usage & Disadvantages

The use of thermowells (also known as dry wells) is common in industrial applications involving the need to remove a temperature sensor from a tank or line without shutting down the system. Thermowells, however, have some basic disadvantages which must also be taken into consideration for any temperature application:

1. Transmission Time: The added mass, and the type of material has a slowing effect upon how quickly the actual temperature reading will show up on the indicator (brass has fastest transmission time).

2. Fit: Proper, very tight fit is essential, as air gaps create insulation, and therefore inaccurate readings. Unfortunately, there are few standards in the industry for Remote Reading Gas and vapor Filled Thermometers, with every brand, every style and every range with a different diameter and length and connection. Bulb lengths and dimensions, internal & external thread requirements etc must be carefully measured for the specially ordered well, so that the internal diameters and lengths and connections match the sensing bulb. Heat transfer compounds should be used whenever an absolutely tight fit is not possible; an inexpensive compound consists of a paste containing 1/3 water and 2/3rds magnesium hydroxide (available from us, or from chemical suppliers).

Thermocouples and RTD's can also come in any size and shape; a common size, however, is 1/4" OD, and with a 1/2" NPT Spring Loaded Male Fitting, these can fit into inexpensive and commonly found bimetal thermometer wells, which have a .260 bore.

3. "Lagging" thermowells take into account the insulation, pipe fittings, or walls etc. through which a sensor might have to pass.

B. Installation Considerations

The most common method of installing a well is to purchase and install a "tee" from a plumbing supply house and use a standard threaded well; ASA 150#, 300# and 600# Flanged Wells, Van Stone Wells, and Socket Weld types are also readily available. All threaded wells are made in easily welded or brazed materials. This is important for installations requiring sealing; the pipe thread provides the mechanical strength, while the brazing or welding provides the seal.

The object is to measure the temperature of the medium, so the insertion should be to the point in the pipe where the measurement is desired, usually in the middle of the pipe. However, the sensing portion and range of the instrument will often determine the minimum insertion length of the well. The "U" dimension of a well is the insertion length of the sensing bulb (the distance from the tip of the internal bottom of the well to the first thread or other connection means) should be entirely immersed in medium being measured. A properly installed element will project into the liquid an amount 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. Some low range bi-metal thermometers, for example, are not available without at least a 4" length stem. Normally, bi-metal thermometers have a sensitive length of 2.5"; RTD's usually have a sensitive length of 1" or so; thermocouples have sensitive lengths of 1/4" or so; grounded thermocouples are tip sensitive, and have a faster response to temperature changes than ungrounded types (but ungrounded thermocouples help prevent current loops and induced voltages that often destroy the thermocouple millivolt signal). Industrial liquid-in-glass thermometers come standard with either a 2" stem (Submarine Thermometers) or more commonly, 3 1/2" stems (Standard Industrial and Retort); sometimes 6", 8", 9" 10" and up to 48" types can be found. Careful measurement of the "U" dimension is necessary for a correct well fit.

C. Velocity Rating Factor

Tapered shank wells provide greater stiffness for the same sensitivity. The higher strength-to-weight ratio gives these wells a higher natural frequency than the equivalent length straight shank well, thus permitting operation at higher fluid velocity. Another consideration might be materials of construction; some wells made of stainless steel, for example, may take higher temperatures, pressures and velocities than a brass one. Fluid, flowing by the well, forms a turbulent wake (the "von Karmen" trail) with a frequency based upon the diameter of the well and the velocity of the fluid. If the wake frequency equals the natural frequency of the well, the well will literally shake itself to pieces and break of f from the piping. Velocity tables are available from us for most types of standard wells, materials, pressures and temperatures. For simplicity sake, brass is rated at 350⁰F, steel and stainless @ 1000⁰F, monel @ 900⁰F service. Slightly higher velocities might by possible at lower temperatures. Typical ratings for straight stepped thermowells in maximum fluid velocity feet per second:

D. Materials of Construction

To prevent electrolysis, the well should ideally be constructed of the same material as the piping. Another consideration is the corrosive conditions the well will face, as well as strength necessary to face these conditions. Wells are often cut from bar stock in brass, monel, 304 and 316 SS, or other special grades of stainless steel, inconel, hastelloy B & C, Nickel, and Titanium. The least expensive are steel and brass constructions as well as 304 and 316 SS. For high temperature thermocouple use, 302 SS sheaths, silicon carbide or porcelain sheaths are common well solutions.