How to Select a Suitable Thermowell
In many pressurised processes it is often not possible or advisable to insert the temperature instrument directly into the media being measured. A thermowell protects the temperature instrument, permits the instrument to be exchanged or calibrated, and keeps dangerous or expensive media within the process installation.
To make a proper selection of a thermowell one must also take into account the thermal lag (response time), the sensing accuracy as well as the service condition, i.e. pressure, temperature, velocity, corrosiveness etc.
In the following different subjects are highlighted for correct thermowell selection.
Instrument Connection
The thermowell is provided with a 1/2″ NPSM internal thread connection. This female connection may be used with NPT or NPSM mating threads. The NPSM thread gives better quality in mechanical connection without seizing or galling.
Shank Configuration
For general purpose, where additional strength is not required, the straight shank or stem is used. The shank diameter is 20 mm (3/4″). The last 63.5 mm (2 1/2″) of the shank steps down to 13 mm (1/2″) to provide faster response.
Tapered thermowells are designed for use in high velocities, where extra strength is required. Special attention shall be given to the vibration effects caused by the fluid passing the thermowell.
The fluid will form a wake, known as a “Von Karman Trail”. The wake has a specific frequency, which is a function of the diameter of the thermowell and the fluid velocity. If the wake frequency coincides with the natural frequency of the thermowell, the well will vibrate to destruction.
Lagging Extension
Thermowells with lagging extension are used in pipe systems and on vessels, where these are insulated. The length of the lagging is determined by the thickness of the insulation.
The standard lagging extension is 89 mm (3″) except for thermowells with insertion length of 63.5 mm (2.5″), where the lagging extension is 60 mm (2″).
Other lagging extensions are available on request with multiple of 89 mm (3″) to provide for standard instrument insertion length.
Process Connection
The thermowells are provided with 4 main methods of process connections: Threads, flanges, weldings, and clamp.
Threaded thermowells are the most commonly used wells, because of the low cost and the ease of installation.
Threaded thermowells are not recommended for pressure above 70 bar (1000 psi).
The standard thread sizes are 3/4″ and 1″ NPT.
Flanged thermowells are used, when the pipe spec. calls for flange executions and at elevated pressure. The thermowells are supplied with flanges according to B 16.5 with raised face or ring type joint for pressure classes from 150 – 2500 lbs.
The flanged thermowell may either have the flange welded on the shank (stem), or the flange and shank may be forged as an integrated unit for use in severe service conditions.
Flange sizes are : 1″, 1 1/2″, and 2″.
Weld-in thermowells are used at extreme pressure and temperature service conditions, and when ASME codes require welded connections.
The weld-in type thermowell is a low cost item, but the disadvantage is that the well is not removable for inspection or replacement.
The standard weld-in size is 38 mm (1 1/2″), 26.9 mm (3/4″ nominal pipe size) or 33.4 mm (1″ nominal pipe size).
Bore
The thermowells are internally gundrilled to a diameter suitable to match the stem or bulb of the temperature sensing instrument.
The standard bore diameters are 7 and 10 mm, but other bore diameters are available i.e. according to SAMA standard: 1/4″ and 3/8″ nominal bore (0,260″ and 0,385″).
Insertion Length
The insertion length often called “U” is the portion of the thermowell, which is inserted into the process line (not necessarily into the fluid). The length of the insertion is determined by the installation the tip of the well shall be immersed into the fluid, where a representative temperature is ruling and long enough to accommodate the length of temperature sensing devise of the instrument.
The insertion lengths are 63.5 mm (2 1/2″) to 572 mm (22 1/2″)
Instrument Insertion Length
The length of the instrument insertion is standardised to accommodate standard temperature instruments or determined by the instrument, where these are not according to a standard.
The lengths are 102 mm (4″) to 610 mm (24″).
Materials
The thermowells are normally manufactured in stainless steel AISI 316 (L, Ti) or materials to customers’ requirements and service conditions.