· Thermowells are usually inserted in a vessel, pipeline and is used to protect the thermo element from process conditions
· Thermowell are inserted either by
· Flange joint – Most applications used flange joints
· The thermowell is clamped between a piping nozzle flange and the temperature transmitter’s cover flange. There will be a gasket between the piping nozzle flange and the thermowell
· It is important that the thermowell and the piping nozzle flange have a serrated finish to help the spiral wound gasket in between it arrest leaks. Serrated finish are small groves between 3.2 to 6.3 micro meter. There will be around 45 to 55 groves per inch.
· There was an incident in PPTSB where the thermowell was covered with satellite material to prevent erosion. The satellite cover came all the way to the connection joint. There should be a serrated finished here instead of the satellite coating. The thermowell was sent to a machining workshop so the satellite material is removed and groves are installed instead
· Thermowell cover flanges are typicall 1 ½” in size. The pressure rating will usually be a pressure rating higher than the piping spec. For example if the pipe rating is 300#, the thermowell cover flange (and gasket) will be 600#.
· Threaded joint – This is not used in high pressure applications. Also, thereded joints require teflon tape. This tape usually melts at high degrees.
· Threaded joint with seal weld – This is used since welding alone is not strong enough to hold the well. A thread is used to ensure stronger well contact
· Before installing a new thermowell, the following values should be calculated:-
· Natural frequency (fn) of vibration when installed in its service
· Vortex shedding frequency (fp) for full range of process flow rates
· For safe applications fn > 2fp
· The insertion length (from flange top to end of thermowell) of a thermowell according to PTS standard:-
· 230 mm for DN80 and DN80 and DN 100
· 255 mm for DN 150
· 305, 355, 405, 455 mm for equipment
· For small pipes (<3”) , the thermowell can be inserted in the elbow of the pipe
· The standard set is to take into account the mechanical strength of the thermowell due to flow and vibrations
· The thermowell should be immersed as much as possible to allow proper conduction of heat to the element i.e. to ensure that the measured temperature is exactly the same as the actual temperature.
· The sensor must touch the end of the thermowell and is inserted into a ½” NPTF thread.
· Thermowell should not be used in high fluid velocity and high vibration
· General Rule of Thumb : The length of the thermowell must be slightly more than half the pipe length.. This is to ensure that the thermowell will always be touching the process liquid. Why not just make it long enough? The length cannot be too long to avoid vibrations
· Temperature elements are put inside a thermowell
· The layer structure inside a temperature element
· The outer well layer – Ceramic type for high temperature or just plain SS
· Magnesium oxide powder (MgO) layer - ~0.5 mm
· Secondary sheath – Made of SS to protect the temperature element wires. Usually 6 mm
· The element tip must touch the thermowell. This is often ensured by having a spring
· Insertion of an element is typically screwed on a cover flange.
· The thermocouple use in our plant is type K, chromel-alumel. Type K is the most common used because it has he highest range
· Thermocouple are usually used for high temperature measurements
· If a thermocouple with no transmitter’s wire disconnects, the reading would go maximum high, this is because thermocouple measures voltage. An open circuit would of course give the highest voltage.
· The 2 Thermocouple wires are colored as
· Red is positive and is Chromel
· Yellow is negative and is Alumel
· Tip types
· Grounded tip – good heat transfer
· Ungrounded tip – No noise (EMI)
· Exposed – Highest Sensitivity
· If you do not know the color coding of the Thermocouple, there are 2 methods to identify which one is positive or negative :-
· Short the wires to create a junctions and use a voltmeter to know which on is +Ve
· Use a magnet to detect chromel, since chromel is ferrous and response to magnets. Chromel is positive and alumel in negative
· RTD’s are used because of its extreme accuracy and sensitivity up to 0.001 degree C
· 3 Wire RTD’s are used in process plants to reduce the connection wire error. The connection wires used are usually of copper type.
· When verifying the direct resistance of an RTD, one needs to compensate the wire errors. This can be done by directly measuring the resistance of the 2 common connections. The Sensor resistance is therefore the resistance of the 2 terminals minus the resistance of the 2 common terminal wires divide by 2.
· 3 wires RTD are used to compensate the resistance from the transmission wire. Since there are 2 transmission wires, we assume the wires are of same resistance. Having a 4 wire RTD will compensate each wires differently
· Most Transmitters compensate the wire error from the sensor to the transmitter. For example, in the Machine Monitoring System (MMS), the RTD is a kilometer length from its’ transmitter to the system. This would of course bring some resistance. To prevent this, RTD transmitters compensate the value of this resistance. Therefore, to measure the direct resistance of the RTD, one has to measure the sensor directly at the nearest terminal to the RTD.
· The most common RTD material used is platinum and nickel. Platinum is much more accurate and has less span shift (linearity).
· Copper RTD are sometimes used to eliminate thermocouple effect on the copper wire extensions.
· The error of RTD for old type is +0.15% and for SMART type is +0.05% of full span
· Causes of RTD error : trapped moisture, vibration, contact with sheath.
· RTD hardly drifts. Only 0.1% span shift in 6 months and most o the time longer.
· RTDs must not be used in vibrating services.
· RTD should not be used on high temperature measurements. This is because at high temperature, the RTD has a self heating. In PTS it is stated that RTDs must not be used above 650 degrees C. However, based on experience, even service at 400 degrees C would reduce the life time of the RTD and later cause problems.
· 4 Wire RTD’s are never used for high temperatures