response time of RTD


Thread Starter

Ayesha Nasser

We have a case where customer specified a stepped type TW with dia 25mm at cold end (below flange) and tip dia 16mm (i.e for U=470mm, 25mm dia for a length of 390mm below flange and remaining 80mm length will be of 16mm dia). RTD is 3-wire, duplex Pt100, Medium gas, max temp 60degC, max pressure 62 barg, velocity 8.4m/sec, density 73.35kg/m3. For better stability, we need to increase thermowell dia to 35mm and 27mm at flange and tip respectively instead of 25mm and 16mm. Can anyone please advise the impact on response time due to the increase in TW dia

Dave Bartran

Calculation on an air basis shows that at 8.4 m/s your first order response time is 18.8 seconds. It will take about 4-5 times that to reach equilibrium. The replacement well on the other hand, has a first order response time of 49.7 sec and will reach equilibrium in about 3-4 minutes.

Have assumed 2" pipe insulation and ignored radiation effects.

By the way it is clear why the first well design was not acceptable as it experienced flow induced resonance in the range of 5.4-8.2 m/s with sufficient stress levels to fail the thermowell at the root connection almost as soon as it is installed and brought up to service conditions. The replacement well is not so exposed so you gotten good design advice in that regard.

Suggest that you reduce the tip diameter to the 16 mm diam to obtain better time response.