Thermowell U-LENGTH Calculation | Thermowell Insertion Length (MOST SIMPLE EXPLANATION)

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Why do we it call U-Length ?
The U standards for unsupported length in thermowell
The U Length of thermowell starts from the tip of the thermowell to the edge of connection ( i.e flange or threaded connection)

How to calculate the U-Length of Thermowell ?
No matter what engineering project you are executing in any part of the world, the basic design rule remains the same.

Thermowell U-Length is the summation of Standout and Immersion Length

a) Standout Calculation

b) Immersion length Calculation

Criteria 1 : Nozzle Standout calculation

First let us look at standout Criteria of Nozzle.

In usually petrochemical project usually the Pipes have Insulation (Hot/cold/acoustic etc)

In order to remove the thermowell nuts, it is recommended to have a clearance space of 100mm

Thus if we assume a standout of 150 mm then with 100 mm insulation the Standout might have to increased to 200mm

So 200 (standout) – 100 (insulation) mm = 100 mm (Clearance space)

Similary we for 150 mm insulation we will increase standout to 250 mm so we have clearnace of 100 mm to easily remove bolts without having to cut through the insulation.

Criteria 2 : Thermowell immersion length rule of thumb

The immersion length is the length of the thermowell that protrudes into the process fluid past the edge of the pipe or vessel

In simple words the length of thermowell that is in contact with process.

See below picture

Imersion length criteria varies from project to project and Standard to Standard.

The optimum immersion length is a tradeoff between accuracy and response time with mechanical strength requirements

API RP 551 recommends a minimum immersion length of 50 mm (2 in.).
The thermowell tip should not be inside the edge of the nozzle. The tip should always be in contact with the flow stream.