Thermowell Flange Class Evaluation

1. Purpose

This procedure establishes the Inflection Point Engineering design requirement for determining the flange class of all flanged thermowells based on ASME B16.5 data, thermowell flange metallurgy and the mechanical design pressure and temperature of associated equipment.

Use the 2003 edition of ASME B16.5 for both piping and vessel thermowells.

2. General

Inflection Point Engineering uses Type 316 stainless steel as the general service metallurgy for flanged thermowells.

Prior to 1977, the pressure-temperature relationship of ASME B16.5 generally allowed the selection of thermowell flange class directly from the associated equipment flange class, because the pressure-temperature relationship of Type 316 stainless steel, at any temperature, was equal to or higher than that of all carbon steels and all chrome-moly alloys.

After 1977, the pressure-temperature relationship of ASME B16.5 rated Type 316 stainless steel lower than carbon steel at temperatures below 800°F (427°C) and lower than various chrome-moly alloys at temperatures below 850°F (454°C).

3. Procedure

The Instrument Engineer is responsible for determining the correct flange class for each thermowell.

The flange class to be specified for a thermowell CANNOT (in general) be directly obtained from the associated equipment flange class.

Determine the flange class for each thermowell, regardless of metallurgy type, from the mechanical design pressure and temperature requirements of the appropriate associated equipment, from the thermowell flange metallurgy and from the ASME B16.5 data. DO NOT assume the thermowell flange class is the same as the flange class of the associated equipment or piping.

The following flange class evaluation procedures cover most of the process services for thermowells. For other process services, use good engineering judgment and process knowledge to select the appropriate design temperature and pressure.

3.1 Process Piping

Generally, determine the flange class of thermowells connected directly to process piping on the basis of the mechanical design pressure and temperature of the FIRST piece of equipment (vessel, reactor, exchanger, filter, etc.) upstream of the thermowell flange.

3.2 Heat Exchanger (design pressure by 10/13 rule)

For shell and tube heat exchangers with high differential pressures, a complete tube failure is considered a viable contingency when the design pressure of the low-pressure side is less than 10/13 of the design pressure of the high-pressure side. If the low-pressure side is liquid phase, Inflection Point Engineering's design procedure dictates increasing the low-pressure side design pressure to 10/13 of the design pressure of the high-pressure side (the 10/13 rule). As a result of applying the 10/13 rule, overpressure protection on the low-pressure side is not required for the low-pressure side of the exchanger.

Determine the flange class of thermowells, for liquid service only, in either the inlet or outlet piping of a shell and tube heat exchanger with a design pressure determined by the 10/13 rule, from the exchanger design pressure and temperature for the following flange locations:

• Thermowell is located between the exchanger flange and a block valve in either the exchanger inlet or outlet piping.

or

• Thermowell is located between the exchanger flange and a piping branch (tee) in either the exchanger inlet or outlet piping.

or

• Thermowell is located between the exchanger flange and a piece of equipment (vessel, exchanger, filter, heater, etc.) in either the exchanger inlet or outlet piping.

3.3 Vessels and Reactors

Determine the flange class of thermowells connected directly to vessels and reactors on the basis of the mechanical design pressure and temperature of the vessel or reactor.

If the required thermowell flange class is higher than the mating equipment nozzle flange class, advise the Design Engineer to increase the equipment nozzle flange class to match the required thermowell flange class.

3.4 Pump and Centrifugal Compressor Suction

Determine the flange class of thermowells located in the suction of a pump and a centrifugal compressor on the basis of the mechanical design pressure and temperature of the suction vessel.

3.5 Pump Discharge

Determine the flange class of thermowells located in the discharge of a pump on the basis of the mechanical design pressure of the first piece of equipment downstream. This equipment generally is based on the shutoff pressure of the pump. If there is a valve between the pump discharge and the piece of equipment then there may be a pipe class break. In that case, use the shutoff pressure of the pump. For the design temperature generally use the maximum process operating temperature plus 50 °F (28 °C).

3.6 Centrifugal Compressor Discharge

Determine the flange class of thermowells located in the discharge of a centrifugal compressor on the basis of the mechanical design pressure and temperature of the first piece of equipment downstream.

3.7 Reciprocating Compressor Suction and Discharge

Determine the flange class of thermowells located in the suction snubber or discharge snubber or piping (upstream of the discharge block valve) of a reciprocating compressor on the basis of the mechanical design temperature of the snubber and the discharge relief valve setting.

3.8 Fired Heaters (individual passes)

Evaluate the flange class of thermowells in the tubes (inside the heater terminal flanges) of multi-pass fired heaters, on the basis of the heater coil design pressure and the maximum process operating temperature plus 50 °F (28 °C) as the design temperature.

3.9 Fire Heaters (combined outlet)

Evaluate the flange class of thermowells in the combined transfer line (outside the heater vendors scope) based on the downstream vessel design pressure and design temperature, provided a valve (either control or isolation) does not exist in the transfer line downstream of the thermowell nozzle.

For the applications where a valve does exist in the downstream transfer line, base the thermowell flange class on the heater coil design pressure and the maximum process operating temperature plus 50 °F (28 °C) as the design temperature.

3.10 Electric Heaters

Determine the flange class of thermowells at the outlet of an electric heater on the basis of the design pressure and design temperature of the downstream equipment.

3.11 Column Bottoms and Liquid Full Vessels

Frequently, the conditions used for flange selection are the design conditions of the equipment. There are exceptions, however. Because the design conditions are defined as the conditions at the top of the equipment, the actual design conditions at a particular flange location may be greater. A common example is a fractionating column. Due to the tray pressure drop and liquid hydrostatic head, the pressure at the bottom of the vessel may be significantly higher than the pressure at the top. THE ACTUAL DESIGN PRESSURE (AND TEMPERATURE) AT THE LOCATION OF THE FLANGE MUST BE USED TO DETERMINE THE FLANGE CLASS. You can use the design pressure and temperature of associated equipment such as reboilers, bottoms exchanges, etc.

3.12 ASME Section I

Thermowells in ASME Section I have a mandatory reduced pressure rating. The tool, , should be used to determine the correct flange class.