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[Saml]

API 560 that tubes support for fired heaters design temperature should be " the flue gas temperature
to which the supports are exposed plus 110°C (200°F)"
 
Those 110°C are a "safety margin" for normal operation fluctuation? Or those 110°C margin should be maintained during normal operation?
 
As an example:
Design case is 940 °C flue gas temperature for the shield tubes supports.
Tube Support Design Temperature 1070 °C made of  A351 Gr HK40 
 
Does it means:

 

- That the fired heater should be protected from reaching 1070 °C in that section? (provided that other limits like tube wall temperature or process temperature are not reached before) ?

 

- Or that it should be protected from going above 960 °C (maintaining the 110 °C margin) ?
 

 
Update: After additional search in API Interpretations. There is this one that seems to inidicate that the margin is intended for usual deviation from the design point and that the first option listed above mithg be the rigth one
 
"We have a specific query on the guidelines laid down for  design of tube support castings. We would like to draw your attention to the recommendation made in respect of design temperature for tube sheets in the convection section (Section 6). It is stipulated in clause 6.1.1 that the temperature allowance of 200 degrees F for radiant section, 100 degrees F for convection section shall be used. Kindly clarify if these temperature allowances cater for end of run conditions in the fouled convection section.
Also, please clarify if any more temperature allowance shall be kept over and above the values specified in the
standard"
 
"In responding to this inquiry, the Engineering Consultant is reminded that API Standard 560 is written to cover fired heaters used in General Refinery Service, wherein the bridgewall temperature is not likely to exceed 1600 degrees F. As such, the highest metallurgy of choice is cast 25 Cr/20 Ni, unless dirty fuels are being used. The additional design temperature allowances of 100 degrees F and 200 degrees F are necessary in consideration of such factors as: EOR conditions; some fouling of radiant and convection coils (which results in less cooling effects on the tube supports); calculation safety margins; etc.
No further temperature allowances are presumed to be necessary, but the Equipment Designer must make that decision. The Equipment Director must also include for some corrosion allowance and a casting factor when designing the tube support itself"

Edited by Saml, 25 January 2018 - 11:40 PM.

[Dazzler]

Hello Saml, 

 

Thinking generally about this, I would say that the MAXIMUM operating temperature needs to be decided on (which therefore includes fluctuations from normal operating temperature cases) and then the 110dC is added to arrive at a design temperature for that location. 

 

Dazzler

Edited by Dazzler, 24 February 2018 - 03:50 AM.