Design temperature Vs design pressure 1

[amar122amar]

For every material there is a specified design pressure and design temperature. Now if i am operating at a pressure less then design pressure, can I go on operating at higher than design temperature. Example: for a refractory lined killed carbon steel design pressure is 4.4 Kg/cm2 (g) and design temperature (internal) is 742 deg C. Now I operate at around 3.6 kg/cm2 (g)then can I have operating temperature more than 742 deg C say 750 deg C.

 

[JoeTank]

For many carbon steels, the allowable design stress is unchanged over a fairly wide range of temperatures. As steel approaches its upper limit of use in The ASME Code there are significant reductions in allowable stress. So the answer is "maybe". More info would be need to assess that situation. I'm not aware of any carbon steels suitable for 700+C, so I'm assuming that the refractory brick must be limiting the temperature of the steel to a much lower temperature. It's the steel temp that governs, not the internal temps of the vessel.

Joe Tank

 

[amar122amar]

ya killed carbon steel design temp is 342 deg C. But vessel internal design temp is 742 deg C due to thermal resistant refractory lining.

 

[corus]

The design temperature gives you the design stress, so if you operate at a higher temperature then your design stress reduces. This doesn't mean that your lower operating pressure is acceptable though as the design pressure may be for extraordinary circumstances. If you have a refractory lined furnace then the refractory will expand and exert a pressure on the shell. This may well be added to your operating pressure to give you the design pressure.

corus

 

[gerhardl]

As always when design and safety limits are passed, the question has a safety responsibility side in addition to the purely technical side.

Some additional questions should be asked and answers considered before any decision:

1. If anything unforeseen happens, for instanc to some part of construction or added additional or attached part not taken into consideration, who will pay the damage and bear the responsibility?

2. What person or administrative level has the authority to decide?

3. What is the consequences for yourself?

In this case you are actually asking a question that alters the general view of already set limits. With temperature as the x-axis and pressure at the y-axis your allowable limits describes a rectangle ending (presumably) in a point on the allowable falling pressure/temperature curve(or perhaps somewhat below, safety limits considered).

You ask to extend the rectangle to include the area above the set pressure limit, but below pressure temperature curve.

Por a single item, for a single purpose, this should be OK, if all possible application details and possible accidental occurences are known.

On the other end of the line: for larger projects or aplications consisting of an assemblies or multiple components and a large set of possible risk variables 'someone' usually has put down limits for a purpose.