4 replies to this topic

[CPChem]

Hi All,

 

I came across ethyl benzene column system with operating pressure is 1.7 kg/cm2g, design pressure is 3.7 kg/cm2g @ 250 deg C & PSV set at 3.7 kg/cm2g. However, the column MAWP is 10.32 kg/cm2g @ 250 deg C. Reboiler MAWP is 7.5 kg/cm2g @ 250 deg C. All other associated equipment has MAWP same as column MAWP.

 

Overfilling is a credible overpressure scenario on this system as upstream pressure is 7 kg/cm2g. Is this a reason why equipment's has MAWP 3X of design pressure. 

 

Please guide me on above system understanding.

 

Best Regards,

 

ChetanP

 

 

 

[Bobby Strain]

Who could possibly know?

 

Bobby

[latexman]

Talk to those who were there before the system was designed and built.

[Pilesar]

Calculated MAWP is usually higher than the design pressure. It is my common practice to have equipment stamped for the calculated MAWP and use that pressure for relief design when possible. When manufacturer uses standard sizes, the wall thicknesses are incrementally sized and not continual. For example, if the manufacturer uses 1/4 inch plate he has on hand, there is no incentive to manually thin that plate to reduce MAWP. Also, the vacuum rating may set the wall thicknesses which push up the calculated MAWP.

[fallah]

 

I came across ethyl benzene column system with operating pressure is 1.7 kg/cm2g, design pressure is 3.7 kg/cm2g @ 250 deg C & PSV set at 3.7 kg/cm2g. However, the column MAWP is 10.32 kg/cm2g @ 250 deg C. Reboiler MAWP is 7.5 kg/cm2g @ 250 deg C. All other associated equipment has MAWP same as column MAWP.

 

Overfilling is a credible overpressure scenario on this system as upstream pressure is 7 kg/cm2g. Is this a reason why equipment's has MAWP 3X of design pressure. 

 

Please guide me on above system understanding.

 

 

Hi,

 

Because there is a credible case (overfilling) which can pressurize the column at 7 kg/cm2g and the MAWP of the column is higher enough than this value (10.32 kg/cm2g), the design pressure of the column could be increased to 7 kg/cm2g hence the set pressure of the relevant PSV as well.

With this change the difference between the design pressure and MAWP of the column becomes reasonable and its operating pressure range will be 2X of the before.