Dear All,

I have to choose the heat exchanger bolting material suitable for Sour Service.
The boltings are INTERNAL and so directly in contact with the process fluid ( completely in sour enviroment ).

The material of this exchanger is carbon steel and we are in HIGH PRESSURE SERVICE and LOW TEMPERATURE ( approx minus 30°C ).

I preliminarily used SA 320 L43 bolting (even if it would be correct to install L7M ) and I got 3.1/2" diameter.

The allowable stress of SA 320 L7 is approx. by 25% higher than L7M so using L7M I would get bigger diameters.

The ASME Code VIII Div.1 - Section II, Part D, limits the size of L7M (also B7M) to 2.1/2" diameter.
I do not understand why the reading the Section II, Part A, while for the L7M the limit is 2.1/2" diameter for the B7M in Table 2 "Mechanical Requirement" are stated also diameter up to 7" (but the allowable stress available are for 2.1/2" max.)!!

I though to provide for the L7 bolts as HOT DIP GALVANIZED per A 153 but I do not know if this will work.

I could use high strength corrosion resistance material with allowable stress close to that of SA 320 L43 but this high strength material is often not accepted by the final users.......

Could you help me please ? Do you have any syuggestion ?

Do you have experiance with high pressure internal boltings with Sour enviroment ???

Thanks in advance for you help and suggestion.

regards

GENGI

USE SA 193 GR B