Interpass

IPA [Interpass absorption] Also called Double absorption, and Double catalysis. An improved version of the Contact process for making sulfuric acid, by which the efficiency of the conversion of sulfur to sulfuric acid is increased from 98 percent to over 99.5 percent. 

Preheat and interpass temperature shall be as specified/qualified per the applicable WPS/PQR. ... 

The contact process was invented by Phillips in England in 1831 but was not used commercially until many years later. Today 99% of all sulfuric acid is manufactured by this method. It was developed mainly because of the demand for stronger acid. All new contact plants use interpass absorption, also known as double absorption or double catalysis. This process will be described in detail in Fig. 2.3. 

Maintain interpass temperature between 177X1 and 232X (350X and dSOT). 

The permissible maximum carbon equivalent for butt welds in line pipe using cellulosic (EXX10) electrodes based on minimum preheat and interpass temperature, pipe wall... 

Preheat is not required for grades up to API X52 when the CE is less than 0.5 to 0.55 [by Equation (5.1)]. Equation (5.2) is commonly used in specifications. Limiting the Equation (2) CE to 0.42 for X60 through X70 grades is usually required. The maximum CE, based on preheat and interpass temperature, wall thickness, and heat input (travel speed), can be estimated from Figure 5.10. For example, for 0.42 CE and 1 in. (25 mm) wail pipe, a preheat of 68° F (20° C) would be required for a 12 KJ/cm heat input however, if the heat input was reduced to 9 KJ/cm, then a 140 F (60° C) preheat would be required. 

FIGURE 9.1 I Schematic diagram of the converter section of a contact sulfuric acid plant employing an interpass absorption system for both better sulfur conversion and emission abatement. Product acid (or oleum) is cooled indirectly with process water prior to storage for sale. 

It may also be necessary to decide limitations on the level of preheat and interpass temperature which can be used for example, welding manually within an enclosed space may preclude high temperatures. 

For hardenable steels the carbon content is used to select preheat, interpass and postheat temperatures. 

Diagram for selecting minimum preheat, interpass and postheat temperatures for alloy and lean alloy steels giving fully hardened HAZs. Note relaxation of preheat temperatures may be possible with low hydrogen processes and thin sections, particularly at low C contents. Confirmation should be obtained by joint simulation tests. For class of steel refer to Table 4.3... 

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