Corrosion allowance

The corrosion allowance is the additional thickness of metal added to allow for material lost by corrosion and erosion, or scaling (see Chapter 7). The allowance to be used should be agreed between the customer and manufacturer. Corrosion is a complex phenomenon, and it is not possible to give specific rules for the estimation of the corrosion allowance required for all circumstances. The allowance should be based on experience with the material of construction under similar service conditions to those for the proposed design. For carbon and low-alloy steels, where severe corrosion is not expected, a minimum allowance of 2.0 mm should be used where more severe conditions are anticipated this should be increased to 4.0 mm. Most design codes and standards specify a minimum allowance of 1.0 mm. 

The minimum corrosion allowance to be considered for a piece of equipment depends on the required service life multiplied by the expected corrosion rate under process conditions. 

According to Equation (4.2), the classes shown in Table 4.4 should be considered for equipment with a design life of 20 years. 

Where the corrosion rate is more than 0.3 mm/yr, or the total corrosion over the design life exceeds 6 mm, other alternatives should be evaluated. These alternatives may include the following  

Per TEMA Standards, for carbon steel pressure parts, using 1/8 corrosion allowance for IHMA class R heat exchangers, use 1/16 corrosion allowance for TEMA class B and C heat exchangers. Heat exchanger tubes, its parts don t stand pressure, or alloy pressure parts do not require any carrosion allowance. See TEMA Standards for more details. 

Based on service, heat exchangers can be classified as cooler, heater, condenser, and vaporizer such as evaporator, boiler, or reboiler. Cooler is used to cool a stream and heater is used to heat a stream. The stream can be vapor or liquid. Condenser is ased to condense a vapor stream partially or totally. Vaporizer is used to vaporize part of the liquid to vapor. There (s no phase change for cooler and heater, but there is phase change for condenser and vaporizer. 

P = maximum allowable working pressure, psig r = inside radius before corrosion allowance is added, in. d = inside diameter before corrosion allowance is added, in. 

E = joint efficiency, see Table 12-4 (Most vessels are fabricated in accordance with Type of Joint No. 1.) 

Typically, a corrosion allowance of 0.125 in. for non-corrosive service and 0.250 in. for corrosive service is added to the wall thickness calculated in Equations 12-1 to 12-4. 

All ASME Code vessels are inspected by an approved Code inspector. The manufacturer will supply Code papers signed by the inspector. The 

It must be pointed out that a Code stamp does not necessarily mean that the vessel is fabricated in accordance with critical nozzle dimensions or internal devices as required by the process. The Code inspector is only interested in those aspects that relate to the pressure-handling integrity of the vessel. The owner must do his own inspection to assure that nozzle locations are within tolerance, vessel internals are installed as designed, coatings are applied properly, etc. 

---

Institut Eransais du Pntrole dimethyl sulfoxide (DMSO) solvent contains up to 2% water to improve selectivity reflux con-sist of aromatics and paraffins ambient rotating-blade extractor, typically 10—12 stages low corrosion allows use of carbon steel equipment solvent has alow freezing point and is non-toxic two-stage ex-traction has dis-placement solvent in the second stage... 

Formex pro-cess, Snam-progetti /V-formyl-morph o-line (FM) water is added to the FM to increase its se-lectivity and also to avoid high reboiler temperatures during solvent recovery by distillation 40 perforated-tray ex-tractor, FM density at 1.15 aids phase separation low corrosion allows use of carbon steel equipment... 

When the design temperatures are significantly below ambient temperature, the primary threat to tank integrity is failure of the material by britde fracture. The tank design codes usually provide thorough treatment of this topic to prevent catastrophic failure. Additionally, there is the consideration of corrosion allowance, defined as extra thickness added beyond that required for strength. Corrosion allowance is not discussed herein. 

Corrosion is probably the greatest threat to vessel life. Partially filled vessels frequently have severe pitting at the liquid-vapor interface. Vessels usually do not have a corrosion allowance on the outside. Lack of protec tion against the weather or against the drip of corrosive chemicals can reduce vessel life. Insulation may contain damaging substances. Chlorides in insulating materials can cause cracking of stainless steels. 

Carbon steel is easily the most commonly used material in process plants despite its somewhat limited corrosion resistance. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. It is also used routinely for the storage of concentrated sulfuric acid and caustic soda [up to 50 percent and 55°C (I30°F)]. Because of its availability, low cost, and ease of fabrication steel is frequently used in services with corrosion rates of 0.13 to 0.5 mm/y (5 to 20 mils/y), with added thickness (corrosion allowance) to assure the achievement of desired service life. Product quahty requirements must be considered in such cases. 

The process engineer gets involved in many mechanical aspects of vessel design such as thickness, corrosion allowance, and internals. Here are some pitfalls to watch for along the way ... 

C = Souders-Brown allowable mass velocity factor of corrosion allowance, in. 

Cone roof and floating roof tanks are usually correlated using vs. volume, w ith materials of construction as another variable. The cost of internal heat exchangers, insulation, unusual corrosion allowance, and special internals should be separated from the basic cost of the tank in the correlations. 

Corrosion Allowance—Shell Side Tube Side ... 

Mild steels are rapidly corroded by mineral acids even when they are very dilute (pH less than 5). However, it is often more economical to use mild steel and include a considerable corrosion allowance on the thickness of the apparatus. Mild steel is not acceptable in situations in which metallic contamination of the product is not permissible. 

Corrosion allowance Tube diameters Square pitch cleaning lane Mm. shell thickness Transverse baffle and support plate thickness Min. thickness longitudinal battles... 

Tube diameter plus tw ice corrosion allowance Chaintcred Required for certain sizes. -inch... 

Factoi less than tube diameter plus twice corrosion allowance... 

Maximum working pressure has been rounded up to the next higher unit of 10 psig. No corrosion allowance is assumed same formula as ANSI B31.3... 

Maximum operating temperature = 2(K)°F Corrosion allowance Material Diameter... 

Water was injected into an oil stream using the simple arrangement shown in Figure 9-6. Corrosion occurred near the point shown, and the oil leak caught fire [5]. The rate of corrosion far exceeded the corrosion allowance of 0.05 in. per year. 

Consideration shonld be given to the corrosion allowance for flame arrester elements and honsings. Element corrosion will rednce the effectiveness of the arrester to qnench a flame and withstand the destmctive pressnre effects of a detonation. Also, increased element thickness will allow for greater periods between element replacement. 

GENERAL MATERIAL MAXIMUM DESIGN PRESSURE and TEMPERATURE LIMITS UMITED BY CORROSION ALLOWANCE CONSTRUCTION Carbon Steel 275 PSIG at -20/I00°F 100 PSIG at 750°F 150 Flanges See Table, This Spec. ly" and Smaller—Socket Welded 2" and Larger—Flanged and Butt-Welded... 

Press./Temp. Limits 175 PSIG/-20 to 150°F 125 PSIG/350°F Corrosion Allowance 0.05 inches ... 

R = inside radius of vessel, no corrosion allowance added, in. 

Universal gas constant = 1544 = MR individual gas constant = MR/M inside radius of vessel, no corrosion allowance added, in. 

Tube Sheet Materiol lerspev f ioixsi, Gry pH t-W,.. Corrosion Allowance - Shell Side ... 

The main febricated parts of the units are carbon steel, with suitable corrosion allowance for the conditions of the chilled and condensing water. When brackish or sea water is used in a barometric condenser, steel construction with a V4 -in. to -in. corrosion allowance is suggested, and minimum wall plates of V2 -in. to -in. may be justified. Internal splash plates should be V2 -in. to -in. minimum, because the atmosphere of water vapor-air is very corrosive. Alloy construction is not justified except in exceptional cases. 

---