Pumps and Piping Systems handle liquids in almost all process industries to and fro and through heights and lengths during the process. Processes require the pumps even to perform and handle high flow rates even during extreme operating conditions of high pressure and temperature. In addition, pumps are subjected to highly corrosive environments that often result in premature failure of pumps and piping systems. It has been observed that premature failure or breakdown of pump and piping systems takes place because of erosion attack, corrosion and / or cavitations (impingement attack). Proper design and selection of material of construction can help overcome these problems.

• Selection of Material of Construction
  Chemically inert materials are used as construction materials for corrosive and erosive industrial applications in phosphatic fertilizer plants, especially in phosphoric acid and other corrosive acid plants. Performance of these materials has been satisfactory in the harsh environmental conditions and premature failure of plant and piping systems have been prevented. Some of these materials are Fiber Reinforced Polymers (FRP) and Composites, Alloy Steel, Rubber lined Steel, PVC/ CPVC, Polypropylene and Special higher resistant alloys (CD4MCU, DURIMET 20, HASTEALLOY-C, SANICRO-28 and H V 9 etc.). Ultra high Molecular Weight Poly Ethylene has been developed as engineering plastic and is commonly used for pumps and pipelines etc because of their advantages over the metals. These plastics have replaced most of the metals used in the machine components. Other engineering plastics developed foe corrosive applications are Poly ethylene, Poly propylene, Polyamide and Poly Vinyl Chloride (PVC) 
• Erect Pump on Stand
  To facilitate installation of piping, cleaning and maintenance, the pump must be kept off the floor and placed on a stand or foundation. This protects the pump from any kind of damage due to corrosion if any system-failure causes flooding of the area. Process and Fertilizer Plants strictly adhere to this practice to save pump and piping system
  
• Support for Piping
  All suction and discharge piping of pumps must be supported independently to avoid any undue stress on the system that may lead to damage. At the designing stage of pump and piping system extra care is taken to prove desired support to avoid any undue stress.  This practice is strictly followed to avoid premature damage to the system.
  
• Avoid Cavitation Damage
  While lying out and sizing the suction piping and component, the NPSH® requirement established by the pump manufacturer must be met in order to avoid the damage due to problem of cavitation.
  
• Easy Maintenance and Replacement
  The pumps and piping systems valves and flange joints etc. are prone to failures while in service. The system must be designed in such a way that it offers ease in replacement of all worn out and corroded parts. It must be ensured that excessive mechanical stresses and stress concentrations are avoided in components (mainly flange joints etc.) to avoid stress corrosion cracking (SCC). This rule must be followed especially while using materials that are susceptible to SCC as in case of stainless steel and super alloys.

Problem of Corrosion in Pumps
1. Problems of corrosion that occur in the pumps used for corrosive and erosive acid service are different in nature due to change in chemical composition of the material of construction of pumps, process conditions i.e. temperature and pressure and other properties of various chemicals used in the process. Other factors include the plant environment, process fluid conditions (solids etc.) and chemical behavior of process fluid whether it is acidic or basic in nature.
2. Electrical contact between two dissimilar metals/ alloys can help overcome the problem of galvanic corrosion. If possible, similar materials should be used throughout the entire structure. In case it is not possible then dissimilar conducting materials should be insulated from one another by using neoprene or Teflon etc.
3. It is advisable to use composite materials or their coatings such as combination of copper with AI203, Ti02 etc. In piping system along bends, elbows and tees, valves, pumps and blowers where there is a change of erosion and corrosion because of high velocity etc.
4. Oxygen reduction is one of the most common cathodic reactions (O2 + 2H2O +  4e = 4 OH) or oxygen serving as depolarizer in presence of H+ ions (2H+ +1/2 O2 + 2e- = H2O). It must be ensured that the systems are designed to exclude air entrainment. If oxygen is eliminated from aqueous environment then corrosion can often be reduced or prevented.
5. While designing the chemical plant or equipments, particular attention must be paid to agitators, liquid inlets and other points like flange joints, systems working at reduced pressure etc. where possibility of air entrainment exists. Use of air-tight gaskets can yield highly satisfactory results. However, passive metals and alloys are exceptions in this case. Titanium and stainless steel are more resistant to acids containing dissolved  air or other oxidizing agents. Use of polymers and FRP linings etc. are highly recommendable.
6. Scratch marks, uneven weld mass, notches etc. result in stress concentration that leads to SCC.  While designing, an attempt should be made to ensure that all welded metal heat treatment, annealing and quenching should be done to normalize stresses or conditions throughout the system in most uniform manner to avoid damage due to corrosion.
7. The system must be designed to avoid failure due to corrosion fatigue i.e. vibration and flexing of tubes and piping etc. Providing rigid supports or extra heavy wall thickness can minimize corrosion fatigue.  It can also be  reduced even by using flexing member of low fiber stress design or shock absorbers.
 
Erosion in Chemical Pumps
Erosion is the mechanical counterpart of corrosion. It is the physical loss of metal alloys due to various reasons like friction, abrasion and lack of lubrication etc. This can be reasonably controlled by proper selection of materials of construction to suit the adverse process parameters.
It is important to ensure that the system is prevented from cavitations erosion and impingement attack through design modification.
Cavitations corrosion may be prevented in many ways, like passing high pressure feed of liquid, designing large diameter suction lines of pumps, introducing inert gas cushion and streamlining shapes. Eliminating high frequency vibration of equipment, reducing high turbulence effects, using large bends or expandable wear plates etc are some of the other ways to avoid corrosion.
While designing piping systems (cooling coils etc.), sharp bends must be avoided, as these are more prone to erosion and corrosion (for velocity > 39ft/s) because of rapid change in fluid direction. This is particularly important in systems that are susceptible to erosion corrosion such as equipment or piping made of lead copper and their alloys.
 
Installation of Separator
In case of process streams that either contain liquid or solid particles entrainment or entrainment is anticipated, then a separator must be installed ahead of feed to pumping system to prevent impingement attack and erosion or corrosion to avoid premature system failure. This has been installed in all process plants for enhanced reliability of the industry.
 
Installation of Strainer
It is essential to install one strainer before the pump and pumping system in case of process stream that contains solid or abrasive particles so that premature failure can be arrested by avoiding corrosion and erosion in the system. This is strictly implemented in all chemical or process plants.
In case of slurry containing particulate abrasive particles the inner side of piping should be suitably coated or piping is made of FRP to prevent damage due to erosion.

Conclusion
Metallurgists and Process Design engineers work consistently to develop new corrosion and erosion resistant materials by applying their practical experience to design and fabrication aspects to avoid premature failures of equipments, pumps and pumping systems. They also plan to prevent premature failure and damage of equipments etc.
Every year industries incur an estimated total loss of almost Rs. 35000 crores because of corrosion. Every year, design modifications and new materials are introduced in the market for commercial use to save the financial losses and also for drastic improvement in environment footprint.

References
1. Phosphates & Phosphoric Acid  - By Pierre Becker
2. Phosphoric Acid, Phosphates & Phosphatic Fertilizers
- By Wm.H. Waggaman
3. Metering Pump Hand Book
- By Robert E. McCabe & William V.Dwyer

R.P.Sharma, dy .General manager, r.C.F.Ltd., Mumbai