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Calcite Contactors for Corrosion Control

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Presentation on theme: "Calcite Contactors for Corrosion Control"— Presentation transcript:

1 Calcite Contactors for Corrosion Control
Lee Odell, P.E. Vice President CH2M HILL

2 Overview 1 – Corrosion Background
2 – Treatment Systems Design, Operation & Maintenance

3 Corrosion Control Purpose protect public health improve water quality
extend plumbing equipment meet regulations

4 Corrosion Battery Analogy Anode Cathode Electrical Circuit
- Battery Analogy Anode Cathode Electrical Circuit Metal lost at anode Anode Cathode The corrosion process is just like the operation of a battery. There is an anode and a cathode, and an electrical circuit between them. In the corrosion cell, metal is lost at the anode. Electrolyte

5 Simplified Corrosion Cell
OH- STEP 4 O2 O2 STEP 1 Water with Dissolved Minerals Fe 2+ STEP 3 CATHODE Base Metal e - e - Your cooling water system provides the ideal environment for the reversion of metal back to the oxidized state. The corrosion process is an “electro-chemical reaction” that can best be explained using the diagram. A four step process occurs. Step 1: At the anode, the pure iron begins to break down in contact with the cooling water. This step leaves behind electrons. Step 2: The electrons travel through the metal to the cathode. Step 3: At the cathode, a chemical reaction occurs between the electrons and oxygen carried by the cooling water. This reaction forms hydroxide. Step 4: The dissolved minerals in the cooling water complete this electrochemical circuit back to the anode. The corrosion process will continue and more and more metal will be destroyed. The only way to stop it is to break the circuit. ANODE e - e - STEP 2

6 Major Factors Influencing Corrosion
pH Temperature Dissolved Solids System Deposits Water Velocity Microbiological Growth The amount of corrosion or the corrosion rate is affected by numerous factors.

7 Types of Corrosion All water systems experiences some degree of corrosion. The objective is to control the corrosion well enough to maximize the life expectancy of the system... All cooling system metallurgy experiences some degree of corrosion. The objective is to control the corrosion well enough to maximize the life expectancy of the system. There are three types of corrosion.

8 General Etch Uniform Attack
General Corrosion Base Metal General Etch Uniform Attack Water Thickness Original Preferred situation Take a small amount of metal evenly throughout the system Anode very large General Corrosion Since all metal corrodes, the ideal situation is to take a small amount of metal evenly from throughout the system. This is called general corrosion. In general corrosion, the anode is very large.

9 Localized Pitting Attack
Pitting Corrosion Base Metal Localized Pitting Attack Water Original Thickness Metal removed at same rate but from a much smaller area Anode very small Often occurs under deposits or weak points Leads to rapid metal failure Localized Pitting Corrosion In some situations the anode is very small. Even though the metal loss is at the same rate, instead of losing metal from the entire metal surface, it is being removed from a small area. The only place for the corrosion to go is down into the metal. The result is a deep hole or pit. This type of corrosion occurs under deposits or at weak points in the metal. This type of corrosion is very serious because it results in rapid failure of the metal.

10 Affects of Corrosion Potential regulatory non-compliance
Shortened pipeline life Water usage increases Corrosion product deposits in hot water tanks Heat transfer efficiency is reduced by deposits Leaks in equipment develop Process side and water side contamination occurs Maintenance and cleaning frequency increases Equipment must be repaired and/or repaired Unscheduled shutdown of plant Uncontrolled corrosion is devastating to your plant. Corrosion is destructive to the cooling system metal and results in costly repairs. The affects of corrosion can be summarized as in the slide.

11 Effect of pH on the Release of Copper into Solution

12 Effect of pH and Alkalinity on Lead Solubility
Alkalinity (mg/L CaCo3)

13 Calcite Contactors Calcite Contactors Use Limestone to Add Calcium Carbonate to Water, Raise pH and add Alkalinity to water. Benefits: Easy to Operate Easy to Maintain No Risk of Overdosing Chemical Operate in Upflow Mode No Need for Controllers/Motor Actuated Valves or backwashing Agenda Ceu’s Encourage Questions/Comments Ball Game

14 What information is Needed to Design a Calcite Contactor?
pH Alkalinity Calcium TDS or Conductivity Flow Rate

15 Calcite Contactors 1200 College St

16 Calcite Contactors Limestone contactors may offer advantages:
easier and safer to operate, reduces operating cost, self adjusts the water pH without risk of alkali overdose, requires minimal maintenance and operator skills, and does not require continuous feed of chemicals

17 Process Description In a calcite contactor, water flows through a bed of crushed sieved limestone in a similar way as it would flow through a sand filter. The pH of water that flows through the limestone bed will be adjusted until it nears equilibrium with calcium carbonate (CaCO3(s)). The components of a contactor include: a contact tank, limestone bed, inlet line, outlet line, overflow line, access lid, backwash line. There are two types of contactors: (i) open and (ii) closed system contactor. The former is exposed to the atmosphere and the latter is covered from the atmosphere. There are also contactors that are built in pressurized vessels. Limestone contactors are typically located at the end of the treatment train – after filtration, primary disinfection and chlorine contact.

18 Contactors

19 Contactor Arrangement

20 Contactors

21 Limestone CaCO3 H+ + HCO3

22 Species Distribution Diagram CaCO3 -> H+ and HCO3

23 Design of Contactors Candidate Systems: pH<7.2 Calcium<60 mg/L
Alkalinity<100 mg/L Iron <0.2 mg/L Manganese <0.05 mg/L

24 Limestone Contactor

25 Contactor Feasibility Decision Tree
Design contactor length using EPA DESCON program Parameters needed: pH Alkalinity (DIC) Calcium Iron Manganese Temperature Velocity % Calcium Carbonate Particle Size Available at Raymond Letterman’s website

26 Descon Design Tool Filter Tank Sizes and Velocity (gpm/sq ft) 24" 36"
24" 36" 48" 60" 25 gpm 8.0 3.5 2.0 1.3 50 gpm 15.9 7.1 4.0 2.5 100 gpm 31.8 14.2 5.1 150 gpm 47.8 21.2 11.9 7.6 200 gpm 63.7 28.3 10.2

27 Design Considerations
Vessel Type – Open, Pressure Up-flow, Down-flow Need Backwash Disposal? Site Glass Pressure gauges Y-Strainer pH Monitoring?

28 Operations & Maintenance

29 Calcite Dissolution Estimate
Ray Letterman/EPA Model Excel Spreadsheet Model Converted RTW to limestone dose to achieve pH 7.5 Used duty cycle of 12 hrs/day Estimate as cm/month of bed depth

30 Questions?

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