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| | | Forum Newbie
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| | We are finding in our submersible stations holes in the pump piping. This occurs in our bigger stations (1000 gpm & larger). Some of it we could contribute to corrosion by h2s but not all. The pipe material is ductile iron class 53. Has anyone experienced these type of leaks before and some suggestions on best type of material to use in a wet well would be helpful. |
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| | | Supreme Being
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| | It appears to be electrolysis deteriation. Whenever you have any metals below water it is best to put sacraficial anodes, such as zinc plates strapped to the metal. |
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| | | Supreme Being
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| | I don't sell this stuff so I can mention a brand name. Anyway I just ordered 40 plates at about $25.00 each with copper fasterers from Boatzinc.com. We will be using these in our clarifiers and lift stations and whereever we have metal under water. |
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| | | Supreme Being
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| We are finding in our submersible stations holes in the pump piping. This occurs in our bigger stations (1000 gpm & larger). Some of it we could contribute to corrosion by h2s but not all. The pipe material is ductile iron class 53. Has anyone experienced these type of leaks before and some suggestions on best type of material to use in a wet well would be helpful.
If Jerry's supposition that the holes are caused by electrolysis, you are in an area where finding real expertise amongst the typical engineer is difficult. I used to know something about it (30 plus years ago), but have forgotten most of the basics. I would suggest that you Goggle "cathodic protection," and get some standard municipal specifications on how large utilities handle the installation of ductile iron piping. Many years ago I worked for the Port of Los Angeles, which had many miles of metal piping buried in sand that had been dredged from the seabed. The average elevation of the port land was only about ten to fifteen above sealevel, so that sometimes pipe was actually below the water table during high tides. This, obviously, meant that piping needing a good cathodic protection system or it would disappear quickly. We used copper-copper sulfate cells to test the electric potential differential along the pipes, where a set of test leads was installed at specific distances along the pipeline. If you have metal piping inside a wetwell, you will have a similar situation to piping buried in soil. In fact, you have the same situation with household piping. Many of you will recall having galvanized steel or iron piping in the house, which might have brass or copper fittings joined to the galvanized pipe. The galvanized pipe will eventually corrode due to the difference in electrical potential in the two metals, where the zinc galvanizing, and then the iron or steel, will become "sacrificial" compared with the copper or brass. I'm not sure what the ductile iron would become sacrificial to, be it cast iron, or whatever materials you might have in the system. Or, possibly, the wastewater or soil might be causing your problem.
Just areas that you might want to look into.
From Wikipedia:
From Wikipedia, the free encyclopedia
Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell.
It is a method used to protect metal structures from corrosion. Cathodic protection systems are most commonly used to protect steel, water/fuel pipelines and storage tanks; steel pier piles, ships, offshore oil platforms and onshore oil well casings.
A side effect of improperly performed cathodic protection may be production of molecular hydrogen, leading to its absorption in the protected metal and subsequent hydrogen embrittlement.
Cathodic protection is an effective method of preventing stress corrosion cracking.
Contents
[hide]
* 1 Origins
* 2 Galvanic CP
* 3 Impressed Current CP
* 4 Testing
* 5 Galvanized Steel
* 6 External links
[edit] Origins
The first use of CP was in 1824, when Sir Humphry Davy, of the British Navy, attached chunks of iron to the external, below water line, hull of a copper clad ship. Iron has a stronger tendency to corrode (rust) than copper and when connected to the hull, the corrosion rate of the copper was dramatically reduced.
[edit] Galvanic CP
Today, galvanic or sacrificial anodes are made in various shapes using alloys of zinc, magnesium and aluminium. The electrochemical potential, current capacity, and consumption rate of these alloys are superior for CP than iron.
Galvanic anodes are designed and selected to have a more "active" voltage (technically a more negative electrochemical potential) than the metal of the structure (typically steel). For effective CP, the potential of the steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction is halted. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode. The driving force for the CP current flow is the difference in electrochemical potential between the anode and the cathode.
[edit] Impressed Current CP
For larger structures, galvanic anodes cannot economically deliver enough current to provide complete protection. Impressed Current Cathodic Protection (ICCP) systems use anodes connected to a DC power source (a cathodic protection rectifier). Anodes for ICCP systems are tubular and solid rod shapes or continuous ribbons of various specialized materials. These include high silicon cast iron, graphite, mixed metal oxide, platinum and niobium coated wire and others.
A cathodic protection rectifier connected to a pipeline
A cathodic protection rectifier connected to a pipeline
A typical ICCP system for a pipeline would include an AC powered rectifier with a maximum rated DC output of between 10 and 50 amperes and 50 volts. The positive DC output terminal is connected via cables to the array of anodes buried in the ground (the anode groundbed). For many applications the anodes are installed in a 60 m (200 foot) deep, 25 cm (10-inch) diameter vertical hole and backfilled with conductive coke (a material that improves the performance and life of the anodes). A cable rated for the expected current output connects the negative terminal of the rectifier to the pipeline. The operating output of the rectifier is adjusted to the optimum level by a CP expert after conducting various tests including measurements of electrochemical potential.
Telephone wiring uses a form of cathodic protection. A circuit consists of a pair of wires, with forty-eight volts across them when the line is idle. The more positive wire is grounded, so that the wires are at 0 V and -48 V with respect to earth ground. The 0 V wire is at the same potential as the surrounding earth, so it corrodes no faster or slower than if it were not connected electrically. The -48 V wire is cathodically protected. This means that in the event of minor damage to the insulation on a buried cable, both copper conductors will be unaffected, and unless the two wires short together, service will not be interrupted.
If instead the polarity were switched, so that the wires were at 0 V and +48 V with respect to the surrounding earth, then the 0 V wire would be unaffected as before, but the +48 V wire would quickly be destroyed if it came into contact with wet earth. The electrochemical action would plate metal off the +48 V wire, reducing its thickness to the point that it would eventually break, interrupting telephone service. This choice of polarity was not accidental; corrosion problems in some of the earliest telegraphy systems pointed the way.
[edit] Testing
Electrochemical potential is measured with reference electrodes. Copper-copper(II) sulfate electrodes are used for structures in contact with soil or fresh water. Silver chloride electrodes are used for seawater applications.
[edit] Galvanized Steel
Galvanizing (or galvanising, outside of the USA) generally refers to hot-dip galvanizing which is a way of coating steel with a layer of metallic zinc. Galvanized coatings are quite durable in most environments because they combine the barrier properties of a coating with some of the benefits of cathodic protection. If the zinc coating is scratched or otherwise locally damaged and steel is exposed, the surrounding areas of zinc coating form a galvanic cell with the exposed steel and protect it from corrosion. This is a form of localised cathodic protection - the zinc acts as a sacrificial anode.
Jeff Naumann
(310) 540-0045
FAX (310) 540-0337
http://www.jeffnaumannassociates.com/ |
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| | | Forum Newbie
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| Are you talking about e,g, some area of discharge piping within the wet well? Also, what (prime and finish etc.) coating system was actually provided for the piping that is encountering problems?
Randy Conner |
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| | This is exposed piping in a wet well. It was originally coated with coal tar epoxy. We have also tried some high solids epoxy. We are looking at c-905 plastic but are having problems finding fittings. Monty |
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| What are the velocities in the piping system? If the velocity is above 7 or 8 feet per second, this could lead to piping failures. Are you seeing any issues with your check valves? The failures that are not attributable to HS, is there erosion on the pipe interior that you can see or does the hole appear to happen from the outside.
Marc |
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