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Interior coating of welded steel penstock
2

Interior coating of welded steel penstock

Interior coating of welded steel penstock

(OP)
Hello everybody:

For a hydroelectric power station, when the penstock (pressurized water pipe) thickness is calculated, one of the parameters taken into account is the so called "corrosion allowance"; that is an amount of millimeters added to the plate thickness calculated on a strength of material basis.

It is intended that this corrosion allowance will cover the pipe material lost during a good part of the power plant operative life.

I would like to know the criteria followed to apply an interior lining (coating) to the penstock instead of selecting a pipe with a greater plate thickness.
 
As a matter of fact, I can not avoid to think that the answer has to do with the conveyed water quality, the costs for the paint and manpower for the paint application, the cost of plates with extra thickness, etc., but, even so I will appreciate any comments.

If a standard or technical specifications is available to support the decision to choose, please let me know.

Thanks a lot in advance.
 

RE: Interior coating of welded steel penstock

if you get a pinhole in your paint and the pipe corrodes anyway or it corrodes from the outside instead of the inside, what then? I think it may be false economy to think that paint can be completely relied upon to extend the life of such a critical element in the plant.

RE: Interior coating of welded steel penstock

Paint will reduce corrosion - if the coating is complete and thorough, and has no pinholes or scrapes or rubbing areas.  In a static tank with a clean fluid of known characteristics, plait can work well over time - if you regularly open, inspect and maintain the system

But a penstock has significant corrosion and erosion problems: the paint will get rubbed off and the metal itself eroded by the water.   

If you want a coating to protect the metal, it must be stronger than the metal you are protecting.  A weld deposit could be applied, but that is more expensive than a thicker pipe.

RE: Interior coating of welded steel penstock

With regard to the lining issue, there are some specific guidelines within the ASCE MOP 79 - Steel Penstocks (Section 10.3) for selection, application, and monitoring of linings and coatings.  The manual is currently undergoing substantial revision, and this particular chapter will change, but most of the material is still valid.

Although there are still various forms of corrosion allowances remaining in some design and manufacturing standards, many engineers now see the use of a corrosion allowance as a temporary patch rather than a permanent solution as corrosion can continue to occur indefinitely in certain situations if left unchecked.  There should be adequate regional data to establish whether or not corrosion will even be an issue (many unlined and uncoated steel penstocks have been operating for a century or more with no significant corrosion).  Also, if internal or external corrosion is anticipated, there are some pretty sophisticated and cost-effective solutions out there in the form of linings/coating/cathodic protection equipment that would arrest corrosion as opposed to simply lengthening the distance that the corrosion needs to penetrate.

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