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Corrosion Allowance in Plate Heat Exchanger

10/05/2017 12:40 PM

In API 662 which is the design code for plate heat exchanger (gasketed type) it is mentioned that the
"Corrosion allowance shall apply to connections only."

What does this statement mean?
Does it mean that the corrosion allowance at the connection points (i.e. at the ends of the plate) is to be complied with? What happens if corrosion occurs at the points where fluid is in contact with the plate?

Please shed some light on this.

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#1

Re: Corrosion allowance in plate heat exchanger

10/05/2017 1:05 PM

The plates are tested and maintained, this is regular maintenance procedure...corrosion allowance is determined by the design engineer for purpose based on materials and usage...These plates are replaced in the field as needed determined by testing...The connections by code have a minimum corrosion allowance regardless....

http://www.eng-tips.com/viewthread.cfm?qid=204375

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#2
In reply to #1

Re: Corrosion allowance in plate heat exchanger

10/05/2017 2:17 PM

Would that not be maximum corrosion allowance? I.e. - the depth of pitting allowed or general metal loss per interim without compromising the connections?

Usually the plates are made of highly corrosion resistance metals, usually specific types of stainless steels for the service conditions.

Some parts maybe alloy metals that are actually in the yellow metals class.

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#3
In reply to #2

Re: Corrosion allowance in plate heat exchanger

10/05/2017 2:22 PM

Yes a minimum maximum allowance....

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#6
In reply to #3

Re: Corrosion allowance in plate heat exchanger

10/06/2017 1:51 AM

Yes and yes. But to be clear there are two separate issues. One, the specification of required wall thickness to satisfy design pressure PLUS an allowance for expected wear and tear (aka service conditions) is a decision taken during design and material selection. Hence there is a minimum required wall thickness + some extra (engineer's judgement) and this dimension is defined in the purchaser's data sheet (API662 app C).

Secondly, thru service life while ongoing verification of integrity is performed, the minimum required wall thickness is verified as present and intact hence capable of rendering required service. API, ANSI and NACE have remnant strength assessment processes for judging pitting types and groups on piping and connections for verifying fitness for service. By having a corrosion allowance incorporated as part of the design, when the inevitable corrosion is discovered in service, there's a better chance one doesn't need to condemn the piece as the added material provided for at the beginning still provides the required strength for pressure containment.

So it's not about allowing corrosion to happen, its about having capacity to tolerate corrosion that WILL happen.

This notion is only allowed to be utilized at the connections, as API662 quite plainly states.

Given the nature of HE plate materials, design duty and PHE configurations, plates are expected to be serviced/changed while supporting connections typically are not, hence the need for incorporation (specification) of a corrosion allowance at the front end to give some service life to the connections themselves.

It should be noted that excess of material provided under auspices of "corrosion allowance" are not to be taken account of in the nominal mechanical strength assessment.

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#7
In reply to #6

Re: Corrosion allowance in plate heat exchanger

10/06/2017 3:12 AM

Thanks for fleshing that out....well done...

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#4

Re: Corrosion Allowance in Plate Heat Exchanger

10/05/2017 2:28 PM

In my View-

No corrosion is acceptable on whole of the heat ex changer. And what whatever is allowed (corrosion allowance) must be limited to joints at the end points.

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#5
In reply to #4

Re: Corrosion Allowance in Plate Heat Exchanger

10/05/2017 2:31 PM

True statement, if the designer wants any kind of service life out of the equipment.

Further, if water treatment were really paid close attention to, and not ignored as is so often the case, corrosion is reduced to a non-issue. If ignored, it grows to a serious issue, that worst case can result in loss of life accidents.

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#8

Re: Corrosion Allowance in Plate Heat Exchanger

10/06/2017 4:31 AM

To possibly add to what others have said - corrosion allowance relates to the mechanical design. From a heat transfer viewpoint, a fouling facto is usually used, to account for material build-up on the plates.

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#9

Re: Corrosion Allowance in Plate Heat Exchanger

10/06/2017 1:48 PM

Connection - Connection interface corrosion is where stresses are higher. These stresses can become out of design specifications therefore are considered critical spots.

Plate - Hot fluid interface is determined by design and should be resist the corrosion just per designed parameters.

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#10

Re: Corrosion Allowance in Plate Heat Exchanger

10/07/2017 5:13 AM

Yes, API 662 is clear:

8.5.1 Corrosion allowance, if specified, shall apply to connections only.

8.2.2 Plate material shall be selected based on an assumed zero corrosion allowance.

If you need more information check TEMA Class R

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