System – Steam Condensate

• Overview

Steam systems have been in existence the 1800’s and are still used to heat and power building properties to this day.  Earliest building heating systems were single 1 pipe layouts where the steam supplied upward to the cast iron radiator, released its heat, condensed, and the condensate returned down the same line back to the boiler.  Many oldest New York City buildings from around the year 1900 and before had this design, although most have since been converted to hot water heating in order to provide better control.

While associated together, steam condensate return pipe suffers a substantially greater corrosion impact, and is typically the first of the two piping systems to develop leaks and failures.

• Common Problems

Steam condensate is separated from the steam supply for various reasons, one of which is to safeguard the higher pressure steam line from corrosion.  Since steam condensate is essentially pure distilled water, it is not immediately clear why it causes such higher corrosion to the same steel pipe.  The answer is due to the release of carbon dioxide gas (CO2) when turned into steam, and the migration of that gas into the condensate lines where it produces carbonic acid.  Even though carbonic acid is a relatively weak acid, it has the capacity to reduce the pH of the condensate down to the range of 5-6, which is very corrosive to carbon steel pipe.

Preventative actions such as the use of neutralizing and filming amines greatly help reduce corrosion activity.  Injected into the boiler feedwater or boiler itself, such chemical additives carry over with the steam to provide a protective film to the pipe as well as neutralize the carbonic acid.  The chemical level is easily maintained by monitoring the pH of the returning steam condensate at 8.0 ore higher.

In response to the well known higher corrosion occurring to steam condensate lines, extra heavy or schedule 80 pipe has been traditionally installed.  Like all other building piping system, however, this standard has been reduced today to favor standard or schedule 40 pipe of much lower wall thickness.  The result, again never surprising, is to find lower service life for newer steam condensate systems compared to much older properties.

• Major Threats

Most steam condensate systems are gravity based and rely on the gradual movement of the condensate back to the boiler or building outlet.  In most cases the pipe remains only partially filled, with a layer of air and oxygen at the top to further drive the corrosion process.  For larger facilities having a central boiler plant and multiple buildings, the condensate is pumped back to the boiler, which then completely fills the pipe resulting in generally lower corrosion.

Oxygen pitting is also a common form of corrosion caused by inadequate deaeration of the boiler feedwater and and higher oxygen levels, as shown at left.  As the condensate cools to produce a slightly negative pressure, oxygen can also enter through any openings to the system.  The net result is very deep and aggressive pitting to some areas while the surrounding surface remains unaffected.

In the absence of adequate grade and in those examples where the pipe is pitched negatively, condensate can collect.  This can result in the force of the steam constantly splashing the condensate against the same area of pipe to produce significant deterioration.

For underground lines protected within a Rickwell casing and buried underground, the casing itself was rarely protected from external corrosion.  Given sufficient time, this outer casing, typically fabricated of standard schedule 40 carbon steel will fail to then introduce a second corrosion front against the condensate pipe inside, as shown at left.

Such threat exists to the steam condensate piping as well as the steam supply lines, although due to the smaller size of the condensate return line, less wall thickness exists leading to an earlier failure.

Areas of interest for any UT inspection of a steam condensate system should include:

• • • All threaded pipe
    • Drip legs
    • Condensate collection basins
    • Extended horizontal lines
    • Any standard or schedule 40 pipe
    • Improperly inclined gravity return lines

• Testing Focus
  

The most important single action for any steam condensate system is the use of amines to raise its pH.  Any UT investigation of a steam condensate system should include:

• • • All above cited problem areas
    • Differentiation between pumped and gravity piping
    • Smallest diameter pipe
    • Schedule 40 threaded pipe if it exists

Corrview maintains a large photo gallery specifically related to steam and steam condensate piping in addition to other corrosion related issues. To visit our gallery on steam and steam condensate piping, please click here

© Copyright 2023 – William P. Duncan, CorrView International, LLC