Z włoskiego:
vapore di stabilizzazione
http://www.proz.com/dictionary/pegging_steam/1345303/2950363
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I think pegging steam comes from pegging valve, where pegging refers to something held still stable) or regulated. (prices are often pegged, interest rates are sometimes pegged, etc.) I think pegging is just an old fashioned way of referring to a feedback regulating loop for steam pressure, in this case, that holds a process variable fixed to (or near) a setpoint. It is a pretty arcane label, today, I presume, since process control is not at all special or unusual, anymore. Pressure control in a deaerator is pretty important, not only for the deaeration process, but to keep the inlets of the feedwater pumps stuffed and not cavitating and to avoid blowing out the pressure relief trap.
https://answers.yahoo.com/question/index?qid=20100605174016A...
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An alternative solution is to provide a 'pegging' steam supply which would maintain the de-aerator pressure from an alternative steam supply (say cold reheat) in the event of an unacceptable rate of pressure decay being detected
http://mda139.net/feedwaterheat/de-aerator-system.html

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sometimes referred to as pegging steam as it is used to “peg” the deaerator pressure or temperature at some reference value, usually around 110kPa or 120.
http://link.springer.com/chapter/10.1007/978-1-4614-4256-1_1...