Metering, monitoring feedwater systems

One of the most common misunderstandings sees in boiler water treatment programs is the confusion surrounding the measurement of condensate return and the correct configuration and dosing point for chemical injection.

09/18/2013


One of the most common misunderstandings sees in boiler water treatment programs is the confusion surrounding the measurement of condensate return and the correct configuration and dosing point for chemical injection.

How to determine condensate return

The amount of condensate return is an important aspect of understanding the water/steam relationship of any steam generating plant. From a water treatment perspective, it determines the cycles of concentration, the level of make-up dilution and therefore the untreated quality of feed water.

Mass balance equation: FW = MU + CR

By placing meters on the make-up (MU) and feedwater (FW) pipelines, we can quickly determine the amount of condensate return by mass balance.

When combined with steam metering (S) we can also determine the blow down rate (BD) of the boiler.

Mass balance equation: FW = S + BD

These two critical parameters determine the level of chemical dosing required and the adequate level of blow down, and the estimated chemical consumption and therefore cost of the program.

Chemical dosing—where and how?

Now that we can quantify the chemical demand of the feed water with adequate metering, where do we dose the chemicals? The chemical dosing point can be a complicated decision based on the sophistication of pre-treatment plant and the nature and number of chemicals used. For this discussion we will restrict our interests to simple feed tank (no economizer or deaerator) and simple chemical programs based on oxygen scavenger, alkalinity and non sequestrant inhibitors. This would adequately represent 90% of industrial steam plants.

Ensure that the chemical containing oxygen control is dosed close to the feed water outlet at the feed tank away from incoming make-up water agitation. The most common mistake is to dose this chemical into the middle or upper part of the feed tank, this results in inadequate corrosion protection of the boiler, or excessive chemical use. For the preferred two- to four-pump system, depending on the chemistry, most scale inhibitors and alkalinity agents may be dosed in the feed line. There are no one-drum programs that outperform the multi pump alternatives.

Content provided by Spirax Sarco, originally published in Steam News Magazine.



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