PTSA: Trust But Verify
Stephen M. Kessler
Technical Sales Leader
Chemical treatment of cooling water systems has been an effective way to:
- Conserve Water
- Maintain Asset Life and Reduce Service Maintenance
- Increase and Maintain Heat Exchange Efficiency
- Decrease Energy Costs
Common approaches utilize a package of inorganic phosphate(s) and/or phosphonate in combination with an azole and either a sulfonated acrylic acid copolymer or terpolymer. Inorganic phosphates are effective mild steel corrosion inhibitors. The phosphonates also provide effective corrosion inhibition, especially under alkaline conditions, and are also superb calcium carbonate deposit control agents. The azoles are utilized in mixed metal systems which also contain yellow metals, i.e. copper, brass, etc. Finally, the polymer component ensures that the phosphates/phosphonates remain soluble and will not cause unacceptable deposit control and negatively impact exchanger efficiency.
For any cooling treatment approach, whether it be a neutral or alkaline program, it relies on the fact that all components remain soluble to be effective. In the past, this was conducted with testing of the phosphate and/or phosphonate component of the cooling water to ensure it meets the target use concentration but also to ensure all were soluble. A typical method would be to do a “delta-Phosphate” analysis.
Today, the monitoring programs have shifted significantly away from “active” ingredient monitoring to “inert” ingredient monitoring relying heavily on PTSA.
“Delta-Phosphate” is the difference between a filtered and unfiltered phosphate analysis. This is considered “active” ingredient monitoring. The filtered sample being that solution which passes through a 0.2 micron filter. All that passes through this filtration is considered soluble. So, if an analysis shows a significant “delta-phosphate” difference, some portion of that phosphate has precipitated and can become a foulant, negatively affecting heat transfer equipment.
Today, the monitoring programs have shifted significantly away from “active” ingredient monitoring to “inert” ingredient monitoring relying heavily on PTSA. This is an inert fluorescent dye (1, 3, 6, 8 pyrenetetrasulfonic acid, sodium salt) fed at ppb levels, is non-toxic, and chemically stable. It is measured quickly and directly through fluorometry. But it is still only an inert tracer. It does not track what is soluble verses insoluble from an active chemistry perspective. If the cooling system is not in chemical control, your inert tracer analysis would not discern that. It will only be correct if the program is effective and control is being attained.
That being said, PTSA is a useful tool, from an ease of use standpoint. It is quick and would yield accurate results with a cooling system in treatment control. The only caveat would be that it should be coupled with an actives analysis periodically to ensure that this is no loss of corrosion and/or deposit control which PTSA would not detect. We at Delta highly recommend this and do provide this service also for our customers.
Trust But Verify.