2 replies to this topic

[NOEC]

The client I work for wants to install some type of pH, conductivity, or fluoride ion monitors on the alkylation unit high pressure (55 psig 300 F) and low pressure (25 psig 230 F) condensate headers. This is to monitor for HF leaks in the condensate system. The acid coolers already have pH analyzers on the cooling water streams, so my initial reaction was to use similar technology. One vendor suggested that since the condensate operates at a high temperature, conductivity sensors would provide better reliability compared to the pH probes. Does anyone have suggestions for one monitoring method over another? What would be the conductivity ranges that I should be monitoring? I've also considering installing sample coolers before the probes to help with taking off-line samples, and it seems this would help with the high temp issue. Any information or recommendations would be greatly appreciated.

Thanks,

Eric

[kkala]

I asked "why conductivity meter instead of pH meter" in the online monitoring of low pressure condesate (to detect any ingoing acid leakage) in the phosphoric acid evaporation unit (1977). The Director said that a conductivity meter is more reliable than a pH meter in this case. I did not ask for more explanations in that meeting, but the Director was keen at technical matters.
This may be true in the query case, see http://www.yokogawa....tExchanger.pdf'> http://www.yokogawa.com/eu/an/analytical/eu-support-files/HeatExchanger.pdf, even though there are no explanations either.
Trying to guess why, a pH meter at high temperatures may be more prone to wrong measurements or damage. Automatic temperature compensation (to reduce values to 25 oC) can be possible for both conductivity meter and pH meter, but easier for conductivity meter, since the pH meter senses H⁺ exchange (complicated procedure).
Sample coolers would add difficulty to online monitoring.
Nevertheless pH metering is also applicable, see http://www.arvanitak...condensate.htm'> http://www.arvanitakis.com/en/food/why_treat_condensate.htm. Probably pH is more suitable in long term monitoring of corrosion trend, conductivity in detecting leakage. Further advice on this topic would be welcomed.

[kkala]

Relevant data concerning industrial applications, from a fluid dynamics university book of 1966 (in Greek):
A. pH meters (can be online, with resistance thermometer to reduce measurement to 25 oC)
A1. Glass electrode: wide use, fast responce, endurance to almost all substances, temperature limit 135 oC.
A2. Antimonium electrode: long endurance, satisfactory precision, temperature limit 100 oC, pressure limit 6 kp/cm2 g.
B. conductivity meters (can be online; temperature compensation not reported, but quite useful).
Electrodes made of steel, graphite, Pt. Alternative current applied to avoid electrode polarization, which would bring errors in measurement. Widely used in boiler water, aqueduct water, sugar industry, H2SO4 industry, etc.

According to above old data, pH meter installed on HP header would have problem (high temperature), and so would have antimonium pH metter on LP header.
Conductivity meters are simpler apparatus. So they are expected to have less maintenance trouble and less temperature restrictions.

Edited by kkala, 16 September 2011 - 09:29 AM.