Kolmetz.com Cooling Tower Monitoring 10 Page 9
In addition to the above VOC sampling requirements, the operator also has to conduct daily
liquid samples (analyzed by gas chromatography) on each cooling water return for benzene at a
0.013
ppmw detection limit. This sampling for benzene using a mass spectrophotometer must be
conducted every month. If the analyzed cooling water detects a benzene concentration greater
than 0.013 ppmw, the analyzer must be used to help determine the area of the plant site from
which the leak into the cooling water system has occurred. A sampled benzene concentration of
greater than 0.013 ppmw on five consecutive days shall be considered a leak.
The above special hydrocarbon leakage sampling and detection requirements will not be found in
a written Federal or State regulations, but yet can be imposed by the State authority on case by
case basis during the permit evaluation and approval stages, especially in an Ozone non-
attainment area.
As a good engineering practice and to ensure that the hydrocarbon leakage can be detected early
in a cooling water system, an on-line TOC analyzer may be installed in the common cooling
water return header, and provided sample connections at different cooling water return sub-
headers for easy leak identification. The TOC analyzer detection limit must be set based on the
TOC concentration in the raw make up water to the cooling tower. For example, if the TOC at
the makeup water is 10 ppm, with 7 tower cycles operation, the maximum background TOC in
the re-circulating cooling water should be about 70 ppm or less for a new non leaking cooling
water system, and must be verified during system startup. Any substantial increase in TOC
concentration could be due to hydrocarbon leakage from process equipment.
In addition to a TOC analyzer, an on-line pH analyzer and a conductivity meter may be installed
at the cooling water return header to monitoring the tower operating cycles. The pH meter is
used to control acid injection into the tower that is not only for the operation, but also ensures
that the blowdown can be discharged within a permit allowable pH range of 6 to 9. Also,
temperature, flow rate and residual chlorine are normally monitored at the cooling tower
blowdown line before discharged to the outfall for permitting reporting purpose.
The on-line residual chlorine analyzer can be used to control the addition of de-chlorination
chemicals so that the residual chlorine at the blowdown will not exceed the permitted limit at the
outfall.
An Ontario Canadian refinery has installed an on-line process gas chromatograph to
continuously monitor the cooling water discharge for benzene, toluene, xylene and ethyl
benzene. The ability to rapidly identify even trace amounts of hydrocarbons means that
corrective actions can be taken immediately. The current state-of- the art gas chromatograph can
easily detect hydrocarbons in ppb range.
Westlake is a light hydrocarbon plant and it currently discharges its cooling tower blowdown to
the outfall. At all of the Westlake plants the cooling tower blowdown is sampled four times a
day for pH, chlorine residual, temperature and conductivity. In addition at WPT2, the newest
plant, the cooling tower blowdown is continuously monitored for TOC, pH, and temperature.
