Hydrocarbon leaks in cooling tower heat exchangers have been commonly known to cause corrosion, fouling and loss of heat transfer due to the presence of hydrocarbon itself and the resulting increased microbiological growth. However, finding these leaks is a difficult and tedious process. Common methods of checking heat exchanger inlets and outlets for hydrocarbon or oxidant residual often are unable to pinpoint the leaking exchanger. Fortunately, two new methods have been developed that can provide quick results and greatly shorten the quest.
According to a recent paper developed by Chevron Products Co., San Francisco, and BetzDearborn Water Management Group, Trevose, Pa., two methods exist to detect hydrocarbon leaks involving activated carbon or a gas trap followed by gas chromatography:
Activated Carbon. Activated carbon has the ability to absorb a wide spectrum of hydrocarbons. When a leak is suspected in one of the cooling towers, an activated carbon sampler is attached to a return water stream for one to three days. At the end of this time, the sampler is removed and the carbon is extracted with carbon disulfide, which then is injected in the fingerprint gas chromatograph.
Gas Trap. A gas trap is simply a five-gallon plastic carboy with a three-hole threaded cap. To use the trap, the bottle is placed near a return water sample point and filled with water. The cap is installed and cooling water is routed into the top of the jug. Because the water can only escape through the dip tube on the bottom, any gas entering the bottle is trapped at the top. When sufficient gas is in the bottle to fill the sample syringe or bag, the clamp is removed and the sample taken. The sample then is run on the chromatograph. This method has the further advantage of being able to pinpoint the leaking exchanger.
Hydrocarbon gas chromatographs can be used to identify various hydrocarbon streams. The traces that are detected are called "fingerprints" and can be used to identify the boiling range of an unknown hydrocarbon stream to place it within a particular product category such as gasoline, jet, diesel, gas oil and so on. With higher resolution, the gas chromatographs also can differentiate within a particular product category such as gasoline streams of hydrobate, reformate, alkylate or FCC gasoline.
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