Caring for Cooling Water Systems

Hydrocarbon leaks can disrupt recirculated cooling-water systems. Here’s an outline of effects and remedies

Water is often the fluid of choice for cooling systems in the chemical process industries (CPI). Recirculated cooling-water systems are used to control the temperature of process fluids, which is necessary for process control and to achieve target product yields and specifications. Hydrocarbon leaks into recirculated cooling-water systems are of particular concern in oil refineries and the petrochemical industry. Contamination in the water can cause a multitude of problems including biofouling, scaling and microbiologically induced corrosion. These fouling and corrosion mechanisms can result in operational inefficiencies, equipment failure and environmental concerns.

Hydrocarbons as nutrients

Petroleum hydrocarbons are compounds consisting of carbon and hydrogen in various configurations. Since living cells use carbohydrates as sources of energy; microbiological organisms — such as bacteria, algae and fungi — use hydrocarbons as their food for growth. Petroleum hydrocarbons, therefore, become natural sources of nutrition for these microbiological species.

Hydrocarbons range from very simple and volatile paraffinic compounds to long-chain waxy compounds. Microbes produce enzymes to break down hydrocarbon chains into small, easily digestible parts. The simpler the configuration, the easier and faster the breakdown. Gasoline, naphtha, kerosene and benzene are broken down very easily. Diesel oil is also primarily composed of paraffinic chains and is therefore absorbed very easily by microbes. Heavier oils, high-viscosity oils and high boiling-point fractions comprise long, polymeric chains. These are very difficult to break down by the microbial enzymes and the organisms take a long time to use these products as their food.

Leak detection

Hydrocarbons, as oils and gases, are typically introduced into cooling water systems due to leaks in coolers and condensers in the plant. Tube and gasket failures in the coolers and condensers are contributing factors to these problems.

Since hydrocarbons are sources of high-caloric and easily digestible food for microbes, the growth of the organisms increases exponentially when hydrocarbons are introduced into a cooling water system. Due to such unwarranted proliferation of microorganisms, the requirement for routine oxidizing biocides like sodium hypochlorite, chlorine and bromine, and chlorine dioxide greatly increases. The immediate effect of hydrocarbon leaks is the gradual lowering of free chlorine content (FRC) in the recirculated water, particularly in the return header. Observations subsequent to process leaks typically include the following:

• Visually, an oily layer can be seen on the sump and a rainbow-colored interference pattern may be observed

• A hydrocarbon-like odor may be noticed in water samples collected from the sump

• In case of gas leakage, bubbling in the sump may be seen. This can also be felt during sample collection from return headers

• Sometimes, the pH of sump water decreases slightly. H2S ingress takes place with most of the petroleum compounds, thereby forming a weak acid in water

• FRC in the system will come down and possibly reach zero if the leak remains undetected for a few days, or if biocides remain ineffective

• The turbidity of the sump water will gradually increase, particularly during heavy oil ingress. Turbidity may exceed 300 NTU (Nephelometric turbidity units) with contamination of heavy and viscous materials

• The sulfate-reducing bacteria (SRB) counts will steadily rise to 10 ³ – 10 ⁴ counts/mL

• The total bacteria count (TBC) will also show an upward trend (more than 10 ⁵ counts/mL)

• The oil content in the sump will be significantly higher than the allowed values

Leaving leaks unchecked will cause problems, the effects of which are discussed in the next sections.

The effects of leaks

Biofouling. Bacteria and algae stick to almost any surface in cooling water systems, particularly where water velocity is low. The microorganisms produce a polysaccharide-layer matrix, which is called slime or biofilm. This film further entraps inorganic matter, precipitates and corrosion products. Numerous problems posed by the biofouling are given below.

• Loss of transfer and operational efficiency as these biofilms are four times more insulating than even calcium-carbonate scales

• Microbes produce localized concentrations of metabolites, such as corrosive gases and acids, which manifest in the form of pitting and grooving

• Biofilms promote scale formation

• Restriction of flow inside the cooler and condenser tubes

• Typical chemical treatments can become ineffective when biofilms grow in volume, as the biocides cannot penetrate the impermeable structure

• Biofilm promotes development of biocide-resistant strains due to sessile growth under and within deposits (sessile refers to microorganisms that are attached to the surface)

• Biofilms also harbor some harmful species that cause environmental and human-safety related concerns

Scaling. Organic acids and polymers produced by bacteria in biofilms combine with calcium and magnesium ions to form insoluble oxalates, acetates and calcium-magnesium polymer complexes. These insoluble-compound deposits are scales, which make biofilms even more impermeable. As a result, heat-transfer efficiency drops significantly.

Microbiologically induced corrosion (MIC). This is another deteriorating effect of oil leaks in cooling water systems. Depending on the function of bacteria, they are grouped as aerobic or anaerobic. SRB is a typical example of anaerobic bacteria. Nitrifying bacteria, which produce nitric acids in the presence of ammonia, are aerobic.

MIC results from various causes, including: a) Cathodic depolarization of sulfur-reducing bacteria, such as Desulfovibrio and Desulfurican. Corrosion typically manifests in the form of localized pitting and grooving; b) The production of corrosive metabolites, such as acids by Thiobacillus and Thiooxidans and other organic acids by various bacteria and fungi species; c) Sometimes bacteria, such as Gallionella and Clonothrix, cause direct oxidation of metal, for example ferrous to ferric, and cause tubercles on metal surface. These are called iron-oxidizing bacteria. Since areas under the tubercles are deficient in oxygen, they act as corrosion cells and result in deep internal grooving; d) Some bacteria are acid-producing bacteria (APB) and thus corrode metals.

MIC may be prevented by routine monitoring of TBC and SRB counts in the cooling water system. However, a common mistake is to measure the planktonic count (microbes present in the bulk water), which shows poor correlation with the sessile count on the metal surface. Sessile-count monitoring, and identification of low-velocity zones and fouling-prone coolers and condensers are a must for formulating an effective water-management program. It is important to select proper biocides to kill unwanted microorganisms and equally important to use biodispersants to disengage organisms from surfaces so that the biocides can act effectively.

Ammonia and hydrogen sulfide sometimes accompany hydrocarbon gases to contaminate cooling water. These chemicals also cause the demand for chlorine to increase to very high levels and may also lead to both fouling and MIC.

Remedial measures

In addition to fixing the actual leak (see box, p. 49), parallel measures are also taken to ward off the deleterious effects of leaks in the cooling water system:

• Dosing of oxidizing biocide is increased to a higher level. Chlorine dioxide, bromo-compounds and ozone are also used

• Biodispersants are dosed at a higher-than-normal rate. They cause faster disengagement of organisms from the surface so that biocides act effectively. This enhances both planktonic and sessile efficacy

• Shock dosing of other non-oxidizing biocides, such as quaternary ammonium compounds, methyl bis-thiocyanate and glutaraldehyde, is also done to kill the microbes. Selection of particular biocides and the dosing rates is important because biocides possess different levels of efficacy against various microorganisms, and each cooling water system has its unique microbiological population

• An overflow and controlled blowdown of the cooling water sump eliminates oil, biomass and froth from the sump, which otherwise would circulate in the system and clog the coolers and condensers. Any blowdown, however, directly affects cost

Before a good cooling-water treatment program is formulated, consideration must be given to the various types of process leaks that are possible, and microbiological populations that are present in the system. In the past, selection of biocides and dispersants was typically based only on their cost and effectiveness against the spectrum of bacteria. Today, the selection is also governed by environmental concerns. Research is being pursued in this area and newer strategies are evolving for the control of biofouling, scaling and MIC due to hydrocarbon leaks.

Edited by Dorothy Lozowski

Finding and fixing the leak When hydrocarbon leaks are confirmed, first and foremost it is important to isolate the source of leakage. The following are typical methods: Measure oil content in the supply and return headers of the main units to determine the unit that is leaking Once the main unit that is leaking is identified, oil content and oxidation-reduction potential (ORP) should be measured at the inlet and outlet of coolers and condensers. An ORP level greater than 450 millivolts (mV) indicates that biofouling is prevented in the system. Values of 500 – 600 mV indicate an almost clean system, while a sudden dip in ORP value indicates hydrocarbon leaks. ORP measurements are quite effective in identifying the source of leaks, and installation of online ORP meters at unit return headers, and critical coolers and condensers is recommended. Leaks sources should be confirmed through laboratory measurements of oil content in the water sample Once the leak source is confirmed, the unit should be isolated immediately. This sometimes causes production losses After cleaning, the leaking heat exchanger is pressure tested to identify and repair the leak