Water Treatment

Cooling Innovations for the Oil & Gas Industry

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Mechanical Stresses for Refinery Cooling and Water Systemsn

  • In refineries, the crude unit overhead condensers are the most critical cooling water exchangers in the whole plant. These condensers are located as close as possible to the vacuum towers to minimize the pressure drop. This arrangement makes the condensers vulnerable to low cooling water supply pressure, thus making them prone to fouling.
  • Air-finned coolers and condensers often used in the refinery light end fractionation units for both overhead condensers and product coolers. Generally, these are located ahead of cooling water exchangers. If the
  • performance of the air-finned coolers drops off, it places more demand on the cooling water condensers resulting in high skin temperature and potential for scale formation. A similar situation can also arise
  • in separator condenser/cooler in the hydrotreater unit due to the performance drop in upstream fin-fanned coolers.
  • Another threat in refinery fractionator overhead condensers is that the process side carries light hydrocarbon containing mercaptan sulphur compounds similar to H2S. Leaks into the cooling water system
  • will form zinc sulphides, consume chlorine and hence lead to corrosion and microbiological excursion.
  • In ethylene plants, the maximum heat duty is taken up by a few critical heat exchangers, namely quench water coolers, acetylene converter intercoolers, depropanizer overhead condensers, C3 splitter overhead condensers, propylene refrigerant condenser and cracked gas/refrigeration surface condenser. In many of these heat exchangers, the cooling water outlet is throttled since temperature control in the process side is highly critical. This affects the cooling water flow and velocity leading to increased skin temperature and high potential for scaling.

Operational Stresses – When the Going Gets Tough...

  • Considering the variable system stresses in different refinery and petrochemical processes (as described above), control on water chemistry becomes extremely essential. However, in today’s highly competitive scenario, manpower resources are heavily optimized. Thus, conventional monitoring and control methods become inadequate as they are based on a few routine testing of grab samples and are slow in response to unusual upset situations.
  • Overfeeding of chlorine/hypochlorite (for microbiological control) and acid (for pH control) due to manual operational error can create highly corrosive environments in the water system.
  • Poor phosphate control is another critical stress factor in cooling water system. Maintaining too low phosphate levels will cause corrosion, but too high phosphate level will lead to hard Ca3(PO4)2 or FePO4 deposition and violation of discharge limits.
  • In a polyphosphate-based treatment program, the reversion of polyphosphate in a high-temperature situation increases the total inorganic phosphate (TIP) in the circulating water independent of cycles of concentration; thus, leading to potential for scaling.
  • Unattended passing blow-down valves or pump glands may cause uncontrolled water leakage from the system, thereby bringing the chemical inhibitor levels down below the safe limit leading to scaling, corrosion or microbiological problem.

Discover how chemical properties can affect cooing assets - especially in petrochemical installations on page 3...