Potable Water

Both potable and irrigation water are supplied through the utility water distribution system in the refinery. The refinery may draw its potable water supply from a municipal water supply or a desalination plant in coastal refineries. For supply to refinery drinking water and safety showers, further treatment is required. The treatment consists of passing through a bed of activated carbon in the form of extradâtes and supported on a bed of graded quartz. Filtered water is next cooled to 85°F in a water cooler and chlorinated. The treated water must have a residual chlorine content of no less than 0.5 mg/liter.

In case the refinery potable water supply comes from a desalination plant, the total dissolved solids (TDS) may be very low, on the order of 2 mg/liter. The water must be chemically conditioned to make it potable and increase the TDS to potable water requirements (80 mg/liter minimum). Chemical conditioning consists of dosing measured amounts of calcium chloride, sodium bicarbonate, and sodium carbonate into the feedwater stream. Typical dosage of chemicals are these:

Calcium chloride: 28 mg/liter

Sodium bicarbonate: 62 mg/liter

Sodium carbonte: 12 mg/liter

WATER FILTER

F-101

WATER FILTER

F-101

Layout Bicarbonate Water

PORTABLE WATER SYSTEM

WATER FOR IRRIGATION & MlSCELLANEOUS USES

Figure 9-4.

PORTABLE WATER SYSTEM

WATER FOR IRRIGATION & MlSCELLANEOUS USES

Figure 9-4.

Refinery utility water system.

Table 9-3 Potable Water Quality Requirements

PARAMETER

UNITS

TOTAL DISSOLVED SOLIDS

ALKALINITY

CHLORINE, RESIDUAL

mg/liter, minimum mg/liter as CaC03

mg/liter, minimum

80 60-80

APPROXIMATELY 8.7 0.5

If a refinery has an assured supply of carbon dioxide, an alternative conditioning treatment is possible. In this case, carbon dioxide is directly injected into feedwater, and this water is passed through a bed of limestone, where calcium bicarbonate is formed. Excess C02 is necessary for the correct hardness level. The remaining C02 is neutralized with a nonlimestone-based alkali (caustic soda) to avoid forming turbidity. Typical potable water quality requirements are shown in Table 9-3. The average potable and irrigation water demand is estimated at 35-75 gal/ day per person working in the refinery, the higher figures being for hotter climates.

The instantaneous demand for potable water is based on two criteria. The first is the computed peak demand resulting from the number of potable water fixtures in the refinery. The second consideration is the simultaneous use of safety showers. An average safety shower requires water flow of approximately 65 gal/minute. The designed irrigation water usage is generally based on approximately 3 in. water per week applied to the irrigated area of the refinery.

FIRE WATER SYSTEM

Many hydrocarbon processing plants are located along waterways, so that availability of fire water is essentially guaranteed. Similarly, coastal refineries may use sea water as backup fire water in an emergency. If, however, fire water is obtained from municipal sources, storage of the water would be required, for it is unlikely that a municipal water supply system would permit the volumes of draw-down required to fight a fire. The wide variety and varying intensity of fires possible in hydrocarbon processing facilities make precise calculation of fire water requirements difficult. Instead, fire water usage in case of actual fires from historical data of the industries is relied on.

The size of the storage tank should be sufficient to provide a 4-6 hr supply at the estimated maximum fire water rate. A good general figure for estimating fire water is 2500-3000 gpm (gallons per minute) in case of an actual fire. Another important consideration is system pressure. The knowledge of the system capacity as well as the suggested layout of the hydrants, monitors, and water spray systems are the basic input required to determine the size and configuration of the distribution system. The other major input is the residual pressure requirements. The minimum pressure desirable at the extremity of the system is 70 psig, with 100 psig required anywhere in the process area. The maximum pressure that can be tolerated on a handheld hose is 100 psig nozzle pressure and that is a consideration in setting maximum system pressure.

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