Municipal water feed to the refinery must be upgraded to boiler feed-water quality by further treatment (see Figure 9-3). Boiler feedwater is used as makeup to fired and nonfired steam generators located in the utility plants and process units. Boiler feedwater is also used for process purposes.
Fresh water, as received from an outside supply, may not have enough pressure to fill tank TK-101. Booster pump P-101 is used to elevate the water supply pressure. Feedwater is stored in tank TK-101, holding approximately 72-hr supply. Water is pumped by P-102 to water filter V-101, which removes most entrained solids. Filter aids are added at the discharge of P-102, using a dozing pump to facilitate removal of suspended solids. Filtered water next passes through a bed of cation and anion resins in V-102 and V-103, which remove all anions and cations in the water by an ion exchange process. A high-purity water, which is essentially free of salts, is produced and stored in tank TK-102. Demin-eralizer beds are exhausted after 3-7 days operation, and they are regenerated by treatment with dilute sulfuric acid (cation resins) and caustic soda (anion resins). Each demineralizer bed is equipped with automatic regeneration facility. The typical feedwater and BFW water qualities are shown in Table 9-2. The demineralized water is supplied to all low-pressure deaerators as makeup to LP boiler water system.
cation anion exchanger exchanger v-102 ^ v-103
to deaerators cation anion exchanger exchanger v-102 ^ v-103
pousheo condénsate p-106
backwash water pousheo condénsate p-106
Treatment for boiler feedwater (BFW). HP = high pressure.
Table 9-2 Properties of Various Water Streams
FRESH DISTILLED LP BOILER CONDENSATE POLISHED UNITS WATER1 WATER2 FEEDWATER3 RETURN4 CONDENSATE5
CaCo3 CaCo3 CaCo3 CaCo3 CaCo3
CaCo3 CaCo3 CaCo3 CaCo3 CaCo3
Si02 Fe Cu jtu umho/cm
10 40 0.04
5 10 20
'WATER RECEIVED FROM MUNICIPAL SUPPLY.
2WATER FROM A DESALINATION PLANT.
3DEMINERALIZED WATER FEED TO LOW-PRESSURE BOILERS.
^CONTAMINATED CONDENSATE RETURNED FROM THE PROCESSING UNITS.
'POLISHED CONDENSATE FEED TO HP BOILERS.
LP = LOW PRESSURE; TDS = TOTAL DISSOLVED SOLIDS; JTU = JACKSON TURBRIDITY UNITS; ALL DISSOLVED SALT CONCENTRATIONS IN PPMW.
Condensate is returned to the water treatment system from two sources:
1. Steam condensate condensed in surface condensers is considered clean and returned directly to condensate storage tank TK-103. Conductivity monitoring is provided on the surface condenser condensate header. This measurement detects any saltwater leakage into the condensate that may occur due to condenser failure. To save as much water as possible, two set points are provided on the conductivity controller. If the conductivity exceeds 100 micro-mho, the surface condenser condensate is diverted to a utility water storage tank. If a severe leak occurs and the condensate conductivity exceeds 500 micro-mho, the condensate is considered too contaminated to save and is diverted to a sewer.
2. Condensate is also collected from refinery's process equipment and returned to the water treatment system. This condensate is considered potentially contaminated with oil. The condensate passes through an activate carbon filter V-105 before entering condensate storage tank TK-103. The carbon filters are typically designed to process feed with 20 ppm oil and reduce it to less than 0.3 ppm oil in the filtered condensate. The condensate is next passed through condensate polisher V-103, which removes any dissolved salts contamination picked up.
The process condensate is returned to the water treatment unit at approximately 205°F. Since the ion exchange resin in condensate polisher V-104 is extremely temperature sensitive, this stream is cooled to 130°F. BFW heat exchangers E-101 and E-102 by exchanging heat with LP boiler feedwater.
During the operation of the steam system, the demand for HP boiler feedwater exceeds the returned condensate from all sources. To make up for this shortfall, demineralized water from BFW storage tank TK-102 is used. The design flow rate for this service is dictated by the loss of condensate.
Polished condensate is supplied as makeup to all high-pressure dear-eators as HP boiler feedwater. In addition, the process condensate and surface condenser condensate are returned from the steam system as feed to condensate polisher system.
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