SPM-6120 Dissolved Air Flotation Unit

Process Description

Sea Water Pumps

Screened sea water from the sump at battery limits is pumped to the Dissolved Air Flotation Unit (DAF) and the Cooling Water System by Sea Water Pumps P-101A/B. These are electric motor-driven centrifugal pumps. Normally only one pump is in operation. The bearings are cooled with circulating cooling water to preserve their mechanical integrity. Cooling water for the pump bearings is supplied from the A and B cooling water headers and returned back to the cooling water system.

The design flow of sea water to the unit is 503 GPM at a temperature of 56 DEG F. The salinity of sea water is 3.5 weight %. Sea water flow to the DAF is normally 245 GPM and flow to the Cooling Water System is normally 206 GPM. The sea water distribution header normally operates at 40.0 PSIG.

Cooling Sea Water Filters

Sea water from P-101A/B and destined for the Cooling Water System is first filtered in Cooling Sea Water Filters F-101A/B. These are cartridge filters designed to remove any suspended solids from the feed water. The sea water supply to the unit has had debris and large particles removed prior to entering the unit. However, suspended solids remaining in the sea water can easily foul and plug the heat exchangers in the Cooling Water System heat exchangers. Normally, only the “A” filter is in operation.

The turbidity (cloudiness) of the sea water due to suspended solids is indicated by its NTU analysis (Nephelometric Turbidity Unit). The sea water feed normally has a 15.0 NTU. F-101A/B normally reduce the turbidity to 3.5 which is sufficiently low for use in the Cooling System’s heat exchangers.

Flocculators

Prior to entering the Flocculator compartments of the Dissolved Air Flotation Unit, sulfuric acid from Sulfuric Acid Pump P-111 is injected into the feed sea water from Sea Water Feed Pumps P-101A/B to decrease its pH from 7.90 to 6.51. This represents the optimum pH of the sea water to maximize the coalescence efficiency of the flocculant to produce sludge in the DAF Basin. A flocculant is a chemical that promotes coalescence of the suspended solids into larger particles so they can be more easily separated from the sea water. The flocculant used in the DAF process is ferric chloride (FeCl3). It is injected into the feed sea water by Ferric Chloride Pump P-112. The flow rates of the sulfuric acid and the ferric chloride are very small compared to the sea water flow rate. These chemicals are intimately mixed with sea water in In-line Chemical Mixer MX-101 prior to entering the Flocculators.

The Flocculators R-101A/B are chambers within the DAF Basin T-101. The DAF Basin is a large box that provides sufficient residence time for the coalescence of the suspended solid particles. Sea water first flows down along and under the feed baffle of Flocculator R-101A. Agitator AG-113A circulates the sea water and flocculant mixture within the chamber of R-101A to ensure intimate contacting of the flocculant and the suspended solids. This mixture then passes into the chamber of Flocculator R-101B in the same way. This chamber is agitated by AG-113B. The flocculated sea water from R-101B then flows over its exit baffle and down into the Dissolved Air Flotation Basin’s (T-101) main chamber.

Dissolved Air Flotation Basin

Flocculated sea water from Flocculator R-101B flows into the main chamber of the Dissolved Air Flow Basin T-101 through the channel formed by the baffles separating the two compartments. As the sea water enters, it mixes with an upward flow of recirculated sea water containing saturated air which is distributed along the bottom of the DAF Basin. An angled baffle along the width of the basin forces the sea water to rise up to the top of the basin. As the hydrostatic pressure decreases toward the top, the air is increasingly released and forms small bubbles that rise to the top of the basin. Agglomerated solid particles (sludge) from flocculation have a somewhat lower bulk density than water so they will float up to the top of the basin, assisted by the small, rising air bubbles. The sludge will accumulate and float on top of the water in the basin and the sea water will flow over the angled baffle.

The floating sludge on the top of the basin will then tend to move toward and then over a baffle at the far end of the main DAF Basin chamber and into the Sludge Trough. The movement of the floating sludge toward the trough is greatly assisted by a motor-driven Sludge Scraper outfitted at the top of the main chamber of the DAF Basin (not shown on Schematic #6 for clarity of the instrumentation). The scraper is a slowly rotating belt with rubber scraping blades that skim the sludge along toward the Sludge Trough. Sludge is intermittently removed from the Sludge Trough by DAF Sludge Pumps P-103A/B which are electric motor-driven slurry pumps.

Clarified sea water then flows down and through a set of perforated lateral draw-off pipes near the bottom of the main chamber under the Sludge Trough. Clarified sea water then flows up and over a baffle and into a collection well at the end of the basin assembly. Clarified water is drawn from here by the UF Feed Pumps, P-105A/B to be further filtered in the UF Section.

A portion of the clarified sea water passing through the perforated pipes is circulated back to the main chamber of the DAF Basin by DAF Recycle Pumps P-102A/B. These are electric motor-driven centrifugal pumps. Normally only one pump is in operation. The normal flow of recirculated sea water is 100 GPM. These are electric motor centrifugal pumps. Normally only one pump is in operation. The recirculated sea water then flows into the top of the DAF Air Saturator Tank V-101.

Dissolved Air Flotation Basin

Flocculated sea water from Flocculator R-101B flows into the main chamber of the Dissolved Air Flow Basin T-101 through the channel formed by baffles separating the two compartments. As sea water enters, it mixes with an upward flow of recirculated sea water that contains saturated air which is distributed along the bottom of the DAF Basin. An angled baffle along the width of the basin forces the sea water to rise up to the top of the basin. As the hydrostatic pressure decreases toward the top, air is increasingly released and forms small bubbles that rise to the top of the basin. Agglomerated solid particles (sludge) from flocculation have a somewhat lower bulk density than water so they will float up to the top of the basin, assisted by the small, rising air bubbles. The sludge accumulates and floats on top of the water in the basin and the sea water flows over the angled baffle.

The floating sludge on the top of the basin will move toward and then over a baffle at the far end of the main DAF Basin chamber and into the Sludge Trough. The movement of the floating sludge toward the trough is greatly assisted by a motor driven Sludge Scraper outfitted at the top of the main chamber of the DAF Basin (Not shown on Schematic #6 for clarity of the instrumentation). The scraper is a slow, rotating belt with rubber scraping blades that skim the sludge along toward the Sludge Trough. Sludge is intermittently removed from the Sludge Trough by DAF Sludge Pumps P-103A/B which are electric motor-driven slurry pumps.

Clarified sea water then flows down and through a set of perforated lateral draw-off pipes near the bottom of the main chamber under the Sludge Trough. Clarified sea water then flows up and over a baffle and into a collection well at the end of the basin assembly. Clarified water is drawn from here by the UF Feed Pumps, P-105A/B to be further filtered in the UF Section. A portion of the clarified sea water passing through the perforated pipes is circulated back to the main chamber of the DAF Basin by DAF Recycle Pumps P-102A/B. These are electric motor-driven centrifugal pumps. Normally only one pump is in operation. The normal flow of recirculated sea water is 100 GPM. These are electric motor centrifugal pumps. Normally only one pump is in operation. The recirculated sea water then flows into the top of the DAF Air Saturator Tank V-101.

Compressed air from DAF Air Compressor C-101 is injected into the side of the DAF Air Saturator Tank and distributed into the recycle sea water entering the top of the tank. The injected air flow is relatively small (160 SCF/H) because only enough air to assist in gently lifting sludge to the surface is needed. Internal spargers are used to distribute the air evenly into the water. The DAF Air Compressor is an electric motor driven rotary compressor.

The DAF Saturator Tank V-101 contains a large bed of packing that provides surface area and mixing for the saturation of the injected air into the circulating sea water. The air-saturated water and any unabsorbed air flow out the bottom of the tank and into the bottom of the entrance to the main chamber of the DAF Basin. A set of distribution spargers at the bottom promotes release of air from the circulating sea water as microbubbles which will enhance the lifting of the agglomerated particles from the Flocculators to the surface of the basin to form a floating sludge phase.

UF Feed Pumps

Clarified sea water from the outlet well of DAF Basin T-101 is pumped to the UF Autostrainers F-102A/B by UF Feed Pumps P-105A/B. A small amount of clarified sea water is dumped to the sea for purposes of level control of the DAF Basin’s outlet well. These are electric motor-driven centrifugal pumps. Normally only one pump is in operation. The bearings are cooled with circulating cooling water to preserve their mechanical integrity. Cooling water for the pump bearings is supplied from the A and B cooling water headers and returned back to the cooling water system.

The design flow of sea water to the UF Section is 230 GPM. The normal flow of dumped clarified sea water is 15.0 GPM. The outlet pressure of the UF Feed Pumps is normally 65.0 PSIG.

UF Autostrainers

Clarified sea water from UF Feed Pumps P-105A/B is first strained in UF Autostrainers F-102A/B. These strainers prevent any larger particles (e.g. sludge) that might be present in the clarified sea water from passing on to the downstream ultrafiltration cartridges in the UF Unit. The internal strainer is a large mesh-coated cylinder. Sea water passes through the mesh to remove any of these particles and is collected in the bottom of the strainer vessel and passed on to the UF Units X-101A/B.

The mesh can be periodically cleaned by means of a rotating rubber wiper assembly which will gently dislodge any accumulated solids on the outside of the mesh. A manual purge valve is provided on each Autostrainer for the removal of this material from the vessel for disposal. The purge valve is normally only opened when the mesh is being wiped. Normally only the A Autostrainer is in operation.

Ultrafiltration Units

Strained, clarified sea water from the UF Autostrainers F-102A/B flows to the Ultrafiltration Units X-101A/B. These units consists of modular racks of ultrafiltration cartridges that remove very small solid particles from the sea water, so they do not plug the downstream Reverse Osmosis Unit. These smaller particles are continuously removed from the ultrafiltration cartridges. Ultrafiltration cartridges use permeable membranes that have small pores sized such that most of the sea water will pass through the pores while most microscopic solid particles will not. A continuous purge of sea water at the end of the UF cartridges ensures removal of these solid particles that will not pass through the membrane.

The flow rate of sea water to the UF Units is normally 230.0 GPM. The purge flow rate is normally 30.0 GPM. Normally only the A unit is in operation. The pressure drop across the operating UF Unit is normally 19.8 PSI. The outlet of each UF Unit is combined and then sent to the RO Feed Pumps P-201A/B.

Over time, the pores of the UF membranes will tend to plug and capacity of the unit will be reduced resulting in an unacceptable high pressure drop. The UF Units are outfitted with connections to a cleaning system which is shared with the downstream RO Units. The cleaning system allows the UF and RO Units to be flushed, cleaned and sanitized. Because the connections of the UF and RO Units to the cleaning system are permanent, the cleaning system is referred to as a clean-in-place (CIP) system.

Instrumentation

Sea Water Pumps

The temperature of the sea water feed is indicated on TI-100. The status of the motors of Sea Water Pumps P-101A/B is indicated on switches HS-101A/B. These switches are also used to start/stop the pumps.

Switches FV-109A/B are used to open the cooling water valves to pumps P-101A/B, respectively. The temperature of the pumps’ bearings are indicated on TI-109A/B, respectively. Interlocks I-109A/B will generate trip alarms THH-109A/B if the bearing temperatures become too high. See the “Interlocks” section below for functional details of I-109A and I-109B.

The total flow from the pumps is indicated on FI-101. The discharge pressure of the Sea Water Pumps P-101A/B is controlled by PIC-101 which adjust the position of control valve PV-101. FIC-102 controls the flow of sea water to the DAF Unit by adjusting the position of control valve FV-102. The PV of controller FIC-102 is used by interlock I-102 (Low Sea Water Flow Interlock). FLL-102 is a switch that indicates the status of the interlock. See the “Interlocks” section below for functional details of I-102.

The flow of sea water to the Cooling Sea Water Filters F-101A/B is indicated on FI-103.

Cooling Sea Water Filters

AI-102 indicates the turbidity (cloudiness) of the water entering the Cooling Sea Water Filters F-101A/B and is measured in NTU (Nephelometric Turbidity Units). An NTU value less than 5 represents a very clear fluid, which is normally produced by the Sea Water Filters. Visually discernible cloudiness starts between an NTU of 10 to 20. Values above 100 are obviously cloudy. AI-103 indicates the turbidity of the sea water leaving the filters.

PDI-101A and PDI-101B indicate the pressure drop across F-101A/B, respectively. Normally only the A filter is in service.

HV-102A and HV-103A are switches to open and close the block valves of filter F101A. HV-102B and HV-103B are switches to open and close the block valves of filter F-101B.
Flocculators

AI-111 indicates the pH of the sea water feed to the In-line Chemical Mixer MX-101. AI-113 indicates the pH of the sea water leaving MX-101.

Switch HS-111 is used to start/stop the motor of Sulfuric Acid Pump P-111. FIC-111 controls the flow rate of sulfuric acid to MX-101 by adjusting the speed of P-111.

Switch HS-112 is used to start/stop the motor of Ferric Chloride Pump P-112. FIC-112 controls the flow rate of ferric chloride to MX-101 by adjusting the speed of P-112.

Low Sea Water Flow Interlock I-102 will stop the flow of sulfuric acid and ferric chloride to MX-101 in the event of very low sea water flow to MX-101. See the “Interlocks” section below for functional details of I-102.

LI-113A indicates the level of sea water in the chamber of Flocculator R-101A. Switch AG-113A is used to start/stop the motor of the agitator for R-101A.

LI-113B indicates the level of sea water in the chamber of Flocculator R-101B. Switch AG-113B is used to start/stop the motor of the agitator for R-101B.

Dissolved Air Flotation

Switch HS-114 is used to start/stop the motor of the Sludge Scraper of the DAF Basin T-101.

LI-114 indicates the level of sea water in the main chamber of T-101. LI-115 indicates the level of sludge in the in the Sludge Trough of T-101. This indication is used by interlock I-103 to automatically start and stop the motors of either of the DAF Sludge Pumps P-103A/B. See the “Interlocks” section below for functional details of I-103. Switches HS-115A/B are used to control the functions of interlock I103.

Switches HS-103A/B are used to manually start/stop the motors of DAF Sludge Pumps P-103A/B.

LIC-116 controls the level of clarified sea water in the collection well at the end of the DAF Basin assembly by adjusting the position of control valve LV-116 which dumps clarified sea water from the discharge of UF Feed Pumps P-105A/B to the sea. AI-116 indicates the turbidity of the clarified sea water drawn from the collection well.

Switches HS-102A/B are used to start/stop the motors of DAF Recycle Pumps P103A/B. FIC-117 controls the flow of DAF recycle sea water by adjusting the position of control valve FV-117. Switch HS-116 is used to start/stop the motor of DAF Air Compressor C-101. FIC-118 controls the flow of air to the DAF Air Saturator Tank by adjusting the position of control valve FV-118.

UF Feed Pumps

The status of the motors of UF Feed Pumps P-105A/B is indicated on switches HS105A/B. These switches are also used to start/stop the pumps.

Switches FV-129A/B are used to open the cooling water valves to pumps P-105A/B, respectively. The temperature of the pumps’ bearings are indicated on TI-129A/B, respectively. Interlocks I-129A/B will generate trip alarms THH-129A/B if the bearing temperatures become too high. See the “Interlocks” section below for functional details of I-129A and I-129B.

The flow from the pumps to the UF Autostrainers F-102A/B is indicated on FI-121. The discharge pressure of the UF Feed Pumps P-105A/B is indicated by PI-121. The temperature of the sea water from P-105A/B is indicated on TI-121. FI-122 indicates the flow of sea water through control valve LV-116 which is controlled by the DAF Basin collection well level controller LIC-116.

UF Autostrainers

Switch HV-125A opens/closes the inlet hand valve HV-125A of UF Autostrainer F102A. Switch MOV-125A opens/closes the main outlet motor-operated valve of F102A. Switch POV-125A opens/closes the pilot-operated purge valve POV-125A.

PDI-125A indicates the pressure drop across F-102A. Switch HS-125A stops/runs the motor of the rotating rubber wiper of F-102A to loosen accumulated material on the filter mesh basket when indicated by high pressure drop. Normally, HV-125A and MOV-125A are open and POV-125A is closed. POV-125A is opened to purge loosened material while the rotating wiper is operating.

UF Autostrainer F-102B is identically instrumented to F-102A. Instrument tags differ by the suffix A or B. Normally, F-102B is on standby service with all its valves closed and its motor stopped.

The outlet pressure of sea water from F-102A/B is indicated on PI-125. The flow of sea water to the Ultrafiltration Units X-101A/B is indicated on FI-125.

Ultrafiltration Units

Switch HV-127A opens/closes the sea water inlet hand valve HV-127A of UF Autostrainer Unit X-101A. Switch HV-128A opens/closes the sea water outlet valve HV-128A from X-101A. Hand controller HIC-129A adjusts the position of concentrate control valve HV-129A. Switch HV-711A opens/closes the cleaning solution supply valve HV 711A. Switch HV-712A opens/closes the cleaning solution outlet valve HV712A.

In normal operation, the sea water inlet and outlet valves HV-127A and HV-128A are open and the concentrate control valve HV-129 is open 30% while the cleaning solution valves HV-711A and HV-712A are closed.

Ultrafiltration Units

Ultrafiltration Unit X-101B is identically instrumented to X-101A. Instrument tags differ by the suffix A or B.

Normally, X-101B is normally in cleaning mode (circulation from/to Cleaning Tank TK-701 with potable water). In this situation, the two sea water valves HV-127B and HV-128B are closed and the concentrate control valve HV-129B is fully closed (0% open). The cleaning solution valves HV-711B and HV-712B are open to allow circulation of cleaning water through the cartridges of X-101B.

The cleaning lines from X-101A and X-101B are combined and pass-through control valve FV-712. FIC-712 controls the cleaning solution circulation flow through the UF Unit being cleaned.

The combined flow of sea water from the Ultrafiltration Units to the RO Feed Pumps P-102A/B is indicated on FI-128.

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