Dry Chemical Feed Systems (1)
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Transcript of Dry Chemical Feed Systems (1)
DRY CHEMICAL FEED SYSTEMS
Characteristics1:
ComplexSimplest method of feeding is by handSolid chemicals may be preweighedSolid chemicals may be poured into tank
Typical Kinds of Chemicals used:
LimeAlumActivated Carbon
Description of Equipment:
A dry installation consists of the following:FeederA dissolver tankStorage bin or hopper
Types of dry feeders:
Volumetric typeGravimetric type
Volumetric feeders
-used where low initial cost and low feed rates desired
-less accuracy acceptable-deliver constant, preset volume of chemical-do not respond to changes in material density-trial and error calibration2
-positive displacement category
Positive Displacement3
-use some form of moving cavity of a specific or variable size
-belt, screw or auger can provide cavity-chemical falls into the cavities-chemical fed governed by the rate at which
cavity moves and empties and the cavity size
Flooding4
-occurs when chemical is forced through the feeder in an uncontrolled fashion
-especially important for those feeding from bins or large hoppers
PreventionRotary valves or other devices required
upstream
Figure 1. Typical Screw Type Volumetric feeder5
Gravimetric or Weigh Feeder6
-used for extreme accuracy and reliability-automatically compensates for difference of
form, size or density of material-not “official”scales-feeder feeds first and then checks and adjusts
the feed through weighing-should be checked regularly
GF classifications:
Pivoted-belt groupRigid-belt groupLoss-in-weight group
Installations between the bin/hopper and the feeder7:
Slide gatesKnife gate valvesOther devices
Installations below the gate or valve8:Flexible coupling
Dissolvers9- key component of dry-feed systems
Small dissolving tank- for material discharging
Typical Types of Dry feeder:
Belt typeGrooved-disk typeScrew typeOscillating-plate type
Typical material flow control devices for feeders:
Movable gateRotary inlet
Operational considerations:
Dry chemical feeder output should be checked periodically by taking a “catch10”
Feed curve11 should be developed
Figure 3. Typical dry feed system
LIQUID CHEMICAL FEED SYSTEMS
Characteristics:Require pumps for conveyance
Typical chemicals fed as liquids:Sodium hypochloriteSome polymersPhosphoric acidFerric chlorideCaustic sodaHydrogen peroxide
NaOCl solution has become prevalent in many plants today as a replacement for chlorine gas as disinfectant
Advantages:-more safe to use than chlorine gas-less toxic
Disadvantages:
-massive volume required-decompose over time-requires suitable storage-high pH-severe skin and eye irritant-decomposition by action of metals
Description of Equipment:
A typical solution-feed system consists of the following:
Bulk storage tankTransfer pumpDay tank (sometimes used for dilution)Liquid feeder
DilutionWhen not diluted:• Day tank is not necessary
When diluted:• Day tank is necessary• Prevents plugging• Reduce delivery time• Help mix the chemical with the wastewater• Disadvantage: can cause scaling when water is
not softened
Liquid feeders are typically metering pumps-positive displacement type-use of plungers or diaphragms-chemical addition rate can be set manually-obtaining or develop a set of calibration curves
showing %full stroke vs pump discharge
Automatic feed systems can be designed to control feed flow based on a process variable such as influent flow, residual chlorine concentration, or pH
Points to remember:
Pressure relief should be provided for positive-displacement metering pumps to prevent line failures if all discharge valves or pump isolation valves are closed.
Most metering pumps should have 34 to 69 kPa (5 to 10 psi) differential across the valves.
Discharge line pressure gauge is used for monitoring pressure at the pump.
Pulsation dampeners are located in discharge piping as close to the pump discharge connection as possible or in the suction side of the pump.
Polymers added to aid settling require special attention for adequate mixing.
Operational considerations:-presence of calibration cylinder for feeder
delivery monitoring-strainers are helpful in removing large
particulates but require periodic flushing-piping and valve arrangements should allow the
system to be isolated-allowance for tees(T) and wye(Y) cleanouts -specialized metering pumps to handle
offgassing
Figure 3. Typical Solution feed system
Figure 4. Diaphragm pump