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