2.10 Chemical Feed Systems: Metering Pumps, Dry Feeders & Calibration

Chemical Feed Systems: Metering Pumps, Dry Feeders & Calibration

The ABC Water Treatment outline lists "chemical feed pumps," "chemical feed equipment," "mix batches of chemical solutions," "add chemicals to hoppers and feed equipment," and "calibrate inline instrumentation" as separate job tasks. Getting the chemistry right in a jar test is worthless if the feeder does not deliver what the operator thinks it is delivering.

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1. Liquid Chemical Feeders

Diaphragm metering pumps

The workhorse of water treatment. A reciprocating diaphragm displaces a fixed volume per stroke through ball or poppet check valves.

• Output = stroke length x stroke frequency. Both are adjustable, but turndown is much better on frequency than on stroke length. Below roughly 20 to 25 percent stroke length, accuracy collapses because the check valves do not seat consistently. When a low dose is needed, reduce speed and keep stroke length near 100 percent, or dilute the chemical.
• Priming problems are the most common failure. Off-gassing chemicals - sodium hypochlorite in warm weather is the classic - form a gas bubble under the discharge check that the diaphragm simply compresses and re-expands each stroke. The pump runs, sounds normal, and delivers nothing. Cures: degassing valve heads, flooded suction, a foot valve, and pumping from the bottom of the day tank.
• Back pressure valve is required when discharging into a low-pressure or open point, otherwise the pump siphons.
• Pulsation dampener smooths the sawtooth output when a steady rate matters (for example, ahead of an in-line static mixer).

Peristaltic (hose) pumps

A roller squeezes a flexible tube. The chemical touches only the tube, so there are no check valves to foul - ideal for slurries, polymers, and lime. Output is directly proportional to speed. The tube is a wear item with a predictable life; output declines gradually as the tube takes a set, which is why peristaltic pumps must be recalibrated on a schedule rather than only after a failure.

Progressive cavity and gear pumps

Used for viscous chemicals such as neat polymer or high-concentration ferric. Both are positive displacement, so both require a discharge pressure relief valve - a positive displacement pump against a closed valve will burst the pipe.

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2. Dry Chemical Feeders

Volumetric feeders are vulnerable to bulk density variation: a lot of lime that arrives 15 percent less dense delivers 15 percent less mass at the same setting. Gravimetric feeders are immune because they weigh.

Dry feeders discharge into a dissolver or slurry tank with a mixer, and then to the application point. Common problems:

• Bridging and rat-holing in the hopper - solved with bin vibrators, air pads, or hopper agitators.
• Lime scaling in slurry lines - lime slurry deposits calcium carbonate; lines must be flushable, and sharp elbows should be replaced with sweeps or crosses with removable plugs.
• Hygroscopic caking - soda ash and calcium hypochlorite absorb moisture. Store sealed, in a dry room.

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3. Gas Chlorination

Gas chlorine systems are built around one safety principle: the entire system downstream of the cylinder operates under vacuum.

1. The vacuum regulator mounts directly on the cylinder valve and will not open unless the ejector downstream creates a vacuum.
2. The ejector (injector) uses motive water through a venturi to create that vacuum and to dissolve the gas into solution water.
3. If a vacuum line breaks, the vacuum is lost, the regulator closes, and air is drawn in rather than chlorine released.

Consequences the exam tests:

• Loss of motive water stops chlorine feed. A plugged ejector, a fouled strainer, or low booster pump pressure shows up as loss of chlorine residual, not as a leak.
• Feed rate is limited by cylinder withdrawal, not by the chlorinator setting alone. A 150-lb cylinder at 70 degrees F will sustain roughly 40 lb/day; a one-ton container roughly 400 lb/day. Exceeding that draws liquid over or freezes the cylinder, and frost on the cylinder is the visible warning.
• Never apply heat to a cylinder. Add cylinders in parallel instead.
• Never install a gas line in a low spot where liquid chlorine can collect and reach the regulator.

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4. Day Tanks, Batching and Solution Strength

A day tank holds roughly 24 hours of chemical and is filled from bulk storage. It is worth its cost because it caps the volume that a feed failure or overfeed can release, and because the level trend across a day is an independent check that the pump is feeding.

Batching arithmetic

The controlling relationship for dilution is:

Volume1 x Strength1 = Volume2 x Strength2

To make 300 gallons of 2.0 percent hypochlorite from 12.5 percent stock:

V1 = (300 x 2.0) / 12.5 = 48 gallons of stock, diluted to 300 gallons with 252 gallons of water.

Always add chemical to water, never water to chemical, particularly for acids, where the exotherm can flash the water to steam and eject the contents.

Sodium hypochlorite decay

Sodium hypochlorite is not a stable inventory item. Its available chlorine declines with time, temperature, light exposure, higher initial strength, and trace metals (iron, copper, nickel catalyze decomposition). A 12.5 percent solution stored at 80 degrees F may lose half its strength in three to six months, and decomposition also produces chlorate. Practical controls: buy smaller, more frequent loads; store cool and dark; use diluted (10 to 12.5 percent rather than 15 percent) product; use HDPE, not metal; and verify strength on delivery rather than trusting the label.

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5. Drawdown Calibration - the Core Skill

A metering pump's dial setting is a guess. A drawdown test is a measurement.

Procedure

1. Install a graduated calibration cylinder on the pump suction, valved so the pump can draw from either the cylinder or the day tank.
2. With the pump running at the setting to be verified, switch suction to the cylinder and note the starting level and the exact time.
3. Run for a measured interval - long enough that the drop is a large fraction of the cylinder, typically 5 to 15 minutes.
4. Record the ending level. Switch suction back to the day tank.
5. Feed rate (gpd) = (Volume drawn, mL / Time, min) x (1 gal / 3,785 mL) x 1,440 min/day

Worked example. A pump draws 640 mL in 10 minutes.

• 640 / 10 = 64 mL/min
• 64 x 1,440 = 92,160 mL/day = 92,160 / 3,785 = 24.35 gallons per day

Now verify the dose. If the chemical is 12.5 percent sodium hypochlorite (specific gravity 1.17) and the plant flow is 1.8 MGD:

• Weight per gallon = 8.34 x 1.17 = 9.76 lb/gal
• Available chlorine per gallon = 9.76 x 0.125 = 1.22 lb/gal
• Chlorine fed = 24.35 gal/day x 1.22 = 29.7 lb/day
• Dose = 29.7 / (1.8 x 8.34) = 1.98 mg/L

Calibrate at more than one setting - a pump can be accurate at 60 percent and badly off at 20 percent - and recalibrate after any repair, any tube or diaphragm change, and any change in chemical supplier or strength.

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6. Feed Control Modes

Feedback-only chlorine control is unstable on variable flow because analyzer lag means the correction arrives after the flow has changed again. Compound loop control gets the bulk of the correction right instantly from flow and uses the residual only for fine trim.

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7. Safety Interlocks and Containment

• Anti-siphon device (spring-loaded injection quill) on every injection point that can be at lower pressure than the day tank. Without it, a shut-down plant siphons the entire tank into the main.
• Feed-on-flow interlock. High-hazard feeds - fluoride above all - must be wired so the metering pump cannot run unless water is flowing, using the flow meter contact or the well pump starter circuit. This is what prevents the classic fluoride overfeed in which a pump keeps running into a static main overnight.
• Secondary containment sized for the largest tank plus freeboard, with chemically compatible coating.
• Segregate incompatible chemicals. Never store or place fill connections where sodium hypochlorite and acid could be cross-connected - the reaction releases chlorine gas. The same applies to hypochlorite with ammonia (chloramine vapor). Use unique, labeled, keyed fill couplings for every bulk chemical, and supervise every delivery.
• Eyewash and safety shower within 10 seconds' travel of any chemical handling point, tested weekly.