12.5 Chemical Feed Equipment & Calibration

12.5 Chemical Feed Equipment & Calibration

Chapter 14 teaches you to calculate a dose. This section is about whether the plant actually delivers it. The gap between the two is where most chemical-related compliance problems live.

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1. Liquid Chemical Feed

Metering pump types

How output is adjusted

Essential accessories — and why each exists

The interlock rule: a chemical feeder must never operate when the process flow has stopped. Without an interlock, a stopped high-service pump plus a running fluoride or hypochlorite feeder delivers a concentrated slug into the first water that moves. This is a recurring cause of real overfeed incidents.

Drawdown calibration — the standard field procedure

1. Open the valve so the pump draws from the graduated calibration cylinder instead of the bulk tank.
2. Note the starting level and start a stopwatch.
3. Run for a measured interval — typically 5 to 15 minutes so the volume is large enough to measure precisely.
4. Record the volume drawn down.
5. Calculate:

$$\text{Feed rate (mL/min)} = \frac{\text{Volume drawn (mL)}}{\text{Time (min)}}$$ $$\text{gal/day} = \frac{\text{mL/min} \times 1{,}440}{3{,}785}$$ $$\text{lb/day of chemical} = \text{gal/day} \times 8.34 \times \text{Specific Gravity} \times \text{Purity}$$

Worked example. A hypochlorite pump draws 780 mL in 10 minutes. The solution is 12.5% available chlorine with a specific gravity of 1.17.

• Rate: 780 ÷ 10 = 78 mL/min
• gal/day: (78 × 1,440) ÷ 3,785 = 29.7 gal/day
• lb/day chlorine: 29.7 × 8.34 × 1.17 × 0.125 = 36.2 lb/day

At a plant flow of 3.0 MGD, that is a dose of:

$$\frac{36.2}{3.0 \times 8.34} = \textbf{1.45 mg/L}$$

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2. Dry Chemical Feeders

Calibration by catch and weigh: divert the discharge into a container for a measured time, weigh it, and compute lb/min or lb/day. Compare with the setpoint and adjust.

Dry feed problems

Lime slakers

Lime feed is uniquely troublesome. Quicklime (CaO) must be slaked — reacted with water to make Ca(OH)₂ — and the reaction is strongly exothermic. Slakers must control the water-to-lime ratio and temperature, and grit from the lime must be separated and removed. Lime lines scale aggressively and require routine acid cleaning.

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3. Gas Chlorinators

Vacuum-operated systems are the standard, because the entire system downstream of the cylinder valve operates under vacuum — if a line breaks, air is drawn in rather than chlorine gas escaping. Components:

• Vacuum regulator mounted directly on the cylinder valve
• Rotameter to indicate feed rate
• V-notch or metering orifice for rate control
• Ejector (injector) — a venturi that creates the vacuum and mixes chlorine into the water stream

The most common failure is a loss of vacuum, caused by insufficient water supply to the ejector, a plugged or worn ejector throat, or a leak in the vacuum tubing. Symptoms are a rotameter that drops to zero or fluctuates wildly, and loss of residual.

Cylinder cautions:

• Feed rate is limited by evaporation. A 150 lb cylinder is generally limited to about 40 lb/day, and a one-ton container to about 400 lb/day, at normal room temperature. Exceeding this freezes the cylinder — frost forms and feed rate collapses.
• Never apply heat directly to a cylinder. Warm the room, not the container.
• Always chain cylinders upright, keep valve protection hoods on when not connected, and use a new gasket at every connection.

Section 13.2 covers chlorine emergency response in full.

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4. Chemical Storage and Handling

The hypochlorite degradation trap: a 12.5% delivery stored warm for two months may test at 10% or lower. A plant feeding to a calculated dose based on the label value will silently underdose. Verify strength on receipt and periodically thereafter, and recalculate feed rates accordingly.