Chemical Feed Equipment & Calibration

Learn chemical feed equipment and calibration, including metering pumps, dry feeders, suction and discharge components, calibration columns, actual output testing, calibration curves, maintenance, and troubleshooting.

Chemical feed equipment must deliver the correct amount of chemical consistently and safely. A feeder can appear to be running normally while actual output is too high, too low, or completely interrupted. Operators therefore need to understand feeder components, calibration, actual output testing, and common mechanical and hydraulic problems.

Reliable chemical feed depends on more than the feeder itself. Suction piping, discharge pressure, check valves, tubing, injection points, chemical condition, and control signals can all affect actual delivery.

Why Chemical Feed Equipment Must Be Calibrated

A feeder setting is only an indication of expected output.

Actual feed can differ because of:

equipment wear;
chemical viscosity;
backpressure;
air or gas in the pump head;
check-valve condition;
suction conditions;
chemical strength;
control-signal error.

Calibration measures actual output so operators can compare it with the expected value.

Metering Pumps

Metering pumps are commonly used for liquid chemical feed.

Many are positive-displacement pumps designed to deliver a controlled volume of liquid during each pumping cycle.

Common Metering Pump Types

Common designs include:

diaphragm pumps;
peristaltic pumps;
plunger or piston-type pumps;
other positive-displacement designs.

Diaphragm Metering Pumps

A diaphragm pump uses a flexible diaphragm to change the volume of the pump chamber.

Check valves control flow through the:

suction side;
discharge side.

Peristaltic Pumps

A peristaltic pump moves liquid by compressing flexible tubing with rollers.

Advantages can include:

simple flow path;
good isolation of chemical from pump components;
easy tubing replacement.

Tubing wear is a major maintenance consideration.

Dry Chemical Feeders

Dry chemicals may be fed using:

screw feeders;
volumetric feeders;
gravimetric feeders;
batch solution-preparation systems.

Volumetric Dry Feeders

A volumetric feeder delivers material based on volume moved by the feeder mechanism.

Actual mass output can change if bulk density changes.

Gravimetric Feeders

A gravimetric feeder measures chemical mass directly, commonly using:

load cells;
weight-loss measurement;
controlled feed mechanisms.

This can provide more direct mass control than purely volumetric feeding.

Dry Chemical Feeding Problems

Dry feeders can be affected by:

bridging;
rat-holing;
moisture;
clumping;
changing bulk density;
screw wear;
empty hopper conditions.

Bridging

Bridging occurs when dry chemical forms a stable arch over the feeder opening.

The feeder may continue operating while little or no chemical reaches the feed mechanism.

Rat-Holing

Rat-holing occurs when material flows through a narrow channel while surrounding chemical remains in place.

This can produce inconsistent feed and misleading hopper-level observations.

Metering Pump Suction Side

The suction side must deliver chemical to the pump reliably.

Important components can include:

storage or day tank;
suction tubing or piping;
isolation valve;
foot valve;
strainer;
calibration column.

Suction Problems

Common suction problems include:

empty tank;
closed valve;
plugged strainer;
collapsed tubing;
air leak;
crystallized chemical;
excessive suction lift.

Flooded Suction

When practical and compatible with system design, a flooded suction arrangement can improve metering-pump reliability because chemical pressure is available at the pump inlet.

Suction Lift

A pump operating with suction lift must pull chemical upward to the pump.

Excessive suction lift can reduce feed reliability and increase priming problems.

Priming

Priming removes air or gas from the pump head and suction line so chemical fills the pumping chamber.

A pump can run mechanically without delivering chemical if it has lost prime.

Gas Lock

Gas lock occurs when gas trapped in a metering-pump head prevents normal liquid displacement.

Possible causes include:

air entering the suction line;
chemical off-gassing;
empty tank;
poor suction conditions.

Chemical Off-Gassing

Some chemicals can release gas during storage or pumping.

Gas can accumulate in:

pump heads;
high points in tubing;
calibration columns.

Suction Check Valve

The suction check valve allows chemical to enter the pump chamber during the suction stroke while limiting reverse flow.

If it is dirty or worn, pump output may decrease.

Discharge Check Valve

The discharge check valve allows chemical to leave the pump chamber while limiting return flow during the suction stroke.

Failure can cause:

backflow;
loss of prime;
reduced output;
erratic feed.

Check-Valve Fouling

Check valves can become fouled by:

scale;
crystallized chemical;
debris;
precipitates.

Discharge Side

The discharge side may include:

discharge tubing;
backpressure valve;
pressure-relief valve;
pulsation dampener;
injection check valve;
injection quill.

Backpressure

Many metering pumps operate more consistently when discharge pressure remains within the intended range.

Insufficient backpressure can cause inaccurate output or siphoning in some system designs.

Backpressure Valve

A backpressure valve helps maintain controlled pressure on the pump discharge.

This can improve:

feed consistency;
check-valve operation;
metering accuracy.

Pressure-Relief Valve

A pressure-relief valve protects the chemical-feed system from excessive discharge pressure.

Excessive pressure can occur if:

a valve is closed;
the injection point plugs;
discharge tubing becomes blocked.

Never Deadhead a Chemical Feed Pump Unnecessarily

Operating against a blocked discharge can damage:

tubing;
fittings;
pump components;
chemical containment.

Injection Check Valve

An injection check valve helps prevent process water from flowing backward into the chemical-feed line.

Injection Quill

An injection quill introduces chemical into the process stream at a controlled location.

Proper placement can improve:

mixing;
distribution;
protection of pipe surfaces.

Plugged Injection Point

Injection points can plug because of:

scale;
chemical crystallization;
reaction with process water;
debris.

A running pump does not prove that chemical is entering the process.

Pulsation

Positive-displacement metering pumps may produce pulsating flow.

Pulsation can affect:

pressure;
flow measurement;
injection consistency.

Pulsation Dampener

A pulsation dampener can smooth discharge flow and reduce pressure fluctuations where system design requires it.

Stroke Length

Some metering pumps adjust output by changing stroke length.

Stroke length changes the volume displaced during each pumping cycle.

Stroke Frequency

Some pumps adjust output by changing stroke frequency.

More strokes per unit time generally increase feed rate.

Speed Control

Motor-driven feeders may use speed control to vary output.

Electronic controllers can also receive:

4-20 mA signals;
pulse signals;
digital commands.

Percent Setting Does Not Equal Percent Output

A pump set at 60 percent stroke or speed should not automatically be assumed to produce exactly 60 percent of maximum rated output.

Calibration should confirm actual performance.

Feeder Capacity

A feeder should normally operate within a practical portion of its range.

A feeder that is greatly oversized may be difficult to control accurately at very low settings.

Turndown

Turndown describes the usable ratio between maximum and minimum controllable output.

A wide turndown range can provide better control as process flow changes.

Calibration Column Method

A calibration column can be used to measure actual liquid chemical consumption over a known time.

A basic relationship is:

Feed Rate = Volume Used ÷ Time

Calibration Column Example

A metering pump removes 600 mL from the calibration column in 10 minutes.

Feed Rate = 600 ÷ 10

Feed Rate = 60 mL/min

Convert to Hourly Output

60 mL/min × 60 min/hr = 3,600 mL/hr

3,600 mL/hr = 3.6 L/hr

Convert to Daily Output

3.6 L/hr × 24 hr/day = 86.4 L/day

This represents measured solution volume delivered under the calibration conditions.

Calibration Procedure

A practical calibration commonly includes:

confirm chemical system is operating safely;
set the feeder at the desired operating condition;
isolate or configure the calibration column as designed;
record starting volume;
operate the feeder for a measured time;
record ending volume;
calculate actual output;
restore normal valve lineup.

Use Real Operating Backpressure

Calibration should represent actual operating conditions as closely as practical.

A pump calibrated with no discharge pressure may deliver differently when connected to the real process.

Calibration Curve

A calibration curve relates feeder setting to measured output.

For example, calibration may be performed at:

25 percent;
50 percent;
75 percent;
100 percent

of the normal adjustment range.

Why a Calibration Curve Helps

A calibration curve can reveal:

nonlinear output;
equipment wear;
poor performance at low settings;
changes after maintenance.

Rated Capacity Versus Actual Capacity

Nameplate capacity represents expected equipment performance under specified conditions.

Actual capacity can differ because of field conditions.

Calibration Frequency

Calibration frequency should reflect:

chemical importance;
equipment stability;
maintenance history;
process requirements;
facility procedures.

Recalibrate After Maintenance

Calibration should be considered after work such as:

check-valve replacement;
diaphragm replacement;
tubing replacement;
pump-head repair;
control changes.

Calibration and Chemical Strength

Feeder calibration measures solution output.

It does not automatically verify active chemical concentration.

If chemical strength changes, the same solution flow can produce a different active chemical dose.

Example of Strength Change

If a pump delivers the same gallons per day but hypochlorite strength decreases during storage, actual chlorine mass delivered decreases.

Calibration Plus Inventory

Chemical inventory can provide an independent check of feeder performance.

Compare:

calibrated feed rate;
expected daily use;
actual tank-level change.

Unexpected Inventory Difference

If expected usage is 80 gallons per day but tank level decreases by only 40 gallons, investigate:

feeder calibration;
tank-level measurement;
operating hours;
leaks;
control settings.

Dry Feeder Calibration

A dry feeder may be calibrated by collecting chemical for a measured period and weighing the collected mass.

A basic relationship is:

Dry Feed Rate = Mass Collected ÷ Time

Dry Feeder Example

A feeder delivers 5 lb in 10 minutes.

Feed Rate = 5 lb ÷ 10 min = 0.5 lb/min

Hourly rate:

0.5 × 60 = 30 lb/hr

Bulk Density Effects

For volumetric dry feeders, a change in bulk density can change mass output even if screw speed remains constant.

Solution Make-Up Systems

Dry chemicals may be mixed with water before final feed.

Reliable solution preparation requires correct:

dry chemical mass;
water volume;
mixing;
solution concentration.

Feeder Accuracy and Process Flow

A correctly calibrated feeder can still produce the wrong process dose if the process flow value is wrong.

Dose depends on both:

chemical feed rate;
process flow.

Flow-Paced Feeder

A flow-paced feeder receives a process-flow signal and changes output to maintain an approximately constant dose.

Control Signal Verification

If feed does not track flow correctly, verify:

flow meter;
signal scaling;
controller output;
feeder response.

4-20 mA Feed Control

A chemical feeder may receive a 4-20 mA command signal.

Incorrect scaling can cause:

underfeed;
overfeed;
incorrect response to process flow.

Loss of Control Signal

Operators should know what the feeder does if the control signal is lost.

Possible configured responses include:

stop;
hold last output;
move to a preset output.

The correct response depends on system design.

Local and Remote Control

Chemical feeders may operate in:

local manual mode;
remote manual mode;
automatic mode.

Unexpected feed should always prompt a check of control mode.

Standby Feeders

Critical systems often have duty and standby feeders.

Standby equipment should be:

maintained;
calibrated;
exercised;
ready for transfer.

Automatic Changeover

Some systems transfer automatically to a standby feeder when the duty unit fails.

Operators should verify that:

changeover occurred;
standby output is correct;
process response remains acceptable.

Leak Detection

Chemical-feed areas may use:

containment alarms;
leak switches;
gas detectors;
visual inspection.

Small Leaks Matter

A small leak can:

reduce delivered dose;
damage equipment;
create chemical exposure;
indicate deteriorating tubing or fittings.

Preventive Maintenance

Routine maintenance may include:

checking tubing;
cleaning strainers;
inspecting check valves;
checking diaphragms;
cleaning injection points;
verifying calibration.

Peristaltic Pump Tubing

Peristaltic tubing gradually wears as rollers repeatedly compress it.

Worn tubing can:

reduce output;
leak;
rupture.

Diaphragm Condition

A damaged diaphragm can cause:

loss of feed;
chemical leakage;
pump failure.

Crystallization

Some chemicals can crystallize when:

water evaporates;
temperature changes;
chemical remains stagnant.

Crystals can plug:

valves;
tubing;
injectors.

Flushing Chemical Lines

Where approved by facility procedure and compatible with the chemical, flushing may help prevent deposits and crystallization.

Do Not Flush Incompatible Chemicals Together

Flushing arrangements must prevent unintended mixing of incompatible chemicals.

Calibration Safety

Calibration may expose operators to concentrated chemicals.

Use appropriate:

PPE;
containment;
ventilation;
safe valve sequencing;
spill-response procedures.

Pressure Must Be Controlled

Before opening chemical piping or removing components:

isolate the system;
relieve pressure;
follow applicable lockout and chemical-isolation procedures.

Common Chemical Feed Equipment Problems

Loss of prime.
Gas lock.
Plugged suction strainer.
Dirty check valves.
Plugged injection point.
Weak or damaged tubing.
Incorrect backpressure.
Incorrect control signal.
Dry feeder bridging or rat-holing.
Feeder calibration drift.

A Running Feeder Can Still Deliver Nothing

Always distinguish between:

motor or actuator running;
actual chemical movement;
chemical reaching the process.

Troubleshooting Low Output

If measured output is too low, check:

chemical supply;
suction valve position;
suction strainer;
air or gas lock;
check valves;
diaphragm or tubing condition;
discharge blockage;
injection point.

Troubleshooting High Output

If output is too high, check:

feeder setting;
control signal;
siphoning;
incorrect calibration;
process pressure change.

Troubleshooting Erratic Output

Erratic feed can result from:

air bubbles;
gas lock;
dirty check valves;
unstable backpressure;
control-signal fluctuation;
dry chemical flow problems.

Verify Process Response

After equipment calibration or repair, confirm the expected process response.

Examples include:

chlorine residual;
pH;
alkalinity;
orthophosphate;
turbidity;
another treatment indicator.

Calibration Records

Useful calibration records include:

date;
feeder ID;
chemical;
feeder setting;
measured output;
chemical strength;
operating pressure;
person performing calibration.

Trend Calibration Results

Calibration history can reveal:

gradual capacity loss;
equipment wear;
changes after maintenance;
need for more frequent service.

Common Equipment and Calibration Mistakes

Assuming feeder setting equals actual output.
Using nameplate capacity instead of measured field output.
Calibrating under conditions very different from normal operation.
Ignoring backpressure.
Ignoring chemical strength.
Failing to recalibrate after maintenance.
Assuming a running pump proves chemical is reaching the process.
Ignoring dry feeder bridging.
Ignoring worn peristaltic tubing.
Ignoring dirty check valves.
Failing to restore correct valve lineup after calibration.
Ignoring safety and pressure isolation during maintenance.

A Practical Liquid Feeder Calibration

Confirm safe operating conditions.
Record feeder setting and process conditions.
Place the calibration column in service according to procedure.
Record starting chemical volume.
Run the feeder for an accurately measured time.
Record ending volume.
Calculate volume per unit time.
Convert to daily output if needed.
Compare measured output with expected output.
Restore normal valve lineup.
Verify downstream process response.
Document the calibration.

A Practical Dry Feeder Calibration

Confirm safe conditions and correct chemical.
Set feeder at the desired operating point.
Collect chemical for a known time.
Weigh the collected chemical.
Calculate mass feed rate.
Repeat if needed to confirm consistency.
Compare with expected feeder output.
Adjust or service the feeder if necessary.
Document the result.

A Practical Low-Feed Troubleshooting Sequence

Confirm chemical inventory.
Confirm feeder is in the correct control mode.
Check suction and discharge valve positions.
Inspect suction tubing and strainers.
Check for gas lock or lost prime.
Inspect check valves.
Inspect discharge tubing and injection point.
Calibrate actual feeder output.
Confirm chemical strength.
Verify downstream process response.

What to Remember for the Exam

Chemical feeders must be calibrated because feeder setting does not guarantee actual output.
Metering pumps commonly use positive-displacement operation.
Common liquid feeders include diaphragm and peristaltic pumps.
Dry chemical systems may use volumetric or gravimetric feeders.
Bridging and rat-holing can interrupt dry chemical feed.
Suction problems, gas lock, dirty check valves, and plugged injection points can reduce liquid chemical feed.
A running feeder does not prove chemical is reaching the process.
Backpressure can affect metering-pump performance.
A pressure-relief valve protects against excessive discharge pressure.
A calibration column measures actual liquid chemical volume delivered over a known time.
A dry feeder can be calibrated by collecting and weighing chemical over a known time.
Calibration should represent actual operating conditions as closely as practical.
A calibration curve relates feeder setting to measured output.
Feeder calibration measures solution output but does not verify active chemical concentration.
Chemical strength changes can alter active dose even when solution feed rate stays constant.
Flow-paced chemical feed depends on an accurate flow signal and correct control scaling.
Standby feeders should be maintained, calibrated, and ready for operation.
Calibration records help identify equipment wear and changing performance over time.
Maintenance on chemical-feed equipment requires chemical isolation, pressure relief, PPE, and applicable lockout procedures.
After calibration or repair, operators should confirm the expected downstream process response.