How to size a chemical metering pump
To size a chemical metering pump, calculate the chemical feed rate from your process flow, target dosage, chemical density and concentration. Then confirm the pump covers your minimum and maximum feed with room to spare, handles the system pressure, uses compatible materials and has the right pulsation dampener and back pressure valve.
What formula calculates chemical feed rate?
The standard formula converts dosage in mg/L and process flow in million gallons per day into gallons per day of chemical solution. The constant 8.34 is the weight of one gallon of water in pounds, which converts mg/L to pounds per million gallons. Dividing by pounds of active chemical per gallon gives solution volume.
Gallons per day = (Flow in MGD x Dosage in mg/L x 8.34) / (Chemical density in lb/gal x Concentration)
- Flow: process flow in million gallons per day (MGD).
- Dosage: target dose in mg/L, which equals ppm in water.
- 8.34: pounds per gallon of water.
- Chemical density: weight of the chemical solution in pounds per gallon.
- Concentration: active chemical as a decimal (50 percent is 0.50).
The top of the formula gives pounds of active chemical needed per day. The bottom gives pounds of active chemical in each gallon of solution. Divide to get gallons per day, then divide by 24 for gallons per hour.
What does a worked example look like?
Here is a pH adjustment example using 50 percent sodium hydroxide (caustic soda). The plant treats 2.0 MGD at a 10 mg/L dose. Fifty percent caustic weighs about 12.76 lb/gal, so each gallon holds about 6.38 pounds of active sodium hydroxide. The required feed is about 26 gallons per day.
Step by step:
- Pounds of chemical per day: 2.0 x 10 x 8.34 = 166.8 lb/day.
- Pounds of active chemical per gallon: 12.76 x 0.50 = 6.38 lb/gal.
- Solution feed: 166.8 / 6.38 = 26.1 gallons per day.
- Hourly feed: 26.1 / 24 = 1.09 gallons per hour.
Now check the range. If flow can drop to 0.5 MGD at a 5 mg/L dose, the minimum feed is 0.5 x 5 x 8.34 / 6.38 = 3.27 gallons per day, or about 0.14 gallons per hour. If flow can rise to 3.0 MGD at a 15 mg/L dose, the maximum feed is 3.0 x 15 x 8.34 / 6.38 = 58.8 gallons per day, or about 2.45 gallons per hour.
A common practice is to size the pump so the maximum feed falls near 80 percent of rated capacity. That points to a pump rated around 3 gallons per hour, which leaves margin for pump wear, a stronger dose during upsets or a weaker chemical delivery.
Always confirm density and concentration from the supplier's safety data sheet. Values vary with temperature and product grade.
If your flow is measured in gallons per minute instead of MGD, convert first: multiply gallons per minute by 1,440 to get gallons per day, then divide by 1,000,000. For example, 1,389 gallons per minute is about 2.0 MGD.
How much turndown do you need?
Turndown is the ratio between the highest and lowest feed rate a pump can deliver accurately. Find yours by dividing your maximum required feed by your minimum. In the example above, 2.45 divided by 0.14 is roughly 18 to 1, but relative to a 3 GPH pump the minimum sits near 4.5 percent of capacity.
That means the pump needs better than about 22 to 1 turndown to stay accurate across the whole range. Things to keep in mind:
- Accuracy usually drops near the bottom of a pump's range, so leave margin.
- Pumps with both stroke length and stroke speed control offer wider turndown.
- If your range is very wide, two pumps of different sizes may work better than one.
Pulsafeeder's PULSAPro hydraulic diaphragm metering pump offers 100 to 1 turndown, which covers wide ranges like this one with room to spare.
What are the most common sizing mistakes?
The most common sizing mistakes are oversizing the pump, ignoring the minimum feed rate and forgetting that chemical strength changes. An oversized pump spends its life near the bottom of its range, where accuracy suffers. A pump sized only for average conditions can fall short during peak flow or a process upset.
- Sizing for average only: always calculate minimum, normal and maximum feed.
- Using the wrong concentration: a 12.5 percent hypochlorite delivery that weakens in storage needs more volume over time.
- Mixing up units: keep flow in MGD, dosage in mg/L and density in lb/gal throughout the formula.
- Forgetting dilution: if you dilute the chemical in a day tank, use the diluted concentration and density.
- Skipping the control signal: decide early whether the pump will follow a flow meter, an analyzer or a manual setting, since that affects the drive and controls you need.
How does system pressure affect sizing?
The metering pump must deliver its rated flow against the highest pressure at the injection point, plus losses in the discharge piping and accessories. Size against the maximum discharge pressure, not the normal one. Then confirm the suction side provides enough pressure to fill the pump head on every stroke.
Points to check:
- Injection pressure: the process line pressure where the chemical enters.
- Piping losses: include tubing size, length, fittings and the injection quill.
- Acceleration head: reciprocating pumps create flow pulses that add pressure loss, especially in long or small diameter lines.
- Suction conditions: flooded suction is preferred, particularly for chemicals that off gas.
Our metering pump installation guide covers suction and discharge piping in more detail.
Which materials are compatible with your chemical?
Choose wetted materials based on the chemical, its concentration and its temperature. Wetted parts include the liquid end, diaphragm, check valve balls and seats, gaskets, tubing and fittings. A material that works with a dilute solution may fail with a concentrated one, so always check the specific conditions.
Common choices include:
- PVC and CPVC: widely used for many water treatment chemicals at moderate temperatures.
- PVDF: strong resistance to many acids, oxidizers and solvents.
- PTFE: common for diaphragms and seals due to broad chemical resistance.
- 316 stainless steel and alloys: used for higher pressure, higher temperature and some solvent services.
Some chemicals need extra care. Sodium hypochlorite releases gas that can vapor lock a pump, so degassing liquid ends are often used. Fifty percent caustic can crystallize in cold weather, so tanks and lines may need heat tracing. Ask the manufacturer for a compatibility check on any critical service.
Do you need a pulsation dampener and back pressure valve?
Most metering pump installations benefit from both. A pulsation dampener smooths the pulsing flow from a reciprocating pump, lowering pressure spikes and acceleration head. A back pressure valve holds a steady pressure on the pump discharge so the check valves seat properly and the pump delivers a consistent, accurate dose.
A back pressure valve is especially important when:
- The injection point is at low pressure or below the chemical tank, which can cause siphoning.
- The discharge line is short, which can cause overfeeding.
A pressure relief valve protects the pump and piping if a discharge valve is closed or a line plugs. Griffco pulsation dampeners and relief and back pressure valves are built for these duties. See our pulsation dampeners for options.
When should you specify an API 675 pump?
Specify an API 675 metering pump for critical service in refineries, petrochemical plants, gas processing and other demanding applications. API 675 is the American Petroleum Institute standard for positive displacement controlled volume pumps. It sets requirements for accuracy, linearity, repeatability and construction that support reliable, long running dosing.
The Pulsafeeder PULSAPro is API 675 compliant and offers plus or minus 0.5 percent steady state accuracy, 100 to 1 turndown and a 5 year drive train warranty. For general water treatment, a well chosen solenoid or motor driven pump such as a Pulsatron is often a good fit.
Browse our full range of metering pumps, or let us build a complete chemical feed skid with tank, pump, dampener, valves and instrumentation.
Sources
Want a second set of eyes on your numbers? Send us your flow, dosage and chemical and we will size the pump and accessories for you. Request a quote.
Frequently asked questions
What does 8.34 mean in the chemical feed formula?
It is the weight of one gallon of water in pounds. Multiplying flow in MGD by dosage in mg/L and 8.34 gives pounds of chemical needed per day.
How much bigger than my maximum feed should the pump be?
A common practice is to have the maximum required feed fall near 80 percent of the pump's rated capacity. That leaves margin without pushing the minimum feed too far down the pump's range.
Is ppm the same as mg/L?
For water and dilute water solutions, 1 mg/L is effectively equal to 1 ppm by weight, so the two are used interchangeably in feed calculations.
Why does my metering pump overfeed?
Overfeeding often happens when the discharge pressure is too low or the injection point is below the tank, which lets fluid siphon through the check valves. A back pressure valve usually corrects it.
What is API 675?
API 675 is the American Petroleum Institute standard for positive displacement controlled volume pumps. It is commonly specified for critical metering service in refining, petrochemical and gas processing plants.
