How Can We Calculate Chemical Consumption or Gram Per Meter in Continuous Scour Bleach
Lesson 27:
Chemical Feed Pump Calculations
Objective
In this lesson we will learn the following calculations:
- Chemical feed rate, lb/day
- Chemical feed pump setting, % stroke
- Chemical feed pump setting, mL/min
- Cycle time
Lecture
Chemical Feed Pumps
A chemical feed pump, also known as a dosing pump, is a compact positive displacement pump that injects a precise amount of chemical or other substance into water, gas or steam. It is typically part of a chemical feed system that comprises a pump or multiple pumps, storage tank, pipes and control panel. There are different methods and types of pumps that can be used, but the general procedure is to draw a preset amount of the chemical into a holding container and then inject it into a pipe or tank that holds the water or other material to be dosed. Chemical dosage is dependent on the desired purpose. For example, the addition of chlorine serves many purposes in wastewater treatment, as illusted in the table below.
Once the chemical dosage has been determined, the feed rate can be calculated, which is the measure of how much chemical is added to the treatment process per unit of time. Feed rate and dosage are two different things. Typical expressions of feed rate include gallons per minute (gpm), gallons per hour (gph), milliliters per minute (mL/min), pounds per day (lb/day) or kilograms per day (kg/day). We have already covered how to determine different dosages, depending on what you are testing for. Now let's show you how to calculate feed rate, pump setting and cycle time for the pumps.
Process Control Calculations
Feed Rate
Once the dosage is determined, you need to determine the feed rate the pump needs to be set on in order to achieve that. That can be determined with the following formula:
or
Example:
How many pounds per day of 55% calcium hypochlorite are requried to disinfect activated sludge effluent flowing at a rate of 0.5 MGD, with a desired chlorine dose of 6.5 mg/L?
Simply plug the numbers into the formula for feed rate:
If you need the answer in kg/day, you will need to convert the flow of 0.5 MGD to cubic meters per day. In looking at the common conversions you will see that 1 cubic meter of water (m3) is equivalent to 1000 kg, 1000 L and 264 gal. We know the flow in MGD, so let's go with the unit conversion that is closest to that unit, which is gallons.
Now use the formula provided to determine the feed rate in kg/day:
Feed Pump Setting, % Stroke
Chemical feed pumps are generally positive displacement pumps (also called piston pumps). This type of pump displaces, or pushes out, a volume of chemical equal to the volume of the piston. The length of the piston, called the stroke, can be lengthened or shortened to increase or decrease the amount of chemical delivered by the pump. As mentioned, each stroke of a piston pump displaces or pushes chemical out. The percent stroke of a pump can be calculated using the following formula:
Example:
The required chemical pumping rate has been calculated as 6 gpm. If the maximum pumping rate is 85 gpm, what should the percent stroke setting be?
If you are given the feed rate in lb/day and need to determine the rate in gpm use the following conversions:
If the example above, we determined we needed a feed rate of 49.28 lb/day for a system. What if we needed to know the feed pump setting in regards to percent stroke? We would do the necessary conversions:
You would do the same if the maximum pumping rate is in lb/day and then plug those values into the equation above.
Let's look at one more.
Example:
A diaphragm pump used to meter sodium hypochlorite has a maximum output of 10 liters per second (L/s). What % stroke should be selected to deliver 2.3 L/s?
Even though the measurements are given in L/s, you simply plug the values into the formula, only because the units are identical. If the two flows were different you would have to do some preprocessing first. The unit L/s will cancel in the numerator and denominator and by multiplying by 100, it gives you the percent:
Feed Pump Setting, mL/min
Some chemical solution feeders dispense chemical as milliliters per minute (mL/min). To calculate the mL/min solution required, use the following equation:
or
Example:
What is the chemical feed rate, in mL/min, needed to achieve a chlorine residual of 1.5 mg/L if the flow is 1.5 MGD and the density of the chemical used, sodium hypochlorite is 1.6 g/cm3?
According to the formula, you need the density in mg/mL so you will need to convert g/cm3 to mg/mL. If you were given the percent chemical, you could skip this step and use the second formula given above:
The only conversion factor on the formula sheet that may help is the fact that 1 cubic meter (m3) is equivalent to 1000 kg and 1000 L.
You should remember that 1 meter (m) is the same as 100 centimeters (cm), so that means that 1 cubic meter (m3) is the same as 100 cm x 100 cm x 100 cm, which is 1,000,000 cm3. Use this to convert from g/cm3 to mg/mL (there are multiple steps):
Now determine the feed pump setting in mL/min now that we have the density in mg/mL:
Example:
A treatment plant is feeding caustic soda at a dosage of 35 mg/L. The plant flow is 2.25 MGD. The caustic soda used is a 50% solution and has a density of 12.8 lb/gal. What is the feed rate in mL/min?
For this one you are given the chemical density as well as the chemical purity, so you will use the second formula given for feed rate:
We still need to do some preprocessing before we can enter all the values into the formula. For instance, the problem gives the flow in MGD, but we need it in cubic meters per day, so let's do that conversion first:
The chemical density is given in lb/gal, but we need it in grams per cubic centimeters (g/cm3), so let's do that conversion now:
*You will need to remember that 1 kg = 1000 g and that 1 m3= 1,000,000 cm3, which we explained in the previous example. You will keep doing conversions until all you are left with, unit wise, is g/cm3.
Now that we have all the values in the form represented in the formula, we can plug in the values and solve for the feed pump setting:
Cycle Time
Establishment of appropriate pumping cycle times is important to protect the life of electrical motors and to prevent the development of septic conditions in wet wells, clarifiers and sumps. The cycle time can be determine by using one of the following formulas:
or
Example:
Calculate the cycle time for a wet well that holds 2500 gallons if the inflow to the wet well is 375 gpm and the lift pump has a capacity of 450 gpm.
Since everything is given in gallons, use the first formula provided and simply plug in the values:
Example:
Calculate the cycle time for a wet well that is 4 m in diameter and 4 m deep if the inflow to the wet well is 0.65 m3/min and the lift pump has a capacity of 35 L/s.
The measurements are given in meters and liters, so we use the second formula. We will need to do some preprocessing for this example:
Fist determine the volume of the wet well:
Volume, m3 = πr2h
Volume, m3 = π(2 m)2(4 m)
Volume, m3 = 50.24 m3
Now convert the pump capacity from L/s to m3/min:
Since the inflow is already in the proper units, simply plug in the values:
Summary
A chemical feed pump, also known as a dosing pump, is a compact positive displacement pump that injects a precise amount of chemical or other substance into water, gas or steam. It is typically part of a chemical feed system that comprises a pump or multiple pumps, storage tank, pipes and control panel. There are different methods and types of pumps that can be used, but the general procedure is to draw a preset amount of the chemical into a holding container and then inject it into a pipe or tank that holds the water or other material to be dosed. Chemical dosage is dependent on the desired purpose. Once the chemical dosage has been determined, the feed rate can be calculated, which is the measure of how much chemical is added to the treatment process per unit of time. Feed rate and dosage are two different things. Typical expressions of feed rate include gallons per minute (gpm), gallons per hour (gph), milliliters per minute (mL/min), pounds per day (lb/day) or kilograms per day (kg/day). We have already covered how to determine different dosages, depending on what you are testing for. Chemical feed pumps are generally positive displacement pumps (also called piston pumps). This type of pump displaces, or pushes out, a volume of chemical equal to the volume of the piston. The length of the piston, called the stroke, can be lengthened or shortened to increase or decrease the amount of chemical delivered by the pump. As mentioned, each stroke of a piston pump displaces or pushes chemical out. Establishment of appropriate pumping cycle times is important to protect the life of electrical motors and to prevent the development of septic conditions in wet wells, clarifiers and sumps.
Assignment
Complete the math worksheet for this lesson and return to instructor via email, fax or mail.
Source: https://water.mecc.edu/courses/ENV148/lesson27b.htm
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