Q. When is a metering pump used in a water or wastewater treatment plant?
A. Specialized feed systems supply chemicals for various stages in water or wastewater treatment. The process requires optimal synthesis of new biological growth, so the addition of chemicals may be required to maintain a favorable growth environment. It is also important to maintain the pH of the wastewater in the process and sufficient alkalinity to provide for this operating pH range.
As a result, it may be necessary to add either acid or caustic for pH control, ferric chloride or alum for nutrient removal, or supplemental carbon sources such as methanol, glycine or acetic acid for process development.
Each chemical feed system is different depending on the type of chemical to be pumped, its concentration and the necessary feed rate. Small feed rates will require metering-type pumps that can provide a specific dose of the chemical to the receiving stream (see Figure 4.14).
Metering pump flow rate can be manually adjusted by a micrometer dial, varying the speed of the drive or a combination of the two. Flow rate can also be controlled automatically in response to a process signal.
Users should be careful when selecting a metering pump for this stage of the wastewater treatment process because many disinfectants, such as chlorine, are degassing chemicals. Air bubbles produced by a degassing chemical can travel through the suction line into the pump's head. Once the bubbles are trapped inside the head, the pump may lose prime or gas lock because of the bubbles.
Metering pumps with auto-degassing capabilities are preferred to prevent gas-lock situations. Gas-lock could prevent accurate injection of the disinfectant parameter. Corrosion inhibitors may also be used to prevent damage to the piping and equipment.
Except with chemicals such as quick lime, abrasion is minimal because there is no grit in most of the chemicals. However, feed concentration rates and chemicals must be reviewed to ensure the pump and piping system materials do not corrode when the full or diluted concentration of the chemical is applied.
Selecting the appropriate metering pump for a wastewater application usually requires an examination of the required chemical injection rate, which is normally determined based on wastewater analysis.
Once the injection rate is known, the next step is to determine the maximum and minimum rate of flow (cubic meters per second or gallons per hour), pressure rating (kilopascals or pounds per square inch), desired control options (manual, timer, pulse, analog or on-off), materials of construction (stainless steel, PVC, PVDF, PP, FKM, EPDM, ceramic, etc.), and type of connection (half-inch NPT, quarter-inch tubing, etc.).
For additional information on controlled-volume metering pumps in wastewater treatment, refer to Hydraulic Institute Wastewater Treatment Plant Pumps: Guidelines for Selection, Application and Operation.
Q. What procedures should be followed to ensure proper testing of controlled-volume metering pumps?
A. Uniform procedures for the setup and testing of controlled-volume metering pumps and for recording the test result data are available in ANSI/HI 7.6 Controlled-Volume Metering Pumps for Test.
The test procedures cover:
- functional testing of production units
- mechanical integrity test at rated speed and specified pressure
- rate of flow and mechanical integrity at rated speed and specified pressure
- verification of performance to the manufacturer's specifications
Optional testing for consideration (based on specific application or criticality of service) includes the following:
- steady-state accuracy
- net positive suction head or net positive inlet pressure
The items the pump test may use include, but are not limited to, the following:
- Factory or purchaser furnished driver
- Drive motor of proper voltage or appropriate design to operate metering pump
- Safety relief valve to protect the pump from overpressure
- Closed tank or open sump, properly sized for the pump being tested
- A discharge pressure gauge suitable for measuring not more than two times the complete range of pressures being tested, located as close as possible downstream of or near the pulsation dampener
- If required, dampening devices, such as pulsation dampeners, needle valves or capillary tubes, may be used to dampen out the pressure pulsations at the discharge pressure gauge.
- For variable speed applications, a means for measuring input speed to the pump should be provided and should be suitable for measuring the complete range of speed over the turndown range.
- A means for measuring pump rate of flow in liters/hour (gallons/hour) or other agreed-upon units
Test setups that do not conform with respect to intake structure, piping and measuring equipment may not duplicate test facility results.
For additional information on controlled-volume metering pump testing, refer to ANSI/HI 7.6 Controlled-Volume Metering Pumps for Test.