By Mike Dowse
Chemical metering pumps and chemical feed systems must function under a variety of harsh conditions. Among the more demanding environments is the municipal potable water and wastewater treatment plant.
Municipal water treatment plant applications require long-term, trouble-free operation with pumps and other equipment operating essentially non-stop from the moment they are installed. The large variety of chemicals used in municipal water processing presents a number of application requirements, which must be addressed in achieving this goal. These chemicals include: sodium hypochlorite, sulfuric acid, polymers, ferric chloride, sodium bisulfite, lime, alum, potassium permanganate and ammonia.
In addition to corrosion resistance, the equipment selected must be protected from "plugging" - which occurs as a result of slurries or the viscosities of some chemicals used in municipal water treatment. Additionally, systems often have accessory systems to measure flow, create back pressure, smooth flow and provide relief. The equipment in each of these systems must function at peak efficiency while meeting the same selection criteria.
The high degree of automation in municipal water and wastewater treatment plants requires attention to the control of chemical feed rates in response to changes in flow or other water quality variables. The task of properly selecting and installing chemical metering pumps/accessories therefore should be preceded by a thorough investigation of all operating environment variables. The municipal professional should rely on the expertise of a reputable chemical metering pump supplier who understands the wastewater treatment business.
By preparing a list of pointed questions and concerns, the utility professional's job becomes easier and the resulting chemical metering pump and chemical feed system is far more likely to meet performance goals. The following information will help the water worker define project variables and ask the right questions during pump/accessory selection and installation.
Sizing & Selecting Metering Pumps
The first step is identifying the required capacity in terms of flow rate and discharge pressure. Avoid over-sizing a metering pump. Size the pump so the maximum expected flow rate is 85 percent to 90 percent of the pump's total capacity. This ensures additional capacity if needed. The minimum capacity should never be less than 10 percent of the pump capacity to maintain accuracy.
Next, ask questions about the construction of the pump. Metering pumps are available in a variety of materials. Among the more important selection criteria is resistance to corrosion, erosion and solvent action. Solvent-based chemicals may dissolve plastic headed pumps. Acids and caustics may require stainless steel or alloy liquid ends. Abrasive slurries can erode equipment unless the right pump construction materials are used.
Some chemicals in the water treatment process are viscous or form a slurry. Others release gas during the treatment process. The equipment supplier should recommend special "liquid ends" to accommodate these applications. Whereas standard metering pumps handle clear liquids with viscosities ranging from water-like to 1500 cps, using liquid ends increases viscosity capabilities to 5,000 cps and light suspensions. For true slurries or higher viscosity, tubular diaphragm heads can be employed which permit pumping chemicals to 20,000 cps or slurries containing up to 10 percent solids. Liquid ends also will vent accumulating gases automatically.
Protection against leaks is another area where the municipal worker should seek expert advice. Double diaphragm heads featuring leak detection and alarms are available. Where leaks must be detected immediately, these devices are invaluable. Example applications include those where contact between the process fluid and the pump hydraulic fluid cannot be tolerated or where, due to the toxic or hazardous nature of the fluid being pumped, leakage cannot be tolerated.
Ask questions about pump driver selection. This selection must be based on matching available utilities, which may include electric, air, gas or other means of driving the pump. Identify hazardous area requirements when selecting the driver. Certain types of dust can ignite and so can many fumes and vapors.
Tell the chemical metering pump provider if the pump is indoors or outdoors. The motor should be sheltered from direct sunlight. Pumps will operate in freezing temperature provided the fluid to be pumped will not freeze and the correct lubricants are selected. Freeze protection and heat tracing may be required. Corrosive environments may require a special coating on the equipment.
What method of control will be used? Manual continuous operation? Simple on/off operation? Control by process signal?
Metering pump flow rates can be manually adjusted by a micrometer dial. This manual control allows pumps to be operated between 10 percent and 100 percent of nameplate capacity by changing the stroke length. A manual variable speed drive changes the stroke speed. A combination of the two may allow additional adjustability or turndown over the range of the drive, depending upon the stroking speed of the pump. For example, a pump operating at 75 spm (which could be turned down to 15 spm) would allow a 5:1 turndown on speed using the variable speed drive and a 10:1 turndown on stroke length using the micrometer dial.
Metering pump flow rate also can be controlled automatically (in response to a process signal) by electric or pneumatic positioners which change the pump stroke length, or by variable speed drives which change the stroking speed. Using a positioner provides a full 10:1 turndown, the complete adjustable range. Using a variable speed drive will provide only as much turndown as the ratio of the pump stroking speed divided by the minimum operating speed of the pump.
It is not practical to use a variable speed drive on motor driven pumps that normally operate at less than 100-150 spm or there would not be a wide adjustable range. Slowing the motor causes each stroke to take longer from start to finish and, as a practical matter, motor driven pumps should not be operated at less than 15 spm. Electronic diaphragm pumps, which are pulsed by a solenoid, can operate at less than a single stroke per minute because the characteristic and timing of each stroke, from start to finish, is the same at all stroking speeds. The moving parts in modern diaphragm pumps offer long, reliable service at all stroking speeds. The highest stroking speeds should be avoided with viscous or abrasive chemicals.
When a metering pump is controlled by automatic, electric or pneumatic stroke positioners, the number of doses remains constant and the size of each dose is reduced, thus keeping the doses uniformly distributed in a constantly flowing line. Use of a variable speed drive changes the stroke speed. The size of dose injected on each stroke remains the same, but doses are less frequent. This can produce an undesirable process result in a constantly flowing line as the discreet slugs of chemical are more widely separated than if a constant dose interval were maintained. Choice of control can be an important process consideration.
Becoming familiar with the chemical metering pump sizing and selection concerns examined here will ensure that discussions with metering pump suppliers focus on the pertinent issues involved in municipal water treatment applications. Once the metering pump is selected, the municipal water and wastewater treatment worker needs to focus on installation. Again, the nature of the operating environment dictates special requirements which a reliable chemical metering pump supplier will be able to accommodate.
Be sure to plan the entire installation -from the day tank, or other liquid source, up to the injection point - and determine which accessories will be included.
Metering pumps will "push" against great pressures but they will not "pull" for very great distances. For this reason, installations designed for flooded suction are always preferred and are easier to prime than suction lift, top-mounted designs. And remember, flooded suction must be used for fluids where the vapor pressure could be less than the suction lift.
Chemical metering pump installation includes a myriad of accessory parts and components. This fact underscores the importance of relying heavily on the proven expertise of a reputable chemical metering pump supplier-who has worked in the municipal water treatment field - to help ensure a successful installation.
A Few Final Points
When replacing equipment or changing chemical programs, it is best to ask a few questions. Will the new program operate at the same feed rates as the previous program? Is the equipment properly sized for the new products? How well has the equipment been operating? Any problems with reliability, accuracy, unusually high maintenance requirements?
There is no better start to a new chemical feed program than to ensure that chemical is delivered accurately with trouble-free equipment. Municipal potable water and wastewater treatment professionals faced with chemical metering pump and chemical feed installations can best ensure a safe, effective and cost-efficient system by partnering with a chemical metering pump supplier with proven wastewater treatment expertise.
Authors Note: Mike Dowse is Vice President of Neptune Chemical Pump Company, Lansdale, Pa. The company manufactures metering pumps, mixers, tanks, panels, chemical feed systems and water treatment accessories, along with custom chemical feed systems.