Key points
Diaphragm metering pumps serve as ideal acid-handling technology in water/wastewater applications
Water serves as an essential part of life. Nothing, be it human, animal or plant, can survive without it. That same rule applies to several industries, such as mining, food and beverage processing or steel manufacturing.
Typically, water helps facilitate and optimise the production process, from cooling equipment to cleaning totes, barrels and storage tanks.
When used in these applications, clean water quickly transforms into wastewater, which is full of impurities. Given the nature of regulatory requirements linked to wastewater handling, it must be disposed of or sustainably recycled. A wide range of acids make this possible, turning the wastewater into a safe, usable product again.
But the acids aren’t the only piece of this puzzle. Operators require the proper pumping technology that can not only process wastewater but also a variety of acids. This paper will highlight the pumping technology—specifically, diaphragm-type metering pumps—that most successfully meets the challenges inherent in most wastewater-handling applications that use a range of acids.
Wastewater Processing Pumps: The Challenge
Acids are an industrial jack-of-all-trades and are used in a variety of unique applications and processes, including the manufacture of agricultural fertilisers, cosmetics productions, fluoridation of drinking water, pH adjustment in cooling towers and boiler water pretreatment. This versatility also makes acids a critical component in water and wastewater-handling operations.
A few of the more commonly used acids in these applications and their characteristics include:
- Hydrofluosilicic Acid: Occasionally referred to as “silly” acid, hydrofluosilicic acid is often used in the fluoridation of drinking water, as a wood preservative agent and in the production of ceramics. Hydrofluosilicic acid is corrosive to most metals and tends to find leak paths.
- Hydrochloric Acid: Also known as muriatic acid. This acid is used to remove scale from boilers, in chemical cleaning and to regenerate water used in treating deionisation units.
- Phosphoric Acid: Used in metal processing, as an etching agent in semiconductor processing and as a nutrient for biological growth.
- Sulfuric Acid: Commonly used for pH adjustment in water treatment, manufacture of car batteries, as well as the pickling, cleansing and plating of metals. Because sulfuric acid is highly exothermic, it is one of the more challenging acids to pump.
- Citric Acid: A relatively weak acid, it is used as a preservative and flavoring agent in food and beverage processing, and as an ingredient in the manufacture of cosmetics, pharmaceuticals and household cleaning products.
These acids have a commonality; they must be safely contained and controlled through the pumping process to help ensure the health and safety of the workers and the integrity of the environment. Some acids, though, are more aggressive than others, and their corrosive nature and temperature can change dramatically at different concentrations or when diluted.
Sulphuric acid, for example, becomes more corrosive when diluted and has a high specific gravity relative to water, so slower pump stroke speeds are desirable when handling it.
Because of the aggressive nature of acids and the need to accurately control the dosage rate to avoid overfeeding, they need to be fed via a metering pump with a high level of repetitive accuracy, generally +/-1%. The material that the pump is constructed of also requires thoughtful consideration.
While C-20, PVDF and PVC are the most used housing materials in acid-metering pumps, operators must ensure that all wetted components are compatible with each specific acid because there isn’t a single material that works well with all acids.
Choosing the wrong construction materials can lead to, at least, pump failures and, at worst, acid leaks that can harm personnel or the environment.
Wastewater Processing Pumps: Solution
The best choices for acid injection in wastewater-handling applications are hydraulic, mechanical or electronic diaphragm metering pump technologies. Let’s look first at hydraulic metering pumps, which are ideal for acid handling because they can inject a precise and controlled amount of chemical.
This leak-free, minimal maintenance, low-energy option also features an internal relief valve, can handle higher pumping pressures, and can be constructed with an automatic stroke control actuator or variable-speed motor. This pump technology can also include a micrometer dial that can be adjusted whether the pump is running or stopped.
Hydraulic diaphragm pumps also have a flow-rate deviation of less than 1%. With a micrometer dial, an operator can calibrate it to 1% increments from 10% to 100% of flow capacity. Additionally, the pump’s stroke mechanism moves only when adjusted, reducing unnecessary wear. At the same time, the piston is powered through the complete stroke length at all capacities to help remove concerns about excessive stress, wear or shock.
Another feature that’s possible on hydraulic diaphragm pumps is a stroke adjustment option that delivers better valve performance than metering pumps with variable-linkage designs. The valve checks optimise performance because they have extra time to seat since they are idle during the by-pass portion of the suction and discharge strokes.
To address material-compatibility concerns, hydraulic diaphragm pumps can be constructed using a variety of materials, such as 316 stainless steel, C-20, PVC and PVDF. PVC & PVDF check valves come in glass or ceramic internals.
While performance varies between pump manufacturers, this pump technology is generally capable of producing up to 240 gallons per hour (908 liters per hour) of flow at 100 pounds of pressure. Operational temperatures range from 125ºF to 200ºF (52ºC to 93ºC) with maximum pressures up to 3,000 psi (206 bar).
Mechanical diaphragm metering pumps are mechanically actuated and designed to handle clear liquids with viscosities ranging from water-like to 5,000 cPs, making them ideal for use in municipal water and wastewater treatment applications.
This ability is a result of the mechanical design that eliminates the use of contour plates on the liquid side of the diaphragm, combined with a straight-through valve and head configuration that delivers improved flow characteristics.
Pump capacity can be manually adjusted using a micrometer dial while the pump is running. This allows the pump to typically produce flow rates ranging from 9 gph to 300 gph (34 lph to 1136 lph) at head pressures up to 235 psi (16 bar).
Liquid ends can be constructed of 316 stainless-steel, PVC & PVDF materials with explosion-proof, variable-frequency drives and wash-down duty motor options. The pumps are self-priming and provide superior performance when pumping chemicals containing small impurities, such as sulphuric acid.
When choosing between a hydraulically or mechanically actuated diaphragm metering pump, the operator must consider the specific needs of his operation. The hydraulic option uses a PTFE disc that flexes between two perforated plates.
Because the diaphragm is hydraulically balanced, the pump can operate against higher discharge pressures than a mechanical diaphragm pump. Still, the tradeoff is that the pump doesn’t have the same suction-lift capabilities as a mechanical diaphragm pump. On the other hand, hydraulic diaphragm pumps offer built-in relief protection, which is especially desirable in high-pressure applications.
Electronic metering pumps are a viable option in acid-handling operations because they can offer stroke length and speed adjustments that make them particularly suited for low-flow acid-feed applications, which can require the pump to possess a wide turndown capability.
These pumps can feature a metering design that operates on any single-phase voltage from 94 VAC to 264 VAC, which is a crucial consideration if the pump operates on solar power. Particular models have manual stroke-length adjustment ranges from 20% to 100%, with analog, pulse & timer control, built into the unit. These pumps are available with PVDF with PTFE seals suitable for handling most acids.
In addition to the pumps, when feeding acids, ancillary piping and accessory components must be carefully considered and selected, including:
- Skids: When practical, acid-feed skids should be constructed of corrosion-resistant structural FRP or polypropylene. If metallic skids are required, they should include an appropriate paint coating that will resist the specific acid to be handled. Integral or secondary containment is also usually desirable.
- Strainer: Diaphragm metering pump check valves operate best when the acid being pumped is clean and free of particulates, so the suction piping system should always include an appropriately sized strainer. When pumping concentrated sulfuric acid, a sludge trap is desirable for removing sludge particles, which will help minimise the possibility of pump failure.
- Calibration Columns: These provide an inexpensive yet reliable and accurate means of verifying the pump’s flow rate. A calibration column of clear PVC is suitable for many acids, although borosilicate glass columns are available and required for use with some acids. (A notable exception is hydrofluosilicic acid, which will etch glass.)
- Injection Quill: An injection quill or corporation stop injects the acid into the center of the pipeline, tank or process for more rapid and uniform chemical dispersion. This helps to protect the pipe or tank walls from the corrosive effects of the acid.
Wastewater Processing Pumps: Conclusion
As mentioned, water is critical to all forms of life and the successful operation of a wide range of industries. In conjunction with its use, the safe, clean and responsible treatment, conservation and disposal of water is becoming an increasingly critical issue across the country. Acids, in many forms, play a correspondingly critical role in treating water and wastewater.
Like water itself, proper care and handling of acids must also be observed. While operators of wastewater-treatment facilities are aware of the responsibilities they have in helping ensure safe water sources, they also know that they must take every measure to optimise the cost-effectiveness of their operations.
That’s where identifying and utilising the most efficient and reliable pumping technology enters the picture. For operators, they have three primary options to select from – hydraulic, mechanical or electronic diaphragm metering pumps. Each of these technologies has their advantages, allowing operators to make the best choice for their water and wastewater processing needs.
