Water treatment and distribution operations are rigorous by nature, involving the movement of water from one place to another, the addition of pressure for filtration, the injection of chemicals for water treatment and the distribution of clean water to its point of use. Accuracy and reliability are particularly important when employing controlled-volume metering pumps as part of chemical and additive injection systems within water treatment processes. Electromagnetic flow meters can be an effective solution to validate the correct operation of this equipment to ensure the utmost efficiency of chemical dosing procedures.
Requirements for Chemical Injection
Specialized feed systems are used to supply chemicals for various stages of water and wastewater operations. The water treatment process requires optimal synthesis, so the addition of chemicals may be necessary to establish a favorable biological growth environment. There is also a need to achieve sufficient alkalinity to maintain a desired pH operating range.
As part of chemical injection, it is often 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. When injecting expensive chemicals into the water treatment process, plant operators must ensure the right amounts are added to the process as part of quality control. The use of too much or too little chemical can result in high operating costs, increased corrosion rates, frequent equipment maintenance and other adverse outcomes.
Role of Metering/Dosing Pumps
Each chemical feed system is different depending on the type of chemical to be pumped, its concentration and the necessary feed rate. Metering pumps may be used as part of a process to inject chemicals into water treatment systems. This is typically found with well water operations. Small feed rates will require metering-type pumps that can provide a specific dose of the chemical to the receiving stream.
In many cases, the metering pump used in a water treatment plant is a positive displacement chemical dosing device with the ability to vary capacity manually or automatically as process conditions require. This type of pump offers a high level of repetitive accuracy and is capable of pumping a wide range of chemicals, including acids, bases and corrosives or viscous liquids and slurries.
Importance of Pump Validation
Water treatment plants are always looking for ways to optimize their operations by minimizing maintenance, downtime, failures and other issues. Each factor can impact efficiency and productivity. But when combined, they can severely affect a plant’s capacity and bottom line.
The only way to know that the right amount of a given chemical is being injected into the water treatment process is to ascertain the actual dosage rate that the metering pump is maintaining. The challenge is that many pumps used for chemical injection do not have an absolute setting that allows a user to dial in a specific dosage rate.
Experience has shown that pump performance validation with flow meters can provide valuable information regarding how accurate the pump is performing versus manufacturer specifications. It can also identify operating issues and efficiency degradation due to worn parts or other conditions. By adding a flow meter and a valve between the pump and the process, users can obtain information to evaluate actual equipment performance, highlight any discrepancies and, if needed, adjust the speed of the pump.
Choosing the Right Flow Meter
Many types of flow meters measure liquid, and some are better suited to water and wastewater treatment environments than others. Some meters are more accurate and repeatable than others. Some require less frequent or more complex maintenance and some last longer than others. It is important to consider all the selection criteria rather than focusing on one aspect alone, such as price. Low purchase price can often be a misleading indicator considering required performance and maintenance activities. A better criterion would be total cost of ownership (TCO), which takes into account not only purchase price but also the cost of installation, maintenance and meter replacement.
Electromagnetic flow meters can be an ideal choice for demanding water treatment applications when considering cost, accuracy and longevity. The electromagnetic measurement technique eliminates the need for moving parts, which can lead to performance and maintenance issues when used in fluids with high solids content. Electromagnetic flow meters measure virtually any conductive fluid, including process water and wastewater. The meters provide low pressure drop, extended turndown and excellent repeatability. They are known for delivering a high accuracy rate at a reasonable cost.
Electromagnetic flow meters operate under Faraday’s Law of Electromagnetic Induction to measure liquid velocity. The law states that a conductor moving through a magnetic field produces an electrical signal within the conductor, which is directly proportional to the velocity of the water moving through the field.
Depending on the fluid media and/or water quality, the standard stainless steel (AISI 316) electrodes used in many electromagnetic flow meters may be sufficient. However, these electrodes are subject to pitting and cracking in corrosive environments, which may cause flow meter accuracy to shift over time. Some meter manufacturers have switched to Hastelloy C electrodes as a standard material to provide improved corrosion resistance and extended service life. This superalloy has high resistance toward localized corrosion, which is an advantage in chloride-containing environments at high temperatures. It has a high level of all-round corrosion resistance attributable to the content of chromium and molybdenum. Chromium increases the resistance to oxidizing conditions and molybdenum increases the resistance to reducing environments.
The use of Teflon liners rather than hard rubber liners by some manufacturers provides a highly temperature-resistant material with strong chemical properties.
Benefits to Plant Operators
Electromagnetic flow meters have proven to be well suited for critical chemical injection applications in water treatment facilities. They enable plant operators to accurately measure the volume of fluid that passes through them. The meters can serve as part of a closed-loop system, sending outputs to a programmable logic controller (PLC) where they are used to determine the amount of chemical dosage over any period of time. This information is helpful in managing chemical costs as well as addressing applicable environmental regulations. They also offer important life cycle advantages to water treatment and distribution facilities. They are designed to achieve +0.25% accuracy in less than ideal fluid flow conditions. At the same time, the nonintrusive, open flow tube configuration virtually eliminates pressure loss. The meters are relatively unaffected by viscosity, temperature and pressure when correctly specified, and with no moving parts to impede the flow stream, service and repair are kept to a minimum.
In a demanding water treatment plant environment, even an optimally sized metering pump may be subjected to different operating conditions than expected. Process adjustments over time can shift the fluid densities, flows, pressures, temperatures and viscosities the pump has to handle.
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