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Environmental regulations require pretreatment of waste water prior to discharge into municipal sewage systems. This is because raw industrial waste streams, especially those with low pH values, can damage sewage system piping and destroy the micro-organisms used in the sewage treatment plant itself.
Many variables contribute to the level of corrosion that can take place within a water supply system. The material of construction of the system itself, or components used in the system, are key factors. Chemically inert materials, such as gold, silver, platinum, or noble metals, greatly reduce the rate of corrosion. Base metals, such as magnesium, aluminum, and zinc have a high corrosion rate; while steel, lead, and copper fall somewhere in the middle. In addition, the amount of exposed area to the water increases the corrosion rate as well. Controlling the level of dissolved oxygen in the water is one tool for controlling the corrosion rate in the system. Another, is to control the Oxidation Reduction Potential (ORP) of the water supply.
Additional factors influencing corrosion include the quality and velocity of the water flowing through the system. Salt content increases the electrochemical conductivity of the water, which in turn, increases corrosion. Adding calcium carbonate will cause a film to form on the metal surfaces slowing the corrosion effect of dissolved oxygen in the system. Excessive flow velocities produce an effect known as Flow Accelerated Corrosion (FAC), a result of the relative motion of the process wearing on the metal surface. It is characterized by a directional erosion pattern on the metal surface. FAC or Erosion corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys).
Industrial plants use a batch neutralization system to treat their chemical waste products. Acid-base reactions are among the most common used in water conditioning systems. Commercially available acids and bases can be used, but often the chemicals wastes from the plant are used for neutralization.
Treating acidic water is relatively easy because the allowable effluent pH range is fairly broad, usually 5 to 9 pH. FIGURE 1.0 shows a typical batch neutralization system for spent acid. In this scheme, a level controller opens the inlet valve to the reaction tank admitting the acid waste, and then closes the valve when the tank is full.
A pH loop is installed in the reaction tank to measure the waste water as it is mixed, and to control the addition of the alkaline reagent whenever the pH drops below the designated set point. The level controller and the pH analyzer are interlocked to assure the discharge valve does not open until the tank is full and the proper pH value has been reached.
A second pH loop is typically installed in the discharge line as a final check that the correct pH value of the water is being discharged back to the environment.
An inherent problem for all batch neutralization applications is "overshoot" of the chemical feed. This is when more chemical is added to neutralize the waste water than is necessary to achieve the desired pH set point. The result is the need for additional chemicals, of an opposite pH value, to bring the pH back in line.
The source of the problem is allowing insufficient retention time for the neutralizing chemical to mix adequately before accepting the pH measurement. The size of the tank, poor mixing, and location of the pH sensor with regard to the point where the chemical addition all contribute to this problem.
A simple solution is to provide a timer that allows the chemical feed for a set period of time (i.e. 30 seconds, one minute, etc),; then shut it off for a period of time to allow adequate mixing; and then release control back to the pH Analyzer. Depending on whether the set point has been reached determines whether more chemical will be added. Such a system optimizes mixing and chemical reaction times so that the pH measurement is a true representative of the batch value.
For control of batch neutralization, a pH measurement coupled with a timer-controlled chemical feed scheme provides very satisfactory results.
This system can be adapted for either acid waste or alkaline waste neutralization.
Any facility that discharges effluent to municipal sewer systems must comply with these pH requirements.
PH202 two-wire pH transmitter (intrinsic safety)
PH450 four-wire pH transmitter (general purpose)
FU20 Four-in-one pH sensor
FF20 Flow-thru pH fitting
FS20 Insertion pH fitting
FD20 Immersion pH fitting
PH87/97 Retractable pH fitting
The SC25F is a digital SMART pH sensor in a 12 mm design that includes an integral temperature element and a Liquid earth electrode.
The FLEXA™ series analyzers are used for continuous on-line measurements in industrial installations. With an option for single or dual sensor measurement, they are the most flexible two-wire analyzer available.
The PH450 pH and ORP analyzer is a multivariable analyzer that combines pH with temperature and ORP (Redox) measurement, which can be utilized through different output functions: two mA current outputs, four independent SPDT contact outputs and HART.
All-In-One pH and ORP digital SMART sensor that keeps the motto "Simple is best" while combing the sensor with built-in intelligence and direct digital communication.
The FU24F is a digital SMART pH/ORP sensor made with a chemical resistant PPS 40GF body for harsh pH applications.
The PH20, FU20 and FU24, all-in-one pH and ORP, sensors show how Yokogawa applies the motto "Simple is best" to sensor technology.
Yokogawa has invested considerable design and development time in producing a full range of fittings with particular emphasis on designs that reduce installation and maintenance time and consequently save operation costs.
On-line measurements often present extra challenges, especially when routine maintenance is required. The PR10 is ideally suitable for applications where the sensors must be removed without interrupting or shutting down the process. Without any special tools the PR10 can be retracted safely from the process at pressures up to 5 bar (72 psi). Using the PR10 allows us to place any dissolved oxygen sensor that has a PG13.5 connenction into a retractable assembly.
The immersion fittings are designed for either pH or ORP (Redox) measurements in tanks, open vessels and drains. They have a "hoisting cable" for easy maintenance.
The heart of a pH measuring loop is the electrode system. Yokogawa has designed a wide range of electrodes to ensure this heart keeps beating under the most severe conditions.
The PH87 retractable assembly is suitable for "Hot-Tap" service, allowing for removal from a pressurized process line or vessel without interrupting the flow or draining the tank. The housing assemblies are constructed of chemically resistant titanium sheathes and are used with Model PH97 rugged sensors.
The SENCOM™ PC Software, SPS24, offers a unique means to optimize the performance of pH/ORP sensors for enhanced reliability and process safety. The software allows the operator to monitor pH/ORP sensor performance, calibrate and configure various parameters.
pH and ORP meters, analyzers and transmitters are used for continuous process monitoring of pH and ORP to ensure water/product quality, monitor effluent discharge, batch neutralization, pulp stock, scrubbers, cooling towers, chemical, water/wastewater treatment and many other applications.
pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.
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