Chemical dosing control

Products for small cooling systems tend to become progressively more dilute. This practice is often necessary to ensure adequate profitability at low revenue-generating sites (because smaller sites require proportionately more service time than larger sites, and technical service time is expensive). Also, where only a limited amount of actives is required to be injected per day, the addition of a more dilute product ensures better chemical dosing control and minimizes the risk of under- or overdosing. 

It thus becomes necessary to preempt the problem by tightly controlling the FW pH, temperature, and residual hardness levels where MU water source contains a high silica level (say, over 20-30 ppm Si02). The location of FW line chemical injection points and the type of chemicals dosed also may influence the risk of silicate sludges and scales developing, and these factors may also need to be considered. 

Variable and often low levels of chemical reserve, leading to a variety of product performance problems. Often as a direct consequence of having no chemical dosing pumps on-site, or inadequate, automatic control systems, or insufficient attention to the quality of makeup water employed, or maintenance of cycles of concentration. 

Does the customer s management fully understand what is required from their side and can they actually deliver the necessary supervisory and manpower support For example, changing from a system of simple chemical dosing pumps to a microprocessor-controlled product residual tracing and addition system will probably require more equipment inspection and calibration time. It will also require a high degree of instrumentation skill. 

Without chemical dosing pumps and adequate program control measures, the matching of chemical usage to operational needs cannot be properly achieved, which may well produce the same result as if no treatment had been used. 

Chemical dosing and control systems are often supplied fully assembled and prewired on a skid, and increasingly dosing tanks are mounted inside secondary tanks to prevent the risk of chemical spillage or leaks causing pollution problems. 

There is a growing trend for many operators not to have any direct involvement with their cooling system chemical dosing equipment (with regard to the addition of chemical, mixing and chemical dilutions, pump adjustments, and possibly even periodic equipment inspections and maintenance). The equipment may also not be owned by the customer. Rather, the water treatment service companies may lease the equipment to the customer, or provide free-on-loan equipment, together with all chemical dosing and control functions, as part of their services to the facility. 

The dose levels for repeated dermal toxicity studies in animals should have at least three concentrations, including an appropriate vehicle control. Except for treatment with the test chemical, the control group should be handled in a manner identical to the test group. The highest-dose level should not produce evidence of toxicity attributable to the test chemical. 

At least three dose levels with a control and (where appropriate) a vehicle control should be used in a subchronic dermal toxicity study. Expect for treatment with the test chemical, the control group should be handled in a manner identical to the test group. The highest-dose level of the test chemical should result in toxic effects but not produce fatalities, which would prevent a meaningful evaluation of the results. The lowest-dose level of the test chemical should not produce evidence of toxicity. Where there is a usable estimation of human exposure, the lowest level should exceed this. Ideally, the intermediate-dose level(s) of the chemicals should produce minimal observable toxic effects. If more than one intermediate dose is used, the dose levels should be spaced to produce gradation of toxic effects. In the low and intermediate groups and in the controls, the incidence of fatalities should be low to permit a meaningful evaluation of results. 

When used in powder or crystal form, dechlorination chemicals (ascorbic acid and sodium thiosulfate) dissolved rapidly causing water-quality concerns, although physical methods (tablets) have been developed since to slow down dissolution rates. Sodium sulfite, when used in tablet form, was very effective in dose control. One tablet was sufficient to dechlorinate 2 mg/L of chloraminated water to below 0.1 mg/L for 45 min when water was released at 100 gpm. Finally, these field tests also indicated that the flow rates of chlorinated waters can significantly impact the efficiency of dechlorination operations. 

Experimental aerosol research frequently requires the controlled generation of an aerosol. A particular property of the aerosol, such as a certain size distribution, may be required to ascertain its transport properties. The control of aerosol generation may extend beyond size distribution and concentration to the physical and chemical properties of the particles. In particular, the effective dose in aerosol therapy is a function of the physical and chemical properties of the aerosol particles in addition to the mass concentration delivered. The size, shape, and structure of the aerosol particles determine their aerodynamic or transport properties and, hence, affect the site and efficiency of deposition. After deposition, these same physical properties of the particles, in addition to the chemical properties, control the surface area of the particles and, hence, the rate of dissolution and absorption of the drug. Consequently, the control of the physical and chemical properties of the aerosol particles and of the number or mass concentration is a prerequisite for the accurate determination of the effective dose in aerosol therapy. 

An additional potential application of particulate counting is process control and monitoring. With the improvement in aqueous particle counters, on-line measurement of number concentrations and size distributions for particulates larger than 1-2 pm is now feasible. Both feedforward and feedback process control applications can be envisioned. Feed-forward control could be used to estimate the coagulant chemical requirements needed for particle destabilization based on measurement of particle count and estimation of particulate surface area (32). Feedback control possibilities include control of the particle size distribution entering a filter, control of chemical dosing prior to granular-media filtration, and control of filter operation. 

In addition, the refinery consumes water for cooling, for boilers, and for domestic purposes, and according to the source (surface water, ground water, reused water), the makeup water needs a specific treatment such as sand filtration, iron removal, (partial) softening, desalination, dosing of chemicals to control corrosion, and biofouling. 

Chemical treatment consists of three weU-known treatment processes for ensuring rehable operation of RO membranes. First, a 20% sodium bisulphite (SBS) solution is injected by the chemical dosing pump (one pump is on standby). The iiyection rate is proportional to the RO feed water flow rate and is controlled by the PLC based on the chlorine concentration monitored by a chlorine analyser downstream of the in-line mixer. SBS like sodium sulphite and sodium metabisulphite is a reducing agent commonly used to dechlorinate RO feed or lower the chlorine concentration to less than 0.05 mg/1 in RO plants that use polyamide aromatic membranes. It takes 7.33 mg/1 of 20% NaHS03 solution to remove 1 ppm of residual chlorine in water on a stoichiometric basis. 

Caustic soda (50% NaOH solution) may be injected into the RO feed water line by the chemical dosing pump (one pump is on standby) to maintain the pH of RO feed water between 7.5 and 8.5 if there is no hkehhood of carbonate scaling. The iiyection rate is controlled by the PLC based on the pH value measured downstream of the in-hne mixer. At alkaline pH conditions, dissolved CO2 gas is converted to bicarbonate ions, thereby enhancing membrane rejection (lower permeate conductivity). 

Under normal operation, the chemical and volume control system is coimected to reactor coolant system cold leg No.l, and returns to the steam generator outlet chamber. The chemical and volume control system is made up of two sub-systems a chemical control subsystem, located inside containment, which consists of regenerative and letdown heat exchangers, mixed bed demineralisers and filters and a makeup subsystem, located largely outside containment and consisting of makeup pumps, mini flow heat exchangers and chemical dosing tanks. Containment isolation valves are provided on both sides of the containment penetrations. 

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