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Pollution Prevention and Control Technologies for Plating Operations
Section 2 - General Waste Reduction Practices
2.5 RINSEWATER REDUCTION
2.5.2 Controlling the Flow Rate of Rinsewater Use
2.5.2.1 Flow Restrictors
2.5.2.2 Manual Control of Water Flow
2.5.2.3 Conductivity Controls
2.5.2.4 Solenoid Valve on Automated Plating Machines
2.5.2.5 Timer Rinse Controls
2.5.2.6 Flow Meters and Accumulators
2.5.2 Controlling the Flow Rate of Rinsewater Use
Regardless of the type of rinse tank arrangement employed (e.g., single overflow, counterflow), water use reduction can be achieved by coordinating water use and water use requirements. When these two factors are perfectly matched, the rinse water use for a given work load and tank arrangement is optimized. Four methods of coordinating water use and water requirements were identified during the Users Survey and literature search. Each of these methods is discussed in the following subsections. Some methods are applicable to a range of plating operations while others are more relevant to specific conditions (e.g., small manual operations, large automated machines). Some of the methods can be combined to optimize water use.
Flow restrictors are inexpensive devices that are connected in-line with the tankís water inlet piping to regulate the flow of water through the pipe. They are typically an elastomer washer that flexes under pressure such that the higher the water pressure, the smaller the hole available for flow passage. Therefore, they maintain a relatively constant flow under variable water pressures. Flow restrictors are available in a wide range of sizes (0.1 gpm to more than 10 gpm). The smaller sized restrictors are most commonly used with multiple counterflow rinse tank arrangements and the larger ones are commonly used with single overflow rinses. Some restrictors aerate the water as it passes through, in a manner similar to a kitchen faucet (venturi effect).
Flow restrictors are applicable to nearly all rinse systems. A possible exception is a rinse tank equipped with a conductivity controller (see Section 2.5.2.3). With conductivity controllers, the instantaneous water flow rate is unimportant since the controller stops water flow based on the low conductivity set point of the controller and the conductivity of the water in the rinse tank. Therefore, restricting the flow will only reduce the time needed to dilute the rinse water to the conductivity set point and will not effect the total volume of water used.
Flow restrictors as a stand alone method of rinse water control are only effective with plating lines that have constant production rates, such as automatic plating machines. Even in such cases, to use water efficiently, the plater must have a means of stopping water flow during non-production periods. With variable production rates, flow restrictors alone will not provide the necessary coordination of rinse water need and use. One method for improving this coordination is to install a timer rinse control (see Section 2.5.2.4).
Generally, the size of a flow restrictor is selected to provide adequate rinsing for all parts. This means that the maximum rinse water flow requirement is the governing factor and that on the average, the flow will be higher than necessary for good rinsing. This fact is a sufficient reason for supplementing the control provided by a flow restrictor.
Flow restrictors are widely used by the respondents to the Users Survey. A total of 222 (or 69.8%) of the respondents employ this method of water use reduction. The average success level of this method is 4.10.
Only one survey respondent cited any problems with flow restrictors. That shop indicated that their platers usually remove the flow restrictors when they need to fill an empty rinse tank quickly and that the platers occasionally forget to replace them (PS 173).
2.5.2.2 Manual Control of Water Flow
Manual control of water flow simply refers to manually opening and closing water valves to adjust flow or to turn the water flow on or off. This method of control is obviously dependent on the operator and usually results in inconsistent water use. Several shops emphasized that this was a particular problem at their facility (PS 133, PS 176, PS 180, PS 265, and PS 276).
Combining manual control with flow restrictors reduces the variability of water flow; however, it does not address the problem of water use during idle production periods. Manual control can be improved by installing a main water valve for an entire plating line that stops water flow to all rinse tanks in that line.
A manual control method reported by one shop involved dumping and refilling all rinse tanks at one time, at the end of the day (PS 055). This shop is relatively small and has an average discharge of 5,500 gpd.
Of the survey 318 respondents, 209 (or 65.7%) use manual water control. The average success level for the respondents is 3.63.
These units consist of three components: (1) probe or sensor located in the rinse tank that senses the conductivity of the rinse water; (2) transformer box that houses the solid state circuitry that controls the system; and (3) a solenoid valve that opens and closes in response to signals from the circuitry. In use, when drag-out is introduced to the rinse tank, the probe senses a rise in conductivity above a set-point which is picked up by the circuitry and the solenoid water valve is opened. The value remains open until the probe senses a drop in conductivity below a set-point. The set-points are operator-adjustable to permit use over a range of desired water qualities.
Conductivity rinse controls have been effectively used to reduce rinse water use (ref. 316). However, in some cases, they have been removed from service due to maintenance problems (ref. 21). The results of the Users Survey show a moderate level of usage for conductivity controllers (51 respondents or 16.0%) and the lowest success rating among rinse water reduction methods (3.25).
A report prepared for the military (ref. 379) listed three criticisms of conductivity controllers. First, the units are sometimes disabled or overridden by operators who object to the appearance of a controlled rinse, which may be less clear than a free-flowing rinse. Operators often override the units by placing the probe into a process tank or a bucket of process solution (i.e., causes solenoid to remain open). This problem can be controlled by shortening the length of the probeís cable or by running the cable through a PVC pipe (if using the later method, be certain the piping arrangement permits access to the probe for periodic cleaning). Second, the controllers do not sense non-ionic contaminants and rinse tanks may become contaminated with particulates such as dust. Third, the units require frequent preventive maintenance to remain operable. In particular, the probes become encrusted or fouled, especially in alkaline rinse waters. For example, one shop that rated the success of their conductivity controllers as ì1î, reported that they are ìdifficult to keep cleanî (PS 086). Another shop that gave the controllers a rating of ì2î indicated that they are a high maintenance item and susceptible to fouling (PS 114). Also, probes sometimes become entangled with plating racks and are damaged or disconnected.
One shop reported that they tried conductivity controllers and found them to cause high water use (PS 069). Presumably, the range of settings available on their units did not satisfy their rinse criteria. The shop discontinued the use of these devices and reverted to manual control and flow restrictors to which they gave a success level of 3 and 5 respectively. A different problem was reported by another shop that indicated their controllers were not triggered by acid at a concentration that obviously impacted rinse quality (PS 124).
Altmayer suggests that conductivity controls using probes to measure the resistance (conductance) between two wires are very troublesome to operate, but that the controllers using inductive loop type probes, which are less susceptible to fouling, can be very successful. Altmayer also related to the experience of PS 069 and PS 124 (see above paragraph), saying that controllers must be specified to control within a certain range of conductivity and that choosing this range is the hardest part of the design. He stated that this problem is mainly caused by the lack of information and data available. Most guidance is based on total dissolved solids (TDS) as opposed to metal ion content, which is more familiar to the plater (ref. 482).
2.5.2.4 Solenoid Valve on Automated Plating Machines
One large job shop reported that they have installed solenoid valves on the water lines feeding each of their automated plating machines (i.e., semi-automated return-type). The valves automatically close when power to the plating machine is turned off.
Timer rinse controls consist of a push-button switch and timer mechanism and a solenoid valve. These units operate in a manner similar to conductivity controllers; however, rather than regulating rinse water flow on the basis of rinse tank water quality, the timer controls simply turn water on and off based on a pre-set time period.
In operation, a plater lowers parts into the rinse tank and pushes a button (alternatively, a momentary switch could be used that is activated by lowering a rack or barrel). The button or switch activates a timer and opens the solenoid valve for a preset time period. After that time period has expired the solenoid valve automatically closes.
The timer setting is selected through trial and error. It is best to select a time period that provides consistently clean rinse water, without excessive waste. Once set, the time period is not changed unless the general trend of production changes.
Fewer of the Users Survey respondents have employed timer rinse controls than conductivity controllers. The number of respondents using timer rinse controls was 36 (or 11.3%). The average success rating was 3.78, which is significantly higher than for conductivity controllers. Several shops indicate that they have employed both. One such shop rated their success with the timer rinse control as ì5î, while rating their success with conductivity controllers as ì1î.
2.5.2.6 Flow Meters and Accumulators
These devices by themselves do not reduce water use. However, they make the metal finisher aware of water use rates and are useful in identifying excessive water use.
Flow meters and accumulators are most useful when installed on fresh water lines feeding individual rinse tanks or, at a minimum, on pipes feeding individual plating lines. Meter readings taken over an extended time period will show trends in water use. Using these data, shop management can identify specific locations where excessive water use occurs and can correct the problem before long-term wastage has resulted.
One shop that installed flow accumulators indicated that they were useful for specific studies, but in general were found to be a rather expensive way to generate usage numbers of ìquestionable validityî (PS 124).