Spill control, Part II: reducing spills: when it comes to spill control, prevention is better than cure. A well-trained operating staff offers the best defense against spill problems.
The previous article in this series discussed the background to spill control, and provided some guidelines on assessing you own mill's performance. This month we will look at ways of reducing spills.
First and foremost, prevention is always better than cure. Appropriate instrumentation is important, but the key is the knowledge and motivation of the mill staff, particularly at the level of operators and maintenance tradesmen. The principal difference between the three mills in Figure 1 and Table 1 is operator skill and attitude.
[FIGURE 1 OMITTED]
Since prevention is never completely effective, spill recovery sumps with automatic activation are required in critical areas. Simple, single line mills typically require 3 to 6 sumps, though some mils require a dozen or so. Actual requirements are very site specific, and usually involve some compromise between the ideal configuration and the costs of retrofitting.
Most mill personnel, particularly at the level of operator/maintenance trade level, have only a hazy idea of what is important in spill prevention and control. Once made aware of the type and size of spills that are significant, many are extremely resourceful in developing improved operating techniques to reduce the frequency and magnitude of the spills.
Training programs should be tailored to the mill's specific systems, and to the level of knowledge of the personnel. Training should explain the key parameters, and how the department worked in affects the effluent discharge. Each piece of equipment or operating procedure that can generate spills should be identified and explained, and corrective action defined. In all cases, training should encourage feedback from operators and maintenance people, since they know many local details well.
A good spill control systems provides operators with continuous, rapidly updated, data on key factors of plant operation. Operators must understand these data, be able to diagnose causes, and take corrective action. Initial training requires several hours of class time for each student, with a couple of hours refresher each year for most operators and maintenance personnel.
Continuous feedback helps workers learn from mistakes. Mills where spills are controlled successfully normally have a report of all major incidents at daily production meetings, and advise all operators in relevant departments about what happened and how to avoid repeat incidents.
Much of the data required is the same as is needed to run an efficient plant; but additional information-including levels of all major tanks and equipment, overflow alarms and conductivity in individual floor drains-is also required.
Mills should continuously measure conductivity in each operating area of interest. Locations should be selected so that they will serve to locate spills in a reasonably small area (such as an evaporator set or the digester department) all under the control of one operator. Sensors should not be located where false positives will occur, such as when an ion-exchange water treatment system is regenerated with caustic. If this is unavoidable, operators must be trained to interpret the data.
Where tank overflows are a problem, an overflow detector is useful. Some mills monitor temperature in the overflow pipe, since this will detect foam overflows that fail to show on the tank level monitor.
Data must be immediately available to the operators who control the system. In mills with modern distributed control systems, it is best to make the values available on the mill's data bus so that environmental staff, supervisors, and management can also review the data on their desktop computers. In a simple, single line mill, half a dozen conductivity monitors are normally appropriate; a very complex mill may require up to about 30.
To recover spills while corrective action is being taken, the normal approach is to install sumps in the mill's floor drains equipped with pumps that start automatically on high conductivity, and pump the contents of the floor drain to either the weak black liquor storage tank or to the blow tank, depending on whether there is likely to be fiber present in the recovered liquor or not.
There are no rational design criteria for sizing these sumps and pumps, which leads to some rather bizarre designs. Mills build sumps of varying sizes, and some are as large as 20-ft by 20-ft, with a depth of 6 feet or more, and two continuous duty vertical pumps. This is not necessary. Successful spill recovery systems often have sumps as small as 4-ft square, a couple of feet deeper than the floor drain they are installed in, and one submersible or light-duty vertical pump.
Water is a major enemy of spill control systems, and is actually a contaminant, since it dilutes the spill, perhaps preventing cost effective recovery. Many mills make the mistake of arranging systems to recover every drip in the areas of concern. Unfortunately, this leads to dilution of the spills so that the many small events pass unnoticed; the dilution of larger spills causes unnecessary loads on evaporators, and perhaps even failure of the system.
Continuous clean water discharges must be kept separate from the floor drains in the areas protected by spill recovery. Some mills run a stainless or plastic pipe as a clean sewer down existing floor drains, so that they can maintain gravity flow.
Table 1 (below): Statistical summary of color discharges at entry to Waste water system, showing that Mill S also has the lowest color Discharge. The data underlying the "the better looking" results in Figure 1 exhibit lower coefficients of variance, but the reader is Cautioned against making a simple assumption that this value is a Good measure of spill control. Mill S Mill C Mill L Mean color discharge, kg/ton 39 52 59 Standard deviation 6 19 25 Coefficient of variance 15% 36% 42% "kg/ton" refers to kilograms of color measured each day divided by the mill's average production rate, both measured over a six month period in 2000.
This is one of a series of columns prepared by the Bleaching Committee of TAPPI's Pulp Manufacturing Division. For more information on TAPPI's next Process Closure Course, contact Tony Johnson by email at email@example.com or Neil McCubbin at firstname.lastname@example.org
Neil McCubbin is president, N. McCubbin Consultant Inc., Foster, Quebec, Canada. Reach him at email@example.com
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|Title Annotation:||Water Treatment|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Dec 1, 2001|
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