Disk of Knowledge

September 2006 - Water Treatment

Submitted by K L Petrocelly; RPA, CPE, CFM, PMP.

There's nothing more refreshing than a cold glass of water in a hot machinery room: But the water we can't live without could prove fatal to your equipment there. The water commonly found in most places has the potential to corrode metal, choke off fluid passages and cause pressure vessels to overheat to the point of failure. In the physical plant that equates to rotted out cooling towers and condensate lines, unscheduled shut downs of your chillers and blowing up the boilers.

CONSTITUENTS OF WATER

Depending on where it's found, water may be teeming with life or completely devoid of it as the result of the manipulations of man. Prior to treatment, it is a veritable soup made up of salts, dissolved gasses, organic and inorganic compounds. The attached table lists the more common impurities which can affect operations in your plant. Regardless if the water you purchase from your local authority is aerated, chlorinated, fluoridated or carbonated, it's quality still doesn't meet the strict operating standards required by your equipment. Though most of the suspended solids have been removed, the dissolved salts and gasses remain, waiting to be manifested as scale or corrosion.

EVERYDAY MALADIES

The five major problems frequently associated with water and machinery are scale formation, corrosion, fouling, foaming and embrittlement; any of which can quickly lay waste to your operations.

SCALE is an extremely hard substance (though it does come in a soft variety) which is created when mineral salts come out of solution as their solubility decreases with an increase in the temperature of the water in which they're contained. The scale forming salts tend to adhere directly to heating surfaces forming layer after layer of insulation on the metal, thereby decreasing it's heat transfer efficiency. Scale results in metal fatigue and failure due to overheating, energy waste, high maintenance costs and unnecessary safety risks. One-sixteenth inch thickness in a fire-tube boiler can cause an eleven percent increase in fuel consumption, but any accumulation is problematic.

CORROSION is the attacking of metals by acids or electrolytic action. The metal can be eaten away in a fashion such as the generalized rusting away of an automobile fender or by localized pitting. Corrosion increases down time and maintenance costs, results in premature replacement of equipment and causes unnecessary safety risks. It occurs where there are high levels of oxygen or carbon dioxide present, low pH values, where contact is made between dissimilar metals and in damp environments or corrosive atmospheres. Corrosion is basically an electrochemical process in which electricity flows through a solution of ions between areas of metal. Deterioration occurs when current leaves the negatively charged metal (anode) and travels through the solution to the positively charged metal (cathode) completing an electric circuit, much like the action in the cell of a battery. The anode and cathode can be two different metals or different areas of the same piece of metal. Corrosion occurs when a difference in electrical potential exists between them or as the result of their physical contact. The solution containing the ions is called the electrolyte, which in most cases is essentially comprised of water, it's ions and the ions generated by the anode.

FOULING is a condition whereby restrictions develop in piping and equipment passages resulting in less than efficient flow. Although these accumulations are undesirable, they don't usually pose the same dangers as scale or corrosion and can easily be remedied. The major consequences of fouling are energy waste and increases in operating and maintenance costs. But if such conditions are allowed to continue and proliferate, such as in cooling towers, heat exchangers or other critical devices, subsequent mal-operation of system components could result in disruptions to department operations and/or the emergence of health related issues such as Legionnaires Disease.

FOAMING is a condition where frothy bubbles, resembling the head on a beer are created in the steam space of a boiler by concentrations of soluble sodium salts aggravated by grease, suspended solids or organic matter. Priming is a consequence of foaming, whereby the frothy bubbles break and the resulting liquid combines to form slugs of water which are carried over into the steam system. Pressure from the steam can impart velocities as high as 80 to 100 miles per hour to slugs of water discharged into steam lines which can wreak havoc with turbine blades, actuating devices and piping downstream of the boilers from whence it is issued.

CAUSTIC EMBRITTLEMENT occurs when small hairline cracks appear in highly stressed areas due to high concentrations of alkaline salts that liberate hydrogen which is absorbed by the iron in the steel effectively changing its physical properties. Characteristically, the condition manifests itself in the high temperature areas of boiler watersides and is caused in large part by carrying boiler water pH values at too high levels (over 11). Embrittlement problems are easily avoided simply by maintaining water pH values within the range prescribed by the manufacturer of your equipment.

Unless these problems are constantly monitored and controlled, they will take their toll in higher fuel costs, increased safety risks, unnecessary downtime, expenditures for equipment replacements and overtime premiums.

THE REMEDIES

The three tools used in the physical plant for improving water quality are demineralization (the removal of inorganic salts from solution by ion exchange), de-aeration (a process in which dissolved oxygen and carbon dioxide are removed by heating of the water) and internal treatment ( a process whereby a variety of chemicals and techniques are used to condition the water to predetermined values). Though each of them has its own level of importance in the scheme of things, it's the latter issue we'll be concentrating on. Whereas the first two can be accomplished easily enough through the purchase of support equipment appropriate to the task, the third calls for a more concerted effort.

Most organizations, large enough to employ viable operating engineers, take advantage of their professional knowledge to help defray operating costs in the physical plant. Such is the case with water conditioning. Analysis and treatment is an area that can pay back handsomely if properly performed. The by-products of a well implemented program are increased heat transfer efficiencies, lower fuel expenditures and decreased consumption of chemicals. Except for steam trap maintenance, it has the most potential for reducing annual operating costs in the power plant. If you don't have the expertise in-house, by all means contract it out to a reputable vendor.

Whichever company you choose to analyze your waters and supply you with chemicals, make certain that your agreement with them is in black and white. The agreement should stipulate exactly what was agreed to verbally. It should state the number and frequency of visits to expect from the representative and the work to be performed during those visits. It should address callbacks for emergencies, tell you what training will be provided for your people and spell out the technical assistance available for problems you may encounter. At a minimum, the vendor should:

* Become familiar with your equipment and its frequency of operation.
* Survey your water systems and supply you with a written proposal indicating your problem areas.
* Provide you with a written analysis, indicating what constituents were found and in what amounts.
* Recommend treatments and proper feeding methods for each system.
* Train your personnel to test your waters in-house and provide log sheets for recording the results.
* Establish parameters to guide your operators in the use of chemicals and methods of introduction.
* Make feed, bleed and blow-down recommendations.
*I nterpret the results of your in-house tests and make recommendations to improve your operations.
* Perform internal inspections on your equipment.
* Establish a chemical inventory and determine reorder points.
* Provide laboratory support such as metallurgical and microbiological analyses.
* Modify your program to meet federal, state and local regulatory requirements.

The investment you make in a service agreement with a reputable chemical vendor may well pay dividends in short order when you consider the potential for loss you have without one. But no water treatment program can succeed without the commitment of both the vendor and the owner. If the vendor only wants to sell chemicals, you should toss him out into the parking lot and if you aren't prepared to do your part, you might as well join him. And even if your organization is bottom line oriented, the decision from whom to buy shouldn't be based on cost alone, but on the services you receive from the vendor. Quote me! "Chemicals are chemicals", regardless if they are referred to by there generic nomenclature, their brand names or an alphanumeric designation. Quote me, again! "Any company that provides service is only as reputable as the representative who services your account". Once you find a good one, don't let him get away. The extra money you spend to get and/or keep him will be returned many fold.