Writing Water Treatment Specifications
Precise planning on the part of the design professional can prevent serious start-up and operational problems, and ensure system longevity, when a proper water treatment program is specified and provided.
Vendor QualificationsWater treatment firms fall into the following categories, and their strengths and weaknesses should be considered when specifying and approving vendors:
National sales and service companies. While providing the stability of major corporations, they are only as good as the local representatives. These companies are probably the best choice for major projects, but are not normally interested in smaller work. The area representative often holds a degree in chemical or mechanical engineering, and is knowledgeable and reliable.
Regional sales and service companies. Many times overlooked in lists of approved vendors, these smaller regional firms usually provide the best service for small to medium projects. As smaller firms, the principals are often involved in the day-to-day operations, ensuring reliable and cost-effective operation. Service is usually a priority with this type of company, making them an excellent choice for facilities that may not have a full maintenance staff. Consult the Association of Water Technologies, Inc. directory (www.awt.org) for qualified regional firms.
Specialty chemical sales companies. This type of organization, consisting of commissioned sales representatives and having no service capabilities, should be excluded from new construction project work.
The service capabilities of the firm should be a priority in selecting a vendor. Many water treatment sales companies have little or no service department, and a salesman is not often of much help when a system goes down. Select a firm that has a full-time, local service department that covers the area of the project. Service capabilities should include equipment installation and repair, on-site water analysis and problem remediation, and should be performed by well-trained service technicians.
Consult the building owner during the specification process, and inquire as to their current water treatment provider. If they are satisfied with that vendor, and if you feel the firm is capable, it is best to leave the water treatment out of the job specifications. This avoids the problems associated with "low-balling," both on the part of the existing service provider and competitors looking to get a foot in the door.
Cleaning of New Boilers and Piping SystemsNew boilers and piping are layered with oil used in the manufacturing process, and often applied as a surface corrosion deterrent. Failure to completely remove this oil can result in priming and foaming in steam boilers, as well as water carryover into the steam system. In hot water systems, this oil film prevents the desired corrosion inhibitor from fully protecting the metal surfaces, and promotes biological growth.
Cleaning hot water systems is accomplished by addition and circulation of a chemical cleaning compound, traditionally a blend of sodium hydroxide and tri-sodium phosphate. Sodium carbonate may also be used. This solution should be introduced at a rate recommended by the manufacturer, circulated, and then removed from the system. Removal can be by draining and refilling the system several times, or, as I prefer, simply flushing the system, under pressure, while providing fresh make-up water. This method allows for system operation during the process, and does not require a second procedure of air removal.
During the cleaning process, it is important that all portions of the system are open and circulating. Any equipment, such as coils that have small tubing, may be bypassed to prevent deposition of debris from the piping. Water balancing should not be scheduled until the completion of the cleaning and treatment process.
In steam boilers, particularly cast iron sectionals, it is essential that the boiler be given a full "over-the-top" boil-out prior to being placed in service. While some chemical manufacturers claim that their cleaning compounds will hold the oil in suspension while the solution is drained from the boiler, the time proven boil-out method is still the best method of oil removal. A mixture of sodium hydroxide and tri-sodium phosphate, at a rate of 1 pound of each per 50 gallons of water, has been used for many years as a reliable boil-out compound. The procedure requires firing the boiler, maintaining a slow, rolling boil, and preventing steam pressure from building during this process, while the surface of the water is continuously skimmed to remove impurities. The boiler manufacturer's recommended procedure should always be followed.
Treatment of Hot Water SystemsHot water boilers and recirculating closed loop systems--despite the fact that they are often considered to be tight, zero make-up systems--need protection from waterside damage.
Scale and corrosion inhibitors can be of the oxidizing category, including blends of sodium nitrite, molybdate, borate, silica, and azoles, such as tolytriazole. Reducing inhibitors include sulfite and neutralizing amines. The chemical used should be determined by considering relevant criteria, such as materials of system construction, operating temperature, make-up water quality, and possibility of potable water contamination.
Sodium nitrite, generally combined with borate for pH control and azole for non-ferrous metal protection, has been a staple of the water treatment industry for many years. Nitrite provides good corrosion protection when maintained within the optimum levels, usually 800 to 1200 ppm NaN02, but actually increases corrosion when maintained at levels below 500 ppm. Higher concentrations of this product, mainly because of the resulting higher pH, have been found to cause pump seal damage. Nitrite is also a contributor to bacterial growth within the system, and should be replaced with a non-nitrite product if bacteria is encountered. Antimicrobial agents are also often used in conjunction with closed system inhibitors; but must be products formulated and labeled for use in hot water recirculating systems.
Molybdate is perhaps the most effective corrosion inhibitor now available, and is becoming widely used. Molybdate inhibitors also will use azoles and some method of buffering pH, usually sodium hydroxide. Molybdate is run at far lower levels than nitrite, normally in the 80 to 120 ppm MoO4 range, and does not contribute to the bacterial problem.
Many new systems are being designed that require freeze protection, hence the use of glycol is becoming common. Glycol specifications need to address the issue of corrosion protection as well as freeze protection. In most cases, good quality hvac inhibited glycols will provide sufficient corrosion protection in any concentration above 25%. Care should be taken to follow manufacturers' directions for blending and dilution, which usually requires the use of deionized make-up water. Both ethylene and propylene glycols are suitable, but again, system design criteria should dictate choice of fluids. Additional corrosion inhibitors should not be added without consulting the glycol manufacturer. Hvac glycols generally use dipotassium phosphate for corrosion control, and should not be mixed with nitrite or molybdate unless approved by the manufacturer.
Hot water systems need some means of adding chemical compounds to the system. This is most often accomplished by installation of a pot feeder. The feeder, also called a shot or by-pass feeder, should be installed in a manner that ensures complete and rapid circulation through the feeder. The preferred method is to pipe across the pump suction and discharge. If this is not practical, the piping can be into the discharge side piping of the recirculation pump, providing a bypass valve arrangement is used to force the flow into the feeder. I do not recommend this piping configuration, as the feeder piping is normally 3/4 to 1 inch, and the main piping will be much larger. To get flow through the feeder, the bypass valve may have to be nearly completely closed, reducing system flow considerably while the feeder is on line (see Figure 1). Filter feeders are becoming quite popular, as a means of providing constant filtration and allowing for chemical introduction in one unit. Replaceable filter bags are easily changed, and work well for either constant or intermittent sidestream filtration (see Figure 2).
As an alternative to pot feeders, many systems can be fed easily by use of a portable hand force pump. This pump, able to overcome system pressures up to 150 psi, can simply be attached to any hose bib or tapping, and the desired amount of treatment added.
Glycol systems should not be allowed to take on make-up water, regardless of need. It is better to let the system go down on low pressure than to dilute the glycol with make-up water. The pressure drop will alert the operator to the loss of fluid. The best arrangement is to provide a glycol feed system, which can add glycol to the system when the pressure drops. The plant operator can monitor the fluid level in the glycol reservoir to ensure that the system is tight. Manual glycol feed systems can also be used, with a hand or electric pump, allowing the operator to add fluid when necessary.
Treatment of Steam SystemsSteam systems, regardless of size or pressure, need a proper water treatment program for protection from scale, corrosion and pitting. This is accomplished with a variety of products, each designed to address a specific problem. While this article is not intended to be a complete water treatment manual, the specifying engineer needs to be familiar with the basics of water treatment chemistry. Water treatment companies are usually happy to provide assistance in writing specifications, but care should be exercised to ensure that the specs are not product specific, thus limiting the ability of competitors to bid.
Oxygen pitting, usually the cause of most premature steel boiler failures, can be addressed mechanically by use of a deaerator. While not practical on small low pressure systems, it should always be considered for high pressure boilers, especially those with a high percentage of steam loss. Once the oxygenated water has reached the boiler, it must then be handled chemically. Oxygen scavengers combine with the oxygen molecules to prevent them from attacking the boiler surfaces.
The most common scavenger in use today is sodium sulfite, or sodium bisulfite, often cobalt catalyzed for rapid absorption. Boiler feedwater temperature is very crucial to the administration of an oxygen removal program, and should be addressed in the design stage to ensure a temperature of approximately 200ÝF for best control. Oxygen scavengers should be fed directly to the boiler water drum, in a manner that will maintain the desired concentration of residual at all times.
Pretreatment of the boiler make-up water is not always necessary, but should be considered for large systems, any system with high make-up requirements, and all systems where the make-up water hardness exceeds 200 ppm as CaCO3. Usual pretreatment consists of a sodium zeolite water softener for removal of hardness minerals. This process, which replaces calcium ions with sodium, allows the boiler operator to maintain higher cycles of concentration, resulting in savings in both water and chemical consumption.
Depending on make-up quality, the alkalinity of the water may have to be adjusted after softening. If the alkalinity of the make-up is too high, this will negate any possibility of raising the cycles, as high alkalinity levels cause surging and carryover.
Mechanical dealkalizers are used to remove alkalinity after softening, and work on the same principle as the softener, but use a different resin.
Internal treatment is the chemical method of removing harmful water contaminants. Several different water management programs can be used, including phosphate, chelant or organic. Selection of the correct program should be based on boiler size and type, make-up water quality and percent of condensate return. Control limits for each system may vary and should be discussed with the water treatment service provider.
Generally accepted practices use some of the following guidelines: Sulfite for oxygen scavenging: 30-60 ppm as SO3; Phosphate for corrosion control: 30 to 60 ppm as PO4. PH of the boiler water should be in the 10.5 to 12.0 range; and Hydrate Alkalinity 200 to 300 ppm as CaCO3. I prefer to use lower levels of phosphate, at least in the Northeastern U.S. where I am familiar with the make-up water quality, often running low pressure boilers at 20 to 40 PO4, and higher pressures, up to 150 psig, at 10 to 20 ppm PO4. Boiler water should contain less than 10 ppm hardness as CaCO3, which assures the operator that the softening process, either mechanical or chemical, is working properly. Chelant and organic programs will carry different levels, and the water treatment company's recommendations should be used.
All chemical programs will use a variety of ingredients to provide adequate protection. An oxygen scavenger will be used for pitting protection; a phosphate, organic or chelant for scale and generalized corrosion protection; and a variety of components for alkalinity adjustment, suspension of solids, and sludge conditioning. Depending on the size and type of boiler, these ingredients may be provided as individual products, or may be combined in one drum for ease of application.
In addition, protection for the steam and condensate system may be specified. In new systems, this chemical protection is beneficial to prevent corrosion and pitting in the system. In older systems, however, where a new boiler is being attached to an old steam system, chemical treatment of the steam system may create problems. Unless the steam system was well protected from day one, it probably contains corrosion deposits. The steam line treatment will effectively break loose these deposits, resulting in plugging of traps and strainers, and the possibility of opening up holes that were covered by these deposits.
Additionally, steam use must be considered. Federal regulations require strict control of steam line treatments, known as amines, when used in systems that use steam contact for operations other than building heat. If the steam is used, for example, for humidification, sterilization, food processing, cleaning or any other direct contact application, there are legal limits to the amount of amine that may be present in the steam. Maintaining this level is difficult and often nearly impossible in small systems. Recent studies are providing additional information on the possible health hazards associated with use of amines in building humidification and the design professional would be well advised to pursue further information before specifying steam line protection in these situations.
Whatever the chemical program desired, application of the chemicals is important. Hand, or slug, feeding is inappropriate for steam systems. Hand feeding results in a series of "highs and lows" of the treatment levels. When hand feeding, the operator adds enough chemical compounds to last for some time, creating a high level condition at time of application. The chemical is then lost by blowdown and diluted by make-up water, falling to a point below the optimum level before the next dose is added. Large steam plant systems may use a combination of hand and automatic feeding, if the operator performs a complete boiler water analysis at a minimum of once per shift.
Automatic feed systems, when properly installed and monitored, provide an even, precise chemical addition rate. These systems can be actuated by several methods, the choice based on system size, configuration, make-up water quality and requirements and plant personnel involvement in the daily operation of the boiler system.
Automatic systems will consist of a chemical metering pump, as well as some method of energizing this pump when desired. Ancillary equipment may include tanks, agitators, monitors and low level alarms. On small to medium systems especially, the rule should be "keep it simple." Additional instrumentation is costly and often unnecessary with the treatment compounds used today.
The simplest system, which is appropriate for low pressure boilers up to 250 hp with low-to-moderate make-up requirements, is to electrically connect the chemical metering pump to the boiler feedwater pump circuit. Thus, whenever the boiler calls for water, the chemical pump is activated.
On systems with high make-up requirements, larger low pressure boilers, and most high pressure systems, it may be advantageous to use a slightly more precise method of chemical addition. A contacting water meter should be installed in the make-up water line, measuring the amount of make-up added to the system, and activating the treatment equipment. Meters and controls can be installed that add chemical for a variable length of time after the meter contacts or pumps that stroke as the meter turns. Either method is acceptable. This system provides precise control of the scale and corrosion inhibitors whose use is directly related to make-up water quantity. Oxygen scavengers, however, may still need to be added based on feedwater pump cycles. In many large plants, the water treatment equipment runs constantly, the output being adjusted daily by the plant operators based on their in-house water analysis.
Installation of treatment equipment is important. Boiler water chemicals should be injected into the feedwater line between the pump and the boiler on most systems. Alternately, chemicals may be fed to the boiler feedwater or condensate tank, if fed below the water line. While this may provide some measure of protection for the tank, feedwater pumps are then exposed to high levels of chemically treated water, which may damage pump seals or impellers. This may be the best choice, however, on small systems where the cost of separate systems (one per boiler), may be prohibitive.
If piped into the feedwater pump discharge, the metering pump must be able to overcome the line pressure. This requires a higher output pump, and should be installed with both a check valve and manual ball valve at the injection point. Pumps connected to the feedwater tank can be of lower output, but should be connected in the same manner.
Boiler water conductivity, which measures the "solids" in the boiler water, is an important issue most often overlooked by the specifying engineer. This measurement is critical to proper boiler operation, especially in watertube and cast iron sectional boilers, which have a very small water surface area, and capacity, compared to firetube boilers. Conductivity is the measurement used to regulate and monitor the cycles of concentration, that is, how much concentration of the make-up water ingredients is allowed in the boiler.
If the conductivity of the boiler water is too low, it results in excessive water and chemical use. This is normally caused by too much manual blowdown of the boiler.
Low conductivity may also signal a low treatment level, as the automatic treatment equipment may not be able to keep up with the extra demand created by the high water consumption. If the conductivity is too high, it indicates insufficient blowdown and possibly excessive chemical compounds in the boiler water. High conductivity can lead to foaming and carryover, especially in watertube and sectional boilers.
On small to medium sized low pressure systems, conductivity should be measured by the plant operator or water treatment service technician at regular intervals. More accurately, a chloride reading is often performed by the water treatment technician, as chemical concentration also affects the conductivity meter reading.
On large systems, and all high pressure process systems, conductivity should be monitored and controlled electronically. Conductivity monitors sample the boiler water, and open the blowdown valve only when the conductivity reaches the desired set point. Piping is from the top of the water side of the boiler (see Figure 3). The cost of the controller will be offset by the savings realized in water and chemicals, but routine maintenance of the sampling sensor electrode is required.
Regarding conductivity, special consideration must be given when specifying cast iron sectional boilers. High output, coupled with small water capacity and surface area, makes it difficult to maintain proper water chemistry. While more resistant to oxygen attack than steel, the sectional arrangement makes these boilers susceptible to sludge deposition. This sludge, normally accumulating in the center and end sections, leads to overheating and failure of the section.
Engineers should make certain that these boilers are piped with bottom blowdowns at all four corners to facilitate sludge removal. In addition, surface blowdown should be provided. This allows the operator to maintain an appropriate concentration (conductivity) level, and is important in removal of solids and contaminants from the boiler water surface.
While I am preaching on blowdowns, let me add this: I have seen thousands of small boiler installations over the years that make no provision for the operator to blowdown the gauge glass. The pet cock that is provided is impractical. Did you ever try to open a pet cock that is at 220ÝF? If you are successful in holding on to it long enough to open it, you are then greeted with a lap full of steam and water discharging directly at you. All boiler gauge glasses should be specified with a ball valve and piping to near the floor. A gauge glass that is not blown down is soon unreliable, and no one will install this extra valve if it is not specified.
Service, Service, ServiceThe best intentions of the designer are of little consequence if the vendor chosen fails to live up to the service expectations and requirements of the system. Careful selection of the vendor at submittal time is important to the successful waterside protection of your system.
Service guidelines should be stringent yet flexible, taking into consideration the system design, size, equipment specified, and the ability of the plant personnel to monitor and maintain the system. Generally, hot water systems should require water analysis at intervals of no more than 90 days, unless the system is taking on make-up. The best practice is to require monthly testing for the first three months, then quarterly thereafter.
Steam systems however, require a minimum of monthly water analysis for small low pressure heating boilers. Larger systems, and high pressure process or utility boilers, should be serviced by the water treatment provider at biweekly or weekly intervals. Plant personnel, if possible, should be required to maintain an in-house water treatment analysis program under the direction of the vendor. Test equipment for their use can be added to the job specifications, but again, care should be taken not to over-specify equipment that may go unused. A simple, portable test kit is usually sufficient. Only in large plants is a wall-mounted test cabinet, complete with titration equipment, warranted. Instructional sessions should be required to familiarize plant personnel with the water treatment system, practices, and procedures.
Water treatment is not, and should not be, mysterious. It is simply a process of choosing the correct product, applying it properly, and monitoring it to achieve the desired result. This result, which should be optimal equipment efficiency and life expectancy, can be achieved by writing the specifications carefully and choosing a vendor that is qualified and competent.