No question: water is the lifeblood of boiler operation. Yet, water can be rife with impurities that can result in rust, scale, corrosion and, ultimately, boiler failure. With water being so integral to the boiler’s ability to produce the steam to be converted into heat, it is imperative that the water be treated properly so the impurities are removed or neutralized.
Boiler-water treatment generally involves the injection of any of an array of chemicals into the boiler water. These chemicals must be dosed at highly accurate rates and speeds so that they themselves do not interfere with the quality of the water and the operation of the boiler. This means that the operator must identify and employ the right type of chemical-injection technology: one that will deliver reliable injection and flow rates no matter the amount of chemical required.
This article will illustrate how diaphragm metering pumps, actuated by hydraulic or solenoid, along with filter feeders, can provide accurate, reliable injection and dosing of the chemicals that are critical to maintaining proper boiler operation.
Industrial Boiler Water Demands Treatment
It is a fact of boiler life that impurities in water will leave solid deposits like scale, rust and corrosion as steam evaporates. Over time, the accumulated deposits will thermally insulate the boiler’s heat-exchanger surfaces, which will not only gradually reduce the boiler’s rate of steam generation but also cause boiler metals to approach failure temperatures.
This is where the treatment of boiler water enters the picture. The different types of boiler-water chemicals are designed to perform different functions depending on the boiler system and water utilized. Some chemicals are added to maintain desired pH levels, which will minimize the water’s solubility, i.e., the ability to be dissolved. Other chemicals can be added to consume oxygen before it can corrode the boiler, prevent foaming, precipitate dissolved solids before they form scale on steam-generating surfaces and remove any precipitates from the vicinity of the steam-generating surfaces.
Some of the more common chemicals used to treat boiler water include:
- Oxygen scavengers.
Coagulants. Boiler water can be treated with ferric chloride or hydrated potassium aluminum sulfate (better known as “alum”) as a way to get any suspended particles in the water to clump together, or flocculate. These heavier clumps of impurities — which often are called sludge — then drift down through the water and accumulate at the boiler’s bottom, where they can be removed during a boiler blowdown procedure.
Phosphates. While the coagulation process removes suspended impurities from the water, the sludge clumps are still corrosive and can damage the boiler as they rest on the bottom of the unit. To combat this, sodium phosphate can be added to the water, which controls the pH level of the clumps and helps prevent harmful caustic corrosion from occurring.
Oxygen Scavengers. Oxygen is a significant cause of corrosion in metals, so it is beneficial to the life and operation of the boiler if any excess oxygen is removed from the water. In this case, oxygen scavengers such as sodium sulfite or hydrazine can be used to help remove oxygen and protect against damaging corrosion.
Chelants. These are specialized molecules that can bind to positively charged metal ions and prevent them from forming insoluble precipitates. Common water-treatment chelants are ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), which are used to form complex ions with calcium and magnesium in the water. The chelant is said to sequester the metal ions as a way to control scale formation in the boiler.
In addition to knowing which chemicals work best with the impurities that are found in general boiler water, operators of power-generating boilers must be aware of the potential pitfalls that can arise when superheated water is required for the operation. By definition, superheated water is that which is used under pressure and at temperatures that range from the typical boiling point of 212°F (100°C) all the way up to the critical temperature of 700°F (370°C).
Special attention must be paid to the operational characteristics of superheated water because at temperatures of more than 570°F (300°C), the water can be more corrosive than at ordinary temperatures. This requires that special care be taken in selecting materials of construction — carbon-steel and stainless-steel piping has been shown to be effective in these applications — as well as the chemicals that may be needed to treat the water.
Metering Pumps Manage Boiler Water Treatment Dosing
Specific types of pump and filter feeder technology can be reliably deployed in chemical-injection activities for the different types of water treatment required and the different types of boilers. Three widely used options include:
- Hydraulically actuated diaphragm metering pumps.
- Solenoid-actuated diaphragm metering pumps.
- Filter feeders.
Hydraulically Actuated Diaphragm Metering Pumps. Hydraulic metering pumps can be suitable for chemical handling because they have been designed to inject a precise and controlled amount of chemical, with a repeatable accuracy of ± 1 percent. This lets the operator know that dosage will consistently meet the predetermined requirements for use as well as limiting the waste and higher costs that can accrue as a result of overdosing. Also, because the pump’s diaphragm is hydraulically balanced, it is capable of operating against much higher discharge pressures than a mechanically actuated diaphragm pump.
Other features and benefits of hydraulic metering pumps include its robustness and longevity (designed for 20 years of service); internal relief valve; leak-free operation; ability to inject chemical at extremely high pressures if necessary for the application (100 to 4,000 psi and higher); automatic control with either an electronic stroke control or variable-speed motor and manual control via adjustable micrometer dial. These pumps can be outfitted with a multitude of motor configurations in order to meet any service requirement.
Some models have variable oil-bypass adjustment that delivers better valve performance than those that have a variable-linkage design. The variable oil-bypass adjustment optimizes performance because it gives the pump’s valves extra time to seat. (They are idle during the bypass portion of the suction and discharge strokes.) It also creates a much smaller footprint than other hydraulic diaphragm metering pump designs.
Hydraulic metering pumps also are available in many different materials of construction — 316 stainless steel, C-20, PVC and Kynar being the most common — which helps quell any material-compatibility concerns that can arise when handling different types of chemicals.
Solenoid-Actuated Diaphragm Metering Pumps. Solenoid-actuated, or electronic, metering pumps are an economical option in chemical injection for low capacity/low pressure injection (usually less than 2 gallons per hour and less than 250 psi) applications for boiler-water treatment. They offer repeatable dosing accuracy of ±3 percent. Many are able to operate at any single-phase voltage from 94 to 264 VAC, or 50/60 Hz, and are generally unaffected by voltage fluctuations. Some have stroke rates of 300 strokes per minute or more, which allows them to provide more even distribution of chemical at low flow rates.
Many models have a manual frequency adjustment or speed control that allows for convenient stroking-speed adjustment. Some models provide both frequency adjustment and stroke-length adjustment, which allows extremely large turndown capabilities.
Typical materials of construction are 316 stainless steel, PVC or Kynar pump heads, Viton or PTFE (Teflon) seals, PTFE diaphragms and 316 stainless steel, and ceramic or PTFE check valves. This gives them the versatility to be used in a range of chemical-injection applications during boiler-water treatment activities.
Filter Feeders. A technology that is growing in acceptance for use in chemical introduction to boiler water — while also possessing filtration capabilities — is the filter feeder. It is suitable for use in low pressure gravity- or pump-return-type steam-heating systems. Namely, the filter feeder combines — in one piece of equipment — the ability to inject various water-treatment chemicals with the capability to perform high capacity filtering of the rust, corrosion, scale and coagulated solids that are produced.
One benefit of using the filter feeder is its ability to protect against harmful particles and debris. This is possible due to the filter bag, which can trap almost all contaminants, rather than using strainers or dirt/air separators. While 60- to 90-mesh screens can intercept larger particles, modern filter bags have been designed to capture impurities that can be as small as one micron, which is five times smaller than the width of a human hair.
Another benefit of the filter bag is that when it becomes full, it can be removed from the filter feeder and thrown away. This means that all of the solid particles captured are removed from the system into the garbage and not reintroduced into the boiler system.
In all, filter feeders are a convenient, reliable way to introduce solid or liquid chemicals into the boiler while at the same time filtering the boiler water. They allow users to remove rust, scale or other contaminants that may compromise the overall boiler performance or any of its sensitive internal components.
In conclusion, boilers are an undeniable necessity when the goal is to produce heat or generate power. The boiler can only perform its job effectively if the water that is used to generate steam is of a quality that does not affect the boiler’s operation. With most boiler water containing some type or level of impurities, it is imperative that the water be treated with chemicals that can make it safe for use. In addition, those chemicals can only perform their assigned tasks if they are used at the proper doses and feed rates. Operators of boiler systems have many options when considering the best way to inject water-treatment chemicals. Hydraulic and solenoid pumps along with filter feeders are reliable technologies in crucial chemical-injection applications.