Boiler water treatment

ENVIRODOS® 200 Series

Why boiler water treatment ?

Inadequate control of oxygen is a major source of boiler corrosion

A common maintenance problem for boiler water treatment is a lack of control of corrosion due to oxygen. Mistakes are made in the mechanical removal of dissolved oxygen or the chemical scavenging of oxygen or both. This results in corrosion pitting attack by oxygen on boiler watertubes and feedwaterlines wich calls for expensive replacement of failed metal and downtime for the boiler.

Suggested Actions

A. Improper mechanical removal of oxygen from feedwater

Mechanical removal of oxygen is accomplished with a deaerator (DA). The DA works by heating the water to a boil at wich point the solubility of dissolved oxygen and other dissolved gasses is at their lowest level. The undissolved gasses are vented to the atmosphere and the deaerated water is used in boiler feedwater. The little remaining dissolved oxygen should be no more than the parts per billion range (ppb) wich is scavenged by chemical treatment.

Proper DA operation requires a vessel pressure of 3 - 4psi above atmospheric and a water temperature measured at the storage section of 8°F above the boiling point for water at the installation's altitude. There should be a visible 18 - 24" steam plume coming from the DA's vent. This contains the unwanted oxygen and other gasses. Three things to check once per shift is 1.) operating pressure, 2.) temperature of water in the storage section, and 3.) an 18 -24" steam plume at the DA vent. Oxygen should then be in the 7 to 20 ppb range depending on the type of DA.

B. Chemical scavenging of dissolved oxygen with ENVIRODOS® 201

The deaerator removes most but not all of the dissolved oxygen. The remainder must be reacted with oxygen scavengers such as ENVIRODOS® 201.

ENVIRODOS® 201 and other oxygen scavengers should be fed into the storage section of the DA - upstream from the boiler. This allows for it to react with oxygen before the oxygen gets into the boiler and it also helps to protect feedwater lines.

ENVIRODOS® 201 should be fed continuously as a function of feedwater flow as opposed to slug dosages. This allows for a proper residual in the operating boiler at all times and avoids conditions of overtreatment and undertreatment swings.

Why Boiler water treatment ?

Many installations across the world are facing problems with boiler systems related to water treatment. One of the most common problems is difficulty in preventing the formation of scale or the occurence of corrosion in the heating systems. As a result of these problems, system life, reliability, efficiency and safety are reduced. Many of these problems are preventable through proper boiler water chemistry maintenance and treatment. To assist installation personnel responsible for operating heating systems, a list of products has been developed over the past years to face the most common problems and to deliver a proper solution.

No service contract nescessary

HIgh costs of operation, increased repair costs, boiler failures, contract disputes, terminations and - on occasion - litigation can many times be traced to the incomplete or inadequate preparation of request for services.

A selection of a good chemical vendor is very important to plant operations. Too often the selection is made on the basis of lowest price per pound of vendor chemicals. What this does is reward the vendor that has the most water in their chemical products! The competitive selection of chemicals and vendor should be based on cost to treat 1,000 gallons of boiler feedwater.

Another fact in buying treatment chemicals is wether to buy with or without service. Most of the companies recognized as boiler water treatment chemical suppliers do not sell chemicals without service. Also, the quality of service provided varies greatly by company and service representative. Service includes monthly plant visits, technical assistance and possibly other services. Conflicts arise because the company may provide advice that results in unnescessary higher chemical use since the company has an interest in selling more chemicals. Typically, chemical products that include service in the purchase price can cost 5 to 20 times more than the generic chemicals.

Inadequate treatment of Carbon Dioxide is a major cause of condensate piping corrosion and boiler deposits

Carbon dioxide comes from the decomposition of the carbonate and bicarbonate alkalinity that is naturally present in the makeup water. The decomposition occurs in the boiler and the carbon dioxide that is formed is volatile enough to escape with the steam and later condense in the condensate system forming carbonic acid wich is acidic and corrosive.

The presence of carbon dioxide can be easily determined by measuring the pH of the condensate. Carbon dioxide lowers the pH. A reading below pH 7,5 indicates corrosive levels of carbon dioxide.

Suggested actions

A. Reduce the amount of carbon dioxide produced in the boiler.

One method is to reduce the amount of makeup water used. This is accomplished by maintaining the optimal cycles of concentration through proper blowdown and also by recovering as much condensate as possible since condensate combines with makeup water to form boiler feedwater. Cycles of concentration, often measured with conductivity or total dissolved solids readings, should not exceed the point where scaling or carryover occurs. The amount of condensate returned can be increased by eliminating wasted steam and of course repairing condensate leaks.

Another method to reduce carbon dioxide is to reduce the amount of alkalinity (bicarbonate) in the makeup water. Dealkalizers remove alkalinity from the makeup water wich is the precursor of carbon dioxide. This is a form of external treatment.

B. Carbonic acid can be neutralized using ENVIRODOS® 203 treatments wich are used to raise the pH to between 7.5 and 8.5.

The proper use of ENVIRODOS® 203 is dependent upon the lenght of condensate piping runs to be protected. ENVIRODOS® 203 should be fed directly into the boiler or into the feedwater piping. It should not be fed into the dearator where it can escape via vent piping.

ENVIRODOS® 203 should be fed continously as a function of feedwater flow as opposed to slug dosages. This allows for a proper pH in the condensate system at all times and minimizes overtreatment and undertreatment swings.

ENVIRODOS® 203 is an organic corrosion inhibitor used for scavenging oxygen and passivating metal surfaces. It can be used as direct replacement for hydrazine in any boiler without the associated safety hazards and handling precautions. It can be used in boilers  of any pressure, up to and including supercritical (220 bar g), and can be fed into feed-waters used for spray attemperation.

ENVIRODOS® 203 is an innovative carbohydrazide, condensate line treatment developed for use as a corrosion inhibitor in steam and condensate systems. ENVIRODOS® 203 may be used at temperatures up to 260°C. ENVIRODOS® 203 contains high, medium and low volatility amines wich provide effective neutralisation of acidic gases, such as CO2, throughout a condensate system.