4 Common Issues in Feedwater Tank Design and Operation

Author: Peter Miller | District Service Manager

Feedwater Tank
Boiler water treatment is critical to ensuring your boiler is operating as efficiently as possible and no proper water treatment program is complete without a thorough understanding of the feedwater setup.  A crucial step in preparing the feedwater for the boiler is preheating and deaerating the water in a separate vessel directly upstream of the boiler.

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These vessels come in two main varieties: deaerators and feedwater tanks.  Deaerators are pressurized vessels that have a designated deaerating apparatus in addition to its feedwater storage section.  Deaerator optimization has already been discussed here, therefore in this blog, we will be discussing proper Feedwater Tank Design.

Feedwater tanks are heated storage tanks that, unlike deaerators, don’t have a specific deaerating section and operate at atmospheric pressure.  In addition to being a feedwater reservoir, these tanks also act as condensate receivers, cold water make up locations, and as a point for chemical injection.  Given the multitude of functions this vessel can have, proper design and operation of the feedwater tank is critical to ensure the boiler sees fully treated feedwater consistently.  A properly designed feedwater tank will help reduce thermal shock and decrease oxygen scavenger usage.  Below are four of the most common design issues with feedwater tanks:

1. Inadequate Heating of Feedwater

Boiler feedwater needs to be heated prior to entering the boiler to prevent thermal shock and for the removal of oxygen from the water.  Most feedwater tanks are equipped with a steam sparger, which heats the tank with steam from the boiler.  The temperature of the feedwater tank should be maintained between 185F and 195F to reduce the amount of oxygen in the water.  By increasing the temperature and reducing the oxygen content, you can greatly reduce your sulfite or oxygen scavenger requirements.  Making sure there is an adequate heating supply and the control valve is functioning properly are the first steps in making sure the feedwater tank is operating properly.  A quick and easy check is to routinely take note of the vent discharge versus the temperature gauge; a well heated feedtank at 190F will have noticeable water vapor coming from the vent, but not excessive.  Heating feedwater is one of the most cost effective ways to remove oxygen and reduce chemical consumption.
Figure 2 - The oxygen content of water is greatly reduced as feed water temperature increases, reducing oxygen scavenger requirement.

2. Improper Cold Water Make Up Design

The cold water entering the feedwater tank is generally 50F to 80F, which means it contains quite a bit of oxygen.  How make up is being fed (on/off or continuous) and where in the tank it’s fed can have a great impact on how easily the dissolved oxygen is liberated. Make up should ideally be fed about 3-6 inches underneath the water line on the side of the tank, preferably through a sparger at a slow and continuous rate.  Adding make up above the water line can cause a splashing effect which reaerates the water. There are also risks with placing the make up inlet over a feedwater pump supply line.  Cold make up water is denser than the heated water in the tank. Make up water fed through the top can quickly sink to the bottom of the tank and inlet of the feedwater pump, causing cold untreated water to go directly to the boiler.

On/Off make up control can also cause issues.   If the make up rate is too great when the feedwater tank calls for water, the feedwater temperature can lower faster than the steam can raise it.  The oxygen scavenger residual in the tank could also be completely consumed as a result.  If the boiler calls for water when the tank is making up, the system runs the risk of sending feedwater with oxygen to the boiler, which can cause oxygen pitting and boiler failure.
Figure 2 - When the make up is placed over the feedwater pump inlet, the system can short circuit and send cold water straight to the boiler

3. Incorrect Feedwater Pump Location and Design

How the feedwater pump is installed in the system can have a great effect on how the system operates.  The water at the inlet of the feedwater pump should be fully heated and devoid of oxygen.  Feedwater tanks are very rarely well mixed with uniform temperature and chemical composition throughout.  The feedwater pump supply should be placed on the opposite side from where the make up enters.  This will give the cold make up water the greatest amount of time to be heated and chemically treated before entering the boiler.  The aforementioned make up sparger is recommended if the feedwater tank has more than one feedwater pump supply line.

Another consideration is the feedwater pump piping for continuous feedwater pumps.  Continuous feedwater pumps have recirculation lines to prevent deadheading, with the water being returned to the feedwater tank.  This recirculation line should be plumbed below the water line.  If the recirculation line is plumbed above the water line, it will reaerate the water and greatly increase oxygen scavenger usage.
Figure 3 - Water needs time to deaerate before being sent to the boiler, and recirculation lines should be returned under the water line to prevent reaeration.

4. Chemical Injection Location

Chemical injection into the feedwater tank is recommended to help protect the feedwater tank and give time for the oxygen scavenger to react.  However, where the chemical is injected can have an effect on treatment quality.  Chemicals should be injected underneath the waterline, preferably through a quill.  The quill should be installed in a spot that bisects the location of the cold water make up and feedwater pump supply.  This will ensure the cold water make up has to travel past the chemical injection spot, and thus being treated prior to entering the boiler.  On most feedwater tanks, the ideal place is in the middle of the tank underneath the water line.
Figure 4 - Untreated make up water should pass through chemical treatment location to ensure water is treated

Conclusions

Feedwater tank design can have a great impact on boiler operation and chemical usage.  While these are the most common issues with feedwater tank design, it is by no means a comprehensive list.  Many other design factors can affect boiler operation such as residence time, venting, condensate return, etc.  Please contact your Klenzoid representative if you have concerns about your feedwater tank design or chemical setup!
Peter Miller has a degree in Chemical Engineering from the University of Michigan and an M.B.A. from Wayne State University. As a District Service Manager, he works with clients to achieve the lowest operational costs for their heat transfer systems through well-maintained water treatment programs. Peter has an interest in brewing and trying all types of beer. His favorite type of beer is free beer, with cold beer being a close second.

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Dealkalizer Technologies

Some important design considerations for the chloride cycle dealkalizer are:

  • Feed water must be softened
    • Calcium chloride can precipitate and foul the beads
  • Minimal impact on total dissolved solids
  • Potential small decrease in blowdown requirements
  • Relatively low capital cost, reasonably effective, simple to operate

 

Some important design considerations for the WAC dealkalizer are: 

  • Additional softening required. WAC can remove as much hardness as there is available alkalinity – any residual hardness needs to be removed before the boiler.
  • Efficiency reduction with increasing flow rate, decreasing kinetics.
  • Handling of acid
    • Sulfuric acid – heat of hydration is a concern (can’t have plastic tanks, plastic piping), higher concentrations are available (up to 93%), calcium sulfate precipitation can be a concern for water sources high in sulfate levels)
    • Hydrochloric acid – fumes, plastic can be used, calcium chloride precipitation is not a concern, lower concentrations available (up to 32%)
  • Higher capital cost, very effective, easy to operate, larger footprint

Ion Exchange Explained



A quick review of ion exchange is required to understand dealkalization and we’ll use the water softening process as an example, as most boiler operators are very familiar with this.  Water softeners use strong acid cation (SAC) resin for ion exchange.  SAC resin has an affinity for divalent ions (Calcium, Magnesium) meaning that the resin wants to grab a hold of these divalent ions as they’re passing through the bed and exchange them with the sodium ions. Once resin is saturated and there are no more available free resin beads for ion exchange, a brute force wash of the SAC bead with sodium chloride (salt) brine is required.

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How to Minimize Amine Requirements



Amines should be dosed at the minimum rate required to neutralize carbonic acid, and to maintain pH levels of 8.0 to 9.0 in condensate.

In situations where incoming alkalinity levels are elevated, the concentration of amine required to neutralize the resulting elevated CO2 levels may exceed OTLs or even PELs. A number of alternatives are available to decrease alkalinity levels from incoming water:
  • Reverse osmosis (RO) Weak-acid dealkalization (WAC)
  • Chloride-cycle dealkalization
  • Demineralization (Demin)
RO, WAC and Demin units remove alkalinity from incoming water sources, and are often implemented to reduce energy and/or water consumption in steam plants because they decrease the overall mineral concentration of dissolved solids from incoming water. However, the chloride-cycle dealkalizer is a standout choice if the goal is to simply reduce incoming alkalinity on a budget. It operates much like a softener unit, and can decrease alkalinity levels by up to 95%.


Chloride-Cycle Dealkalizer Operation

Chloride cycle dealkalizers use strong base anion (SBA) ion exchange resin to swap carbonate and bicarbonate ions for chloride ions.  The footprint is similar a sodium softener, and they also use salt as the primary regenerant.  A small amount of sodium hydroxide if also often used to increase the effective capacity per regeneration.



The reduction of alkalinity in the feedwater, reduces the formation of carbonic acid in condensate, thus reducing the required amount of amines to neutralize the carbonic acid to maintain pH levels of 8.0 to 9.0 in condensate.

Implementation of a chloride-cycle dealkalizer can reduce your amine requirement by up to 90%.

PELs & OTLs



There are 2 important concentration guidelines:
  • Permissible Exposure Limits (PELs)
  • Odor Threshold Limits (OTL)
The following table describes the limits set by Occupational Safety & Health Administration (OSHA) and American Conference of Governmental Industrial Hygienists (ACGIH):


Exceeding PELs poses a health risk to occupants. These PELs should never be exceeded for any period of time. See this link for a related article from the Centers for Disease Control and Prevention (CDC).

http://www.cdc.gov/mmwr/preview/mmwrhtml/00001848.htm

It is best practice to also follow OTLs to minimize the likelihood of complaints from occupants, especially from those with sensitivities.

A More Detailed Look at the Components of Steam



Oxygen

Liquid water always contains some concentration of oxygen (O2). The solubility of oxygen is primarily determined by the temperature of the water. Higher temperatures reduce the solubility of oxygen in water (see graph).
Because oxygen is extremely corrosive in high temperature water, steam boiler treatment programs use chemical and/or mechanical means of eliminating dissolved oxygen in water. An effectively treated steam boiler, and the steam it produces, will have near-zero dissolved oxygen concentrations.

Carbon Dioxide

Carbon dioxide (CO2) is released by the heating of carbonate (CO32-) and bicarbonate (HCO3-) in boiler water. These ions are naturally present in water from lakes, rivers and underground wells, and their concentration determines the alkalinity of the water source. The amount of carbonate alkalinity entering the boiler is proportional to the volume of carbon dioxide gas that will be in the generated steam. Carbon dioxide eventually forms carbonic acid in condensate. Higher alkalinity values result in greater carbonic acid concentrations.


The Release of Carbon Dioxide



The above reactions describe the release of carbon dioxide gas from sodium bicarbonate (1) and sodium carbonate (2).

The heat energy in boiler water is sufficient for the first reaction to proceed to 100% completion.  The completion of the second reaction is dependent on increasing pressure and temperature.

Higher carbonate and bicarbonate levels in boiler feedwater will lead to proportionally higher concentrations of CO2 in steam.

Amines

The amine compounds used in boiler water treatment are selected based on their boiling point, and their distribution ratio. The distribution ratio is a measure of how far the amine will travel before condensing. An optimal blend of amines will protect the entire condensate piping network (near and far). Amines are considered volatile organic compounds, and their concentration must be monitored to prevent exposure to levels beyond permissible limits.


Lesson about Amines to Impress Your Water Treatment Professional



Amines are a functional group in organic chemistry, and are derivatives of ammonia. They are separated into three main groups, primary, secondary and tertiary amines. These groups are defined by the number of hydrogen atoms replaced by organic substituents.

The most commonly used amines for neutralizing carbonic acid in condensate are:
  • cyclohexylamine (CHA)
  • diethylaminoethanol (DEAE)
  • morpholine
These amines are selected for their availability, basicity (ability to neutralize acids), boiling points, and most importantly, distribution ratios.

Distribution ratios (DR) are a measure of the how far amines will travel with steam before condensing. A proper blend of amines will include low DRs to protect condensate piping closest to the boiler, and high DRs to protect piping in longer and more complex condensate networks. Below is a table with the properties of the amines discussed above.

Other Types of Humidification Systems

Pan Humidifiers:

Pan humidifiers are essentially small shallow basins filled with water. The basins are heated with electric elements or steam, with the intent of evaporating water.

Pan humidifiers are found in smaller HVAC systems, and are susceptible to biological and corrosion fouling.

Water Spray Humidifiers:

This design uses an array of nozzles to atomize liquid water directly into the air stream. The phase change from liquid to vapour causes a noticeable drop in air temperature.

This type of system is most susceptible to biological and corrosion fouling. Facilities with year-long continuous cooling loads requiring high RH are best suited for this technology.

Steam to Steam or Clean Steam Generators:

These systems are small steam boilers, specifically designed to produce steam from high purity water sources, such as demineralization, or reverse osmosis. The energy input comes from steam raised elsewhere in the facility by a traditional steam boiler.

This design is typically more costly, and adds complexity, but produces steam with no boiler water treatment compounds.


Clean steam generators can only produce steam at low pressures.  The packaged heat exchangers rely on the higher energy content of higher pressure steam.

Water purity is critical for clean steam generators.
  • Low hardness levels (>3ppm of calcium, magnesium, or iron) will lead to fouling of heat exchange surfaces.
  • Water with even moderate alkalinity levels will release CO2 gas which will corrode any condensate piping components.
  • Moderate levels of total dissolved solids (TDS) will lead to priming or carry over, which may damage the steam control valves and/or contaminate the steam.
Therefore, Reverse Osmosis (RO) systems are ideal for humidifier makeup.  These units are designed to remove nearly all of the minerals from incoming water sources, and produce water with TDS concentrations of 0-5 ppm.

Steam to steam generators do cycle up.  Despite high purity makeup, there are always some dissolved solids.  If the generators do not purge some volume of water regularly, the bulk water will concentrate beyond acceptable levels, causing water discolouration and may lead to fouling and/or corrosion to system components depending on materials of construction.

Effects of Humidification on Occupant Comfort and Building Materials

RH levels have a direct impact on the health of patrons in a facility.

When humidity is too low occupants will get dry skin, irritated sinus, throats and eyes.

When humidity is too high mold/mildew problems can occur in the building, thus increasing the risk of illness to occupants. These health impacts are of increased concern with health care facilities who treat immunocompromised patients.

RH levels also have an impact on building materials.

The amount of moisture the material can hold will determine the extent to which it shrinks and swells with fluctuations in humidity. The effect is especially pronounced in wood and drywall, where gaps and cracks will form over time.

Windows are also prone to condensation in cold climates because they generally have little insulation value. The likelihood of condensation on windows increases as the indoor relative humidity rises, and the outdoor temperature decreases.

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