COOLING WATER TREATMENT

ABSTRACT

Traditionally, chemical water treatment has involved adding corrosion inhibitor and then periodically testing for the residual.  However to be effective, these programs must also include the control of scale and biological growth.  Controlling the 3 sides of the cooling water triangle will drastically increase the effectiveness of your water treatment program.  Failure to control any one component of the triangle will lead to probable failure of your water treatment program and repair or replacement of your valuable capital equipment.

THE COST OF FAILURE

TERMINOLOGY

  • Conductivity- the measurement of how much electrical current  water is carrying.  Used to measure Cycles of Concentration (CoC)
  • Cycles of Concentration- The number of times minerals have been concentrated in a system due to evaporation
  • pH- measurement of free hydrogen in the water.  Represents how acid or alkaline the water is.
  • Blowdown- the removal of water from a cooling tower system to reduce or limit the Cycles of Concentration
  • Corrosion Coupon Rack- piping assembly that is a slip stream from the main system used to hold corrosion coupons.  Flow through the coupon rack should be regulated to 3-5 g.p.m.
  • Pot Feeder- feeder used to introduce chemical into a closed system.
  • Chemical Residual- the amount of chemical measured in a particular system
  • White Rust- the formation of zinc hydroxide deposits in a galvanized cooling tower.  Caused by inadequate cooling tower passivation at start up.
  • Evaporation Credits- a program by-which a city will give you a credit on your water bill for the water evaporated from your cooling system.

pH

pH can tell us whether the water has corrosive or scaling tendencies.  High pH tells us that the water is more likely to cause scale or mineral deposits.  Low pH tells us that the water is more likely to cause corrosion

CONDUCTIVITY

Conductivity gives us an approximate idea of what the mineral content of the water is based on the water source used to supply the cooling tower.  High conductivity is critical and can cause rapid scaling and mineral deposition on heat exchange surfaces (cooling tower fill and condenser tubes).

  • Cooling tower conductivity should run between 2500-3500 umhos.
  • Conductivity above 3500 umhos an cause scaling
  • Conductivity below 2500 indicates water/chemical waste.

ASPECTS OF WATER TREATMENT

Looking at a water treatment program for a condenser water system it is easy to focus on the traditional areas, such as:

  • Corrosion control
  • Microbial control
  • Deposition/scale control

While these are important, they do not take into account the broader scope of what needs to be done to ensure the maximum lifespan of these systems.  In designing a system consideration needs to be given to each of the following aspects:

 

  1. Is it a new or existing system?
  2. If it is existing, are there corrosion products in it?
  3. Is there a risk of organic contamination?
  4. What level of monitoring can be expected (plant or supplier personnel)?
  5. How extensive is the fluid distribution piping?
  6. What are the typical blowdown rates?
  7. What is the metallurgy of the cooling tower?
  8. Does the system run constantly or intermittently?
  9. Are the tubes enhanced?

The objective of a treatment program is to minimize damage and allow the system

to operate as efficiently as possible.  This means that:

 

  • The corrosion inhibitors need to be able to get to the metal surfaces
  • Deposits are not present that can interfere with fluid circulation
  • Microbes are not present that can compromise the corrosion protection

CORROSION INHIBITORS

NOT ALL CHEMICALS ARE THE SAME!!  organic corrosion Inhibitors using polymers and phosphonates are the most effective program. Phosphates are used-good corrosion protection, but can form mineral deposits.

  • Zinc can be used, is not environmentally friendly and can CAUSE corrosion at pH over 8.5
  • Molybdate is often used but is VERY expensive at useful levels
  • New technology-organic base, safer to handle, lower liability, potentially improved performance.

BIOCIDES

Approximately 70 % of the cooling water chemical programs that fail do so as a result of poor biological control. The key to effective biological control is applying the proper amount of chemical with the proper frequency.  A vigorous biocide program will utilize 2-alternating products.

  • Water with pH > 8.0, chlorine is not acceptable
  • Quaternary biocides neutralize most corrosion inhibitors
  • Sulfur based biocides do not perform well in alkaline water
  • Swimming pool floaties do not count as biocide

SYSTEM MONITORING

Considering the liabilities of improperly treating any system, monitoring is one of the simplest ways of ensuring that performance criteria are being met.  The low cost combined with the confirmation of due diligence makes monitoring virtually mandatory.

  • Corrosion coupons (mild steel < 2.0/copper < 0.2)
  • Real time corrosion monitoring
  • Mineral balance
  • Regular evaluation of system water conditions
  • DAILY testing and log sheets
  • Electronic reporting from your controller

CONTROL EQUIPMENT

Water treatment control equipment has evolved significantly over the past 10 years.  Controllers can do everything from notifying designated people of a problem, to emailing service reports and other information daily, weekly, or monthly.

At a minimum, your cooling tower controller should control blowdown, chemical feed, 2 biocides, and have an integrated flow switch.

FILTRATION

The use of any means of filtration can help any condenser water system.  Filtration is particularly important if your chiller has enhanced tubes on either the condenser or the evaporator side

  • Filtration maintains low water turbidity allowing more economical and effective chemical treatment
  • Filtration removes organic matter that can feed biological life in the cooling water
  • Filtration removes environmental debris that can be a breading ground for Legionella Pnumophelia .

CLEANING

Chemical water treatment is only an adjunct to proper maintenance of a cooling water program.   C.T.I. (Cooling Tower Institute) recommends that cooling towers be cleaned twice a year.

Annual inspection and brushing of condenser tubes is also an important part of a good PM program.  Enhanced tube heat exchangers are especially vulnerable to corrosion from even small amounts of sludge, oxidation, or environmental debris.

WATER TREATMENTS MYTHS

  1. Only the”Big Companies “ can offer the latest technology-FALSE
  2. All chemicals are the same-FALSE
  3. If you have soft water you do not need chemical treatment-FALSE
  4. If the tubes are clean, the chemicals are working-FALSE, FALSE, FALSE,
  5. All chemicals are hazardous-FALSE
  6. Magnets-FALSE
  7. Panther Unit-FALSE
  8. Ozone-FALSE
  9. “If it scales up we will clean it for free” FALSE AND DECEPTIVE
  10. Concentrated products that are mixed on site-FALSE
  11. A little white rust can not be prevented-FALSE

“NOT ACCEPTABLE” MAINTENANCE BEHAVIOUR

EXPECTATIONS

From : Various Source Document

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