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My water system is scaled up! What should I do to prevent cooling tower or boiler scale?
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WHO IS RESPONSIBLE FOR THE SYSTEM’S SCALE?
As a water treatment professional, I am used to being blamed for a lot of things. A few months ago, we ran a blog titled STOP! Don’t throw your water treater under the bus. It is an unfortunate reality that chemical treatment is often first to receive blame since the use and application of chemicals is often misunderstood. The truth is usually some type of mechanical failure occurs, resulting in the scale build up. However, this doesn’t mean the water guy is completely out of the woods. In this article we will discuss some of the more common causes and what to do to resolve the problem.
 
WHY DOES SCALE OCCUR?
 
COOLING TOWERS
Most towers are treated with either city or well water with makeup hardness levels ranging from 100 to 1,000 ppm of calcium hardness. Others use softened water from softeners or reverse osmosis units that are either blended with some hard water or fed with straight softened water. A last group includes towers that use recycled, reused, wastewater, or some other source that is not common. We will not get into that group today. 

City/Well Supply
– acid is fed to reduce the alkalinity and prevent the calcium carbonate from causing scale on the tower, condenser coils, or heat exchanger. The pH range and alkalinity to run this need to be determined by your water treater using various scaling indexes such as Puckorius, LSI or RSI.  This will vary depending on make-up water quality and system operation. NOT ALL SYSTEMS AND PROGRAMS ARE THE SAME. Here are common problems that can cause scale.
  • pH probe issues
    • This is typically the sole way acid is fed. When the probes are not checked and monitored regularly, they can get out of calibration. When out of calibration you can overfeed or underfeed acid. Underfeed = scale forming conditions. Overfeeding = bigger problems, too much acid for a long enough time will corrode the metallurgy of the tower and coils.
  • pH probe solutions
    • Having redundancy; two probes in line should not fail at the same time. You can visually see when one probe is starting to get out of spec.
    • Online monitoring. Most controllers can be online now. The MAJOR benefit is an alert via email/texts are sent out immediately to your water treatment representative and staff when something needs attention or is out of range.
    • Set a maximum runtime for the acid pump. This will stop the pump and trigger an alarm
    • Regular manual testing of the pH to confirm the probe readings. If you have the manpower!
  • Pump failures
    • Acid pumps take a lot abuse. If the pump fails and you don’t feed acid, scale will build up quickly
    • Inventory is a critical factor as well. Running out of acid can cause days of high pH/Alkalinity quickly leading to scale
  • Improper Water Treatment program
    •  This goes back to what I mentioned earlier about setting up each system properly from the start, with attention paid to the Scaling Indices. NOT ALL SYSTEMS AND PROGRAMS ARE THE SAME.  Just because one set up works in one City, County, or State it does not mean that is the right program for you. This is where you can place the blame directly on your water treater for not matching the program to your specific water quality. In some cases, the water can be so poor that traditional water treatment will not solve the problem. Additional pre-treatment equipment may be required. Trust me, we have been there!
Softened Supply
In general, if scaling is happening on a system that is designed with softened water make-up its related to the softener not working, however, that’s not always the case. To learn some preventive maintenance tips on maintaining your softener, read The Hard Facts: Water Softener Tune-ups Help Maximize Water Quality and Reduce Costs.
 
Common Soft Water Problems:
  • Process contamination
    • Leaking heat exchanger with hard water getting to the tower.
  • Blend valve issues
    • A lot of systems on soft supply have a blend valve to allow a small amount hardness (10-30 ppm) in the system. If a valve is closed or not functioning that can change the make-up quality.
  • Water Softener
    • No salt in the brine tank.
    • Softener lost power.
    • Softener is in bypass.
    • Softener Control Valves are leaking, not drawing brine, etc. need to be serviced.
 
BOILERS
All steam boilers need to be on a softened supply of less that 1ppm of hardness for make-up, there is no way around it. Iron scale is also something that can occur. Here are some of the most common causes:
  • Softeners or other pre-treatment not working will cause calcium scale, there is no way around it.
  • Condensate contamination with hard water.
  • Condensate chemical treatment or lack of it will cause iron scale to occur. On the left is a scaled-up boiler tube. On the right are clean tubes free of scale.
On the left is a scaled-up boiler tube. On the right are clean tubes free of scale.
 
NOW WHAT?
First things first – you need to understand why this happened and fix the root cause of the issue. Continuous descaling of systems is expensive due to energy and gas loss from inefficiency, down time in production, and labor.  Now it’s time to descale your system and get back to normal operation.  
 
 
The charts above show how even small amounts of scale reduces system efficiency.
 
HOW TO DESCALE?
The process is simple but there are risks associated with various methods of treatment. To remove the scale, specialty cleaners or acid is fed and circulated through the system. The type of acid used depends on the kind of scale you are trying to remove.  For example, if there is calcium scale in tower, an inhibited sulfamic acid could be best. If iron needs to be removed from steam boiler tubes, then nitric acid could be used. The type of acid to be used should be determined with help from your water treatment company. On the left is a scaled-up system. On the right is a system that is managed properly showing no scale. Not all systems can be descaled. If the tower scale is so solid that the coils have no gaps or the fill on the side is too solid, not enough water/acid can flow to penetrate the calcium.  To remove scale water must be able to flow through and on all the areas that need cleaning. Before chemically descaling any system it’s best to try and physically chip away and remove as much scale as you can. Warning! Acid is aggressive on scale and metal! If not applied properly, damage to your metal can occur. Ask your water treatment supplier about inhibited acids or specialty cleaners that are not acid based. 
 
On the left is a scaled-up system. On the right is a system that is managed properly showing no scale.
 
PROCESS
For a successful descale, the system needs constant recirculation. In some cases, the system recirculation pumps can be used. If not, an external skid system with a storage tank and pump is required. About 1lb of acid will dissolve 1lb of scale.  Since we never know how many pounds of scale are in the system it’s best to go with the 10% rule, for every 100 gallons of water held add 10 gallons of acid. 
 
Once the acid is added and the water is circulating it’s best to sample the pH of the water regularly. After initial dosing of the acid, the pH should be between 1-3.  As the acid is dissolving the calcium scale, the pH will rise back to neutral pH of 7.  The goal is to keep adding acid until the pH stays below 2 and does not increase. During that process foam will occur. To prevent a giant foam party from happening, a silicone defoamer should be added to the system as needed.  The cleaning process will take several hours. 
 
The system will need to be drained after, and sometimes during, the cleaning. In many cases you can dump the system to drain. Some municipalities have strict pH discharge limits, so adjustments may be required. Check with your municipality prior to starting the job. 
 
OVERVIEW
Nobody wants to descale a system, but we all know accidents happen. It’s key to understand why it happened and find a way to prevent it from happening again. Whether its increased process checks or improved automation and remote monitoring of your water system, do whatever it takes to minimize the cause. This will help your system run as designed, reduce downtime, and improve efficiency. If you have specific concerns about scale in your systems, reach out to us for a Site Survey and one of our technical engineers will be in touch.
 
 
 
Jason Waack, Territory Manager, Watertech of America, Inc.
Jason has over 15 years’ experience in the water treatment industry. He has been a Territory Manager with Watertech for 5 years, managing accounts in the central part of Wisconsin. Jason frequently leads educational training events and seminars to promote better water management and improve safety.

 
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