If you’re doing process heating in your plant, it’s likely you have a steam-generating system involved in that process. That means you’ve probably got an expensive boiler and a network of carbon steel pipes running through the plant. It is important to protect your investment from oxygen pitting, scale and thermal shock damage. Not doing this is a recipe for costly repairs and downtime if you don’t keep your system corrosion-free.
If you’re operating a steam system already, it has a deaerator to manage that task for you. If you’re specifying a new system, it’s important to understand and weigh the value of that critical component. In either case, you need to make sure deaeration receives the attention it deserves, not only for the long-term health of your system, but also for the efficient operation of your plant. This post has information and advice to help you do just that.
The importance of boiler feedwater deaeration
Let’s start by discussing what the deaerator is and what it does. In the simplest terms, it collects and re-heats the returned condensate from the steam system. The returned condensate is then mixed with softened water that’s chemically treated to remove the oxygen, carbon dioxide and non-condensable gases from the water mixture before it’s pumped into the boiler.
The reason the condensate and make-up water need to be treated is to keep your boiler, carbon steel piping and stream traps from corroding. The oxygen content in that water enables that to happen, so getting as much of it out of the system before it returns to the boiler is imperative.
While these may seem like small, long-term maintenance concerns, it’s important to remember that a repair could take as long as a month, depending on the size of your boiler. That kind of downtime is beyond inconvenient- it’s unacceptable. That’s why deaeration is such an indispensable part of system care and reliability.
How boiler feedwater deaeration is done
Water at 70°F could contain oxygen at 8.6 ppm and water at 150°F could contain oxygen at 4.3 ppm. Oxygen content of 7 ppb is generally accepted by the ASME, but your boiler supplier should specify what level is ideal for operating their equipment.
Heat is the most common method for removing oxygen. Typical deaeration for smaller boilers utilizes a spray or combination spray and tray system to remove oxygen. The feed water is sprayed into a stream of 5 psig steam for efficient heat transfer and subsequent oxygen removal. Return condensate is distributed on a tray to generate flash steam. The flash steam helps remove any residual oxygen in the condensate return, as well as the incoming boiler feed water.
Large boiler systems with higher boiler feed water rates may utilize a vacuum system. These tend to operate at 212 0F or less. A vacuum pump or steam ejector is used to pull nearly full vacuum on a spray tower to remove dissolved gasses. These types of systems are common on very large boilers that operate at high pressures for steam turbine power generation.
One of the most important things to know when specifying a boiler feedwater system is the percentage of returned condensate you can expect. That measure will inform how much make-up water you’ll continuously need to add to the system. This will affect the overall deaerator sizing.
The mechanical deaeration process is often enhanced by adding oxygen-scavenging chemicals to the water. These chemicals maximize oxygen removal by tying up the oxygen to prevent corrosion.
The impact of make-up water on your boiler feedwater
In addition to deaeration, it is vitally important to understand boiler feed water quality. Hard water, most notably calcium or lime, will plate out on the boiler tubes and piping under high temperature and pressure. That can lead to two problems: reduced heat transfer, which limits the system’s efficiency, and/or hot spots on the boiler tubes, which can lead to tube failures. Over time, scale flakes off the pipes and tends to get caught in the steam traps. It has the appearance of a clay-like substance that can plug them up over time.
No system can recover all the boiler condensate, meaning there will be boiler blowdown. Many processes have direct-inject steam heating systems, where the steam is absorbed by the product. That means softened water needs to be constantly added to the system to compensate the difference and remove minerals.
If you have 0% condensate return (the worst case), you’ll be making up 100% of the feed water. That’s inefficient enough, but you’ll also have to re-heat that water, using a lot of the boiler’s energy in the process (as much as 12% in the 0% return scenario). In a 0% condensate return system, you’ll always be adding chemicals, too, to protect the boiler and adjust the pH. That’s more time and money. Make sure you know exactly how much water make-up you’ll need and be prepared to keep a clean supply of it.
Removal of dissolved solids is crucial to boiler longevity and efficiency. This can be accomplished using reverse osmosis or a more traditional resin based water softening system. Again, the system size will depend on the amount of condensate that is recovered and returned to the boiler.
How to avoid boiler feedwater problems
Keeping your boiler, pipes and traps corrosion-free isn’t necessarily a big task, but it is an ongoing one that requires some planning and vigilance. Here are three basic things you can do to make sure your system remains healthy and your process stays up and running:Analyze your source-water
The water that’s fed to your plant might be city water, well water or lake water. Depending on the source, it could contain anywhere from 180 ppm (lake water) to 1,250 ppm (well water) of dissolved solids. That water needs to be softened before it enters the system, and your water-softening system provider needs to know that starting point. That’s why it’s important to take source-water samples and have them analyzed before you engage the system. Then you’ll need the proper water filtering, softening and dealkalinizing systems to deliver what’s known as “zero water” (i.e., water with a hardness close to zero). Together they’ll ensure you get off to a clean start.Monitor your system
At the very minimum, it is important to check the deaeration pressure and temperature daily. The deaerator should operate at 5-7 psig and 225-227 0F. Test boiler feed water and condensate return weekly for dissolved oxygen and total dissolved solids.
Your boiler supplier will likely specify a level of total detectable solids (TDS) they believe to be acceptable in water fed to their boilers. Since dissolved solids in the water are not detectable by the human eye, you’ll need to either manually test or continuously monitor the system to identify them. The best continuous measurement is total carbon, however, this is a very expensive instrument. A good, economical, continuous measurement is conductivity. Conductivity is measured in micro-Siemens per centimeter using a conductivity meter. If it’s very clean, your feedwater will have extremely low conductivity and your boiler, system and plant will be well protected. In most cases these monitors don’t come with the system, so you’ll have to purchase and install them yourself. This is a highly recommended add-on investment well worth making to ensure the longevity of your system.Manage your chemicals
Even with maintaining DO and TDS within recommended limits, these very small amounts can cycle up over time. The concentration of solids and chemical reactives naturally increase over time and if you don’t eliminate them periodically or continuously, they will concentrate in the system. From time to time it’s necessary to remove dissolved solids, suspended solids, sludge and chemical additives from your boiler feed water. When build-up happens, these chemicals need to be sent to either the drain (blowdowns) or the atmosphere (blowoffs). The frequency of these blowdowns and/or blowoffs will depend on the percentage of condensate your system returns and the quality of your raw water. As always, the best way to manage these conditions and decisions is continuous system monitoring.
As we mentioned at the outset, repairs to boilers and steam systems are costly in both dollars and downtime. You likely can’t afford either, so do your research, understand your system and be vigilant with your monitoring and management. If you’re at all concerned about designing, specifying or operating one of these systems, be sure to consult an experienced engineer. It’s the best way to get one that’s scaled, equipped, laid out and running in the most efficient and effective way possible for your budget, your plant and your process.