Deaerator Achieving Performance
My name is Kelly Paffel, I’m the Technical Manager for Inveno Engineering LLC. We’re located in Tampa, Florida. We are an engineering firm working with specific deaerator steam and condensate systems, and today I want to talk about the
Is the plant deaerator system achieving maximum performance or is the plant using excessive amounts of sulfite chemicals to reduce oxygen content in the boiler feedwater system? The deaerator performance is a critical part of the steam and condensate system to prevent corrosion in the steam system.
The deaerator operation provides a high-quality water for the boiler system, non-condensable gases removal and elimination, and ensuring a high steam quality for process applications. The deaerator performance is measured in units or parts per billion. Seven parts (7 ppb) per billion is the international standard, therefore the deaerator operating without sulfite being added for 48 hours needs to be operating at seven parts per billion dissolved oxygen level.
Now, there are two types of deaerators. Tray type, agitation is provided by spilling water over stacked trays, steam is injected into the tray section to bring the temperature up and let the non-condensable gases come out of the solution. The other type is atomizer or spray, condensate makeup is sprayed through a set of spray nozzles, usually hitting a diffuser, and steam is injected into the process to bring the temperature up and again let the gases come out of solution. Either tray or spray should be able to bring the dissolved oxygen content down to seven parts per billion.
Just to comment on deaerator tray design. Trays assist in the non-condensable gas removal, so up here there are trays here where the water comes and drops across the trays and drops into the bottom of the storage tank, and down below it is feed water. Now, the spray type is two types, fixed and variable orifice. The makeup and condensate comes in here and sprays out, hits a diffuser system and the non-condensable gases are removed. Again, both types gets the dissolved oxygen down to seven parts per billion. Scrubbing gas steam, two actions. Preheats the liquid for the boiler operation and increases the water temperature for gas elimination, non-condensable gas elimination, mechanical separation, defusion of gas particles surrounding the atmosphere.
The thing I really want to talk about is steam venting. All deaerators must vent a percentage of steam, and the reason why is that we have to get the non-condensable gases to come out of the solution, and we’ve got to get the non-condensable gases out of the deaerator. So out here on top of the deaerator, there should be steam being vented out, which as you can see in the picture. The thing is, is that we have to know what is the correct amount of steam that we need to vent to make sure we’re removing all the non-condensable gases?
Old technology methods for testing deaerator performance always used to say 14 to 18 inches of a plume. This plume should be 14, 18 inches, and the other thing, a small plume is good, then a larger volume would be outstanding, but that would be a significant energy loss then. Energy, and really the venting has no bearing on the true deaerator performance. So we could have a tremendous velocity going out, and it really has no bearing on the deaerator performance. The other method is sulfite residual. We do that chemical test daily typically, and tells us that we have a sulfite residual, but it doesn’t tell us that we have removed the non-condensable gases.
The deaerator can have sulfite residual but fail the dissolved oxygen test. Therefore, we need to be looking at the deaerator performance, and the thing is, is that the deaerator, we test it to the ASME standard for testing deaerators, which is PTC 12.3. Sulfite is removed 48 hours prior to testing, so we don’t want any sulfite in the deaerator, we want a true performance of the deaerator without sulfite. Now, we achieve or do not achieve seven parts per billion or lower. If the deaerator is running at seven parts per billion, then we use sulfite to drive the dissolved oxygen close to zero. This test should be done at least every six months. And now today, plants have online measurements. So it’s really a critical test.
Conducting dissolved oxygen testing and adjust the steam venting based on dissolved oxygen test. The thing is, is that we know that if we are at seven parts per billion, or excuse me, three parts per billion, we can reduce the amount of venting and we want to hit somewhere around five to seven, and then use sulfite to bring it down to the rest of the way. If we have a 30 parts per billion test, then we can increase the venting to assist in getting rid of the non-condensable gases or the oxygen out of the deaerator, and get our reading down to seven parts per billion.
The other thing then we can do, we can adjust the steam pressure. We always say deaerators should run at six PSI. Not so. I mean, we can run deaerators 150 PSI, but we can adjust up to 10 PSI and see what the effects of that is on our dissolved oxygen testing. So there’s a number of things that we can do with the deaerator operation to achieve this seven parts per billion. Why we stress this is that 30% of the deaerators that we find in operation are not meeting specification. The other thing is you must have a proper sampling system. As shown here, they have all the different samples coming down into a central located sample with a sample cooler here, and it allows the plant personnel to take the test easily.
The other thing is, is that all deaerator internals, spray nozzle, springs, sprays, internal, should be visually inspected. If possible, have the spray action occur. Like this here, this is showing the spray action, that we are getting spray coming out, hitting the diffuser. This deaerator happened not to meet specification. We just then get the water to break down into the smallest molecules possible so we could have proper deaeration. The other thing is, is there is online oxygen testing, which there are several vendors, and they’re very accurate, this one here, 0.1 PPB, and they’re very affordable today and it ensures proper operation of the deaerator.
That’s the conclusion. The comments is make sure you’re testing the deaerator at least every six months, if not every three months. Look at online testing. But it’s very critical that we have the deaerator system in operation. Our approach here is that we do short term impact, so steam system engineering assessments, steam system balancing, steam system performance and steam system engineering training, longterm upgrades, process changes and lowering steam cost. If you have any questions regarding or are looking for more technical information, please contact us down here. Here is our email addresses, and we’d be more than happy to help you. Have a great day.