Boiler Carryover | Rasmussen Mechanical Services
What is Boiler Carryover?
Boiler carryover is when something other than clea n steam leaves the boiler through the header. This often includes things like liquid, contaminants in vapor and even solids. Low pressure boilers are generally more susceptible to carryover but high pressure systems can have these issues as well.
What Causes Boiler Carryover?
Carryover can be caused by either mechanical or chemical means.
Types of Mechanical Carryover
- Improper boiler construction/ design
- High water levels
- Surge firing, on-off firing, or on-off feedwater addition
- Running the boiler in excess of boiler steam capacity
- Improper blowdown
Types of Chemical Carryover
- Improper chemical cleaning of new boilers
- High solid concentration (Creates increased surface tension)
- Organic Contaminants (Oil, grease, soap, fat, milk solids etc.)
- High alkalinity
- Improper water treatment
Luckily the science behind chemical carryover is pretty simple. Surface tension of boiler water rises as the alkalinity and conductivity or TDS increases. The higher the surface tension the more difficult it is for the steam to break through the surface of the water cleanly. This is what causes small droplets to be carries into the header and foaming to occur on the boiler waters surface.
Issues with Carryover
When water is leaving with the steam, this creates two BIG issues. One, boiler water requirements will commonly exceed pump capacity causing water levels in the boiler to fall. This can confuse operator and make it appear like you have a boiler feedwater pump issue. In these situations your boiler will generally shut down as it hits a low water level.
The second issue is that as that water get up into the header, it interacts with the steam already in the system. This causes a phase change to occur and the water converts into steam. This phase change causes the water to expand to 100 times its original size and creates water hammer throughout the system. In extreme cases this can cause steam pipes to be ripped apart and can create a serious safety hazard.
When carryover is mild, it can have some pretty detrimental side effects on the system as well. As small amounts of water enter the system, the water flashes to steam. This causes it to drop all the chemicals and particles it was carrying into the steam piping. This eventually causes traps, strainers and control valves to clog. It can also cause process steam to become contaminated resulting in negative effects on product quality.
How To Test For Carryover
There are a number of ways to test for boiler carryover, this includes condensate conductivity testing, Alkalinity testing, or chloride testing. Chlorides testing is often the simplest way to determine if you have boiler carryover. This is because chlorides are not chemically affected by boiler conditions. According to Viking Water Technology : “Chloride ions…are extremely soluble and do not precipitate or decompose when subjected to boiler conditions. Therefore, chlorides are used as a measure of boiler water concentrations.”
In systems that don’t have returns, you can check for carryover in the main feed line. This involves taking samples from trap discharges or condensate receivers. From the sample you will be able to compute your carryover percentage.
Call the pro’s at Rasmussen Mechanical Services at 1–800–237–3141, email sales@rasmech.com or chat with an agent . Our team is here to help!
Originally published at https://www.rasmech.com on November 12, 2020.
