Frequently a customer calls saying, “The boiler feed pump won’t keep up with the
boiler.” If the pressure and capacity was properly selected, the likelihood of this
happening is remote. However, when the boiler feed pump is running and the water
level is slowly descending to low level cut-off, it is difficult to convince someone that
the fault does not lie with the boiler feed pump.
In nearly every instance, we have found excessive boiler carryover to be the culprit. In
other words, boiler water is leaving with the steam which results in water requirements
exceeding pump capacity. Low pressure boilers are primary offenders although high
pressure units are sometimes responsible.
Carryover can be either mechanical or chemical. Some of the causes of carryover are:
1) Boiler construction-improper internals
2) High water level or improper location of water column
3) Surge firing, on-off firing, or on-off feedwater addition
4) Surge loads where steam demand exceeds boiler capacity
1) Improper cleaning of new boiler
2) High suspended solids
3) High alkalinities
4) Oil in the boiler
5) Improper water treatment and control
The simplest method of determining boiler carryover is the chloride test. Chlorides
are not chemically affected and offer a direst percentage relationship.
For instance, if the raw water has 40 ppm (parts per million) of clorides and the system
has 80% returns, the chlorides in the deaerator should be around 8 ppm. Boiler
chlorides are generally 5 to 10 times raw water or in this case, between 200 and 400
ppm. Suppose under the above conditions your deaerator tests at 200 ppm of Cl with
the boiler at 350 ppm. You can see that the carryover exceeds 100% of the steam
demand. This is not unusual for a high carryover situation, and I have been at jobs
where the daerator water and the boiler water had exactly the same chemical
composition. This includes high pressure jobs as well as well as low pressure. There is
no way a properly selected boiler feed pump can keep up with boiler water
requirements under these circumstances.
On installations where there are no returns, it might be necessary to run tests on
steam samples from the main feed line. Sometimes tests can be run from trap
discharges or condensate receivers located in the plant. Any competent water
treatment analyst is capable of ascertaining the existence of carryover and computing
the exact percentage.
If the water level in the boiler is dropping, or the pump seems to cavitate for no reason,
check for carryover. It may explain what is happening.