Steam System Problems that Cause Dryer Flooding
A previous blog post identified mechanical problems that could cause paper dryer flooding. Dryer flooding occurs when condensate is being evacuated from the dryer at a rate that is less than the rate at which steam is condensing in the dryer. Here are nine problems related to steam systems that could cause dryer flooding.
- Low differential pressure - Most system problems are related to running at a low differential pressure. This may be the result of setting the DP too low or from controlling to an inaccurate DP signal. The best way to troubleshoot steam system problems is to directly measure the differential across the dryer (not just between the headers). This requires pressure taps on both the steam supply side and the condensate drainage side of the rotary joint. (Care must be taken to do this work safely).
- Temporary loss of differential pressure - This problem is most common on high-speed machines operating with rotating syphons. If there is a loss in differential pressure, even a temporary loss, the condensate will cover the clearance between the syphon and the shell. If the differential pressure is restored but is not above the flood-recovery differential pressure, the condensate will not be removed from the dryer. The addition of an aspirator hole in the rotating syphon will significantly improve the flood-recovery performance of the syphon. It is not uncommon to have a temporary loss in dryer differential pressure during a brief stop or during a sheet break, particularly with older steam and condensate systems.
- Differential pressure too low - If the normal dryer operating differential pressure is set and controlled to a value that is too low to handle the current operating speed, steam pressure, and condensing load, then the dryer will tend to load up with condensate. The Tappi Water Removal Committee has published a Technical Information Paper on recommended operating differential pressures. TIP 0404-31.
- Desuperheater flow - Water is sprayed into the steam supply piping to reduce the temperature of the steam to a value closer to the saturation steam temperature. If the instrumentation, controls, or water control valves are not properly functioning, an excessive amount of water can be dumped into the steam header. This will greatly increase the amount of water that has to be evacuated from one or more of the dryers in that section.
- Improper orifice plate size - If the condensate orifice plate is too small, the pressure drop will be higher than expected and the dryer will not drain at the normal operating differential pressure.
- Incorrect differential pressure indication - This is one of the most common problems. If the indicated differential pressure is higher than the actual differential pressure, the dryers may flood even though the differential pressure appears to be adequate. There are several causes of incorrect DP readings:
- Transmitter is not functioning
- Transmitter is not properly calibrated
- Water legs on each side of the transmitter are not equal
- Water leg on one side of the transmitter has air in the line
- Seal pot is located below one of the headers with poor connection lines
- Differential pressure is calculated from two pressure readings (double calibration errors and larger range for errors)
- Pressure and DP readings are “calibrated” against inaccurate gauges
- Improper DP valve operation - An inoperative DP valve can cause dryers to flood. If the controls indicate 50% opening, for example, and the DP valve goes to 20% or if it closes when it is supposed to be opening (reversed operation), the dryers will flood.
- Choked thermocompressor - If the dryer thermocompressor is undersized, it may be running in “choked flow”, where an increase in valve position will result in reduced blow-through steam suction flow rate.
- Improperly sized thermocompressor - If the thermocompressor is undersized, it will not be able to generate enough differential pressure and recirculate the blow-through steam.