Separating throttle wet vapor
WET STEAM can be caused by:
1. Undesired carry-over of un-evaporated water from the boiler.
2. Normal condensation of saturated steam in the distribution network or process equipment.
PREVENTION OF UNDESIRED CARRY-OVER
Carry-over from the steam boiler is basically categorised as foaming or priming. This may cause sometimes severe operating and maintenance problems, which not only impact on plant economics, but can have serious safety implications.
In general terms, this is mainly caused by excessive levels of total dissolved solids, which have to be controlled by proper water treatment and boiler blowdown. However, other chemical imbalances, erratic load conditions, poor water level control and/or mechanical boiler defects may be responsible.
Many times the root cause of an operating problem or maintenance issue is not correctly identified. Take the example of a hospital boiler system where the decision was made to reduce the boiler pressure from 10 bar g to 8 bar g, in the interest of better energy efficiency due to higher latent heat of steam at lower pressure. Apart from many other more or less subtle side effects, the apparently simple action of turning down the burner firing rate resulted in higher steam disengagement velocities during high steam demand periods, which in turn allowed unevaporated water to carry over into the steam distribution system. This resulted in greatly increased maintenance requirements for valves, steam traps and regulators, as well as serious issues with the integrity of the steam sterilisers due to the impurities introduced by residues from boiler water chemicals.
Just so we don't create the impression that the boiler pressure reduction as such was a bad idea: far from it. What is however important is that all modifications to a steam and condensate system must be approached from a WHOLE SYSTEM point of view, which invariably requires extensive practical experience with this subject.
The measure of steam quality is termed Dryness Fraction, expressed as % dryness in steam.
Relatively low cost instruments available for determining the Dryness Fraction are calorimeters (either throttling, separating or combined) or conductivity-based meters which relate the TDS (total dissolved solids) in boiler water to the TDS in the steam sample. For most industrial applications where steam purity measurements are carried out on a grab-sample basis, these methods can be sufficiently accurate to give a meaningful representation of Dryness Fraction (but make sure you can get a representative sample in the first place!). For high pressure steam systems, much more accurate methods such as sodium ion analysers are required, together with continuous monitoring.
The cost of WET STEAM due to carry-over from the boiler(s) can be substantial. If your steam includes say 10% moisture, your steam heated equipment (heat exchangers) cannot achieve more than about 90% of rated capacity.
Carry-over from boilers can scour the pipe internals, increasing both the dissolved and suspended iron content in the steam and condensate network, resulting in greatly increased maintenance costs.
This can be insured against by installing an efficient steam separator. We have available designs which can ensure better than 99% dry steam (dryness fraction >0.99)
It is also possible to incorporate a steam sampling nozzle upstream of the separator.
This will allow the periodic attachment of a calorimeter to measure actual moisture content. A further option is a second sampling nozzle on the downstream side, to facilitate checking and confirming of the separating efficiency.
A separator should also be used upstream of any primary flow meter element, e.g. orifice plate, venturi or vortex meter, to ensure accuracy.
Systems such as these invariably show excellent cost benefits, from the point of better energy efficiency due to dry steam and because of the subsequent lower maintenance and increased productivity. Accurate flow and dryness fraction measurements should no longer be regarded as optional extras, but as important tools to the overall management of steam systems.
Remember, IF YOU CAN'T MEASURE IT YOU CAN'T MANAGE IT.
