Vaisala HMP228 - Manual (Page 8)

HMP228
Operating Manual
M210282en-A
2
ppm) is that the saturation point remains stable regardless of the oil type or the
aging of oil, additives used etc. As water activity exceeds 0.9 aw in any
system, there is a risk for segregation (especially if the temperature decreases).
The water activity is used for alarming at the point of >0.9 aw that the risk for
free water in the system is obvious. The most important advantages of this
system are the fact that water activity is immune to the aging of oil and to
additives, and that the HMP228 transmitter can be used for continuous on-line
measurements. In addition, the HMP228 can be calibrated against salt
solutions and no reference oils are needed.
1.2.2 Lubrication oil in paper machines
Typically, a paper machine contains two or three separate lubrication systems.
Usually, one is located at the wet end and the other at the dry end. There is a
certain amount of free moisture constantly present which means that there is a
risk of this moisture becoming into contact with the machine bearings. The
most common reasons for the entrance of water are an inadequate sealing of
the housing and cleaning with high pressure. However, accidental leakages
from oil coolers and other equipment may also cause damage. In paper
machines, the oil should absorb water while lubricating the bearings and then
release this water when collected into the reservoir. It is to be noted that
bearings should never be exposed to oils that have a high water content; this is
especially important during standstill because the risk for corrosion process
increases as the oil temperature decreases. It is essential to monitor the water
content and keep it on a suitable level.
When measuring the water content of oil in paper machines, it would be useful
to measure the water activity before an oil reservoir and from a pressure line
flow. This way, the performance of dehumidifiers can be kept under control to
ensure that no free water reaches the bearings.
1.2.3 Transformer oil
The determination of moisture in oil is an essential part of a comprehensive
transformer maintenance program. Aging and deterioration increase the
capacity of oil to absorb water. The primary interest in transformers is to
measure the water not in oil but in the cellulosic insulation around the
transformer windings. Heating and cooling have a considerable effect on
moisture levels in oil. While temperature raises, the paper insulation of
transformers tends to loose moisture which is absorbed by the oil surrounding
it. Saturation level is thus a true indicator of moisture present. The HMP228
method provides for a reliable detection of the aging of oil and possible
leakages. Water activity helps to prevent heavy overloadings and to monitor
the transportation of moisture in the transformer.