2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
52
Clean Steam
Overall Requirements of a
“Clean” Steam System
The overall requirements of a
“clean” steam system, irrespective
of the means of generation of
production used, can be very simply
stated:
It is essential that the
steam delivered to the point of
use is of the correct quality
and purity for the process.
In order to achieve this end
goal, there are three key areas of
design which must be considered
once the requirement for clean
steam has been identified.
• Point of Use
• Distribution
• Production
Design and operation of
equipment, piping, components,
etc. in all these three areas will
influence the quality of the final
process or products. It is essential
for the needs of the user
process to be the first concern.
Must the steam be pyrogen free?
Are any boiler additives allowed?
Are products of corrosion going to
harm the process or product?
Must the risk of biological contamination
be totally prevented? It
is by answering these questions,
and perhaps others, which will
indicate the required type of production,
design of the distribution
system, and the operation modes
of the user equipment, including
aspects such as steam trapping.
Specific Requirements of
“Clean” Steam Systems
Clean or pure steam produced
from water of very high purity is
highly corrosive or “ion hungry”.
The corrosive nature becomes
more pronounced as the concentration
of dissolved ions decreases
with the resistivity approaching the
theoretical maximum of 18.25
megohm/cm at 25°C. In order to
recover a more natural ionic balance,
it will attack many of the
materials commonly used in
pipework systems. To combat this,
pipework, fittings, valves and
associated equipment such as
traps, must be constructed from
corrosion resistant materials.
Typically, a “clean” steam system
of this type will have resistivity values
of the condensate in the 2-15
megohm/cm range, resulting in
very rapid attack of inferior quality
components.
Even in some filtered plant
steam applications, such as in the
food, dairy and pharmaceuticals
industries, certain corrosion inhibiting
chemicals may be prohibited
from the boiler and steam generating
system. Again, condensate is
then likely to be very aggressive
and so careful consideration must
be given to material selection.
A common problem encountered
on clean and pure steam
systems in the pharmaceutical
industry is that of “rouging”, which
is a fine rusting of pipes and system
components. This is
encountered most frequently
when low grade stainless steels
are used, and further corrosion
due to galvanic effects can take
place where dissimilar alloys are
present in the same system.
Unless care is taken with material
selection throughout the
system, corrosion can become a
major problem in terms of:
a) Contaminating the system
with products of corrosion,
which are undesirable or
even potentially dangerous to
the process or product.
b) Severely reduce life of system
components, increasing
maintenance time, material
replacement costs, and system
downtime.
In order to prevent these
problems, austenitic stainless
steel should be used throughout,
never of lower grade than AISI
304. For severe duties, the recommended
material is AISI 316
or 3161L (alternatively 316Ti) or
better, passivated to further
enhance corrosion resistance.
In summary, 316 or 316L
stainless steel is essential in
pure steam systems from its production
at the generator right
through to the steam traps. Not
only will inferior materials corrode
and fail prematurely, they will also
lead to contamination of the system
as a whole. Note that
although filtered plant steam will
not necessarily be so aggressive
by nature, the exclusion of many
of the corrosion inhibiting feed
chemicals for end product purity
reasons will still demand the use
of austenitic stainless steel, never
of lower grade than 304/304L, but
preferable 316/316L.
Clean Steam and Condensate
System Design
The proper and effective drainage
of condensate from any steam
system is good engineering practice,
as it reduces corrosion,
erosion, and waterhammer, and
increases heat transfer. This
becomes even more important in
“clean” steam system, where
poor condensate drainage in the
distribution system or at the user
equipment can result in rapid corrosion
and also, under certain
conditions, the risk of biological
contamination. The following
points should be carefully considered:
• Pipework should have a fall in
the direction of flow of at least
1.0 inch in 10 ft., and should be
properly supported to prevent
sagging.
• Adequate mains and service
pipe steam trapping should be
provided, for example at all vertical
risers, upstream of control
valves, and at convenient
points along any extended pipe
length. Trapped drain points
should be provided at intervals
of at least every 100 ft.
• Undrained collecting points
should not be used, as dirt
should not be present and they
provide an ideal location for
bacterial growth where systems
are shut down.
• Condensate should be allowed
to discharge freely from steam
traps using gravity and an air
break. This air break should be
provided at the manifold outlet or
the closest convenient location
(Fig. 57). Where the air break
would otherwise be in a clean
room, the potentially harmful