Chemical Engineering December 2014 - 55

Environmental Manager
cable fire-fighting systems should
there be a fire in a unit.
Fire-water and utility-water systems
have sometimes been combined
in non-critical plants. In the
event of a fire, the connected utility
water system would be tripped.
However, these combined systems
are always risky. Various fire codes
recommend that no utility-water
connections be made to the fire-water
system. In some special cases,
the fire-water system may be used
for emergency process-cooling requirements,
but only as the secondary
(reserve) supply.
Fresh (treated) water is always
preferred (over seawater, brackish
or untreated water, for instance) for
fire-water systems in all onshore
plants. Untreated or brackish water
can cause many issues such as corrosion,
which can potentially wreak
havoc on the system components.
In general, engineers should purchase
or construct the fire-water
pumping system and the fire-water
distribution system using proper
materials (for instance, selecting
suitable corrosion-resistant materials
or proper protective coatings),
because untreated raw water (such
as seawater) could be used as the
secondary source for extra fire-water
capacity, in the case of an unexpected
fire event. If this happens,
the fire-water system should be
flushed with treated water after the
incident, to remove residual traces
of untreated source water.
Selecting fire-water pumps
Centrifugal pumps with a relatively
flat characteristic performance
curve (a graph of head versus
flowrate) are generally selected
for fire-water pumps. Ideally, the
head should rise continuously from
the rated point to the shutoff point,
with only a small increase of head
(say, a 9-15% rise of the head from
rated point to shutoff point). These
pumps can provide a steady, stable
flow of water at a relatively uniform
pressure over a wide range of required
fire-water flowrates.
A relatively flat performance
curve is always encouraged for centrifugal
fire pumps for the following
reasons:
* The control of a fire event usually
requires variable amount
of water at a relatively constant
pressure
* Fire-water pumps are typically
operated in parallel. A relatively
flat curve ensures troublefree
parallel operation
Sometimes, a large amount of
water can be required by the firewater
system to battle a vast fire;
in those cases, the required water
could be considerably larger than
the rated flow of the pump. In
this regard, the fire-water pump
overload point (the end operating
point at the right side of the pump
curve) should demonstrate a capacity
of more that 150% of the rated
capacity at a head more than 70%
of the rated point. In other words,
operation point could move to the
far right side of the rated point and
that point should offer sufficient
flow and head.
A steep pump curve should always
be avoided. As a rough indication,
the average slope of a fire-water
pump curve should preferably
be around 10-20% (for instance, an
average slope of 1/10 up to 1/5).
Fire-water pumps can idle
against closed valves for a short
period of time. In other words, for a
short time, the pump should be able
to operate in a closed water system
without any fire-water application.
Check valves should be provided
at both the discharge and the suction.
The rated pressure of a firewater
pump could be 4-30 barg.
Single-impeller centrifugal pumps
(for applications that require pressure
below roughly 12 barg), and
multi-impeller centrifugal pumps
(for higher-pressure systems) are
also commonly used.
The differential pressure of a
pump is proportional to both the
square of the rotating speed and
the square of the impeller diameter.
A discharge pressure of around 10
barg can be obtained by a relatively
large, single-impeller pump (with a
suitable speed).
Overhung (OH) pumps have been
used for small- and medium-sized
fire-water pumps. Users should
consider the between-bearing (BB)
pump design when size, power rat54
ChemiCal engineering www.Chemengonline.Com DeCemBer 2014
FIGURE 2. Shown here are several
examples of fire-water pumps; an identical
spare pump is commonly used to increase
the reliability of fire-water pumping
systems
ing and power-density exceed a certain
level. As a rough indication,
this limit could be 400 kW.
As noted, the fire-water pumps installed
at any given facility should
be able to operate in parallel. However,
there are some challenges
and issues in ensuring parallel operation.
Even in certain conditions,
pumps designed to operate in parallel
could be subject to overheating
or damage. A well-known danger is
one pump operating at higher flow,
forcing another pump to operate at
lower flow; operation at lower flow
can be damaging to the pump.
When fire-water pumps are operated
in parallel, the pump with the
lowest head may work at a reduced
flowrate. In this way, the pump
could work far from the " best efficiency
point " with a very low efficiency,
high friction and heat generation,
which can result in damage.
Even in identical fire-water pumps,
pumps that have been in use for
more hours (and thus has probably
been subjected to more wear), pumps
with minor defects, and pumps with
slightly lower speed could all be
subjected to a reduced flow, which
can create problems during an actual
fire event. Because of this effect,
operators should rotate pumps
over time, so that each pump works
as the main fire-water pump for
some period of time; this can help to
ensure even wear patterns among
identical pumps in service. Individual
protection against the minimum
flow (to ensure a minimum flow for
each pump) is recommended.
Monitoring of the differential
temperature of each pump can provide
valuable insight for estimating
the parallel operation issue (the re
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Chemical Engineering December 2014

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