2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
48
Condensate Pumping
In nearly all steam-using plants,
condensate must be pumped
from the location where it is
formed back to the boilerhouse,
or in those cases where gravity
drainage to the boilerhouse is
practical, the condensate must be
lifted into a boiler feed tank or
deaerator. Even where deaerators
are at low level, they usually
operate at a pressure a few psi
above atmospheric and again, a
pump is needed to lift condensate
from atmospheric pressure to
deaerator tank pressure.
Electric Condensate
Return Pumps
When using electric pumps to lift
the condensate, packaged units
comprising a receiver tank (usually
vented to atmosphere) and
one or more motorized pumps
are commonly used. It is important
with these units to make sure
that the maximum condensate
temperature specified by the
manufacturer is not exceeded,
and the pump has sufficient
capacity to handle the load.
Condensate temperature usually
presents no problem with returns
from low pressure heating systems.
There, the condensate is
often below 212°F as it passes
through the traps, and a little further
subcooling in the gravity
return lines and in the pump
receiver itself means that there is
little difficulty in meeting the maximum
temperature limitation. See
Fig. II-74 (page 119).
On high pressure systems,
the gravity return lines often contain
condensate at just above
212°F, together with some flash
steam. The cooling effect of the
piping is limited to condensing a
little of the flash steam, with the
remainder passing through the
vent at the pump receiver. The
water must remain in the receiver
for an appreciable time if it is to
cool sufficiently, or the pump discharge
may have to be throttled
down to reduce the pump’s capacity
if cavitation is to be avoided.
See Fig. II-75 (page 119).
The PUMP NPSH in any given application can readily be estimated
from:
NPSH = hsv = 144 (Pa - Pvp) + hs - hf
W
Where:
Pa = Absolute pressure in
receiver supplying pump,
in psi (that is at atmospheric
pressure in the
case of a vented receiver).
Pvp= Absolute pressure of
condensate at the liquid
temperature, in psi.
The absolute pressure at the
inlet to the pump is usually the
atmospheric pressure in the
receiver, plus the static head from
the water surface to the pump
inlet, minus the friction loss
through pipes, valves and fittings
between the receiver and the
pump. If this absolute pressure
exceeds the vapor pressure of
water at the temperature at which
it enters the pump, then a Net
Positive Suction Head exists.
Providing this NPSH is above the
value specified by the pump manufacturer,
the water does not
begin to boil as it enters the pump
suction, and cavitation is avoided.
If the water entering the pump is
at high temperature, its vapor
pressure is increased and a
greater hydrostatic head over the
pump suction is needed to ensure
that the necessary NPSH is
obtained.
If the water does begin to boil
in the pump suction, the bubbles
of steam are carried with the
water to a high pressure zone in
the pump. The bubbles then
implode with hammer-like blows,
eroding the pump and eventually
destroying it. The phenomenon is
called cavitation and is readily
recognized by its typical rattle-like
noise, which usually diminishes
as a valve at the pump outlet is
closed down.
However, since in most cases
pumps are supplied coupled to
receivers and the static head
hs = Total suction head in feet.
(Positive for a head above
the pump or negative for a
lift to the pump)
hf = Friction loss in suction piping.
W = Density of water in pounds
per cubic foot at the
appropriate temperature.
above the pump inlet is already
fixed by the pump manufacturer, it
is only necessary to ensure that
the pump set has sufficient
capacity at the water temperature
expected at the pump. Pump
manufacturers usually have a set
of capacity curves for the pump
when handling water at different
temperatures and these should
be consulted.
Where steam systems operate
at higher pressures than
those used in LP space heating
systems, as in process work, condensate
temperatures are often
212°F, or more where positive
pressures exist in return lines.
Electric pumps are then used
only if their capacity is downrated
by partial closure of a valve at the
outlet; by using a receiver mounted
well above the pump to ensure
sufficient NPSH; or by subcooling
the condensate through a heat
exchanger of some type.
Pressure Powered Condensate
Pump
All these difficulties are avoided
by the use of non-electric condensate
pumps, such as the
Pressure Powered Pump . The
Pressure Powered Pump is
essentially an alternating receiver
which can be pressurized, using
steam, air or other gas. The gas
pressure displaces the condensate
(which can be at any
temperature up to and including
boiling point) through a check