Chemical Engineering May 2022 - 31

Cover Story
Vent Away Condensate
Pump Frustrations In A Flash
The guidance provided here can help to mitigate problems occurring in pumping systems used
for condensate recovery
C
ondensate recovery is crucial to
energy reduction and water conservation
in a steam-using plant.
It reduces effluent discharge and
treatment requirements, as well as the
chemicals and heating costs associated
with preparing raw makeup water. Furthermore,
having a greater percentage of highquality
condensate for boiler feed helps to
reduce corrosion in the piping system [1,
2]. Even with relatively simple condensate
pumping systems like the one shown in
Figure 1, condensate recovery can sometimes
create complex challenges for plants.
A problematic pumping station, such as
the one shown in Figure 2, is a result that
may be all too common in some steam systems
and can occur due to leaking seals or
pump cavitation, ultimately causing condensate
to overflow. Are all such instances due
to normal wear, or have some been caused
by poor design or insufficient system maintenance?
Experience demonstrates that
many comparable reliability issues can be
avoided with improved design and system
maintenance practices, but first it is necessary
to identify probable causes and implement
suitable mitigation action, preferably
during the design stage, when possible.
How does cavitation occur?
Cavitation is normally a key factor, and engineers
should first understand what causes
this phenomenon in order to design around
James R. Risko
TLV Corp.
IN BRIEF
HOW DOES CAVITATION
OCCUR?
FIGURE 2. This pump station exhibits severe spillage, creating
a potential safety hazard and source of problems
FIGURE 1. A simple condensate pump arrangement includes
an inlet, receiver, vent, pumps and overflow loop
its potential occurrence [3, 4].
Figure 3 shows a centrifugal condensate
pump that is experiencing cavitation issues.
This situation happens when flash steam
- formed from the re-vaporization of hightemperature
condensate by the suction-side
pressure drop - rapidly pushes away liquid
volume in the impeller. Condensate is further
accelerated by pump rotation, and this
combination leads to impeller erosion. As a
result, the vapor pockets, which are compressed
by liquid flowing through the prop
troughs, quickly collapse and cause an onrush
of condensate into the void. This can
cause more damage to the impeller and
pump seals. Cavitation is typically not a singular
event, but rather is ongoing throughout
the causal conditions, resulting in capacity
reduction or catastrophic failure of the condensate
pump itself, or resultant spillage
through leaking seals or the overflow.
This leads many engineers to ask how
cavitation can be avoided. An understanding
of both net positive suction head available
(NPSHA) and net positive suction head
required (NPSHR) is needed.
NPSHA is the resultant head at the eye
(central point) of a pump's impeller. It is a
function of all of the fluid variables acting on
the inlet side of the pump. Those include
positive head factors, such as pressure created
by the inlet height of the condensate
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY2022
31
PUMP SELECTION
VENT PROBLEMS AWAY
HEAD OFF PROBLEMS
NON-ELECTRIC OPTIONS
TURN FLASH INTO CASH

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Chemical Engineering May 2022

Table of Contents for the Digital Edition of Chemical Engineering May 2022