Chemical Engineering September 2012 - 61

Solids Processing
Environmental Manager
PI
Suction
Vacuum Systems:
Recommendations
For Safe Operation
Follow this guidance to ensure that steam ejector
Stanley S. Grossel
Process Safety & Design Consultant
T
his article is the first in a twopart
series, in which the types
of process vacuum equipment
(steam ejector systems and
mechanical vacuum pumps) are discussed.1
Emphasis is given on how
they are constructed and operate, and
how to design associated parts of the
systems (such as foundations, suction
piping, discharge piping, and so on), to
ensure safe operation.
Many unit operations - including
distillation, evaporation, drying, crystallization,
filtration, and others - are
often conducted under vacuum conditions.
To achieve the desired vacuum,
the following three process- vacuum
systems can be used:
* Steam ejector systems
* Mechanical vacuum pumps
* Integrated vacuum systems (which
combine steam ejectors and mechanical
vacuum pumps)
Available capacities and operating
ranges for vacuum pumps and vacuum
pumping systems that are most often
used in process operations throughout
the chemical process industries (CPI)
are listed in the Table on p. 60 [1].
This article reviews the hazards associated
with steam ejector systems
and mechanical vacuum pumps, and
recommends design and operating
practices that can be taken to prevent
or mitigate these dangers.
1. Editor's note: This is the first half of a twopart
article. Part 2 is scheduled to appear in the
October 2012 issue.
Steam ejector systems
Steam ejector systems are generally
categorized into one of four basic
types: single-stage, multi-stage noncondensing,
multi-stage condensing,
and multi-stage with both condensing
and non-condensing stages.
For many years, steam ejector systems
dominated process applications
thanks to their simplicity, low purchase
cost, good reliability and their
ability to be designed for very large capacities
(in excess of 1,000,000 acfm)
and be operated at very low pressures
in the micron range (for instance, sixstage
units are routinely designed for
suction pressures in the range of 3-10
microns). They also are available in a
variety of materials of construction.
Steam ejector systems are ideal for
wet-vacuum and highly corrosive applications.
As a result, they are still
being used instead of mechanical
vacuum pumps in applications that
require a reliable vacuum system that
can tolerate corrosive chemicals, liquid
slugs and solids carryover.
Safety considerations. In general,
there are very few safety problems
associated with the operation
of steam ejector vacuum systems.
However, one safety issue associated
with their design is the potential for
back-streaming of steam into the process
equipment.
For instance, steam ejectors normally
use a " steam bleed " to control
the suction pressure. If the process
PI
X
Cooling water
return
T
Y
Z
Condensate
Cooling water
supply
Steam
supply
T
Condensate
TI
TI
To seal pot
systems, mechanical vacuum pumps and integrated
vacuum systems are designed, operated and
maintained to ensure process safety
T
Gate valve
Four-way valve
Steam trap
PI
TI
To seal pot
Pressure indicator
Temperature indicator
Piccolo connection
FIGURE 1. Shown here is a schematic
of a three-stage steam ejector system
with inter-condensers (Reprinted with
permission from Ref. 1)
contains a water-reactive chemical
(such as acetic anhydride), a steam
bleed is not acceptable. If the ejector
should " backfire, " this will push steam
back into the process and could result
in a possibly violent reaction, which
could overpressure the system.
To avoid this problem, a nitrogen
bleed can be used to control the suction
pressure. If the jet should back
fire, nitrogen will prevent the steam
from reaching the process. However,
using nitrogen to control an existing
multi-stage ejector may not be technically
feasible. If nitrogen is to be used
to control a multi-stage jet, the user
must indicate this when writing the
performance and equipment specifications
for the steam ejector.
Using nitrogen to control the suction
pressure of the first stage - that
which is closest to the process - will
affect the design of subsequent stages,
and using nitrogen to control suction
pressure can significantly affect the
design of the inter-condensers. If the
back flow of steam or water into the
process equipment from the ejector
system can cause a serious safety or
quality problem, then a liquid separator
(a so-called " knockout pot " ) may be
installed between the ejector system
and the process equipment.
Another possible safety problem
may result from the loss of the intercondenser
cooling medium (usually
water), which could lead to overpressure
of the system.
CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2012 59
YZ intercondenser
Aftercondenser
http://WWW.CHE.COM
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