Ammonia Plant Steam System Optimization
Steam systems use complex arrangements of turbine drivers, heat exchangers, fans and blowers requiring careful heat
exchanger engineering and process engineering services oversight to sustain optimal plant performance. There are
countless ways to design and arrange steam system components to supply the necessary energy requirements to drive
rotating equipment, provide heating needs and furnish process steam to the plant. This article concentrates on improving
steam system performance. The article illustrates a base case plant operation, followed with improvement in Vacuum
Surface Condenser performance, improving Steam Superheat, and using the most efficient combinations of turbines in
operation and examples of the economics for improvement.
The steam system for an Ammonia plant is the "heart" of the facility. It has been said that Ammonia plants are steam
producers that also happen to make a little ammonia on the side. Typically, ammonia facilities generate 4 to 5 times as much
steam as the product ammonia they produce. Thus, the steam system is vital to an Ammonia plant and the selection and
arrangement of steam system equipment is key to the plant efficiency. There are countless ways to design and arrange
steam system components to supply the necessary energy requirements to drive rotating equipment, provide heating needs
and furnish process steam to the plant. In this brief article, we shall concentrate on improving steam systems, rather than
fundamental design. Figure 1 on Page 2 illustrates a typical modern 1800 STPD Ammonia plant steam system employing
several levels of steam generation as "supply" headers operating in the Base Case at nominal 1500 Psi, 550 psi and 50 psi
pressures.
In this steam system design, an extraction turbine, (103-JAT), recovers mechanical work from all of the high pressure steam
generated in the plant partially driving the synthesis compressor, while providing 550 psi steam, which is then further let down
through back-pressure turbines or a pressure control valve furnishing 50 psi steam for process heating and other uses. A
large portion of the 550 psi steam from the extraction turbine is used as Process Steam, with the remainder being used to
generate power in vacuum turbine driven rotating equipment applications, or supplies the 50 psi header as shown on Figure 1.
Improving Surface Condenser Performance
In the Base Case example in Figure 1, the main (largest) vacuum turbine surface condenser, (101-JC), which recovers the
exhaust steam from the process compressor drivers is operating at 4 Inches HgA pressure (25.9 Inches Mercury Vacuum.)
The operating pressure may be excessively high due to fouling, mechanical problems or overload. With cleaning, modification
or replacement, the operating pressure can be restored to normal design value of 3 Inches HgA. As illustrated in Figure 2, the
vacuum has been restored by renovation or replacement to 26.9 Inches Mercury, (3 Inches HgA pressure) and the steam
balance improves as shown. In this case, high pressure steam generation and boiler water pumping requirements resulting
from replacement of the condenser have reduced by about 1.3 %, resulting in energy savings, (at 82% HHV furnace
efficiency), of 13.7 MM Btu/Hr (HHV), equivalent to about $346,000 per year savings, based upon an energy cost of
$3.00/MM Btu (HHV). Thus even if equipment replacement were needed to resolve the condenser pressure problem the
economics are very favorable to upgrade it.
