SIM 4120 Gas Fired Boiler

Model Description

The de-aerator is fed low-pressure steam, demineralised water and low-pressure condensate. The deaerated water is then either sent to a scrubber (not modelled) or onto one of the two boiler trains. The purpose of the deaerator is to remove oxygen and any other dissolved gases from the feed. This prevents damage to metallic equipment in the form of rusting and corrosion.

The boilers are fed air and fuel gas (mainly methane) via a fuel gas KO drum. The flow rate of these feed streams control both the pressure of the vessel and the %Oxygen in the resulting flue gas. The boilers work in a similar fashion to a double-effect evaporator, using the already heated water to further heat the fresh feed by flowing it back through a heat exchanger, improving overall efficiency.

The resulting steam from the boilers is fed to a high-pressure steam header. A small amount of water from the boilers is fed to a continuous blowdown (BD) drum to remove any impurities and maintain water quality. Water can also be fed directly to an intermittent BD drum if there are any suspended solid impurities or build-ups in the boilers that need to be removed. Waste water from the BD drums is fed through a cooler and into a chemical sewer.

Each boiler is fitted with an ignition system in which a pilot flame can be turned on or off. Whilst the main flame is online, turning the pilot flame on will have no effect. However, when starting up the boiler, a steady flow of both air and fuel gas is needed. Once this is established, turning on the pilot flame will ignite the gas and the main flame will display as ‘online’.

This model also includes an option to enable fouling on the boilers. If this is enabled, boiler efficiency will begin to decrease with time due to a gradual build-up of impurities, which affect heat transfer. This effect can be countered with the use of blowdown systems. This fouling simulation is greatly accelerated so the simulation user can see the effect of build-ups on the boiler systems quickly.

Pressure, temperature and flow alarms are linked to an ESD system to ensure the safe operation of the equipment. Every extreme alarm (LL or HH) will trip the relevant valves and pumps to keep the system in a safe state. The various alarms and their associated trips can be viewed on the two ESD pages accessible via the ESD overview page. The ESD overview page also includes an ESD and a PSD switch which when pressed will trigger the appropriate shutdown and trip all relevant valves and pumps.

Every vessel is linked to a pressure safety valve (PSV) which will open at excessively high pressures to ensure safety is maintained.

Learning Areas