Analysis of HVAC design features for blast-resistant DCS central control rooms
[Abstract] Based on the traits of blast-resistant DCS central control rooms in petrochemical plants, this paper presents the HVAC design features from indoor environment, fresh-air system, cooling and heat sources, air distribution and equipment standby, and summarises common construction problems.
[Keywords] DCS blast-resistant central control room, indoor environment, fresh-air system, air distribution, equipment standby, heat dissipation.
Petrochemical units can leak toxic, harmful or explosive gas that harms operators or causes explosions. The DCS control room must protect against these gases and is usually a fully enclosed blast-resistant structure. Its stable long-term running depends on the HVAC system. With rapid petrochemical growth, recent projects include blast-resistant control rooms whose design differs from usual air conditioning in environment, fresh air, sources, air distribution and standby.
1. Traits of the blast-resistant control room. The whole building is sealed with no external windows. Control consoles and DCS cabinets fill the space. Equipment gives off much heat and runs 24 hours; the cabinet and UPS rooms run 2 to 3°C above the design temperature, summer 26±2°C. Traditional load figures no longer meet cooling needs, so some sites added split units on top of the ducted system. Without them, cabinet or UPS rooms run hot.
2. Cooling source. The control room needs a stable, undisturbed environment for long runs. Per the blast-resistant design code, prefer air-cooled packaged units with their own source, with standby, because air cooling holds a constant temperature. If a water-cooled chiller is used in summer, add a standby unit; if the plant supplies chilled water, the refrigeration station needs a standby. This project, by agreement, used chilled water with a standby refrigerant unit at the station.
3. Fresh-air system. Essential for a sealed room with no windows and 24-hour staffing. A centrifugal exhaust fan sized by the required positive pressure brings in fresh air. Amount: the sealed room needs more than the 30 m³/(person·h) of the heating-and-ventilation code, or staff feel stuffy; per the petrochemical blast-resistant code use 50 m³/(person·h), with centrifugal exhaust sized by positive pressure. Because total fresh air is set by headcount, not area, the actual per-person amount can fall short, so add the unstaffed cabinet rooms; if the headcount-based amount is under 10% of supply, use 10%. Treatment: supply and return air must be cleaned; use a fresh-air purification unit with a chemical filter. Use a C3 coarse filter and a Z2 medium filter for supply, a C3 coarse filter for return, and an activated-carbon adsorber protected by a Z2 medium filter ahead of it, with a coarse filter ahead of that. Blast valve: the reinforced-concrete envelope must resist external blast waves, so any opening undermines it. Openings under 150 mm get a blast valve that snaps shut on an external explosion, cutting harm, with resistance equal to the blast wall. Note some blast valves are directional, so mark and fit them correctly. Where flammable or toxic gas detection exists, fit matching detectors at the fresh-air intake and an electric tight shut-off valve on the intake that closes on alarm, stopping gas entry.
4. Air distribution. The control room holds much DCS gear and must not use water-based central cooling that could leak; it uses an all-air central system. Supply from above and return from above is used; under-floor supply is unsuitable in petrochemical plants because the floor stays dirty, raising dust and sending cold air straight onto cabinets, destabilising electronics.
5. Equipment standby. The HVAC runs all year; besides reliable units, set standby so that if one fails the rest still meet 100% demand. The fresh-air purification unit also needs standby.
6. Duct material. Galvanised steel is common but rusts, has no sound absorption and is slow to install. Blast rooms use composite glass-fibre duct with moisture-proof foil; it is moisture-proof, sound-absorbing, insulating and low-leakage, and quick to modify and install.
7. Smoke control. The blast control room is neither a fire-loaded plant nor a crowded public building; SH/T 3017 classes it as class D. So rooms need no mechanical smoke extraction except possibly the inner corridor. But because it is the plant control centre, fire must be extinguished fast and smoke cleared so staff can return; a post-fire exhaust is needed. After a fire, the fresh-air system pushes smoke into the inner corridor, exhausted by its fan; because the operation hall is tall, over 2 m to the ceiling, a mechanical exhaust is added there.
8. Conclusion. Blast-resistant DCS control-room HVAC draws on many fields; domestic codes cover it little and experience is thin, so problems must be solved and improved as projects run.