Process Engineers normally are not directly involved in Heating, Ventilation, Airconditioning (HVAC) requirements of a Chemical Process Plant. HVAC engineering is a specialized branch of engineering and HVAC engineers normally involve themselves in system design for a chemical process plant. However, adequate ventilation is a matter of concern for all engineers who consider safety as an integral part of any chemical process plant and it is important to know the hazards related to inadequate ventilation.
NFPA 30 defines adequate ventilation from the perspective of a chemical process plant as follows:
An area is adequately ventilated if it is ventilated at a rate sufficient to maintain the concentration of vapors within the area at or below 25% of the lower flammable limit.
This shall be confirmed by one of the following:
– Calculations based on the anticipated fugitive emissions; or
– Actual vapor concentration sampling under normal operating conditions, conducted at a radius of 5 feet from equipment.
An acceptable alternative is to provide ventilation at a rate of not less than 1 cubic foot per minute per square foot of solid floor area.
Ventilation shall be accomplished by natural or mechanical ventilation, with discharge of exhaust to a safe location, without recirculation of the exhaust air.
Completely open (from all 4 sides) outdoor locations are usually considered adequately ventilated.
Process equipment buildings are buildings that contain hydrocarbon piping and equipment. Some examples are: analyzer buildings, metering stations, pump stations, compressor stations, and separation stations. They should have openings on all sides whenever practical to allow for ventilation. If they must be fully enclosed due to ambient conditions, adequate ventilation needs to be provided. to prevent the accumulation of flammable gases. The building exhaust vents should be located to preclude the accumulation of gases at low points such as pipe trenches, as well as at the ceiling.
Process buildings are frequently ventilated with windows, floor louvers, and roof ventilators.
A minimum of six air changes per hour are recommended for process buildings.
If mechanical equipment provides the required ventilation, safeguards are needed to protect against its undetected failure.
Discharge or exhaust must be to a safe location outside the building.
Ventilation rates can be achieved either by continuous introduction of fresh makeup air into the enclosure, or by recirculation of air in the building. A recirculation system should ensure that the air is monitored continuously using a system that automatically alarms, stops recirculation, and provides full exhaust to the outside in the event that vapor-air mixtures over 25% of the lower flammable limit are detected.
Recirculation should be designed with adequate air movement and direction to minimize “dead” areas where flammable vapors such as heavy hydrocarbons may collect.
If conditions include the risk of a large flammable vapor release occurring in a confined space, and the calculated rate of diluent ventilation is not sufficient to dilute and disperse the released vapor to below the “Lower Flammable Limit” (LFL) within a reasonable time (i.e., four hours), then supplemental emergency ventilation should be provided. Emergency ventilation can be natural ventilation through panels or louvers, or switching recirculation fans to full fresh air make-up or exhaust. The travel direction of ventilated vapor should avoid its reaching an ignition source outside the enclosed space being ventilated.
It is recommended that when combustible gas detectors are installed they provide an alarm at 20% of LFL and are interlocked to rotary process equipments such as pumps and compressors so that they cannot be started or they shut down when the detector detects a value of 60% of LFL.
To conclude, adequate ventilation in a chemical process plant is of utmost importance and process engineers should be aware of good ventilation practices for a safe design.
