Mechanical Ventilation – General Rules to live by

Many facilities will use mechanical ventilation to aid in reducing worker exposures, as well as controlling flammable atmospheres from becoming explosive.  There are some general rules of ventilation that should be applied in all cases and this article is an attempt to briefly mention the key points.  Mechanical ventilation systems are ONLY as good as we design, operate, and maintain them.  These following tips are from my “lessons learned”.

1. So as to not “short circuit” a ventilation system, adequate make-up air must be provided to the area that is being ventilated.  A general rule of thumb is that the volume of make-up air must meet or exceed the volume of exhaust air.  
2. Air intakes need to be positioned in relationship to the exhaust so that the system has a “sweeping motion” through the area that is being ventilated.  
3. The area around make-up air intakes needs to be assessed to ensure that the air being drawn in as make-up air is CLEAN so as to not introduce a hazard to the area.  Make-up air can NOT be pulled from a HAZLOC (e.g. Class I, II, or III areas).  In processes where the vapors are heavy than air, it is often best to pull make up air from the rooftop and discharge at ground level – but BEWARE the discharge must be to a SAFE location!  Discharging flammable vapors or gases to to a parking lot is NOT a safe design.  
4. Make-up air should not be pulled from a diked area.  This sort of goes along with the issue stated above, but even toxic storage that has secondary containment can present a hazard to the ventilated area.  Again we want to ensure we are using a clean source of air for our make-up air and pulling this air from a diked area is setting us up for potential problems.
5. Both sides of the make-up air intake must be kept clear.  Both temporary objects and permanent objects must not be placed in front of make-up air intakes.  We have seen hazardous waste drum storage set up in front of the intakes and cardboard and plywood being used to intentionally block the intake so that cold air will not cause freezing problems for the pipes in the area.
6. Air intakes should be located sufficient distance from other ventilation discharge points – roof top intakes can be difficult to manage as most mechanical ventilation systems tend to discharge to roofs.  With other systems discharging to the roof, we just need to make sure that these discharges will not make it to our intake(s).
7. Air in takes should not be near truck unloading bays or loading docks as exhaust from engines can be sucked into the ventilated area. 
8. If we are discharging flammable vapors we must ensure that the area we are discharging them to is designed to receive flammable vapors.  This means we want to be sure that the discharge is set up properly so that flammable vapors are discharged high above our roof line that they can not accumulate in “unventilated portions of our roof”.  Depending on what RAGAGEP was used to design the process, ventilation and the electrical classification, the ventilation discharge area  may need to be classified as a Class I HAZLOC.  If the ventilation system discharges at the ground level, it may be wise to restrict plant traffic and personnel a safe distance from the discharge.  A “Best Practice” is to cordon off the area and post warning signs to notify personnel of the potential hazard.
9. Ventilation systems must be included in our mechanical integrity inspection/testing program.  These inspections must verify more than the system is operational, but we need to at least annually verify the system is moving the amount of air it is designed to move in the time period specified in its design (i.e. how many CFM is being moved needs to be verified).
10. Ventilation systems should be alarmed when they shutdown.  This alarm needs to be both visual and audible so that personnel in the ventialted area are infomed of the “loss of ventilation”.  A “Best Practice” would be to tie the ventilation alarm into an interlock that would stop all processing and transfers of hazardous materials.  NOTE:  a THOROUGH PROCESS HAZARD ANALYSIS is needed to determine the consequences of stopping all processing as in some cases stopping agitation may make matters worse.  Also, if your facility does go with some type of interlock, ensure this interlock is part of the inspection/testing program.
11. Ventilation systems should be on emergency back-up power.  If the process area is designed to drop out all electrical contacts when a flammable atmosphere is detected then the main power source for the ventilation system should be SEPERATE from the rest of the process equipment – as well as be on emergency power. 
12. Operations generating flames, sparks, or hot material such as from grinding wheels and welding/hotwork must not be conducted anywhere that would permit sparks to be introduced into any exhaust system.  Refer to 1910.252(a)(2)(viii)

13. Where ventialtion systems are in place for flammable vapors/gases at concentrations greater than 1% of the LEL, the rotating element of the fan must be nonmetal, or the fan must be constructed so that a shift of the fan blades or shaft does not permit two metal parts to rub or strike.

14. Consider car-sealing your ON/OFF switch for the ventilation system in the ON position so that it can not be turned OFF without a thorough review of the consequences.  (See CAI / Arnel Chemical Plant Explosion)