Precautions To Be Taken To Prevent From Static Electricity As Per NFPA 77

Precautions To Be Taken To Prevent From Static Electricity As Per NFPA 77
Chemical Industry Electrical Consultancy Services Engineering Design

Precautions To Be Taken To Prevent From Static Electricity As Per NFPA 77

 

Static has always been a problem in the chemical industry. It has been discovered to be the culprit of many industrial fires.

Static electricity is the branch of electrical science dealing with the effects of the accumulation of electric charge. NFPA 77, Recommended Practice on Static Electricity, provides guidance on identifying, evaluating, and controlling static electric hazards for the purpose of preventing fires and explosions. Although this recommended practice originated in 1937 and has continuously evolved for the last several decades, static electricity incidents continue to occur.

In the chemical industry, say a storage tank farm, two of these elements are present. The chemical is stored (such as alcohols and chemical solvents) in fuel. Air is always present around us. That leaves only heat or a source of ignition. Once the last element is met, you get fire or an explosion.

Section 12.4.1 in Chapter 12 talks about precautions to be taken to prevent static electricity hazards in Bulk Storage Tanks

For all liquids (from non-conductive to conductive), the following precautions should be taken:

  • The tank and all associated equipment, such as piping, pumps, and filters, should be grounded.
  • Personnel entering or working near tank openings should be grounded.
  • Splash filling should be avoided.

Specific Recommendations for Large Conductive Fixed-Roof Tanks.

For non-conductive liquids, the following additional precautions should be taken:

  • High static electric charge–generating elements, such as pumps and filters, should be located at a suitable residence time upstream of the tank inlet.
  • For uncontaminated single-phase liquids, the inlet flow velocity should be restricted to 1 m/s until the fill pipe has been submerged to a depth of twice the inlet pipe diameter. The fill rate can then be increased up to 7 m/s.
  • For multiphase or contaminated liquids and where it cannot be ensured that water bottoms will not be dis­turbed, the inlet flow velocity should be restricted to 1 m/s during the entire fill cycle.
  • Use of a centrally located inlet pipe that extends to within 150 mm of the bottom of the tank is recommended. A horizontal tee is recommended at the discharge for bottom fill connections.
  • Accumulation of water and sediment in the tank should be minimized.
  • For multistage loading, liquids should be transferred to the tank in increasing order of density.
  • In all cases, the maximum flow velocity should not exceed 7 m/s.

Specific Recommendations for Large Conductive Floating-Roof Tanks and Fixed-Roof Tanks with Internal Floating Covers.

For all liquids, an initial flow velocity no greater than 1 m/s should be maintained at all times while the roof or cover is landed, regardless of the depth of liquid above the inlet connection. Flow velocity can be increased when the roof or cover becomes buoyant. At that time, the flammable atmosphere will be shielded from the potential development during filling by the floating roof or cover, provided the roof or cover is made from a conductive material and is properly grounded.

Flow velocity restrictions for large tanks –

Precautions

With Floating Roof or Internal Cover

With Fixed Roof, No Floating Cover

Keep flow velocities below 1 m/s

Essential until the roof or cover is afloat

Essential during the initial filing period and when loading a contaminated or two-phase liquid or a liquid with a substantially lower density than that already in the tank

Keep flow velocities below 7 m/s

Not essential when the roof or cover is afloat. NOTE: A flow rate limit will often be needed to avoid damaging the roof by the too rapid movement

Recommended in all cases in which the 1 m/s limit does not apply

Ensure adequate residence time between strong charge generators (e.g., microfilters) and the tank

Essential until the roof or cover is afloat. NOTE: The residence time can be calculated using a velocity of 1 m/s in this instance

Essential

Avoid disturbing water bottoms with the incoming product or entrained air or by blowing outlines with gas

Essential until the roof or cover is afloat

Essential

Avoid charging low-density liquids into tanks containing substantially higher density liquid

Unnecessary

Recommended as far as practicable. If unavoidable, keep the flow velocity below 1 m/s

 

Enquire Now


    This will close in 0 seconds