Flare System Design & Considerations

November 19, 2014

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Introduction

A typical flare system is comprised of the following components:

#1.Relief, safety and depressuring valves.

#2.Pressure relieving header(s) that convey discharges from safety and pressure control valves in the process to the flare.

#3.Knock-out drum located before the flare stack in order to separator any condensate or liquid from the relieving vapors.

#4.Flare stack consisting of the riser structure, seal and burner tip.

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Introduction (2)

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Introduction (3)

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Introduction (4) Design of a flare system first requires a detailed analysis

of the possible situation that can cause discharge from pressure relief valve, emergency blowdown, etc. thus establishing the maximum loading for emergency operations.

The maximum load of a system is comprised of the individual loads contributed by the entire process.

A conservative design is one that assumes all contributors for the process are relieving simultaneously under any emergency conditions.

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Design Considerations

• The relief header shall generally not be pocketed.

• Where this requirement cannot be met, a drain line discharging to a knockout pot shall be provided.

  Design of pressure relief valve piping shall conform to API

RP520, Parts 1 & 2.

It is mandatory that the nominal size of inlet piping be the same as, or greater than, the nominal size of the pressure relief Valve inlet flange.

  Pressure relief valve tail pipes shall be self-draining to the

relief header or other destination. 

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Design Considerations (2)

Pressure relief valve tail pipes shall be sized in accordance with the principles of RP520, Part I, Section 8; RP520, Part II, Sections 3.1 and 3.2; RP521, Section 5.3A.1.

In no case shall the size of the tail pipe be less than the size of the relief valves outlet connection.

When pressure relief valves discharge vapours to the atmosphere, the vent line shall terminate at least 3 m above equipment or any service platform located within a radius of 12 m of the valve.

Such valves shall be located at the highest practical elevation on vertical vessels. A 10 mm diameter weep hole in the tail pipe for drainage to a safe location at the low point of the line shall be provided.

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Design Considerations (3)

When pressure relief valves discharge to the atmosphere, the discharges shall be directed away from adjacent equipment, platforms, ladders, manways etc.

Consideration of relief valve reaction forces should be considered in design with suitable support. This is usually not a problem with relief valve discharging to a piping header system. This is always a problem with relief valves discharging to atmosphere.

Piping upstream of a relief device should be designed with as few restrictions to flow as possible and should not be pocketed.

Flow are through all pipe and fitting between a pressure vessel and its relief valve shall be at least the same as that of the valve inlet (i.e. isolation valve shall be full bore). Depending on the actual relief valve capacity, the pressure drop of the inlet piping and fittings shall not exceed 3% of the valve set pressure (to avoid chatter).

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Design Considerations (4)

When two or more relief valve are fitted to one connection, the cross sectional area of this connection shall be at least equal to the combined inlet areas of the valve and the pressure drop requirement shall apply for the combined flow of the valves.

Relief valves on cold process streams shall have an un-insulated inlet line of sufficient length to prevent icing of the relief valve. Alternately heat tracing may be required.

Best practice is to install relief valves as close to main header; minimizing relief valve outlet pipe length.

Ensure operating pressures are maximum of 90% of the relief valve set pressure to avoid leakage and accidental lifting of relief valve.

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Design Considerations (4)