Oct-2012

Optimising sweep gas flow in a flare header

How to calculate the minimum flow of flare header sweep gas needed to maintain safe conditions

SATISH MATHUR
Bechtel India

Article Summary

The purging of flare systems prevents the ingress of air through the open stack, which can create an inflammable mixture of hydrocarbons and oxygen inside the system. Stack purge, together with purge reduction seals, is generally used to prevent this, in addition to a make-up gas for sweeping the flare headers. Some estimates of header sweep gas quantities are suggested by Duggan, Simpson and others.¹

With rising energy costs, there is a need to minimise the amount of gas used for sweeping without compromising the safety aspect, which is that there should be no air ingress in the system. This article develops the correlations for estimating the header sweeping gas requirement needed to compensate for the shrinkage effect due to ambient cooling or rapid cooling of the gases after a hot gas release. It also provides guidelines to facilitate a decision on investing in instrumented intermittent purge systems, which can provide further savings in the quantity of sweep gas. The practice of using rule-of-thumb flare header sweeping gas rates could lead to substantial annual utility costs in large flare networks and is therefore not a desirable practice.

A typical flare network consists of several sub-headers that collect the discharge gases from relief valves, process flaring control valves, emergency depressurising systems and equipment depressurising for shutdown purposes. In large complexes, the sub-headers may terminate in unit knockout drums for liquid removal before joining the main header leading to the common flare stack.

The flare stack can have a water seal to prevent any flashback from the stack to headers (except in cryogenic applications), a seal on the stack to prevent infiltration of air and a dedicated continuous flare stack purge. The quantity of stack purge gas required is dependent on the size of the flare tip, the composition of the purge gas, the composition of the waste gases and the design of the seal.² In most cases, the seal purge gas quantity is specified by the seal supplier.

In addition, the unit area flare headers and sub-headers are provided with a sweeping gas supply in most installations, although some flare systems with a water seal and stack designed for internal explosion may not require continuous header purge.³ This is to compensate for a shrinkage effect due to ambient cooling or rapid cooling of the gases after a hot gas release in the flare header network.

This article deals with header sweeping gas quantity, which is independent of the stack purge flow due to their different objectives, as explained above. Natural gas available from the plant fuel gas system is often used for purging, both at the stack as well as in the sub- headers, with a backup gas supply source for ensuring uninterrupted gas availability.

Basis
Under normal operation, when the plant is in steady condition and if the valves are not passing, there should be no flow of waste gas into the flare headers. In this condition, the stagnant gases in the flare headers can be subjected to volume shrinkage due to cooling. The factors influencing shrinkage are:
• Temperature of gas in the header after stoppage of flaring
• Ambient temperature
• Wind speed
• Flare pipe surface area
• Heat capacity of flare pipe metal
• Thermal conductivity, viscosity and density of gas.

Method
Heat transfer from the hot flare gases to ambient is by the following means:
• Inside the pipe: by natural convection (gases inside the pipe are considered stagnant when not flaring)
• Across the pipe wall: by conduction
• Outside the pipe: by natural convection, forced convection and radiation.
Theoretical analysis of natural convection heat transfer inside enclosed surfaces is provided in the Nusselt equation:⁴

NNu = a (NGr NPr)m

where
NNu = hiL/k = Nusselt number
NGr = L3ρ2GβΔt/μ2 = Grashof number
NPr = cμ/k = Prandtl number.
The factors “a” and “m” for horizontal cylinders are given in the reference. Using these, hi, the heat transfer coefficient for natural convection inside the pipe, is computed.
When a heated flare pipe surface is exposed to flowing air, the convective heat transfer outside the pipe is a combination of forced and free convection. For this mixed convection condition, Churchill recommends the following equation for computing the heat transfer coefficient h:5

(Nu - δ)j = (Nuf - δ)j + (Nun - δ)j

The forced convection Nusselt number for the horizontal pipe of diameter D is given by Incropera and Dewitt5 as follows:

Where ReD = VD/ν = Reynolds number.
The natural convection Nusselt number for the horizontal pipe of diameter D is given by Churchill and Chu5 as follows:

Where RaD = g.β.ρ.cp(Δt)D3/(ν.kf) = Rayleigh number.
The overall Nusselt number Nu is computed using j = 4 and δ = 0.3. Once the Nusselt number is computed, the heat transfer coefficient external to the pipe is computed as: