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Chemical Process Technology

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Tuesday, April 15, 2008

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Pressure Relief Valve (PRV) discharge to a tail pipe, subheader, header, flare knock-out drum and subsequently flare to atmosphere. All of above collective called flare collection and disposal system. One of the phenomenon potential results catastrophic failure of flare system is the hydrate formation or icing in any part of flare collection system.

How hydrate and ice form in the flare collection system ?
There are two scenarios where hydrate or ice can form in the flare collection system. First scenario, when process fluid at high pressure and low temperature passing the PRV, Joule-Thompson (JT) effect results low temperature at downstream of PRV. As the temperature lower than the process fluid hydrate formation or icing temperature, crystal form hydrate or ice will form. Second scenario, when cold and dry relieving fluid mix with warm and wet relieving fluid the mixing temperature drops below mixture hydrate formation or icing temperature.

Once hydrate, it will find a surface to stick on and it will form "nucleus" to stick on it. The hydrate size is gradually increase and it potentially of completely blocking the pipe.

Will Hydrate or Ice formed in Flare system practically ?
Having said that hydrate or ice may form once the condition is right, however, it is unlikely to see hydrate or ice formation in flare header. The reasons could be :

i) As the PRV discharge is normally limited to some period and not sustainable, the discharge inventory (hydrate former) may not sufficient to completely block a large flare line.
ii) PRV discharging fluid at very high velocity. High gas momentum sweep away hydrate or ice formed.
iii) Flare piping is "hot" and contain heat which sufficient to destroy the hydrate or ice stick on it

Good engineering practices to minimize potential of catastrophic failure cause by hydrate or ice formation
No doubt the potential of hydrate or ice formation in flare collection system is low, however, it still remain a high risk cause. Good engineering practices shall be incorporated to minimize the possibility of hydrate or ice formation.

i) Segregate warm & wet fluid with cold & dry fluid into different headers.
ii) Avoid small PRV discharge tail pipe
iii) Make short piece between PRV outlet flange and expander
iv) Minimise elbow and tee before tail pipe end at large subheader
v) Increase reliability of high pressure tripping system to minimise relief events

PRV leaks results Hydrate or Ice formed
PRV leaking or passing is one the phenomenon that very difficult to avoid. Whenever PRV passing, hydrate or ice still potentially form at PRV outlet due to small opening and prevent the PRV to open. In this case, high reliability and availability heat tracing (e.g. High SIL loop, connected to emergency power supply, etc) may be considered to ensure the PRV temperature is maintained above hydrate or ice formation temperature.

Coldness from fluid potentially transfer to the PSV vent chamber and the moisture content in the PSV chamber may potentially friezed and preventing the PRV to open. Piston balanced PRV may be considered in this particular case.

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posted by Webworm, 12:59 PM


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