5 replies to this topic

[Prabhat12]

Hi ,

 

Due to relief of propane or lighter material in flare header  from propane sphere PSV,  calculated temperature at water seal drum is sub zero  . That may cause the freezing. What should be better protection to heat propane vapor ( fro eg. from -10 deg c to +5 deg c _) . Should it be steam tracing or electrical tracing . Relief Flow rate is 10 ton/hr.

 

Any suitable scheme to tackle this issue . Relief scenario is liquid propane overfilling from storage sphere . Storage spheres are not provided with overfilling protection.

 

Thanks,

[flarenuf]

hi Prabhart12

 

you say "Storage spheres are not provided with overfilling protection." 
nothing at all? alarms trips shutoff valves?

[Art Montemayor]

Flarenuf asks a very important question.  Any compressed, saturated liquid that is allowed to exceed the maximum filling % of a storage or transport vessel (usually between 80 and 90% of total volume) is an extremely hazardous situation that should be avoided at all costs.  The potential for a BLEVE (boiling liquid expanding vapor explosion) comes to mind.  Normally, what occurs with a propane or LPG vessel is that a PSV relieves excess GAS pressure – not LIQUID pressure.

 

I believe you are failing to describe your scenario accurately.  Correct me if I am misinterpreting what you have described, but I think you are relieving LIQUID (not gas) propane through a PSV, which is a conventional, free liquid expansion resulting in a refrigerating effect due to the compressed liquid vaporizing at constant enthalpy.  This is analogous to a standard mechanical refrigeration system and what you are dumping into your flare header is LIQUID PROPANE AT SATURATED LOW TEMPERATURE.  No flare system can handle this stream safely and you have a potentially more dangerous situation than just freezing your water seal drum.  You cannot just concentrate on heating the propane vapor.  Your main hazard is the liquid propane that you must first vaporize and subsequently heat up to have a gaseous stream you can flare safely.

 

I think what Flarenuf infers is correct: first mitigate the possibility of overfilling the propane vessel – with instrumentation or operational procedures.

 

Please correct me if I have misinterpreted your description.  This issue is far too important to allow any misunderstandings.

[fallah]

Prabhat12,

 

At first it was better uploading a simple sketch of the system...Then for control the level of the liquid inside such spherical tank you need using tank gauging system...

 

Anyway, if the system is an existing one, at least there should be a knock out drum before seal drum which is equipped with an electrical heater inside to vaporize incoming liquid propane, then the vapor propane conducted to the seal drum....

[Prabhat12]

Dear Art / Fallah/flarenuf,

 

Thanks for Your replies. I have attached a simplified sketch . As a background , Currently spheres/ bullets PSV are routed to atmosphere . Propoasl is to route them to flare header. Few spheres have ESD valves and while others have not. Upstream system detail i am trying to gather in full . But as per operation , liquid overfill have been happened in one spheres in past . So now two options are being considered :

 

1. Option -1 : Design the flare system considering liquid overfill may happen.

2. Option -2 : Provide SIL rated instrumentation to avoid liquid overfill in spheres.

 

No doubt , option -2 is recommended. But considering , option -1 , whether it would be feasible to design the safe flare system.   

 

Below are the two scenarios:

 

1. Propane bullet overfill : With given composition , relief valve outlet tempertaure would be -32 deg c . And taking the heating effect ( atmospheric minimum temp is 5 deg c ) , relief stream temperature would be -9 deg c at KOD , and it would be all vapor. So for this stream , steam coil / or vaporiser would not be helpful . So steam tracing / electrical heat tracing may be required to heat  10 ton/hr propane vapor from  -9 deg to + 5 deg c to avoid water freezing in seal drum. Duty requirement would be 60 KW.

 

2. Butane sphere overfill : Butane relief would be two phase at KOD and temperature would be -1 deg c . Butane liquid can be vaporised by existing steam coil and flared.

 

In attached sketch , vaporiser has been shown as optional. This could be attached to KOD or separate as required. I have seen in cold flare system ( like oleflex unit ) vaporiser attached with flare KOD .

 

Please advise.

 

 

 

 

Attached Files

•  sketch.xlsx   24.19KB   19 downloads

[fallah]

Prabhat12,

 

As per your study result, option 1 appears to be feasible, provided that:

 

-Propane or butane overfill scenario wouldn't be happened simultaneous with the other relieving scenarios terminated to existing flare header,

 

-If there would be simultaneous relieving due to such overfills and the another relieving from other sources of over pressures, the existing flare network and stack can tolerate the new combined relief load and the KOD can accept the new liquid inventory