6 replies to this topic

[Fr3dd]

Hello everybody,

 

I would like to have your insights about an issue I'm currently dealing with.

 

Situation: Imagine a vessel which function is to collect liquids (production fluids, chemicals, water, etc, none of them above 70 deg C) and hold them up to pump them to the drain header. One line is connecting this vessel to a header which is routed to flare, providing this safeguarding to the vessel in case of overpressure (also considering the fact that, under certain scenarios, the vessel will handle two-phase fluids). this header also collects a gas stream that is routed to flare. A simplified scheme is attached.

 

 Vessel.jpg   13.74KB   3 downloads

 

In case of a "catastrophic scenario", this gas stream could reach really high pressures (up to 340 barg) so a restriction orifice is provided to assure a manageable pressure for the flare header (around 0.5 barg). Obviously, such a high depressurization will lead to really low temperatures (since the gas stream is HC, mostly methane) but all the chosen materials can cope with that.

 

The problem: During a HAZOP study, one of the concerns was that, during normal operation, the line connecting the vessel with the flare header will pressurize the system, including the piping connected to the gas stream (and the RO) with gas that may contain water (from production fluids or even water in equilibrium within the vessel (please note that holdup times are high enough to think and equilibrium can be reached). Then, in case of a catastrophic event, the depressurization will cause a temperature drop that will freeze the water vapour leading this to a potential blockage of the line..

 

My opinion: From the process engineering point of view, I think this will not happen. The fluid velocity during the depressurization (that most likely will be at least mach 1) will push all the gas present in the line away (and all the water vapour will go away. However, I was told that this was not an acceptable assumption for a HAZOP. Also, in the case that the vessel holds water in equilibrium, even the flare backpressure will prevent the water vapour (with a Vapour pressure of around 0.09 bara, as its temperature is 45 deg C) to pressurize the system.

 

What is your opinion regarding this?. Can you suggest more arguments to support my position?. Do you see a wrong assumption from my side?.

 

Thanks in advance for your help!.

[shan]

Snow/ice crystals will be formed when the blowdown stream temperature is lower than 32 F.  However, the snow/ice crystals are unable to integrate into big pieces or deposit on valve/piping because high velocity force of the blowdown stream.

[fallah]

Fr3dd,

 

I think the water vapor, if available in the line connecting the vessel to flare header and the line connecting the RO to flare header, can be available at RO upstream as well in normal operation; and on the other hand the gas stream also may be included the water vapor, then hydrate formation at downstream of the RO can be credible in catastrophic scenario due to high pressure (340 barg) gas stream because the water vapor removed by high velocity gas stream can be replaced by the water vapor from the water included in the vessel. Such hydrate formation may lead to line freezing and blocking...

Injection of methanol or MEG at upstream of the RO can remove the problem due to possibility of hydrate formation...

[shan]

MEG injection sounds like an attractive solution.  However, it may not be feasible if you consider the unpredictable emergency blowdown with extreme high initial flow rate.

[Bobby Strain]

You seem to withhold a lot of information. Which prevents us from being of much help.

 

Bobby

[Fr3dd]

Thanks for al your replies guys,

 

shan: what you describe is exactly what I think will happen. I don't see the ice crystals forming a significant formation that could lead to a blockage.

 

fallah: indeed, under normal operation, there is no gas stream and any water that may be pressurizing the line will be at both sides of the RO. However, please note that the gas that could potentially come from the gas stream is considered dry (it is treated to have a dew point sufficiently low to meet export specs). My point is that the amount of water pressurizing the line  will not be that high (so not that much water will be present) and the velocity will be sufficiently high to avoid a significant ice formation (as shan says). Definitely, methanol or MEG injection is not an option, since the gas stream/depressurization is only expected in a catastrophic scenario that will happen real quick.

 

Bobby: I tried to summarize all the relevant information I could think of, which kind of information do you think is missing?

 

Thanks again! Hope to hear more from you soon.

[Bobby Strain]

Volume, composition, function of equipment, size, continuous or intermittent, source of gas and liquids, etc.

 

Bobby