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Slug Flow In Psv Inlet Line


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#1 sanderson231

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Posted 22 October 2018 - 04:09 PM

I am looking at an overfill case for a product separator on a refinery hydrotreating unit.  The separator receives a two phase mixture from the reactor effluent trim cooler.  It separates the feed into three phases; vapor, liquid hydrocarbon, and sour water.  The vapor product flows to an amine contactor to remove H2S, then to a KO drum and finally to the recycle compressor.  In the event the hydrocarbon product valve fails closed, the separator will fill with liquid hydrocarbon, then the amine contactor will fill with hydrocarbon liquid and then the compressor suction KO drum will fill with liquid.  This should not result in the separator exceeding its MAWP.  High-High level in the KO drum should shutdown the compressor (2oo3 voting level transmitters).  The compressor trip also shuts down the charge pump.  Even if the charge pumps fails to shutdown the PSV on the separator is adequate with the compressor down.

 

However to be conservative, one can consider a contingency were the separator PSV needs to relieve all the vapor and liquid hydrocarbon feeding the separator.  I sized a PSV using the HEM method (Annex C API 520) with an inlet line that resulted in an inlet pressure drop of  less than 3% of set pressure.  However when I looked at the flow regimes in the inlet line the horizontal sections are in plug flow and vertical sections are in annular flow.  One of the assumptions for the HEM method is that the vapor and liquid phases must be in mechanical equilibrium i.e. moving at the same velocity.  This is clearly not the case with the inlet line under consideration.  Making the line bigger likely aggravates the slug flow.  I also have concerns that the vertical section of piping will transition to plug flow as the PSV cycles closed.

 

Is it appropriate to design the PSV using the HEM method and also ignore the flow regimes in the inlet piping with the possibility of a liquid slug hitting the PSV?



#2 Bobby Strain

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Posted 22 October 2018 - 08:21 PM

In all my years I never encountered this scenario. And I have designed many systems that you describe.

 

Bobby



#3 shan

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Posted 23 October 2018 - 07:23 AM

For the scenario of the separator liquid overfilling, both the separator hydrocarbon level control valve and the separator water level control valve are assumed fail closed.  Is it a double jeopardy?

 

Usually, the slug size is longer than PSV inlet piping section.  Therefore, you may select largest pressure drop from vapor, liquid or two phase flow to determine your PSV inlet piping.  



#4 sanderson231

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Posted 23 October 2018 - 11:46 AM


 

The separator sour water valve is controlling the interface level.  It will not open when hydrocarbon level builds.

 

Indeed the slug and bubble lengths are long.  Inlet pressure drop is not the issue.  The question is does the HEM method apply and for valve reliability is it ok if the simulation shows slug flow in the inlet line.



#5 shan

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Posted 23 October 2018 - 01:15 PM

Then, the separator pressure control valve should open to discharge the separator content before PSV's reliving .  Is it a double jeopardy case for the simultaneous malfunctions of both the separator hydrocarbon outlet valve and the separator pressure control valve?  



#6 sanderson231

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Posted 27 October 2018 - 09:29 AM

From my experience, the typical assumption for basic process control systems is that the valve either holds last position or it functions normally if the action makes the relief scenario worse.  I've never seen credit taken for BPCS to reduce PSV size or eliminate a PSV.



#7 Bobby Strain

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Posted 27 October 2018 - 07:30 PM

This reminds me of the saying "the blind leading the blind"

 

Bobby



#8 PRVBoss

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Posted 20 November 2018 - 04:01 PM

Pretty late to the party here, but wanted to suggest that the flow regime in the piping is not necessarily the same as the flow regime in the PRV nozzle. Studies have shown that equilibrium is achieved in 4-6" lengths of converging nozzles, which is most any API 526 valve bigger than an "E" orifice. However, the issue of non-equilibrium flow in piping and fittings is not well understood and the AIChE DIERS group is partnering with Purdue University to investigate (looking for funding). 

 

I think you're fine using HEM for the PRV sizing. You could always consider slip, but the HEM will be more conservative for the PRV sizing. If you're interested in non-equilibrium effects you could take a look at ISO 4126 or the papers by Diener/Schmidt on which it was based.






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