6 replies to this topic

[u062268]

Hello everyone;

 

 

 

In our Plant we are using a Screw Compressor to compress Hydrogen (Recycle & make-up) to get the desired discharge pressure for Hydrotreating Reactors. At 09:11:07 am, the compressor got alarm for Low Suction Pressure from three pressure indicators (Part of ESD system) as shown in the attached picture. After 4 seconds only ( at 09:11:11) the compressor tripped due to low pressure of suction (16.04 barg). The normal suction pressure of compressor has to be ( 17.6 barg).

 

Actually the compressor tripped in just seconds after alarm was showing and suction pressure dropped by around 2 bargs in four seconds only.  Nothing was reported abnormal before tripping except the alarm from suction Drum pressure followed by alarm from ESD indicators of suction pressure, other indicators were reading normally. No upset reported before tripping from other parameters in the Area.

 

We expected that the Pressure Relief valve opened for instant causing Low pressure in Suction Drum and re-set itself automatically because it is (Modulating Relief Valve). But How it comes that Flow indicator (46272.1 Nm3/hr ) didn’t capture any reduction in flow while the pressure indicators in its upstream and downstream were reading low pressure four a few seconds??!!!

 

Does the RV capable to reduce the pressure by 2 barg in just 4 seconds ?

(RV Data attached in the pictures) . Flare Header Pressure is 0.013 barg.

 

 

Before the accident, the Controller valve of Recycle Hydrogen wasnt matching the reading in DCS by +30% and one of the operator saw this valve was  fluctuting between the set point by +-20% in the field. Plus the unmatching of reading in DCS and in the Field wasnt the matter for a long time and we are processing normally by this unmatching in readings.

I was suggesting that the Recycle Hydrogen valve closed for instant causing the Low pressure in Sucrion Drum!!! How can I know my idea or their ideas are right !!! Is there any other assumptions?

 

Why we didnt say the RV of Separator vessel pop-up and re-set itself directly (Modulating type) ??

(RV Data attached in the pictures)

 

 

Thank You

 

 

 

 

 

 

 

 

Attached Files

•  K-2901.JPG   52.82KB   3 downloads
•  RV.JPG   46.03KB   2 downloads
•  RV Data of Suction Drum.JPG   85.04KB   3 downloads
•  RV Data of Separator Vessel.JPG   62.64KB   2 downloads

[TS1979]

It seems that you gave a wrong set pressure for the RV (23.56 bara) while the pressure for the drum is controlled at 17.8 psig. In such case, the RV will be constantly lifting. I will assume that your RV set pressure is 23.56 barg, considering 10% blowdown for the RV, the reset pressure should be 21.2 barg. It is unlikely the RV problem. How about the control valve on the off gas line which may fail open?

[u062268]

TS1979:

 

Why not considering 20% Blowdown?!!!

 

 

You mean Off Gas valve opened more than in DCS causing dropping in Suc Pressure !? Can it cause this drop in just few seconds?!! 

 

 

Check the DCS pic bellow:

Attached Files

•  dcs.JPG   52.65KB   3 downloads

[TS1979]

Too much time working on pressure with psig unit, the RV datasheet showing setting pressure at 23.56 bara (or 22.56 barg) is OK for the service.

 

The relief valve blowdown is determined by the valve manufacturer and the value is shown on the valve datasheet. The typical value for the blowdown is about 7%~10%. You cannot assume that the blowdown is 20% of the setting pressure.

 

As to if the suction pressure loss is due to the off gas control valve or not, it is just a brainstorming.

[u062268]

TS1979:

 

 

 

How can I detect if RVs were passing before trip !?

[chandan20]

U062268

what is the position of the line going to J2901?
what is the capacity of this line, can it handle such flow to reduce the pressure?
i had asked this because it is located in the downstream of your flow indicator, so relieving pressure through this isn't going to reduce flow.

[u062268]

U062268

what is the position of the line going to J2901?
what is the capacity of this line, can it handle such flow to reduce the pressure?
i had asked this because it is located in the downstream of your flow indicator, so relieving pressure through this isn't going to reduce flow.

 It is isolated

 

We were suspecting the PC valve of Vapor Line of Reactor Separator because it has the most flow of Hydrogen (around 38000Nm3/hr).

So, if this valve shutted off there will be sudden drop of pressure in Suction drum and suction pressure of compressor as well.

After second trip of this compressor for the unknown reason, we found the RV of Reactor separator was passing ( Inspection Report: Pilot operated relief valve, initial pop  test was carried out in our work shop  and found passing at 0.3 bar after opining  the Teflon soft seat found damage and the disc found with minor corrosion deposit).

 

Once I trend this PC valve of separator,  before both of trips times, the valve opening reached 23% and then followed by the trip. It seems the valve was shutting off in the field while in DCS the reading showing 23% opening. We found the positioner of this valve damaged and already we replaced it.

 

Now, my question; is this logic?! Was this PC valve responsible for sudden drop in pressure?!

Why Flow indicator couldnt catch the reduction in flow!

Hydrogen make-up was normal before trip.