Wet Gas Compressor trip

[Hedewandro Lucredi]

Our FCCU has a wet gas compressor. When this compressor trip by any problem, there are two valves in the suction that open for flare. The failure mode of these valves is one fault closes, the other is stationary. We are studying to change the fault close to open fault. What risks can this bring to the process? Any E-cat carry ? Environmental impact (flare burning)?

[dblewis]

I do not know your exact compressor line up so I will try to answer some of your questions. I imagine that the wet gas compressor takes suction off a Main Column OH Accumulator and there are suction flare lines to PSV’s going to a flare system. Some compressors have anti surge lines coming from the discharge back to the OH accumulator / suction after cooling the discharge gas. Are these the 2 valves (stationary to flare/ & anti-surge valve?)you are talking about?

[mlarson]

The answer will only apply to compressor trip specifically that the rest of the unit is operating normally. it is not uncommon that the suction of the compressor has more than one “dump control valve (s)” to divert suction flow to the flare while maintaining pressure on the column. There are two considerations 1) Control of the pressure of the column so that tower upsets are minimized 2) Issues on the flare load itself that need to be evaluated i.e. WGC is low pressure and back pressure on flare can reduce “relieving” capacity.

Specific to an isolated compressor trip – the controls should be set to dump to flare however with sufficient control as to hold “normal” pressure of the tower. The next step is to reduce overall unit feed rate to minimize flaring and if necessary a controlled shutdown of the unit.

No comment can be made on the impact of catalyst flow or regenerator / reactor pressure balance without knowing unit specific configuration and operating parameters

[Hedewandro Lucredi]

Our FCCU is an orthoflow and modified for external risers. The reactor gases go to the main fractionator. From the top after condensation the gases go to the compressor. In the event of a compressor failure, there are two valves (one redundant from another) that relieve gases to the flare. These valves are not the anti-surge valve of the compressor. Today the failure mode of these valves is a stationary failure and the other failure closes. We want to transform the failure closes to failure opens. What are the problems for the process? What is the typical failure mode for these valves?

[Hedewandro Lucredi]

Sorry. It is a Ortoflow C

[dblewis]

There are some other options with one being the use of multiple pilot operated relief valves that are set at increasing pressures and having an emergency dump valve as a last resort. The FCC I was on, had 4 pilot operated relief valves and one flare dump valve. The flare dump valve failed open on loss of air pressure or electrical failure.

[Hedewandro Lucredi]

If one of the two wet compressor gas suction valves that relieves to the flare to open incorrectly with the compressor running is there a risk of air entrainment from the regenerator to the main fractionator? Is there any risk of damage to these equipment like fractionator, compressor, etc ?

[dblewis]

If your unit SIS system is set up right, that should never happen.

[airmaxpneumatic]

Good to be here, I was also looking for the solution for this one, Basically I am working with Pneumatic Valve Manufacturer.

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