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[Shahriar]

When either of the LP compressors trains on the our AX platform are started we experience high vibrations levels as we pass through each of the LP&HP compressor critical speeds. These vibrations levels only peak for approximately 3 seconds and then return to acceptable running levels.

The compressors passe through the critical speeds during the following activities

• Compressor Start-up
• Compressor Shutdown
• Planned Trip to idle (Offloading Unit)
• Autimatic Trip to Idle (Trip Alarm)

According to our operation team, It has become routine practice to increase ALL compressor (LP&HP comp) vibration trip settings from 4.1 mills to 5.0 mills to overcome the vibration peaks / spikes during the passing through the critical speeds. These adjustments are recorded in the process inhibit register at the time they are applied.

These adjustments are also made to the compressor vibration trip settings, when the unit are being swapped over, tripped to idle or shutdown in a controlled manor (trip with cool down period).

The trip levels are then returned to there normal values once the unit are fully loaded, swapped over, shutdown or the risk of tripping back to idle has passed. This can mean to the duration that the vibration levels are increased can be 30 mins to >1hr

Each time these adjustments are made it requires an instrument tech to alter the settings, if a unit is required during the night they will have to call him out which can add further delays to starting or recovering units. It also means the instrument techs have to wait around to return the trip settings back or else they will be left in place until we can get someone.

From a safety perspective there are following concerns:

• Duration that the trip setting will be adjusted for
• Ensuring adjustments are recorded every time they are applied
• Possibility that the adjustment could be left on
• Potential for human error in applying adjustment to trip settings or wrong channels
• Does everyone fully understand what and why these adjustments are being made and the possible impacts
• Sends out poor a message to new / less experienced techs & Ops with reference to adjusting trip settings

From a production perspective there are following concerns:

• Delays in starting units due to waiting or calling out of techs
• Unnecessary hot stopping of unit when Planned or Automatic trips to idle occur
• Number of failed / unnecessary starts

It is proposed to automate the inhibiting of the trip function whilst the compressor goes through the critical speeds in both directions (ramping up to minimum speed / slowing down to idle).

There are two options proposed, Vibration suppression or the temporary increasing of trip function set points across the compressor train.

I want to know if there is any standard or guidance that addresses this issue and whether these sort of modifications in the control system are safe to implement. Are we compromizing the plant safety by introducing these automated inhibits?

your help is appreciated.