Instrument air header pressure decay with moisture breakthrough
At 02:15 the unit is steady at 100% power. Instrument air header pressure has drifted from 690 kPa(g) to 622 kPa(g) over 40 minutes. Compressor A is in AUTO and has been loaded continuously; its discharge temperature is 96 °C against a normal 78–82 °C, and the dryer outlet dew point indicator reads −8 °C where it normally sits near −40 °C. The low-pressure alarm is set at 600 kPa(g), the standby compressor auto-start at 570 kPa(g), and most pneumatic valve actuators in the plant need at least about 480 kPa(g) to stroke fully against their springs. In the last 20 minutes the feedwater recirculation valve has hunted ±8% and two heater drain valves have drifted toward their fail-open position. On shift are you, a shift supervisor and one field operator; a maintenance technician is on site for unrelated work. Ambient temperature is 34 °C and the air dryers are lined up normally, with no bypass in service.
1. State the most probable cause of the falling header pressure, and explain which indications support it and which rule out the main alternatives.
Read the worked answer
The pattern points to moisture breakthrough past the dryers combined with wet, sticky pneumatic positioners, not a simple loss of compressor capacity.
Reasoning from the evidence:
- The compressor is loaded continuously yet the header is still falling, so instantaneous demand exceeds supply. A failed compressor or a stuck unloader would typically show the machine cycling or not reaching its rated discharge pressure, with a normal dew point.
- Discharge temperature of 96 °C with 34 °C ambient means the aftercooler and separator are being asked to remove far more water than at design conditions; carryover into the desiccant beds shortens the drying cycle. The dew point moving from −40 °C to −8 °C is the decisive indicator: the dryers are no longer drying.
- Hunting and drifting positioners on the feedwater recirculation and heater drain valves are the classic consequence - water and ice in the instrument tubing and positioner relays give slow, sticky, drifting strokes.
- A large external air leak remains possible and must be checked, but a leak alone would not explain the dew point and discharge temperature.
Field checks, in order: compressor amps and discharge pressure at the local panel, aftercooler cooling water flow and temperature, separator and drain trap operation, dryer switching timer and purge flow, dew point at each dryer outlet, then an audible/ultrasonic leak survey of the header and a walkdown of the drifting valves.
2. Header pressure continues to fall. Describe how you sequence your response as it passes 600, 570 and 550 kPa(g), including which end uses you protect and when you would recommend a controlled shutdown.
Read the worked answer
Sequencing matters because the risk changes as margin disappears.
At 600 kPa(g) (alarm): announce the alarm, inform the supervisor, send the field operator to the compressor/dryer area with a radio, and start a timed log of header pressure every 5 minutes. Confirm the standby compressor is in AUTO and its auto-start setpoint is correct. Ask the maintenance technician to stand by, not to start unrelated work that needs air.
At 570 kPa(g): verify the standby compressor actually starts, and if it does not, start it manually. Reduce discretionary air demand - sootblowers, air-operated tools, non-essential service air - and place the affected control valves on manual with conservative positions before they move themselves.
Approaching 550 kPa(g): air is only ~70 kPa above the minimum for full stroke, so any further drop converts a pressure problem into an uncontrolled valve-position problem. Recommend a controlled power reduction to the supervisor while you still have authority over the valves, and be prepared to trip before a safety-related pneumatic valve (feedwater control, letdown, steam generator level control) drifts on its own. A deliberate, orderly shutdown beats an unplanned one triggered by a valve moving uncommanded.
3. Write the turnover entry for the incoming shift. What must be recorded, and what would make the entry inadequate?
Read the worked answer
The entry must let the incoming operator reconstruct the trend and act without re-diagnosing.
Content:
- Time the drift was first noted and the pressure at each logged interval (690 → 622 kPa(g) in 40 min), with the log sheet referenced.
- Compressor A status (AUTO, loaded continuously), discharge temperature 96 °C, dryer dew point −8 °C against a normal −40 °C, ambient 34 °C.
- Affected valves already identified and their current status: feedwater recirculation hunting ±8%, heater drain valves drifting, each placed in manual at a stated position.
- Actions taken: field operator dispatched, standby compressor verified/started, air demand reduced, work order and tag numbers raised, maintenance notified.
- Standing instructions: alarm at 600, standby start at 570, escalate to the supervisor and consider power reduction by 550 kPa(g).
An entry is inadequate if it says only "instrument air pressure low, maintenance aware". That omits the trend, the dew point evidence, which valves are already compromised, what was placed in manual, and the trip/decision criteria - leaving the incoming shift to rediscover all of it during the night.