13d • General discussion
Oil failure on semi hermetic compressors
High pressure and oil pressure switch troubleshooting
🔴 RED CIRCLE
Danfoss High-Pressure (HP) Safety Control
(Common models: Danfoss KP7 / KP15 / KP17 depending on range)
What it is
• Mechanical high-pressure cutout
• Senses discharge pressure only
• Wired in series with the compressor safety circuit
What it does
• Protects the compressor from excessive discharge pressure
• Opens the control circuit when pressure exceeds its setpoint
How it operates
• Internally uses a diaphragm & spring
• Discharge pressure pushes against the diaphragm
• When pressure > setpoint → contacts open → compressor stops
• Some models are:
• Manual reset
• Others auto-reset (not common on racks)
Typical trip range
• Medium temp: ~350–400 psi
• Low temp: ~300–350 psi
(exact value depends on refrigerant)
What causes HP to trip (REAL rack causes)
1. Condenser airflow problems
• Dirty condenser coils
• Failed condenser fans
• VFD fan stuck at low speed
• Winter fan control stuck closed
2. Over-condensing / split rack issues
• Too many compressors running on one side
• Other side unloaded
• Poor heat rejection balance
3. Non-condensables
• Air in system
• Bad evacuation after repairs
• Nitrogen contamination
4. Liquid line restrictions
• Plugged filter drier
• Closed or partially closed king valve
• Collapsed line
How to troubleshoot HP trips
1. Check actual discharge pressure with gauges
2. Compare to HP cutout rating
3. Verify:
• Condenser fans running
• VFD speed command
4. Check coil condition
5. If pressure is normal but HP trips:
• Bad HP control
• Misadjusted setpoint
• Kinked sensing line
Key field rule:
HP trips are almost always a heat rejection problem, not a compressor problem.
🔵 BLUE CIRCLE
Danfoss Oil Differential Pressure Safety Control
(Typically Danfoss MP54 / MP55)
What it is
• Net oil pressure safety
• Compares oil pump pressure vs suction (crankcase) pressure
What it does
• Protects compressor from oil starvation
• Shuts compressor down if oil pressure differential is too low
How it operates
• Two pressure inputs:
1. Oil pump discharge (high side)
2. Crankcase / suction pressure (low side)
• Calculates net oil pressure
• If net oil < setpoint (usually ~10–15 PSID)
for a set time delay → trips
Time delay
• Typically 30–120 seconds
• Prevents nuisance trips on startup
What causes oil safety to trip (THIS is the tricky one)
1. Low oil level in system
• Oil trapped in evaporators
• Poor oil return
• Excessive unloading
2. Unloader problems
• Compressor running unloaded too long
• Low piston velocity
• Oil not returning
3. Low suction pressure
• Split rack imbalance
• Light load conditions
• Frozen or restricted evaporators
4. Oil dilution
• Refrigerant flooding crankcase
• Foamy oil
• Warm oil → pressure collapse
5. Mechanical issues
• Worn oil pump
• Plugged oil filter
• Restricted oil sensing line
How to troubleshoot oil trips (STEP-BY-STEP)
1. Check oil sight glass
• Foamy? Empty? Overfull?
2. Measure:
• Oil pressure
• Suction pressure
3. Calculate net oil pressure
4. Check oil filter temperature (restriction check)
5. Verify unloader state
6. Inspect sensing lines for blockage
7. Temporarily force compressor to 100% loaded
• If oil stabilizes → unloader logic issue
Critical rule:
Never assume the oil control is bad until you verify net oil pressure.
🧠 WHY THESE TWO GET CONFUSED ON RACKS
HP Safety
Oil Safety
Trips fast
Trips delayed
Pressure related
Oil return related
Heat rejection issue
Load / oil balance issue
Usually system-wide
Often compressor-specific
🔗 HOW THEY INTERACT IN REAL LIFE (IMPORTANT)
Here’s a classic rack failure chain:
1. Condenser fans underperform
2. Discharge pressure rises
3. Compressor unloads
4. Oil velocity drops
5. Oil return slows
6. Oil safety trips
(even though HP started the problem)
This is why techs chase the wrong safety.
⚠️ PRO TECH WARNINGS
• Never leave oil safety jumped
• HP trips = fix airflow
• Oil trips = fix oil return, loading, balance
• One compressor tripping oil while others are full = oil migration imbalance
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Oil failure on semi hermetic compressors
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