6.3 Compressor Startup and Operating Parameters

Standard Operating Procedures for Compressor Startup

Starting an industrial ammonia compressor is a critical operation that requires strict adherence to standard operating procedures (SOPs) to prevent catastrophic mechanical failure. The transient period during startup is when mechanical stress, lubrication deficiencies, and liquid carryover risks are at their highest. A Certified Assistant Refrigeration Operator must follow a disciplined pre-start and startup checklist.

Pre-Start Inspection Checklist

Before initiating the startup sequence, the operator must verify the mechanical readiness of the compressor and its auxiliary systems:

1. Energize Crankcase Heaters (12-24 Hours Prior):
   • The Physics of Oil-Ammonia Miscibility: Anhydrous ammonia is highly soluble in compressor lubricating oil. When the compressor is offline and the crankcase is cold, ammonia vapor from the system will migrate to the crankcase and condense directly into the oil.
   • Preventing Foaming: If the compressor starts with liquid ammonia dissolved in the oil, the sudden drop in crankcase pressure during startup causes the ammonia to boil violently out of solution. This creates a thick foam, which is drawn into the oil pump. The pump cannot pump foam, leading to a loss of oil pressure, oil starvation of bearings, and massive oil carryover into the discharge line.
   • Target Temperature: Crankcase heaters must maintain the oil temperature between 100°F and 120°F (or at least 20°F to 30°F above the saturated suction temperature corresponding to suction pressure) to drive off dissolved ammonia and ensure proper oil viscosity.
2. Verify Oil Level: Inspect the crankcase sight glass. The oil level should be between 1/4 and 3/4 full, depending on manufacturer specifications.
3. Verify Auxiliary Systems: Ensure that condenser fans, evaporative water pumps, and cooling jacket water flows are active.
4. Position Stop Valves:
   • Discharge Stop Valve: Must be completely OPEN. Starting a positive displacement compressor against a closed discharge stop valve will cause pressure to rise instantly to dangerous levels, potentially rupturing piping or blowing cylinder heads before safety cutouts can react.
   • Suction Stop Valve: Must be completely CLOSED (or cracked open only slightly if automated bypasses are not present). This prevents a sudden surge of low-pressure gas or liquid from entering the cylinders.

The Startup Sequence

1. Ensure Compressor is Unloaded: Verify that the capacity control system is set to the fully unloaded position (minimum capacity).
2. Start the Drive Motor: Energize the motor. Monitor the oil pressure gauge immediately.
3. Verify Oil Pressure: Net oil pressure must establish within 10 to 15 seconds. If the oil pressure safety switch trips or if pressure fails to rise, shut down the compressor immediately.
4. Slowly Open the Suction Stop Valve:
   • Modulation: The operator must slowly "crack" open the suction stop valve. Do not open it rapidly. Opening the valve too quickly will cause a sudden pressure drop in the suction line, which can boil liquid refrigerant in the suction accumulator, causing it to carry over (slug) into the compressor.
   • Monitor Pressure and Noise: Keep one hand on the valve wheel and monitor the suction pressure gauge and motor current. Listen intently. If you hear a rhythmic metallic knocking, liquid refrigerant or oil is entering the cylinders. Immediately close the suction valve.
   • Gradual Transition: Once the suction pressure stabilizes and the cylinders clear any trace of liquid, slowly open the valve further. Continue this gradual process until the suction valve is 100% open.

Normal Operating Parameters

Once the compressor is running in a steady state, the operator must regularly log and monitor key operating parameters to ensure they remain within safe design envelopes.

1. Suction Pressure and Temperature

• Suction Pressure: Determined by the evaporative temperature required for the process. For example, a high-stage system might run at 20 psig (+5.5°F saturated suction temperature), while a booster system might run at 0 psig (-28°F saturated suction temperature).
• Suction Superheat: Superheat is the temperature difference between the actual gas temperature and its saturation temperature at that pressure. $\text{Superheat} = T_{\text{actual}} - T_{\text{saturation}}$ Normal superheat at the compressor inlet should be maintained between 10°F and 20°F.
  • Too Low (under 5°F): Suggests that liquid refrigerant is carrying over from the evaporator, posing a risk of liquid slugging.
  • Too High (over 30°F): Indicates that the suction line is poorly insulated or suction gas is picking up excessive ambient heat. High suction superheat increases the specific volume of the gas, reducing compressor capacity, and leads to excessively high discharge temperatures.

2. Discharge Pressure and Temperature

• Discharge Pressure: Determined by the condensing temperature, which is governed by ambient wet-bulb temperature and condenser cleanliness. Normal discharge pressures for ammonia systems range from 120 psig to 180 psig. High discharge pressure increases the compression ratio, placing higher electrical loads on the motor.
• Discharge Temperature: Must be monitored closely. For reciprocating compressors, the absolute maximum limit is 275°F (135°C). Above 275°F, lubricating oil carbonizes, causing valve failure and cylinder wear.

3. Net Oil Pressure

• Calculation: Net oil pressure is the actual pressure delivering lubricant to the bearings and activating hydraulic unloaders. It is calculated as: $P_{\text{net}} = P_{\text{pump discharge}} - P_{\text{crankcase (suction)}}$
• Normal Range: Typically 30 to 45 psi above crankcase pressure.
• Safety Cutout: The oil pressure safety switch (OPS) monitors net oil pressure. If it drops below 15 to 20 psi, the switch will trip after a 90-second time delay, shutting down the compressor to prevent bearing seizure.

4. Oil Temperature

• Normal Range: 100°F to 140°F (38°C to 60°C).
  • Too Hot (over 150°F): Oil viscosity drops, reducing its load-carrying capacity and leading to bearing wear.
  • Too Cold (under 90°F): Oil retains dissolved ammonia, increasing the risk of foaming at startup, and has high viscosity, causing high drag and hard starting.

IIAR and Safety Regulations

• IIAR 2 Requirements: Mandates that every compressor must be equipped with high-pressure, low-pressure, high-discharge temperature, and low-oil-pressure safety cutouts. These cutouts must be checked and calibrated annually (or per facility PSM mechanical integrity schedules) to ensure they trip at the correct parameters.
• Safety Relief Valves: Every compressor must have a dual-relief valve assembly installed between the compressor discharge and the discharge stop valve. This prevents overpressurisation of the compressor head if the discharge stop valve is closed while the compressor is running.