Compressor Control Notes

Instrument Technician - Compressor Control

Objective One - Reciprocating Compressors

Introduction to Compressors

A compressor is a machine designed to increase the pressure of a gas. There are two main types of compressors:

• Positive Displacement Compressors: These compressors confine a volume of gas within a mechanical device and reduce the volume, thereby increasing the gas pressure.

• Dynamic Compressors:

Reciprocating Compressors

Reciprocating compressors are a type of positive displacement compressor that utilizes a piston within a cylinder moving back and forth to draw in gas and then discharge the compressed gas.

Single-Acting Reciprocating Compressor

• Uses only one side of the piston for compression.

• Contains a suction valve and a discharge valve, both one-way and spring-loaded.

• During the suction stroke, the piston moves down, causing the suction valve to open and the cylinder to fill with gas.

• During the discharge stroke, the piston moves up, closing the suction valve and compressing the gas. Once the pressure exceeds the discharge valve's pressure, it opens, releasing the compressed gas.

• A crankshaft, continuously rotated by a gas engine or electric motor, drives the piston via a connecting rod.

• Piston rings prevent gas from escaping between the piston and cylinder walls.

Double-Acting Reciprocating Compressor

• Utilizes both sides of the piston for compression.

• A crankshaft rotates a connecting rod, which moves a crosshead back and forth between guides. The crosshead is connected to the piston.

• The piston rod moves through packing material to prevent gas leakage.

• Gas is drawn in and compressed on one side of the piston, while compressed gas is discharged on the other side simultaneously.

• Each crankshaft rotation results in two compression strokes.

Multistage Reciprocating Compressors

• As gas is compressed, its temperature increases. Multistage compressors use intercoolers between stages to remove heat generated during compression.

• Intercooling: Cools the gas to near its initial temperature, reducing power consumption and maintaining temperatures within specified limits.

• Compression and cooling reduce gas volume, leading to physically smaller high-pressure stages compared to low-pressure stages.

Pressure vs. Volume Diagram

The operation of a reciprocating compressor is described by the pressure versus volume diagram:

• Compression Stroke (A): Cylinder is full of gas at suction pressure, suction valves close.

• Discharge Begins (B): Piston compresses gas, pressure exceeds discharge line pressure, and discharge valves open.

• Discharge Stroke (C): Piston pushes compressed gas into the discharge line. Clearance volume exists at the end of the stroke.

• Suction Stroke (D): Pressure drops, closing the discharge valve. Gas in the clearance volume expands until pressure is lower than the suction line, then the suction valves open.

Key Concept: Reciprocating compressors do not build up discharge pressure; the pressure results from the resistance to flow downstream. Each stroke displaces a fixed volume.

Note: At a given speed, a reciprocating compressor is a variable-pressure machine with relatively constant flow.

Objective Two - Reciprocating Compressor Capacity Control

Compressor Capacity: The actual quantity of gas compressed and delivered, measured in volume per unit time.

Compressor Capacity Control: Manipulating the flow through the compressor to match process requirements.

Control Methods

Regulating discharge or suction pressure controls capacity. Methods include:

• On/Off Control

• Constant Speed Unloading

• Recirculation

• Speed Control

On/Off Control

• Simplest control method.

• Compressed gas is stored in a receiver.

• A high-pressure switch (PSH-1) turns the compressor off when a set pressure is reached.

• A low-pressure switch (PSL-1) restarts the compressor when pressure drops.

• Example: A gas distribution system is pressure regulated to $550$ kPag with receiver pressure oscillating between $620$ kPag (PSL-1) and $1030$ kPag (PSH-1).

Constant Speed Unloading

• Altering a reciprocating compressor’s capacity by mechanically adjusting suction valves (suction valve unloading) or cylinder volume (step clearance unloading).

• Used during start-up to reduce torque/power or to reduce capacity.

• Common methods:

  • Suction Valve Unloading

  • Step Clearance Unloading

  • Variable Clearance Unloading

Suction Valve Unloading

• A three-step technique allows compressor operation at 100%, 50%, or 0% capacity.

• Holding a suction valve open during the compression stroke pushes gas back, preventing compression and saving power.

  • 100% capacity: Both suction valves close.

  • 50% capacity: One suction valve is held open.

  • 0% capacity: Both suction valves are held open.

• Two pressure switches control capacity:

  • Above high-pressure switch: 0% capacity (fully unloaded).

  • Below high-pressure switch: 50% capacity (half loaded).

  • Below low-pressure switch: 100% capacity (fully loaded).

Step Clearance Unloading

• A five-step unloading technique enables operation at 100%, 75%, 50%, 25%, or 0% capacity.

• Clearance pockets and valves are opened when unloading is required.

• Gas enters the clearance pocket during compression, expanding back during suction, reducing capacity.

  • 100% capacity (full load): All clearance pocket valves are closed.

  • 75% capacity: One clearance pocket valve is open (unloaded).

  • 50% capacity: Two clearance pocket valves are open (unloaded).

  • 25% capacity: Three clearance pocket valves are open (unloaded).

  • 0% capacity (no load): All four clearance pocket valves are open (unloaded).

• Alternative five-step unloading uses one clearance pocket with suction valve unloading.

• Four pressure switches or a pressure controller can activate unloaders based on discharge pressure.

Variable Clearance Unloaders

• Allow continuous adjustment of clearance pocket volume.

• A hydraulically actuated variable volume clearance pocket unloader is mounted on the outer head of a double-acting compressor.

• Varying hydraulic pressure changes the clearance pocket volume.

• Full hydraulic pressure: Zero clearance volume, full capacity.

• Minimal hydraulic pressure: Maximum clearance volume, approximately 50% capacity.

• Designs exist with variable clearance pockets on both sides of the piston for 0% to 100% capacity control.

Recirculation

• Recirculation involves recycling discharge gas back to the compressor's suction side (recycle or spillback control).

• The spillback valve's opening controls suction pressure.

• If inlet gas capacity drops, suction pressure drops. The spillback valve provides gas to compensate.

• Compressed gas is cooled (aftercooler) before recirculation to prevent overheating.

• Disadvantage: This method wastes power because gas pressure is dropped back to suction pressure and then re-compressed.

Speed Control

• Recommended for medium to large reciprocating compressors where constant flow is not guaranteed.

• Varies the speed of the prime mover to control compressor capacity.

• Combustion engine prime mover: An engine governor controls speed by regulating fuel.

• Air/fuel ratio controllers can improve engine speed control efficiency.

• Basic Speed Control Strategy:

  • Suction pressure controller (PC-1) controls the compressor based on inlet flow rate.

  • Discharge pressure controller (PC-2) controls the compressor based on downstream demand.

  • A low selector (PY-1) selects the lower of the two controller outputs.

  • The output signal goes to both the spillback valve (PCV-1) and engine governor (GOV) in a split range configuration.

  • Spillback valve: Fully open at 0% control signal, fully closed at 55%.

  • Engine speed: Minimum speed at 45% control signal, maximum speed at 100%.

  • If suction pressure drops, PC-1 decreases its output, increasing spillback valve flow or reducing engine speed.

  • If discharge pressure increases, PC-2 decreases its output, increasing spillback valve flow or reducing engine speed.

Reciprocating Compressor System

System Components

A two-stage reciprocating compressor system includes necessary equipment and valves.

• Speed Controller: Extends operating range and efficiency. Compressor speed adjusts to match inlet flow, maintaining pre-set suction or discharge pressure.

• Pulsation Bottle: Dampens pressure pulsations from the compressor's operation on both suction and discharge sides.

• Scrubber: Suction scrubbers remove liquids and solids from the gas before entering the compressor stages. Inter-stage scrubbers remove liquids that condense due to cooling.

• Intercooler: Increases compressor efficiency and prevents overheating by cooling gas between stages using air heat exchangers.

• Blowdown Valves: Depressurize the compressor during shutdowns, releasing trapped gas between stages.

• Shutdown Valves: Suction (SD) and discharge shutdown valves isolate the compressor during emergencies.

• Purge Valve: Increases pressure in the compressor before start-up.

• Check Valve: Prevents backflow from downstream piping during start-up.

• Flare Valve: Protects equipment from over-pressurization by releasing excess pressure.

• Suction Throttling Valve: Protects the compressor from high suction pressure by automatically or manually controlling the pressure.

• Aftercooler: Cools spillback gas to prevent overheating, and cools compressed gas for the next process.

Objective Three - Reciprocating Compressor Problems

Safety Protection

Reciprocating compressors and their prime movers have safety devices to address potential issues:

• Over or Under Pressurization

• Overheating

• Loss of Lubrication

• Excessive Vibration

• Rod Monitoring

Pressurization

• Compressors operate within suction, discharge, and differential pressure limits.

• Sudden pressure changes indicate process upsets, requiring immediate shutdown.

• Pressure switches or transmitters trigger alarms or shutdowns if pressures exceed set limits.

Overheating

• Gas temperature increases during compression. High inlet gas temperature can exceed compressor specifications.

• Temperature switches or transmitters trigger alarms or shutdowns if temperatures exceed set values.

• Lube oil and water jacket temperature must be controlled to maintain running clearances and lubricant viscosity.

Loss of Lubrication

• Results in mechanical friction, generating heat and destroying components. Excessive wear also destroys components.

• Flow switches or transmitters trigger alarms upon loss of lubrication flow.

Excessive Vibration

• Can cause compressor or engine failure due to worn bearings, misalignment, or mechanical breakdowns.

• Vibration sensors (inertia sensors, accelerometers, proximity probes) measure vibration.

• Specialized vibrational analysis systems diagnose machine health before excessive vibration occurs.

Rod Monitoring

• Monitors mechanical internal parts.

• Includes:

  • Rod Drop

  • Rod Loading

  • Rod Reversal

Rod Drop

• Occurs in horizontal cylinders when piston rings wear, causing the piston to drop.

• Pneumatic eutectic sensors or proximity probes measure rod drop.

• Eutectic sensors melt due to friction, venting pressure and triggering alarms/shutdowns.

• Proximity probes sense the distance between the rod and probe, triggering alarms/shutdowns.

Rod Loading

• The force due to differential pressure across the piston and the inertia of moving parts.

• Excessive rod loading can cause compressor failure.

• Alarm/shutdown calculations are based on maximum allowable discharge pressure for a given suction pressure, using rod-loading curves or strain gauges.

Rod Reversal

• Reversal of compression and tension loads at the crosshead pin.

• Insufficient lubrication can cause bushing failure.

• Detected by:

  • Predicting based on cylinder pressures and piston part weight, requiring continuous pressure measurement and software.

  • Monitoring crosshead pin temperature with electronic or pneumatic (eutectic sensor) methods.

Reciprocating Compressor Safety

Interlocks and Permissives

Compressor control systems use shutdown instrumentation, interlocks, and permissives for safe operation.

• Permissives: Prevent operation until specific conditions are met.

• Interlocks: Sense limit conditions or improper sequences, shutting down equipment to avoid undesirable conditions.

Compressor Alarms and Shutdowns

Monitor process variables for safety and maintenance. Alarms alert operators to problems; shutdowns indicate safety issues and trigger automatic shutdown.

• Shutdowns can be:

  • Class A

  • Class B

  • Class b

  • Class C

Class A Shutdowns

• Always armed and continuously monitored.

• Examples: Engine oil level, compressor lube oil, engine jacket water temperature, suction scrubber level, compressor/engine vibration, compressor discharge temperatures, operator shutdown buttons.

Class B Shutdowns

• Armed after the unit is running and the Class B timer has expired.

• Examples: Compressor lubrication without flow, compressor rod reversal, engine jacket water temperature.

Class b Shutdowns

• Armed after the unit is running and the Class b timer has expired (shorter duration than Class B).

• Examples: Compressor oil temperature, engine oil temperature, ignition fault.

Class C Shutdowns

• Armed when the process variable is within operating limits; no time limit.

• Examples: Engine under speed, low suction pressure, low discharge pressure.

Compressor Start-Up

Start and Load Sequences

Start-up involves a start sequence to bring the compressor to a ready-to-load state, followed by a load sequence.

Start Sequence

A five-step process:

1. Operator Initiated Start: Clear Class A shutdowns, open blowdown valves (if pressurized) or open purge valve (if depressurized). Reset suction override and speed controller. Start oil permissive pressure fault timer.

2. Pre-Lube: Start engine and compressor lube oil pumps, which must reach minimum pressure before the oil permissive timer expires. Start pre-lube timer on obtaining oil pressure permissives.

3. Engine Crank: Energize engine crank solenoid. Start the crank timer and engine failed to start timer. Abort if engine turnover is not detected within the allocated time.

4. Ignition: Start engine purge timer when engine speed is greater than a set minimum. Energize engine ignition module and start the fuel delay timer when the engine purge timer expires. Energize the engine fuel solenoid when the fuel delay timer expires. De-energize the engine crank solenoid when the engine speed exceeds the engine crank termination speed.

5. Engine Warm-Up: If the start attempt is successful, the engine accelerates to its engine idle speed setpoint. The lube oil pumps run for a set time after the engine has started and then shut off. The Class B and Class b timers start. Load when engine oil temperature reaches its minimum.

Load Sequence

A three-step process to bring the engine and compressor online:

1. Acceleration to Minimum Load Speed: The discharge shutdown valve opens. The suction shutdown valve opens. The engine speed controller setpoint ramps to the minimum load speed.

2. Initiate Loading: Place the override suction pressure controller and override discharge pressure controller in automatic. Activate the engine speed and recycle valves signals based on the low selected output of these two loops.

3. Online: The compressor is considered online once the minimum load speed has been achieved and the override suction pressure controller and override discharge pressure controller are in automatic.

The step in which the purge valve pressurizes the compressor and the step at which the suction shutdown valve and discharge shutdown valve open may vary based on company policy.

Compressor Shutdown and Unloading

Shutdown procedures vary based on how the compressor is stopped (normal stop, automatic process shutdown, or emergency shutdown (ESD)).

Normal Stop

A three-step sequence:

1. Operator Initiated Stop: Start a cooldown timer. Switch the suction override controller to manual and its output ramps down to 0%. This ramps the engine down to its engine idle speed and ramps the spillback valve to fully open.

2. Engine Stop: When the cooldown timer expires, the engine fuel valve solenoid de-energizes and the engine ignition module de-energizes three seconds later, thus stopping the engine. The discharge override controller is switched to manual and its output set to 0%. The engine speed governor is set to 0%.

3. Post Lube: The post-lube timer is started. The compressor and engine oil lube pumps are started. When the post-lube timer expires, the lube pumps stop.

Blowdown valves are not opened, leaving the compressor pressurized.

Process Shutdown

A three-step sequence initiated by a discrete shutdown sensor, analog setpoint, or sensor failure:

1. Engine Shutdown: The engine fuel valve solenoid and engine ignition module de-energize, stopping the engine.

2. Control Shutdown: Switch the discharge override controller to manual and set its output to 0%. Let the engine speed governor be set to 0%.

3. Post Lube: Start the post-lube timer. Start the compressor and engine oil lube pumps. When the post-lube timer expires, the lube pumps stop.

Blowdown valves are not opened, leaving the compressor pressurized.

Emergency Shutdown

Simultaneous events initiated by an operator ESD button or programmed emergency shutdown:

• Engine Shutdown: The engine fuel valve solenoid and engine ignition module are immediately de-energized.

• Shutdown and Blowdown Valves: The blowdown valve is immediately opened. The suction and discharge shutdown valves are immediately closed.

• Control: All controllers are switched to manual and their outputs are set to 0%.

• Post Lube: No engine post-lube is done.

Objective Four - Centrifugal Compressors

Basic Principle

A centrifugal compressor is a dynamic compressor that increases gas pressure by accelerating gas to high velocity and then converting the velocity to pressure.

• A prime mover rotates the impeller (rotor).

• Gas enters the center (eye) of the impeller and is accelerated, increasing its kinetic energy.

• The diffuser converts kinetic energy to pressure by slowing gas velocity.

• The scroll (volute) captures gas and directs it to the outlet, further increasing pressure.

Compression Ratio

Defined as the ratio of absolute stage discharge pressure to absolute stage suction pressure.

• Calculation:
  $Discharge Pressure / Suction Pressure = Compression Ratio$
  Example: If the suction pressure is $200$ kPaa and the compression ratio is $4:1$, then the discharge pressure is $800$ kPaa.

Multistage Compressors

Single-stage compressors compress gas once. Multistage compressors compress gas multiple times by delivering discharge from one stage to the suction of another.

• Straight-Through Multistage Compressor: Impellers and diffusers are arranged in line along the rotor shaft. Each stage increases final discharge pressure. Multistage compressors typically require intercooling to maintain discharge temperature.

  • Interstage Cooling: External heat exchangers cool the gas. Interstage scrubbers remove condensed liquids.

• Surge: It is possible for the gas to flow backwards against the forward action of the impeller if the discharge pressure becomes too high for a given operating speed.

Choke or Stonewall Point

• The gas velocity cannot exceed the speed of sound; this limits the operation of a centrifugal compressor.

• Occurs when the compressor is operating at low discharge pressure and very high flowrate.

Compressor Map

A characteristic performance curve describing compressor operation. The x-axis measures flow, and the y-axis measures pressure increase.

• For constant speed, flow increases as discharge pressure decreases and vice versa.

Note: At a given speed, a centrifugal compressor is a relatively constant-pressure, variable-flow machine.

Objective Five - Compressor Capacity Control

Control Strategies

Capacity control is achieved by regulating discharge or suction pressure, maintaining constant pressure by varying capacity. Flow control is also used.

Methods:

1. Suction Throttling

2. Guide Vane Positioning

3. Speed Control

Either flow or pressure control can be used depending on the application.

Suction Throttling

• For discharge pressure control, a discharge pressure controller (PC-1) throttles the suction control valve (PV-1) if discharge pressure is too high.

• For flow control, a flow controller (FC-1) throttles the suction control valve (FV-1) if flow is too high.

• Effect: Decreasing inlet pressure significantly reduces flow capacity (at constant pressure) or discharge pressure (at constant capacity).

Guide Vane Positioning

• A discharge pressure controller (PC-1) adjusts inlet vane angle (PV-1) if discharge pressure is too high.

• Effect: Changing vane angle reduces flow capacity (at constant pressure) or discharge pressure (at constant capacity).

• Adjustable inlet vane guides are more efficient than suction throttling.

Speed Control

• A discharge pressure controller (PC-1) sends a remote setpoint (RSP) signal to a speed controller (SC-1), which adjusts speed using a variable speed drive (VSD).

• Effect: Decreasing compressor speed reduces flow capacity (at constant pressure) or discharge pressure (at constant capacity).

• Capacity control by speed variation is the most effective and efficient.

Note: Steam, gas turbines, and electric motors with VSDs are available.

Objective Six - Common Problems and Anti-Surge Control

Surge Phenomenon

Surge is possible in centrifugal compressors because there is no mechanical block in gas passage backward.

• Occurs when kinetic energy from the impeller is less than the gas pressure energy at the discharge, causing flow reversal.

• Can quickly destroy a compressor.

Surge Cycle

• At point B, discharge pressure is too high and gas velocity is too low, causing flow reversal.

• Flow decreases and pressure drops to point C.

• At point C, gas velocity is sufficient to overcome pressure, reversing flow back to point A.

• Process reverts to point B, starting another surge cycle.

Cycle occurs rapidly. Prolonged intense surges cause failures.

Anti-Surge Control

Prevents surge by opening a surge, anti-surge, or recycle control valve, diverting gas back to the suction side.

Surge Ratio Control Strategy

Determines when to take action based on differential pressure ($\Delta P$), head pressure (h) from inlet flow, and surge data.

• The surge line is a straight-line plot of differential pressure versus head pressure.

• A surge control line is parallel to the surge line, separated by a bias value (b).

• The slope (m) is determined from surge data.

• Action is taken when the operating point crosses the surge control line.

• The head pressure from a differential flow meter measures the kinetic energy of the gas stream.

Measures differential pressure and calculates head pressure: $m(\Delta P) + b$.

Basic Ratio Surge Control

• The compressor's differential pressure is measured by PDT-2.

• Its signal is sent to the ratio bias station PDY-2.

• The slope for the ratio bias station is determined from the centrifugal compressors map.

• The bias needs to be large enough so that the surge control system can respond to prevent surge (10%).

• The output of the ratio bias station is the remote setpoint (RSP) for the surge controller FC-1.

• The surge controller compares its RSP against the actual head pressure as measured by FT-1 to determine its control output. A volume booster (1:1) increases the speed of operation of the recycle valve (FCV-1A).

• The surge control is a proportional integral (PI) controller and must have antireset windup. In normal operation the measured inlet flow head pressure is higher than the calculated RSP.

Backup Surge Control Strategy

Open-Loop Backup

• Prevents surge even if the surge controller is not functioning.

• FSL-1 activates interlock one (IL1) if head pressure (h) is less than the ratio station (PDY-2) output by a set value (usually about 5%).

• IL-1 de-energizes the solenoid (S) on the recycle valve (FCV-1A), opening it quickly.

• IL-1 activates function FY-1A, which drops its output to zero. The low select FY-1B selects the zero output of FY-1A, fully opening the recycle valve.

• The open-loop backup is lost if the ratio station's output goes to zero.

Surge Control Systems

• Integrated with operating, safety, or emergency control systems.

• Dedicated anti-surge controllers have multiple inputs for surge line and operating point determination.

• Multiple anti-surge strategies can be programmed.

Instrumentation

• Fast-acting and reliable process transmitters are necessary (e.g., flow oscillation detection at 1 Hz requires fast flow transmitters).

Recycle Valve

• Typically a linear fail-open globe valve, tuned for fast action. Parralel setup often found to increase realibility.

• Volume boosters and oversized ports increase operating speed.

Supporting equipment

• Blowdown Valves: Depressurize the compressor during shutdown.

• Purge Valve: Equalizes pressure before start-up.

• Check Valve: Prevents backflow.

• Flare Valve: Protects against over-pressurization.

• Scrubber: Prevents liquid entry.

• Aftercooler: Required during recycling events.

Startup

Aims to quickly bring the compressor up to speed without overheating. The recycle valve is fully opened during startup, gradually closed as speed increases.

Shutdown

In controlled shutdowns, maintain steady flow and gradually open the recycle valve to remove power while maintaining surge margin.

In emergency shutdowns, immediately open the recycle valve fully.

Normal Opperation

In normal operation, the surge control system needs to prevent surge while keeping the compressor online. As the operating point approaches the surge limit, the surge control system gradually opens the recycle valve to keep the operating point away from surge.

If the operating point is approached too fast or the compressor starts a surge cycle, the surge control system needs to fully open the recycle valve preventing surge or its escalation. A properly designed surge control system can allow total recycle (no process flow) and keep the compressor online allowing the operator to respond as necessary.