Condition monitoring solutions for reciprocating compressors

As one of the oldest compressor designs, reciprocating compressors (recips) enjoy unique capabilities and are quite flexible with features such as broad capacity control ranges and the ability to compress gas regardless of mole weight or k value. Add multiple stages and they can compress a gas to extremely high pressure ratios. Consequently, they are an ideal fit for selected applications—such as hydrogen compression in the de-sulfurization process of many refineries—and as part of the low‑density polyethylene production process in chemical plants.

However, reciprocating compressors suffer from one very serious drawback: high maintenance costs. In fact, compared to similarly sized centrifugals, recips can consume as much as five times the amount in maintenance costs.

That’s why a continuous monitoring system is so important.Specifically designed to meet the unique machinery protection and condition monitoring needs of recips, our solutions, which include safety integrity level (SIL) certified products can help close the gap, bringing your recip maintenance costs closer to the levels associated with centrifugals while simultaneously improving reliability, availability, and safety. And, with patented technology,comprehensive services, and over 55 years of experience monitoring reciprocating compressors, no one is better equipped to help you than Bently Nevada, a Baker Hughes business.

Recip monitoring—in detail

1. Crosshead vibration
Crosshead-mounted accelerometers can detect machinery problems due to impact-type events such as excessive crosshead clearance, liquid ingestion into the cylinder, excessive clearance in the crosshead pin bushing, and loose or cracked nuts, bolts, liners, or pistons.
3500 Monitor: 3500/70M Four-Channel Impulse/Velocity Monitor
Transducer: 330425 Accelerometer
Crosshead shoe temperature
Continuous monitoring of crosshead shoe (slipper) temperature provides useful information on developing slipper problems such as overload, fatigue, and insufficient lubrication.
3500 Monitor: 3500/60 or 3500/61 Six-Channel or 3500/65 16-Channel Temperature Monitor
Transducer: RTD or Thermocouple
2. Frame vibration
Increases in frame vibration can indicate problems such as imbalance due to unusual pressure differential or inertial unbalance, looseness in the foundation attachment (such as deteriorating grout or shims), and high moments caused by excessive rod load.
3500 Monitor: 3500/70M Four-Channel Impulse/Velocity Monitor
Transducer: 190501 Velomitor CT Piezo-Velocity Transducer

3. Main bearing temperature
This measurement indicates problems related to fluid-film bearings, including overload, bearing fatigue, or insufficient lubrication.
3500 Monitor: 3500/60 or 3500/61 Six-Channel or 3500/65 16-Channel Temperature Monitor
Transducer: RTD or Thermocouple

4. Cylinder chamber pressure profiles
The single most effective way of determining the overall health of a reciprocating compressor is by examining the cylinder pressure profile. Online access to the internal pressure for each cylinder chamber enables continuous monitoring of cylinder pressures, compression ratios, peak rod loads, and rod reversal. This provides valuable information on the condition of suction valves, discharge valves, piston rings, packing glands, and crosshead pins. And, interfacing the 3500 system with System 1 software provides valuable diagnostic plots such as Pressure versus Volume (P-V), Log P versus Log V, Pressure versus Crank Angle (Theta-θ) (P-T), and rod load curves. Performance monitoring is a standard feature of System 1 software for chamber pressure measurements and delivers important information such as indicated horsepower, flow balance, and power losses.
Continuous rod load
Combined (inertia and gas) rod load calculated at the crosshead pin provides information about the lubrication condition of the crosshead pin. Insufficient reversal or excessive rod load can be identified and corrected before costly running gear damage occurs.
3500 Monitor: 3500/77M Four-Channel Cylinder Pressure Monitor
Transducer: 165855 Cylinder Pressure Transducer
5. Piston rod/plunger position
Horizontal and vertical proximity probes observing the piston rod provide information on rod bow, rod flex (vibration), and rider band wear. Continuous monitoring gives maximum magnitude and direction of rod movement along with the crank angle at which the maximum occurs. The source of movement can be identified, enabling the proactive scheduling of maintenance for rider band replacement or crosshead repair. Monitoring the piston rod vibration provides early warning for rod break. Monitoring the plunger position in hyper compressors provides protection against deteriorating plungers.
3500 Monitor: 3500/72M Four-Channel Rod Position Monitor
Transducer: 3300XL Series Proximity Transducer
6. Multi-event Keyphasor signal
A proximity probe observing a multi-toothed wheel on the crankshaft provides a precise reference timing signal every 30 degrees of rotation in comparison to a once-per-turn reference point. The torsional vibration of the crankshaft changes from revolution to revolution for each load step on the machine. Having multiple events per revolution improves the ability of the system to capture data with respect to crank position, regardless of the shape or change in shape of the torsional vibration. This allows measurements such as rod position and cylinder pressure to be correlated with a highly accurate crank angle measurement. Rod position can be captured at the same point in the piston rod stroke, and cylinder volume can be computed precisely at each point in the stroke for highly accurate P-V curves.

3500 Monitor: 3500/25 Keyphasor Module
Transducer: 3300XL Series Proximity Transducer
7. Suction/discharge valve temperature
Suction and discharge valves are typically the highest maintenance items on recips. Faulty valves can significantly reduce the compressor’s efficiency as well as increase combined rod load. By recompressing the same gas, a leaky valve becomes hotter than normal, causing the valve cover plate temperature to increase. By monitoring both absolute valve temperature and differential temperature between a group of valves, valve problems can be spotted early and repaired before efficiency losses mount.

Suction gas temperature
Continuous monitoring of suction gas temperature provides a useful baseline against which to compare suction valve temperature. Relative changes in temperature between suction valves and suction gas can give an indication of worn suction valves. In addition, suction gas temperature, combined with the integrated equation of state engine in System 1 software, provides the information necessary to calculate capacity at suction conditions and cylinder flow balance numbers.
Discharge gas temperature
Continuous monitoring of discharge gas temperature can give an indication of worn valves, unloaders, excessive compression ratio, or insufficient cooling water.
3500 Monitor: 3500/60 or 3500/61 Six-Channel or 3500/65 16-Channel Temperature Monitor
Transducer: RTD or Thermocouple

8. Cylinder acceleration
Placing accelerometers on the cylinder monitors the opening and closing events of the valves. The impact-type waveform information can be combined with cylinder pressure data to assess the health of the cylinder valves. Valve related malfunctions such as loose valve cover bolts, valve fluttering, or capacity control malfunctions can be detected. For hyper compressors, the cylinder acceleration changes significantly when valve springs break or valve seats wear out.
3500 Monitor: 3500/70M Four-Channel Impulse/Velocity Monitor

Transducer: 330425 Accelerometer

9. Pressure packing case temperature
Continuous monitoring of pressure packing temperature provides useful information on developing problems related to the packing including excessive wear, insufficient cooling, and inadequate lubrication.
Packing vent line temperature
Continuous monitoring of the vent line provides an indication of leaking pressure packing. With an increase in process flow, the vent line becomes hotter. For years, operators have used the subjective method of touching the line to see if gas is leaking.
3500 Monitor: 3500/60 or 3500/61 Six-Channel or 3500/65 16-Channel Temperature Monitor

Transducer: RTD or Thermocouple

At the heart of our recip monitoring solution is the Bently Nevada 3500 Series machinery protection system

Integrated machinery protection and machinery management
Our powerful 3500/22M TDI rack interface module eliminates the need for bulky external data acquisition on the hardware or special interface modules between the monitor rack and software. Simply plug an ethernet cable into the 3500/22M rack and you’re ready to communicate with our powerful System 1 software for advanced diagnostics, condition monitoring, and other plant asset management functionality, without interrupting or interfering with machine protection. The result is a zero-footprint solution that simplifies System 1 connectivity for existing machinery protection systems.
And, the enhanced data collection technology of our 3500/22M TDI rack allows earlier and improved diagnosis of previously difficultto-recognize intermittent events (such as cylinder liquid ingestion).

• Fully compliant with the requirements of American Petroleum Institute (API) Standard 670 for machinery protection systems and Standard 618 for reciprocating compressors

• Available in 19 in. 14-slot EIA rack or space-saving 12 in. 7-slot mini‑rack

• Available with a variety of high-quality display options ranging from self-configuring VGA touch screens to remote workstations to LCD panels that mount directly on the rack face

• Digital (Modbus) and analog (4-20 mA) interfaces available for connection to control systems and historians

• Safety integrity level (SIL) certified monitor modules are available for use in functional safety applications

• Six- or 16-channel temperature monitors accept a variety of RTDs or thermocouples. Absolute and differential alarming capabilities allow the monitors to be used for valve temperatures, gas, or other process temperatures, bearing temperatures, crosshead shoe (slipper) temperatures, pressure packing case temperatures, and a variety of other machinery or process temperature measurements

• Capabilities for assessing piston rod vibration and position at precisely defined crank angles for repeatable, useful information on rider band wear, rod bow/ flex, and other conditions. Also applicable to hyper compressors using plungers instead of conventional pistons

• All monitor modules are totally software configurable via the 3500’s rack interface module—key lockable to prevent unauthorized tampering

• Keyphasor module accepts multi-eventper-turn signals for greater accuracy—required for reciprocating compressor applications where crank angle must be precisely determined

• Single-cable ethernet connectivity to our System 1 software for advanced diagnostics and condition monitoring functionality

• Highly reliable power supplies accept worldwide AC/DC voltages/frequencies and can be ordered as fully redundant to ensure uninterrupted performance

• Powerful cylinder chamber pressure measurements now available for the most complete picture of reciprocating compressor health. Dynamic P-V curves, rod reversal measurements, and much more

• Special-purpose four-channel vibration modules are designed to address the casing vibration, crosshead acceleration, and cylinder acceleration measurement requirements unique to reciprocating compressors by providing reliable machinery protection without false or missed alarms. Filtering, alarm delay, full‑scale range, transducer input type, and a host of other options are fully software configurable on a per‑channel basis

• Generic four-channel vibration modules accept velocity, acceleration, and proximity transducers. Filtering, alarm delay, full-scale range, transducer input type, and a host of other options are fully software-configurable on a per-channel basis. Used for a variety of measurements on the recip’s prime mover

• Relay modules allow voting of userconfigurable alarms and health statuses across multiple channels. Relays are highly reliable and intended for connecting the monitoring system to external annunciators and machinery control systems for auto-shutdown

• Fully digital communications of all monitored parameters and statuses using a variety of industry‑standard protocols—ideal for connection to process and machine control systems rather than using analog (4-20 mA) outputs. Supports time‑stamping of alarms and clock synchronization.