Understanding Reciprocating Compressor KPIs in Enalysis

In addition to Blowby, Enalysis calculates several key performance indicators for reciprocating compressors that can help users understand cylinder capacity, mechanical loading, and conditions that may affect equipment health.

Three important maintenance based KPIs are volumetric efficiency, rod loads, and degrees of reversal. These values should be reviewed together with the compressor operating conditions, configuration, report history, and manufacturer limits.

Which reciprocating compressor KPIs does Enalysis calculate?

For reciprocating compressors, Enalysis can calculate information including:

• Volumetric efficiency for the head end and crank end
• Gas or static rod load in compression and tension
• Net rod load in compression and tension
• Degrees of reversal
• Net rod load ratio

Other calculated values such as Blowby, expected discharge temperature, cylinder power, and cylinder flow provide additional context when reviewing compressor performance and health.

What is volumetric efficiency?

Volumetric efficiency describes the actual pumping capacity of a cylinder compared with the piston displacement. In practical terms, it indicates how much of the piston stroke is being used to draw new gas into the cylinder.

The Enalysis training material identifies several factors that influence reciprocating compressor volumetric efficiency, including cylinder clearance, compression ratio, gas slippage or Blowby, and gas characteristics.

• Increasing cylinder clearance reduces volumetric efficiency and cylinder capacity.
• Increasing compression ratio reduces volumetric efficiency because more of the piston stroke is required for the gas trapped in the clearance volume to expand back to suction pressure.
• Blowby and other gas losses can also affect how effectively the cylinder moves new gas.

Why does low volumetric efficiency matter?

Low volumetric efficiency means less of the cylinder stroke is available to draw in new gas. The Enalysis 101 training notes that when volumetric efficiency falls below 20 percent, compressor valves may not have enough time to fully open before the piston reaches the end of its stroke. This can increase valve wear and maintenance requirements.

The source training describes changes to cylinder clearance and configuration as possible ways to address low volumetric efficiency. Because these changes affect compressor loading and operating limits, they should be evaluated with your Detechtion Engineering Account Manager and in accordance with the compressor manufacturer requirements.

What are rod loads?

Pressure acting on the reciprocating compressor piston creates forces that are transferred through the piston rod and compressor frame. Enalysis calculates both static rod loads and net rod loads in compression and tension.

Rod loading becomes a concern when the loads on the compressor frame approach or exceed the manufacturer ratings. Static rod load represents the load created by gas pressure, while net rod load also considers the forces associated with the reciprocating components.

 What should I do when rod loads are high?

The Enalysis 101 training recommends additional review when rod loads exceed 95 percent of the applicable limit. The source guidance recommends:

• Contacting your Detechtion Engineering Account Manager
• Increasing reporting frequency
• Monitoring valve health
• Evaluating whether a configuration change, such as additional clearance, may reduce the load
• Low compressor speed
• Single acting cylinders
• Damaged discharge valves
• High compression ratios
• Low volumetric efficiencies
• Small cylinder bores with large piston rods

Do not make compressor configuration changes based only on a single Enalysis value. Review the report, trends, operating conditions, and manufacturer limits with the appropriate engineering resources.

What are degrees of reversal?

A reversal occurs when the net force on the piston rod changes from compression to tension or from tension to compression. The Enalysis training states that two reversals should occur during each full rotation of the compressor crankshaft so the crosshead pin can receive proper lubrication.

Degrees of reversal are measured in degrees of crankshaft rotation for the lesser reversal. Enalysis calculates this value to help identify conditions where the force on the crosshead may remain on one side for too long.

What can cause low degrees of reversal?

The Enalysis source material identifies several conditions that can reduce reversals:

The training material warns that loss of reversals can interfere with proper crosshead pin lubrication and can contribute to serious equipment damage. Review a low reversal condition promptly with the appropriate engineering resources.

How do these KPIs work together?

These KPIs should not be reviewed in isolation. Changes in compressor configuration or operating conditions can affect more than one calculated value at the same time.

• Low volumetric efficiency can contribute to reduced degrees of reversal.
• Valve damage can affect Blowby and may also contribute to loss of reversals.
• Changes in pressure, compression ratio, speed, and cylinder configuration can affect mechanical loading and capacity.

Reviewing the current report together with trends can help determine whether a change is temporary, related to operating conditions, or requires additional investigation.

How should I review a flagged reciprocating compressor KPI?

1. Confirm that the compressor operating data and configuration in the report are accurate.
2. Identify the KPI that is flagged and review the corresponding value and applicable limit.
3. Compare the result with previous Enalysis reports and trends.
4. Review related KPIs such as Blowby, volumetric efficiency, rod loads, and degrees of reversal for additional context.
5. Consider recent operating changes, maintenance, or compressor configuration changes.
6. Contact your Detechtion Engineering Account Manager when a condition is severe, persistent, approaching an operating limit, or difficult to interpret.