What Is Blowby in Enalysis?

Blowby is a calculated Enalysis indicator used to help evaluate the health and efficiency of reciprocating compressor cylinders. Detechtion uses the term Blowby to quantify inefficiency in the compression process, particularly inefficiency associated with valves and piston rings.

In simple terms, Blowby represents gas that is being recirculated and recompressed within a cylinder rather than moving efficiently through the compression process. As more gas is recirculated and recompressed, discharge temperature can increase, additional horsepower may be required, and the cylinder may have less effective capacity.

What does Blowby tell me?

Blowby can provide additional information about cylinder condition and help identify when further investigation may be appropriate. It is intended to support field observations and maintenance decisions, not replace the experience and judgment of operations personnel.

Changes in operating conditions, gas flow, and ambient temperature can affect compressor pressures and temperatures. When those changes occur together with an increase in calculated Blowby, the result can provide additional justification for investigating the cylinder. A relatively constant Blowby level can provide context that other operating changes may be within the normal range for that compressor.

What Blowby levels can indicate

Detechtion’s source guidance provides the following general Blowby guidelines for reciprocating compressor cylinders:

• Up to 10%: Light valve or piston ring wear may be present, and visible damage can be difficult to identify.
• 10% to 15%: Obvious valve damage may be present, including broken or chipped plates and/or springs.
• 16% to 30%: Severe valve and/or piston ring damage may be present, and multiple valves may be affected.
• Greater than 30%: Extreme damage to valve assemblies may be present, with possible damage to the cylinder lining and piston.

These values are guidelines and should be reviewed together with compressor trends, operating conditions, flow, maintenance history, and field observations. A single Blowby value should not be used by itself to determine a maintenance action.

Why can small amounts of Blowby be normal?

The Blowby calculation compares the actual temperature rise across a cylinder with the temperature rise expected from the compression process. Small percentages of Blowby are considered normal and expected because real compressors do not operate under perfectly ideal conditions.

Higher Blowby values indicate increasing inefficiency in the cylinder. The source guidance notes that at Blowby levels below 20%, the calculation can also be used to estimate the amount of gas being recompressed or, for later compression stages, the increase in interstage pressure.

What is Expected Discharge Temperature?

Expected Discharge Temperature is the calculated discharge temperature that would result from adiabatic compression based on the cylinder specifications, running speed, suction and discharge pressure, and suction temperature.

Enalysis uses the relationship between the expected and reported discharge temperatures as part of the Blowby calculation. Reviewing Expected Discharge Temperature together with the actual discharge temperature can provide additional context for an elevated Blowby result.

What can cause elevated Blowby or related maintenance flags?

The source guidance identifies several conditions that can contribute to elevated Blowby or related gauge-maintenance indications:

• Damaged or worn valves
• Worn piston rings
• Unaccounted pressure drops, such as interstage pressure losses
• Clogged inlet suction filters
• Faulty temperature sensors or IR temperature guns
• Faulty pressure sensors
• High lubrication rates
• Improper valve springing

Because data quality and operating conditions can affect the calculation, review the compressor inputs and field conditions before assuming that a high Blowby value is caused by mechanical damage.

 How should I review an elevated Blowby result?

When Blowby is elevated, use the value as one part of a broader compressor review:

1. Confirm that the field pressures and temperatures used in the report are accurate.
2. Review the Blowby trend across multiple Enalysis reports rather than relying on a single result.
3. Review reported flow and compressor operating conditions for additional context.
4. Compare the Enalysis result with observations from operations personnel in the field.
5. Consider recent maintenance, configuration changes, or instrumentation issues that could affect the result.
6. Contact your Detechtion Engineering Account Manager if you need assistance interpreting the condition or determining the next investigation step.

What is Gauge Maintenance?

Gauge Maintenance is essentially the opposite of Blowby. Blowby is calculated when the reported discharge temperature is greater than the expected discharge temperature for a cylinder. Gauge Maintenance indicates that the reported temperature rise is lower than is thermodynamically possible.

When Gauge Maintenance is identified, Enalysis is indicating that the input data and/or the compressor software model should be investigated. This can include reviewing pressure and temperature measurements and confirming that the compressor model and configuration accurately represent the machine.