Reciprocating Compressor Blowby: Symptoms, Causes, and Detection

What blowby is

Every cylinder on a reciprocating compressor seals against its bore with a set of piston seal rings, often called compression rings. On the pressure stroke the piston pushes gas toward the discharge valve, and the rings are what keep that gas on the high pressure side of the piston. When the rings wear, break, or lose their seal, some gas slips back past the piston to the low pressure side instead of leaving through the discharge. That slippage is blowby. The gas never gets fully compressed or pushed out. It stays in the cylinder, gets recirculated, and gets recompressed on the next stroke.

Blowby also moves gas between the two ends of a double acting cylinder. On a double acting design the piston compresses gas on both the head end and the crank end. Worn rings let gas leak from the high pressure end to the low pressure end inside the cylinder. Either way the result is the same. Each stroke moves less gas than it should.

Why blowby means lost capacity

Blowby shows up first as lost capacity. A cylinder is sized to move a set volume per stroke. When gas leaks back past the rings, the effective volume drops, so throughput falls even though the unit runs at the same speed and the same suction and discharge pressure. Operators often see it as a slow decline in flow that no longer matches the compressor performance curve.

The recirculated gas also carries heat. Gas that slips back gets recompressed again, and each pass adds energy. That drives cylinder and discharge temperature up and pushes horsepower per unit of gas up with it. So a cylinder with bad rings does less work while burning more fuel and running hotter. Field data puts some level of ring slippage on roughly one in three reciprocating compressors in gas service, so it is common, not rare.

Symptoms to watch for

• Lost capacity and throughput. Flow falls below the compressor curve for the same speed and pressures.
• High discharge temperature. The cylinder discharge runs hotter than the expected value for that pressure ratio.
• Elevated cylinder temperature. The cylinder body and the gas leaving it climb over time.
• Rising horsepower per unit of gas. The driver works harder to move less.
• On a double acting cylinder, the head end and crank end temperatures drift apart.

Here is the catch. Rising discharge temperature is also the classic sign of a leaking valve. Early on the two faults look identical from the panel. That is why you confirm with trends, a PV card, or an inspection before you pull a cylinder.

What causes blowby

Blowby is almost always the end result of wear or damage inside the cylinder. The usual causes are:

• Worn or broken seal rings. Normal wear opens the ring end gap until it no longer seals. Rings can also chip or break from fatigue, liquids, or debris.
• Worn rider bands. Rider bands carry the weight of the piston. When they wear thin the piston sags, and the piston and rings can touch the liner and score it.
• A scored liner or cylinder bore. Once the bore is scratched, the rings cannot seal against it and they wear faster. Scoring comes from metal to metal contact, debris, or lost lubrication.
• Liquids in the gas. Slugs of liquid wash the oil film off the bore, raise wear, and can break rings. Liquids are one of the most damaging things you can send through a cylinder.
• Dirt and abrasives. Sand, rust, and scale grind the rings and the bore. High temperature can also bake the cylinder oil into hard coke deposits that interfere with the rings.
• Poor lubrication. Too little oil, the wrong oil, or a plugged lubricator point starves the rings and speeds wear.

How to detect blowby from routine service data

You do not need a portable analyzer to catch blowby. Most of the signal is already in the numbers you record on a normal service round. The key is to trend them, not to read them once.

• Trend cylinder discharge temperature against its expected value. Calculate the expected adiabatic discharge temperature from suction temperature, suction and discharge pressure, and the gas k value. When the measured discharge runs steadily above that value and the valves check out, ring blowby is the likely cause.
• Track flow against the compressor curve. Compare measured throughput to what the cylinder should move at that speed and those pressures. A widening gap points to lost volumetric efficiency, which is exactly what blowby produces.
• Watch the trend, not one reading. A single hot reading means little. A slow climb in discharge temperature or a slow fall in flow over weeks is the real tell.
• Add oil analysis. Rising iron and other wear metals in the cylinder lube say the rings, bands, and bore are wearing.
• Use rod drop if you have it. A proximity probe that measures how far the rod has dropped tracks rider band wear, which often runs alongside ring wear.

PV cards and temperatures for confirmation

When you want to confirm the fault and separate it from a valve problem, a PV card is the sharpest tool. A PV card, or pressure volume card, plots cylinder pressure against piston position for one full stroke. It shows the real compression and expansion against the ideal curve.

• Ring blowby sags both curves. The measured compression and expansion lines both fall below the ideal adiabatic line, because gas is leaking past the piston during the stroke. On a double acting cylinder the head end and crank end cards show opposite effects, because gas moves from the high pressure end to the low pressure end.
• Valve leakage looks different. A leaking valve distorts one part of the card, the suction or the discharge event, rather than both curves. The pattern is not symmetric across the two ends.
• Capacity loss is visible on the card. The swept and trapped volume shrinks from the design value, which is the same lost throughput you see in the flow trend.

Pair the card with the temperatures. A higher than expected cylinder discharge temperature next to a card that shows both curves sagging is a strong blowby signature.

How to prevent blowby

• Keep the gas clean and dry. Good suction scrubbers, knockouts, and coalescing filters keep liquids and solids out of the cylinder. This is the single biggest lever you have.
• Lubricate correctly. Use the right cylinder oil and the right feed rate for the gas and the pressure, and check every lubricator point on your rounds.
• Control discharge temperature. Keep cooling and valves healthy so the cylinder oil does not bake into coke that fouls the rings.
• Replace on condition, not on a guess. Trend the temperatures, flow, oil, and rod drop, then change rings and rider bands when the data says they are worn, before they score the bore.
• Inspect the bore at overhaul. Measure ring end gap and bore wear and look for scoring, so a marginal liner does not eat a fresh set of rings.

How EverSense helps

Blowby is a slow, quiet capacity thief. By the time flow is obviously down, the rings and maybe the bore are already worn. The good news is that the warning signs sit in data you already collect: cylinder discharge temperature, flow, suction and discharge pressure, oil analysis, and run hours.

EverSense reads that routine service data and watches for the pattern of blowby: discharge temperature drifting above its expected value while flow slips below the curve. It checks each unit against its own OEM manuals and against an archive of about 25,000 field repairs across 38 equipment makes, so it can separate likely ring blowby from a leaking valve and draft the investigation for your team. It never touches the controls. It just tells you what to look at and why, so you can plan the cylinder work before you lose the throughput. To see it on your fleet, book a demo.