Oil & gas

Reciprocating gas compressor

A leaking valve shows up as heat before it shows up as a trip.

Each compressor stage has a discharge temperature that follows from its pressure ratio. A leaking valve or worn rings push gas back through the cylinder and the temperature climbs above that line. The twin reads it per throw, together with rod load, and names the valve before it breaks.

1,340 hp gas engine, 4 throws, 2 stages, 1,200 rpm, 10.4 MMscfd, 150 to 1,100 psig

Built and run to API 618, API 11P

Start with one station or fieldSee how we prove results
Plan view of a reciprocating gas compressor: gas engine, crankcase frame with four throws, two large stage-1 cylinders and two smaller stage-2 cylinders, intercooler, suction and interstage scrubbers, with numbered sensor points.12345678gas into pipelineintercoolerstage 1stage 2gas engineT1T2T3T4
Capacity
10.4 MMscfd
Brake power
1,225 hp
Interstage
439 psig
Peak rod load
45% of rating

Anatomy and nameplate

Plan view of a reciprocating gas compressor: gas engine, crankcase frame with four throws, two large stage-1 cylinders and two smaller stage-2 cylinders, intercooler, suction and interstage scrubbers, with numbered sensor points.12345678gas into pipelineintercoolerstage 1stage 2gas engineT1T2T3T4
  1. 1Suction and discharge pressure per stage. Sets the pressure ratio each cylinder works against.
  2. 2Discharge temperature per cylinder. Compared with the model for that ratio. Heat above the line is gas going round again.
  3. 3Valve cap temperatures. Where fitted, these point to the one valve that leaks.
  4. 4Frame and crosshead vibration. Loose crosshead, worn pin bushing or liquid in a cylinder.
  5. 5Rod drop or rod position. Where fitted, tracks rider band wear and the piston settling in the bore.
  6. 6Engine speed and load. Speed sets the strokes. Load shows how close the engine is to its rating.
  7. 7Lube oil pressure and temperature. Frame oil and cylinder lubricator health.
  8. 8Scrubber levels. Liquid that gets past a scrubber ends up in a cylinder.
Reciprocating gas compressor nameplate
Duty
ServiceNatural gas gathering, field to sales line
GasSpecific gravity 0.65, k = 1.27
Suction150 psig at 35 °C
Interstage439 psig, cooled to 45 °C
Discharge1,100 psig
Overall pressure ratio6.77 (stage 1 2.75, stage 2 2.46)
Capacity10.4 MMscfd
Brake power1,225 hp, 91% of the engine rating
Specific power117 hp per MMscfd
Discharge temperatureStage 1 109 °C, stage 2 112 °C
Volumetric efficiencyStage 1 75%, stage 2 75%
Machine
Driver1,340 hp natural gas engine, 1,200 rpm, direct coupled
FrameSeparable, 4 throws, 2 stages, double-acting cylinders
Stroke and rod5.5 in stroke, 2.0 in rod, 1,100 ft/min piston speed
Stage 1 cylinders2 × 8.75 in bore, 17% clearance, 447 cfm displacement each
Stage 2 cylinders2 × 5.375 in bore, 20% clearance, 161 cfm displacement each
Rod load rating40,000 lb combined, tension and compression
CoolingEngine-driven aerial cooler: jacket water, intercooler and aftercooler
ScrubbersSuction and interstage, with level switches and dump valves
Rod load at design
Stage 1 throws17,867 lb compression, 17,717 lb tension, 45% of rating
Stage 2 throws15,323 lb compression, 12,374 lb tension, 38% of rating
Reversal, stage 2157° in tension, 203° in compression each turn

From the well to market

Wells lift the fluid. The separator splits it. Oil goes to the tank battery and gas to the compressor station, and both leave by pipeline for processing.

Well on rod pump: Beam pumping unitWell on rodpumpBeam pumpWell on ESP: Electric submersible pumpWell on ESPSubmersible pumpSeparator: Splits oil, gas and waterSeparatorOil, gas, waterTank battery: Crude storage tanksTank batteryCrude storageCompressor station: Reciprocating gas compressorsCompressorstationGas compressorsPipeline: Oil and gas to marketPipelineTo marketProcessing: Crude preheat exchangersProcessingCrude preheat
  • Well fluid
  • Crude oil
  • Gas
  • Has its own twin page

The physics

The equations behind every curve on this page.

Discharge temperature

T₂ = T₁·r^((k − 1)/k)

Above this line means gas is going round again. Stage 2 runs at 112 °C healthy.

Volumetric efficiency

VE = 1 − Cl·(r^(1/k) − 1) − losses

Gas left in the clearance re-expands before new gas can come in.

Rod load

F = P_d·A_head − P_s·(A_head − A_rod)

Must stay inside the frame rating and change direction every stroke.

Isentropic power

P = k/(k−1)·p₁·Q₁·(r^((k−1)/k) − 1) / η

1,225 hp at the coupling for 10.4 MMscfd.

Heat and capacity

A leaking valve heats the cylinder and takes gas away from the line. Both show up against the model before the valve breaks.

Discharge temperature against pressure ratio, stage 2

Gas in at 45 °C after the intercooler, k = 1.27

Discharge temperature against pressure ratio, stage 260801001201401601801.52.02.53.03.5Limit 150 °CHealthy 112 °CThrow 3 leaking 118 °CStage pressure ratio (p_d / p_s, absolute)Discharge temperature (°C)
  • Model, healthy valves
  • Leaking discharge valve, 10% backflow
Stage 2 ratio, healthy
2.46
Stage 2 ratio, leaking
2.37
Throw 3 above the model
8.8 °C

Gas that leaks back through the valve gets compressed again, so the cylinder runs hot for its pressure ratio. The ratio itself falls, because the leak pushes interstage pressure up. That is why we compare against the model at the ratio the machine is actually running.

T₂ = T₁·r^((k−1)/k) leaking: T₂ = T₁·r_t·(1 − f) / (1 − r_t·f)

Capacity and power against suction pressure

1100 psig discharge, 1200 rpm, no unloaders

Capacity and power against suction pressure468101214161820681012141618100120140160180200220Engine rating 1,340 hp10.4 MMscfd at 150 psigSuction pressure (psig)Flow (MMscfd)Brake power (hundred hp)
  • Flow, healthy
  • Flow, one leaking valve
  • Brake power, healthy (right axis)
Capacity lost to one valve
0.14 MMscfd
Load at design
91% of engine rating

Higher suction pressure packs more gas into every stroke, so flow and power both climb until the engine runs out of horsepower. A leaking valve shifts the whole flow line down.

Q = VE·(2·A_head − A_rod)·S·N VE = 1 − Cl·(r^(1/k) − 1) − losses

Rod load

Gas pressure on both sides of the piston and the weight of the moving parts add up to the load on the rod, every degree of the turn.

Rod load across one turn, both stages

150 → 439 → 1,100 psig, 1,200 rpm. Compression positive.

Rod load across one turn, both stages-40-2002040050100150200250300350Rating 40k lb compressionRating 40k lb tensionCrank angle (degrees from head-end top dead center)Rod load (thousand lb)
  • Stage 1 throw, combined
  • Stage 2 throw, combined
  • Stage 1 gas load only
Peak, share of rating
Stage 1 45%, stage 2 38%
Reversal, stage 2
157° tension, 203° compression

The load must stay inside the frame rating and change direction every turn, so the crosshead pin bushing gets oil on both sides. A high interstage pressure or a stuck unloader can take either one away.

F = P_HE·A_head − P_CE·(A_head − A_rod) − m·r·ω²·(cos θ + λ·cos 2θ)

Failure modes

What breaks, what it looks like, what the twin does about it.

Failure modeThe physicsSignals that show itHow the twin catches it
Valve failureA broken plate or spring lets gas flow back through the valve, so the same gas is compressed again.Discharge temperature above the model on one cylinder. Valve cap temperature up. Capacity down. Interstage pressure moves.Compares each cylinder with the temperature its ratio predicts, and points to the end and the valve.
Piston ring wearGas leaks past the rings from one end of the cylinder to the other.Both ends of one cylinder run hot. Capacity falls slowly.Tells ring wear from a valve by the pattern across head end and crank end.
Packing leakWorn packing rings let gas escape along the rod to the vent.Packing vent flow or temperature up. Small capacity loss.Trends packing temperature and vent flow against rod load and speed.
Rod load over rating or no reversalHigh interstage pressure or an unloader out of place raises the gas load, or holds it in one direction.Interstage pressure off the model. Crosshead vibration. Crosshead pin temperature.Works out combined rod load and reversal every degree from the pressures it reads.
Liquid carryoverLiquid from a full scrubber reaches a cylinder and cannot be compressed.Scrubber level high. Sharp crosshead vibration. Valve damage that follows.Watches scrubber levels and dump cycles against gas temperature and flow.

What the twin does not do

  • The compressor panel and the station ESD stay in charge of every trip. TwinEdge is never in the safety loop.
  • We do not change speed, unloaders or recycle valves. Your operators and your SCADA do.
  • We read the panel. We have no write path into it.

How we catch it early

A physics twin and a PINN, working on one reciprocating gas compressor.

The twin is a set of equations for this machine, tuned to its nameplate. Give it the flow, speed and conditions and it returns what every reading should be. It always gives the same answer for the same inputs.

A physics-informed neural network, or PINN, sits on top. It is trained on the plant’s own data and it is penalized whenever it breaks a physics rule. So it can only learn what the equations leave out: wear, fouling, friction. It cannot invent an answer that the machine could never produce.

  1. Step 1

    Measure

    Flow, pressure, current, temperature, vibration. Whatever the asset already has.

  2. Step 2

    Physics twin

    The equations of this machine, set to its nameplate. It says what the readings should be right now.

  3. Step 3

    Residual

    Measured minus expected. Demand, weather and speed are already taken out.

  4. Step 4

    PINN

    A small neural network that learns the cause of the gap, under the physics rules.

  5. Step 5

    Finding

    A named cause, a date, and the kWh or hours at stake. An engineer reviews it.

loss = (measured − predicted)² + λ · (physics residual)²

The second term is the rulebook. It is zero when the network obeys the equations.

Why this beats black-box AI

A black-box model learns patterns from history. It needs examples of each failure, and it goes quiet when the pump runs at a point it has not seen. The twin starts from physics, so it works on day one. The PINN adds what physics cannot know about this particular machine.

That is also what makes the savings countable. The baseline is a calculation, not a guess, so the gap between baseline and meter is a number a customer can check.

What the twin checks on this asset

  • Discharge temperature against the model, per cylinder
  • Capacity against volumetric efficiency: 10.4 MMscfd at design
  • Combined rod load and reversal, every degree of the turn
  • Frame and crosshead vibration trend
Fixed alarmBlack-box AITwin + PINN
Needs failure history to learnNoYes, lotsNo
Says what is wrongNoRarelyYes, by name
Works at a flow it has never seenNoOften failsYes, the equations still hold
Puts a number on the savingNoNoYes, in kWh and dollars
Lead time on slow wearLateVariesWeeks to months
Every number can be tracedYesNoYes, to an equation

What the twin puts a number on

Energy and cost, from the equations.

One leaking discharge valve

35 hp

Spent on gas that goes round again. $6,969 a year in engine fuel.

Capacity lost

0.14 MMscfd

1.4% of the line, $427 a day at $3.00 per Mscf

Throw 3 above the model

8.8 °C

At the stage 2 pressure ratio the machine is running

Unplanned shutdown day

$31,322

10.4 MMscfd not moved at $3.00 per Mscf

What we do for you

We watch it. We catch it. We prove it.

Our reliability engineers run the twin on your reciprocating gas compressor, review every finding, and help your crew fix it. After the repair we compare the meter to the baseline and write the result in the ledger.

Start with one station or fieldSee the 90-day pilot

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