Module 6
Compressor & Pump Stations
Friction and hills steal pressure from a flowing pipeline, so every so often we plug in a machine that pushes the pressure back up. Those machines — compressors for gas, pumps for liquids — are what keep the fluid actually moving the whole way to its destination.
What you'll be able to do
- Explain why stations exist and define suction and discharge pressure.
- Compute a compression ratio the right way — using absolute pressure (psia).
- Tell centrifugal from reciprocating machines, and name the three driver types.
- Reason about how far apart stations sit, and why "it varies."
- Estimate the fuel-gas energy cost of moving gas, and keep it separate from LUAF.
The whole module at a glance: why stations exist, what machines do the work, and what it costs.
Why stations exist
In Module 5 you saw that a flowing fluid steadily loses pressure to friction along the pipe, and loses more when it has to climb uphill. Left alone, that loss would eventually bring the flow to a stop.
A station is the fix: a machine that takes the fluid in at its current low pressure and hands it back at a higher one. We call the inlet pressure suction and the outlet pressure discharge.
🚴 A concrete picture
Think of a long bike ride with gentle downhill drift and headwind. You keep losing speed (pressure), so every few miles you pedal hard for a stretch (a station) to get back up to cruising speed — then coast again until the next push.
Pressure sawtooth. Fluid bleeds pressure with distance, then a station shoves it back up. The fluid enters at suction, leaves at discharge.
The one number: compression ratio
The single most useful number for a station is its compression ratio — how much it multiplies the pressure:
compression ratio = discharge pressure ÷ suction pressure (both ABSOLUTE)
⚠️ Use absolute pressure, not gauge
Pipeline gauges read psig (pounds per square inch gauge) — pressure above the surrounding air. The ratio only makes physical sense with psia (absolute pressure, measured from a true vacuum).
Convert by adding atmospheric pressure: psia ≈ psig + 14.7. Forgetting this is the classic beginner mistake.
🧮 Worked example
A station pulls gas in at 200 psig and pushes it out at 800 psig.
Wrong (gauge): 800 ÷ 200 = 4.0. Right (absolute): (800 + 14.7) ÷ (200 + 14.7) ≈ 814.7 ÷ 214.7 ≈ 3.8. The gauge shortcut overstates the ratio.
Gas compression
Gas is squishy (compressible), so we use compressors to raise its pressure. There are two workhorse designs, and they solve different problems.
Centrifugal ~1.2:1 / stage
- Spins gas outward with an impeller, like a fan flinging air.
- High, continuous flow; modest pressure ratio per stage.
- Favored on big mainlines (above ~5,000 horsepower — hp, a unit of mechanical power).
- Driven by gas turbines or electric motors.
Reciprocating ~3:1–4:1 / stage
- Positive-displacement: pistons trap a fixed slug of gas and squeeze it, like a bike pump.
- High ratio per stage, good turndown (works well at part-load).
- Per-stage ratio is capped by discharge temperature — squeeze too hard and the gas gets too hot.
- Driven by reciprocating gas engines (or motors).
🧭 The big idea
Centrifugal = lots of flow, gentle squeeze per stage. Reciprocating = less flow, a hard squeeze per stage. Need a big jump in pressure? Either chain centrifugal stages together, or reach for a reciprocating machine.
What spins the machine: drivers
A compressor needs something to turn it. The standard three drivers are:
Gas turbines and gas engines often burn a slipstream of the pipeline gas itself as fuel — which leads straight to the energy-cost discussion below.
Liquids pumping
Liquids (crude oil, refined products, NGLs — Natural Gas Liquids) barely compress, so instead of compressors we use pumps. The dominant choice is the centrifugal pump, the liquid cousin of the centrifugal compressor.
Pumps in series
Stacked one after another, each adds more pressure (head) — same flow, higher lift. Use it when you need to push hard.
Pumps in parallel
Side by side, they add up their flows at the same pressure. Use it when you need to move more volume.
PD pumps for thick stuff
For high-viscosity products (heavy, sticky crude), positive-displacement (PD) pumps handle what centrifugals struggle with.
Station spacing
How far apart do stations sit? There's no single answer — it depends on pipe diameter, terrain, throughput, and design pressure.
📐 Why "it varies"
Gas compressor stations sit roughly every 40–100 mi, most commonly 40–70 mi — different sources cite different ranges, so quote a range, never a single magic number.
Liquids pump stations run wider, about 20–100 mi apart. A big-diameter line at high pressure can go farther between pushes; rugged terrain pulls stations closer together.
The energy cost of transport
Pushing gas across a continent isn't free. Compressors burn energy, and for turbine/engine drivers that energy is fuel gas — a slice of the very gas being shipped, siphoned off and combusted to drive the machine.
System-wide, compressor stations burn less than about 3% of throughput as fuel. On a single long-haul line the cumulative burn can reach ~3–5%, growing with distance and tough terrain.
⚠️ Don't conflate fuel gas with LUAF
LUAF (Lost & Unaccounted-For gas) is a broader accounting bucket: it folds in fuel gas plus leaks, blowdowns, line fill, and measurement error. LUAF runs about 1–4% (averaging ~2%, varying by jurisdiction).
So fuel gas is one ingredient inside LUAF — not a synonym for it.
Estimates are illustrative — real staging and fuel burn depend on gas composition, temperature limits, efficiency, and terrain.
Go deeper: why discharge temperature caps a stage optional
When you compress gas, you heat it — squeeze harder and it gets hotter. Every machine and pipe coating has a temperature limit, so there's a ceiling on how much pressure ratio one stage can deliver before the gas gets too hot.
That's why high overall ratios are split across multiple stages with intercoolers between them: cool the gas back down, then squeeze again. The widget above mimics this by dividing the total ratio across stages until each stage's ratio sits within the machine's per-stage limit.
Key takeaways
- Stations restore pressure lost to friction and elevation: fluid enters at suction, leaves at discharge.
- Compression ratio = discharge ÷ suction, always in absolute pressure (psia ≈ psig + 14.7).
- Centrifugal = high flow, low ratio per stage (~1.2:1); reciprocating = positive-displacement, high ratio per stage (~3:1–4:1, capped by discharge temperature).
- The three drivers are gas turbines, reciprocating gas engines, and electric motors.
- Compressor spacing ~40–100 mi (most 40–70); pump spacing ~20–100 mi — always a range.
- Fuel gas is <~3% system-wide (~3–5% long-haul); LUAF (~1–4%) is a broader, separate bucket.
A station takes gas in at 200 psig and pushes it out at 800 psig. What's the (approximate) compression ratio?
Why: Compression ratio is discharge ÷ suction in absolute pressure. Adding ~14.7 to each gauge reading gives 814.7 ÷ 214.7 ≈ 3.8. Using gauge values overstates it.
You need a high pressure ratio in a compact, part-load-friendly machine. Which fits best?
Why: Reciprocating compressors are positive-displacement and deliver a high ratio per stage (~3:1–4:1) with good turndown. Centrifugals give only ~1.2:1 per stage and shine at high continuous flow.
Which set lists the three standard compressor drivers?
Why: The standard three drivers are gas turbines, reciprocating gas engines, and electric motors. Turbines and engines often burn a slipstream of the pipeline gas as fuel.
A colleague says "fuel gas and LUAF are the same thing." What's the right correction?
Why: Fuel gas (<~3% system-wide, ~3–5% long-haul) is one component inside LUAF (Lost & Unaccounted-For, ~1–4%), which also captures leaks, blowdowns, line fill, and measurement error.