Water Dew Point & Hydrate Prevention

The Inevitability of Water in Natural Gas

Raw natural gas emerging from a subterranean reservoir has been trapped in porous rock for millions of years, in direct physical contact with massive saline aquifers. Consequently, the raw gas arriving at the wellhead is always 100% saturated with water vapor at the reservoir temperature and pressure.

As the gas travels up the wellbore and enters surface flowlines, it cools down. Because cold gas can hold significantly less water vapor than hot gas, the vapor immediately begins to condense into free liquid water.

The Dangers of Free Water

Allowing liquid water to persist in a natural gas pipeline causes two catastrophic, industry-ending problems:

1. Acid Gas Corrosion

Natural gas often contains acidic impurities like Carbon Dioxide ($CO_2$) and Hydrogen Sulfide ($H_2S$). In a completely dry state, these gases are inert and safe to transport in standard carbon steel pipes.

However, the moment liquid water is present, the $CO_2$ dissolves to form Carbonic Acid, and the $H_2S$ forms highly corrosive Sulfuric Acid species. This "wet acid gas" will rapidly attack the internal walls of standard carbon steel pipelines, leading to severe pitting, sulfide stress cracking, and eventual fatal pipeline ruptures.

2. Gas Hydrate Ice Plugs

Under conditions of high pressure and low temperature (which are extremely common in subsea pipelines and winter surface operations), liquid water and light hydrocarbon molecules (like methane) physically bond together into a crystalline lattice structure known as Gas Hydrates.

Hydrates look and feel exactly like dirty packed snow or ice, but they can form at temperatures well above the normal freezing point of water (up to 20°C / 68°F at very high pressures). Hydrate crystals rapidly stick together, forming massive, solid ice plugs that can completely block pipelines measuring several feet in diameter. Removing a hydrate plug is a highly dangerous operation that can take weeks of lost production.

The Water Dew Point Specification

To absolutely prevent acid corrosion and hydrate formation, the gas must be severely dehydrated before entering a transmission pipeline. Pipeline tariffs strictly regulate the maximum allowed water content, usually enforced via a Water Dew Point limit.

• Standard Transmission Pipeline Spec: Typically a maximum of 7 lbs of water per MMSCF (approximately 112 $mg/Sm^3$). At a pipeline pressure of 70 bar, this guarantees the water dew point will remain below -10°C, safely preventing condensation during winter.
• LNG Liquefaction Spec: Cryogenic LNG plants require bone-dry gas (< 0.1 ppmv water). The water dew point must be driven down below -100°C to prevent microscopic ice crystals from destroying the cryogenic aluminum heat exchangers.

Try the Calculator

If you are designing gathering networks for wet natural gas, you must account for two-phase flow and increased pressure drops. Try the Weymouth / Panhandle Calculator on this page to size your long-distance transmission lines safely!