The menace of hydrates in compressors

Hydrates are composed of water and light hydrocarbons in a solid matrix that resembles ice. Hydrate composition is water and methane. Other compounds may occur in quantities no greater than 2 % Hydrates are formed at low temperatures such as downstream of throttling a high pressure, saturated gas across a control valve. Hydrates can cause significant damage to compressor internals.

The article contains excerpts from the case study, "Hydrate Occurrence in Centrifugal Compressor Systems" at the 41st Turbomachinery Symposium by Orlando Donda Filho and Macaé Pedro Antonio Lima Donda of Petrobras and Eder Ramalho of Caterpillar Solar Turbines.

Hydrate formation is not easily detectable in compressor systems because instrumentation and protective controls are not normally provided for this purpose. Hydrate leaves no trace. When systems are examined for the potential cause, the temperature and pressure are normally at ambient conditions and the hydrates have evaporated.

Control strategies should be developed and devices should be provided to detect and avoid damaging hydrate formation in compressor systems. There have been a number of compressor repairs required where the root cause of failure has not been clearly identified.

Hydrate formation in the suction side of a compressor may result in the following: Reduction in suction temperature. Reduction in discharge pressure. Reduction in compressor flow. Low energy surge (the most dangerous event). Increase in discharge temperature.

Quite often protection systems are unable to guard against operating with hydrates in the working fluid.

• Failure of high discharge temperature switch
• Pneumatic Anti-Surge Detector didn’t work in presence of low energy surge
• Anti-Surge Control System did not include any fallback strategies for instrument failure
• Vibration not high enough to activate vibration shutdown
• Surge condition not possible to be recognized by Operators
• Low suction temperature control not implemented.

How to prevent hydrate formation

• Maintain operation within the design range
• Provide active suction temperature control with existing coolers
• Provide low suction temperature alarm when operation approaches the border of hydrate envelope
• Use process transmitters instead of switches to improve process control reliability and reduce failure potential
• Review surge control system control strategies including fallback (or instrument failure) strategies