Salt deposits, compressors

Figure 4-27. Ammonium nitrate salt deposits in the inlet of a nitrous gas axial compressor.

The compressor is disassembled. I get the opportunity to accompany the rotor to Dallas, in the back of a van that needed a new suspension. Once there, I watched the manufacturer s machinist crew clean and rebalance the wheels. I noticed that the salt deposits were thickest on the middle wheel. The last wheel was only slightly encrusted while the first wheel was clean, except for some waxy grease. 

Why this sort of salt distribution I reasoned that the entrained brine did not dry out until it reached the middle wheel. But, by the time it reached the last wheel, all the salt deposits that were going to accumulate in the compressor had done so. 

R.L. Ashbrook A Survey of salt deposits in compressors of flight gas turbine engines , NASATN 0-4999,(1969). 

Chemical reactions between these deposits and the protective surface oxide can lead to destruction of the oxide and rapid corrosive attack. In gas turbines, oxidized sulfur contaminants in fuel and chlorides from ingested air (marine atmospheres) tend to react to form salt deposits. The presence of sodium sulfate, potassium sulfate, and calcium sulfate together with magnesium chloride have been reported in such deposits for compressor-stage components [14]. Sodium sulfate is usually regarded as the dominant component of the salt deposits. 

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

Eventually, the fouling deposits on the rotor will become so thick that they start to break off, especially if you shut the compressor down for a few hours for minor repairs to the lube-oil system. When the compressor is put back on line, bits and pieces of grayish salt break off, and unbalance the rotor. At 8000 rpm, the high-vibration trip cuts off the fuel to the gas turbine, and the machine is taken off line for repair. 

Offline cleaning intervals. Irrespective of the compressor performance degradation, a sound combined online and offline washing regime shotdd incorporate at least four offline compressor cleanings per year in order to remove the salt-laden deposits on the downstream stages. This recommendation is also valid for a peak load tmit running only a few hoiu s per year. 

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