Reciprocating vs Centrifugal compressors

[ibrahim]

A compressor is a piece of equipment that compresses gas either to transfer to a specific location or for a certain process requirement. Compressor are manufactured depending on application and can be class into two basic types; positive-displacement and centrifugal.

Integrally geared centrifugal compressors can operate at many times higher speeds than reciprocating compressors. The higher speeds ultimately result in smaller package sizes, requiring a smaller footprint as compared to a reciprocating compressor. The operating speed of a reciprocating compressor is very slow due to mechanical and dynamic limitations. Furthermore, the lower speed of reciprocating compressor lends itself to larger compressor size, heavier weight, and larger plot plan size. Whereas the centrifugal compressor with higher operating speeds results in smaller overall compressor package sizes such as smaller gearing, bearings, seals, lubrication system, and foundation. Smaller packages ultimately lend themselves to saving in lower overall installations as well as lower capital and spare parts costs.

Higher reliability is fully attainable with centrifugal compressors. The rotating aerodynamic components (impellers) have no physical contact with the stationary parts (inlet shroud). On the contrary, the reciprocating compressor moving components such as the piston and valves are physically in contact with the cylinder and other stationary components during operation. The physical contact causes wear and tear of both moving and stationary components, which requires constant maintenance. However, a centrifugal compressor operates for many years with continuous service without overhaul maintenance, resulting in less power plant down time. This eliminates loss of product, provides more profit, lowers risk, and results in lower maintenance cost. Overhaul periods are more predictable by analyzing characteristic efficiency and vibration trends. A typical centrifugal compressor overhaul inspection period is more than 7 years as compared to less than 2 years for most reciprocating compressors.

In addition to the economical advantages of implementing a centrifugal compressor over a reciprocating compressor, many technical advantages are also evident. The centrifugal compressor discharge pressure can be regulated to less than 0.5% per second. The well-regulated compressor discharge pressure provides very steady supply of fuel to the gas turbine. This is an advantage since it does not cause additional burden to the turbine controls. On the other hand, a reciprocating compressor at best can provide 2% or more of pulsating pressure. Unsteady supply of fuel may cause hardship on the turbine control system. A reciprocating compressor would require an impractically over-sized pulsation bottle to minimize supply pressure pulsation to the level as steady as a centrifugal compressor.

Considering there is no physical contact between the centrifugal compressor aerodynamic components, the need for lubrication within the compression components is not required; thus it will not add oil or other contaminants to the process gas. However, a reciprocating compressor requires oil lubricant for the piston rings. This oil eventually ends up in the process gas or it has to be separated to protect the gas turbine. Due to physical contact between the piston rings and the cylinder, the wear of the rings and packing causes particle contamination of the fuel gas. Hence, this contamination could cause premature wear on the turbine blades and other turbine fuel gas passages.

[tosif]

Ibrahim, excellent sharing. What about the usage of Recip-gas-engines as driver rather than Gas-turbines. I think gas engines are more fuel efficient and can eliminate the fuel control issues. Yeap the maintenance issues will then multiple because of higher requirement of PMs in gas engines. What say you?

[ibrahim]

gas engines require very extensive maintenance, just like a CNG driven car. such as it requires an ~ 8 hrs shutdown every second month (1500 hrs ~ 1800 hrs) for full lube oil change along with filters replacement. but yes they are the most feasible drivers for recip engines, especially when installed at wellhead.