COMPRESSOR SURGE BASIC INFORMATION AND TUTORIALS

What is Compression Surge?

Compressor surge is a phenomenon of considerable interest; yet it is not fully understood. It is a form of unstable operation and should be avoided. It is a phenomenon that, unfortunately, occurs frequently, sometimes with damaging results. Surge has been traditionally defined as the lower limit of stable operation in a compressor, and it involves the reversal of flow.

This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually, a part of the compressor is the cause of the aerodynamic instability, although it is possible for the system arrangement to be capable of augmenting this instability.

Compressors usually are operated at a working line, separated by some safety margin from the surge line. Extensive investigations have been conducted on surge.

Poor quantitative universality or aerodynamic loading capacities of different blades and stators, and an inexact knowledge of boundary-layer behavior make the exact prediction of flow in the compressor at the off-design stage
difficult.

A decrease in the mass flow rate, an increase in the rotational speed of the impeller, or both can cause the compressor to surge. Whether surge is caused by a decrease in flow velocity or an increase in rotational speeds, the blades or the stators can stall.

Note that operating at higher efficiency implies operation closer to surge. It should be noted here that total pressure increases occur only in the rotational part of the compressor, the blades.

The surge line slope on multistage compressors can range from a simple single parabolic relationship to a complex curve containing several break-points or even "notches." The complexity of the surge line shape depends on whether or not the flow limiting stage changes with operating speed from one compression stage to another; in particular, very closely matched stage combinations frequently exhibit complex surge lines. In the case of compressors with variable inlet guide vanes, the surge line tends to bend more at higher flows than with units that are speed controlled.

Usually surge is linked with excessive vibration and an audible sound; yet, there have been cases where surge not accompanied by audible sound has caused failures. Usually, operation in surge and, often, near surge is accompanied by several indications, including general and pulsating noise level increases, axial shaft position changes, discharge temperature excursions, compressor differential
pressure fluctuations, and lateral vibration amplitude increases.

Frequently, with high-pressure compressors, operation in the incipient surge range is accompanied by the emergence of a low frequency, asynchronous vibration signal that can reach predominant amplitudes, as well as excitation of various harmonics of blade passing frequencies. Extended operation in surge causes thrust and journal beating failures.

Failures of blades and stators are also experienced due to axial movement of the shaft causing contact of blades and stators. Due to the large flow instabilities experienced, severe aerodynamic stimulation at one of the blade natural response frequencies is caused, leading to blade failure.

CENTRIFUGAL FLOW COMPRESSORS BASIC INFORMATION AND TUTORIALS

What are centrifugal flow compressors?

Centrifugal compressors are used in small gas turbines and are the driven units in most gas turbine compressor trains. They are an integral part of the petrochemical industry, finding extensive use because of their smooth operation, large tolerance of process fluctuations, and their higher reliability compared to other types of compressors.

Centrifugal compressors range in size from pressure ratios of 1:3:1 per stage to as high as 13:1 on experimental models. Discussions here are limited to the compressors used in small gas turbines.

This means that the compressor pressure ratio must be between 3-7:1 per stage. This is considered a highly loaded centrifugal compressor. With pressure ratios, which exceed 5:1, flows entering the diffuser from the rotor are supersonic in their mach number (M > 1:0). This requires a special design of the diffuser.

In a typical centrifugal compressor, the fluid is forced through the impeller by rapidly rotating impeller blades. The velocity of the fluid is converted to pressure, partially in the impeller and partially in the stationary diffusers.

Most of the velocity leaving the impeller is converted into pressure energy in the diffuser. The diffuser consists essentially of vanes, which are tangential to the impeller. These vane passages diverge to convert the velocity head into pressure energy. The inner edge of the vanes is in line with the direction of the resultant airflow from the impeller.

In the centrifugal or mixed-flow compressor the air enters the compressor in an axial direction and exits in a radial direction into a diffuser. This combination of rotor (or impeller) and diffuser comprises a single stage. The air enters into the centrifugal compressor through an intake duct and can be given a prewhirl by the IGVs as shown in figure below.

The inlet guide vanes give circumferential velocity to the fluid at the inducer inlet. IGVs are installed directly in front of the impeller inducer or, where an axial entry is not possible, located radially in an intake duct. The purpose of installing the IGVs is usually to decrease the relative Mach number at the inducer-tip (impeller eye) inlet because the highest relative velocity at the inducer inlet is at the shroud.

When the relative velocity is close to the sonic velocity or greater than it, a shock wave takes place in the inducer section. A shock wave produces shock loss and chokes the inducer. The air initially enters the centrifugal impeller at the inducer.

The inducer, usually an integral part of the impeller, is very much like an axial-flow compressor rotor. Many earlier designs kept the inducer separate. The air then goes through a 90 degrees turn and exits into a diffuser, which usually consists of a vaneless space followed by a vaned diffuser.

This is especially true if the compressor exit is supersonic as is the case with high-pressure ratio compressors. The vaneless space is used to reduce the velocity leaving the rotor to a value lower than Mach number -- 1 (M < 1).

From the exit of the diffuser, the air enters a scroll or collector. The centrifugal compressor is slightly less efficient than the axial-flow compressor, but it has a higher stability. A higher stability means that its operating range is greater (surge-to-choke margin).