FACTORS IMPACTING COMPRESSOR SURGE

BY AMIN ALMASI.

Surge can be a major challenge for turbo compressors. Operation in the surge area will result in instability, exposing the machine to destructive stresses and forces, high vibration, and even serious damage.

Surge during shutdown (trip) has been reported for many turbo-compressors. This is particularly possible if the machine operates at high head and low flow, immediately before the trip, when the operating point can move toward the surge line and even pass it during coast-down (when the turbo-compressor reduces flowrate). When a turbo-compressor experiences a serious alarm, an emergency shutdown is usually initiated. But an immediate shutdown could result in a surge. In this case, the surge happens shortly after the shutdown (trip) and at a high energy level. This could be a surge at a high head (operating point could pass the surge line at high head).

In many cases, there are advantages to not removing the driving power from the turbocompressor (tripping) immediately by delaying for a few seconds so the anti-surge valve can be opened and the discharge pressure (head) can be sufficiently reduced. As soon as the trip is intended, the anti-surge is opened, and the compressor shutdown is implemented with a second or two delay.

Many alarms and malfunctions do not require an instantaneous shutdown. For example, a high bearing temperature and a high vibration (unless it reaches more than 10 times allowable levels). An exception is loss of lubrication oil where it could potentially be worse than a full load surge. A safety study helps determine if such a delay is allowable.

RATE OF SPEED REDUCTION

Another critical parameter is the rate of speed reduction (coast-down time) during trip/shutdown. For many turbocompressors, rapid speed reduction can cause surge to be reached sooner and at a higher head condition. This results in a high energy surge event.

Great care should be taken for gas turbine-driven compressors. As soon as the fuel supply to the gas turbine driver is cut-off, the power is eliminated to the driven turbo-compressor and the speed drops rapidly. Some installations maintain the fuel flow to the gas turbine driver for up to two seconds, while the anti-surge valve (turbo-compressor recycle valve) opens. This delay may generate a safety hazard.

The head-making capability of a turbo-compressor is reduced typically by the square of its running speed, while the pressure ratio is imposed by the upstream and downstream piping and facilities system. Therefore, the unit will surge if the anti-surge valve cannot provide fast pressure relief at the discharge system. The deceleration rate as a result of train inertia and energy dissipation is a decisive factor. The rate of pressure relief at discharge not only depends on the reaction time of the anti-surge valve, but also on the volume of gas enclosed in piping and other systems between the compressor and anti-surge valve.

Speed reduction is fast in aeroderivative gas turbines. An emergency shutdown of a two-shaft or aero-derivative gas turbine-driven compressor can be problematic since train inertia is low and train speed decreases rapidly in a trip. Roughly 20-30% speed reduction can be expected for an aeroderivative-driven machine in the first second after the shutdown. This results in around 50% reduction in head generation capability. An antisurge valve for such a compressor should be able to reduce the pressure across the turbo-compressor by about half during the same period. The worst-case scenario for an anti-surge system is an emergency shutdown of the gas turbine while the turbocompressor is operating at high pressure and close to surge.

Similar scenarios may be applicable for an electric motor trip. However, the inertia of a typical electric motor-driven train is much higher. A typical rotor assembly for an electric motor driver is relatively heavy and large, and a gear unit is often required. A typical electric motor-driven turbocompressor has three to seven times higher inertia compared to a two-shaft or aero-derivative train.

High-pressure gas trapped in the discharge system plays a major role in surge. Large volumes of pressurized gas need time to depressurize. The volume of pressurized, high-energy gas to be dissipated can be reduced by discharge check valve(s) located upstream of large headers or vessels that store significant amounts of high-pressure gas.
Fast-closing check valves are generally specified.

Anti-Surge Controller.

Challenging situations have been reported when the anti-surge valve loop is taken downstream of the aftercooler(s). Usually, in these cases, the discharge gas volume in the cooler and piping is too large and the anti-surge valve (recycle valve) cannot avoid a surge. An additional hot gas bypass valve is often required. This is a short recycle loop without any cooler that only operates for a very short time during trip or emergency.

Vent valves on the discharge piping can effectively reduce discharge pressure and stored energy that contributes to the severity of the surge. This is particularly useful in multi-section turbocompressor installations where recycling around the 2nd stage, for example, results in high-pressure gas being added to the 1^st stage discharge energy. Venting can allow some gas from the 2nd stage to be removed. Such venting should only be used as the last resort. Most vent valves are small and can be opened rapidly.

Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.

Turbomachinery International