MYTH: COMPRESSOR SPEED AND POWER DON’T MATTER

When we size a centrifugal compressor for a single set of operating conditions, the result is a fixed power consumption for a given speed. That should define the size and type of driver. Unfortunately, it’s not that simple. We have to look at the entire operating map of the compressor, site conditions, and driver speed and load range.

Specifically, real live operation at a compressor station involves the requirements to start and stop compressors in all possible conditions. That entails operation at changing ambient temperatures, humidity, and barometric pressure. It also requires the operation at various operating conditions off-design for the compressor.

Let’s start with compressor operating conditions. A common myth states that compressors always operate along a propeller curve. For example, each compressor’s throughput is uniquely matched to a set of suction and discharge pressures, and thus to a power demand. Conversely, the flow through the compressor is independent of suction and discharge pressure.

Neither assumption is universally true. Most compressors are in applications where the suction and discharge pressure may vary independently because of flow off/intakes or bypasses, and the capacity demand is driven by commercial requirements rather than optimal fluid dynamics. There is seldom a simple single operating curve matched to the best efficiency islands. Rather, we see random spots on the compressor map reminiscent of the potholes on a road.

Centrifugal compressors use a wide range of drivers. Some drivers, such as electric motors, force a single operating speed. Other drivers, such as frequency controlled electric motor drives, electric motor drives with a variable speed gearbox, gas turbines, and steam turbines, allow varying the operating speed of the compressor.

If gearboxes are used between the driver and the compressor, matching the speed is as simple as defining the gearbox ratio. However, in many instances, avoiding a gearbox is desired. This in itself is somewhat arbitrary since gearboxes are usually reliable devices, alignment issues are rare, and the additional losses due the gearbox are often more than compensated by a better aerodynamic compressor performance owing to the better speed match.

Similarly, each of the variable speed drivers has a specific performance characteristic that defines how much power it can make available to the compressor at a given speed and ambient condition. A two-shaft gas turbine or a steam turbine’s output power capability drops very little with changes in compressor speed.

A variable speed gearbox, driven by a constant speed motor, shows a similar characteristic, but over a smaller speed range. A single shaft gas turbine will allow very small excursions in speed, and they are accompanied by a significant drop in power.

Variable frequency-controlled electric motors have a large speed range, but the power capability drops linearly (or more, depending on motor cooling capability) with speed. Additionally, the output power of gas turbines and their efficiency depend on ambient pressure, temperature and, to a lesser degree, humidity.

Ultimately, the power and speed demands of the compressor need to be matched to a driver, while meeting all the necessary operating scenarios. Given the number of variables, users often specify margins for each of these to account for degradation. If one adds margins on power output of the driver, additional margins on the compressor power consumption, and then also assumes the most adverse ambient and process conditions, equipment is often oversized.

This means that the operation at the actual operating conditions may be compromised, or at the very least, not optimal. The situation can be avoided by not solely sizing the equipment on the most extreme operating conditions, but rather on a statistical life cycle cost analysis of the expected operating cycles. There may even be cases when it is advantageous to allow the compressor to run in recycle for some of the low-flow, off-design conditions to avoid having an undersized flow capacity staging or casing for the more typical operating conditions.

Another rather fundamental requirement is that the equipment has to be able to start. On two-shaft gas turbines or steam turbines, this is usually not a problem. Single-shaft gas turbines may require a large helper drive to overcome startup torque and inertia.

Constant speed electric motors require special attention, since their startup will tax the electric grid due to the high inrush current demands. Methods to reduce the inrush current, such as soft starters or reduced voltage starters, often also reduce the starting torque, sometimes to the point where the starting time becomes too long and the motor is at risk of overheating. Variable speed gearboxes can only help with this problem if they allow for hydraulically decoupling the motor from the compressor.

Matching the compressor and its driver is not trivial, but it is well understood. Careful planning and an understanding of the issues is the key to success. Prior to investing millions of dollars in a major piece of machinery, it is worthwhile to perform an operational optimization study to make sure the machine functions as expected, as required, and as efficiently as possible. Good luck avoiding those potholes!

Authors

Klaus Brun is the Machinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is the Manager for Systems Analysis at Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and the chair of the IGTI Oil and Gas Applications Committee.