A Note on Compressor Stability and Control (2)

The Role of After Cooler

1. Introduction

A compressor operating region is limited by its maximum and minimum allowable operating speeds, maximum available driver power, choke flow and stability (or surge) limit. It is necessary for a compressor in any application to be controlled and operate safely and reliably in its operating region. This is specifically important in transient operating conditions. Transient operating conditions are changes in process demand, start-up, normal shutdown and emergency shutdown (ESD).

Compressor station stability is a result of an interaction between Compressor, Anti-Surge Devices (control system / method, valves, etc.) and Station Piping Layout (coolers, scrubbers, check valves, etc.).

Different control methods might be employed based on the specific application. Those are Suction / Discharge Throttling, Variable Speed, Variable Inlet Guide Vane and Gas Recycling. However, gas recycling is effectively an integral aspect of safe transient operation.

Aftercooler plays a significant role when it comes to gas recycling. Here in this article, the role of aftercooler in stable operation of a compressor is discussed. 

2. Aftercooler 

Whether to employ hot or cold recycle, or put another way, whether to include aftercooler in gas recycling or not (see Figure 1), is determined by a detailed Dynamic Simulation. What is presented here, can be regarded as a conceptual prediction.

A general layout of a compressor station is presented in Figure 1. Main components – other than the compressor and its driver – are scrubber, aftercooler and Anti-Surge Valves (ASV) together with station piping, main line and recycle lines. As mentioned, all these components affect the compressor stability in a way.

Figure 1: A compressor station general layout 

Basically, including an aftercooler in a compressor station has two effects on the compressor station characteristics; firstly, it cools the gas at compressor discharge and secondly, it adds to the station volume. These effects result in different outcome (and hence might be desirable or undesirable) in different transient operating conditions. 

3. Cold Recycle 

Recycling causes the gas to heat up. Sometimes it is necessary to cool the gas during recycling to prevent from overheating (going over the maximum allowable temperature). Cold recycle line is used in such a situation. This can happen during start-up if takes too long for the compressor to reach its operating point / speed.

An uncooled start-up might be possible with one of the following methods [1]:

a. Quick acceleration

b. Delayed hot gas recycle

c. Throttled hot gas recycle 

4. Hot Recycle 

There might be a transient situation for which a smaller gas volume in the system is desirable. This can happen during an ESD when the pressure needs to be quickly equalized to protect the compressor from very low flow and surge. Hot recycle line is used then so the system volume will be smaller as much as the aftercooler volume and its associated piping. 

5. Final Remark 

One can predict that (based on the above discussion), a compressor station needs to be equipped with both cold and hot recycle for stable operation at all times (including start-up and ESD). This is only a prediction. It is to be emphasized that this prediction needs validation by a Dynamic Simulation. Different results might be achieved in certain situations. It is also to be noted that different compressor drivers (for the same case) result in different layouts because of the difference in their inertia and response time. 

Reference:
[1] Rainer Kurz and Robert C. White, "Surge Prevention in Centrifugal Compressor Systems"

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