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Sep-2016

Refrigeration compressor anti-surge control

A refrigeration compressor requires the same high-performance and fast-acting surge control technology as other gas compression circuits

MEDHAT ZAGHLOUL and WAYNE JACOBSON
Compressor Controls Corporation
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Article Summary
In a ‘normal’ gas compression circuit, the fluid that is compressed in the compressor, flowing in the immediate upstream and downstream process lines and vessels, is all in a gaseous phase without any change of state from liquid to vapour or from vapour to liquid. In this ‘normal’ gas compression circuit, the compressor could experience surge-inducing events due to changes in the process conditions downstream of the compressor (for instance, sudden increase in process resistance), due to changes in the suction of the compressor (such as changes to gas composition or inlet pressure and temperature), or sudden changes in the driver performance (sudden drop in shaft power or speed, for instance).

In the refrigeration circuit however, there is a change of state (from liquid to vapour) that takes place immediately upstream of the compressor in the LP process chiller and another change of state (from vapour to liquid) almost immediately downstream of the compressor in the condenser. In a refrigeration circuit, the process is ‘closed loop’ and the gas composition does not usually change. Also, the change of state in the condenser means that the discharge resistance changes very slowly (in fact seasonally) with the changes in ambient temperature around the condenser. A possible cause of compression surge can be due to a drop in vapour available for compression, which occurs when the process load drops, thus less vapour in the process chiller is produced.

There is therefore a perception by process designers to consider that any surge-inducing events on a refrigeration compressor are limited and slow occurring and require only simple and ‘low performing’ surge control technology. This is not a correct perception as it ignores the surge-inducing events that could occur because of driver problems, which tend to be fast and sudden.

Thus a refrigeration compressor requires the same high-performance and fast-acting surge control technology as ‘normal’ gas compression circuits.

Number of anti-surge control loops
In general, each compressor stage that supplied refrigerant to a process chiller requires a dedicated anti-surge control loop (controller plus valve).

In the example given in Figure 1, where the LP process chiller provides refrigerant vapour to the LP stage of the compressor and the pre-cooler provides refrigerant vapour to the HP stage of the compressor, two dedicated anti-surge control loops would be required.

On the other hand, if a compressor stage is linked to a flash economiser only, then a dedicated anti-surge loop for that stage is generally not required. This is because the flash economiser, unlike a process chiller, provides refrigerant vapour to the compressor stage in a manner that is not dependent on process load changes. In other words, the flash economiser circuit provides continuous forward flow through it at all 
operating conditions of the compressor when it is running. This is illustrated in Figure 2.

The need to provide recycle gas cooling
Method 1 – Quenching

In refrigeration compressors, it is advisable to design the anti-surge (recycle) piping so that it commences upstream of the discharge condenser. The reason is straightforward. If recycle gas was taken from downstream of the condenser, it would be, by definition, mixed phase with mostly a liquid content and so would be unsuitable for use as recycle gas.

In a propane refrigeration application, the temperature of the propane gas at the discharge of the compressor may easily reach 130-140°C, and if recycled back to the LP stage suction drum, would be problematic on two levels:
•    First, the replacement of cold and denser vapour originating in the process chiller with hot recycle gas would initially drive the operating point of the LP stage further into surge
• Secondly, it would be just a matter of time before the machine tripped on high LP stage inlet temperature.

Therefore, good design practice dictates that the recycle gas be cooled to a temperature approximating the vapour coming from the process chiller. A relatively straightforward method of doing this is to apply an evaporative cooling concept in the sense of releasing an atomised fine spray of liquid refrigerant into the hot recycle gas stream downstream of the anti-surge valve. Since this almost instantaneously evaporates, it sufficiently cools the hot recycle gas stream. This is typically achieved by installing a specially designed atomising nozzle system into the recycle piping, downstream of the anti-surge valve. This is illustrated in Figure 3.

When quenching is used as the method to cool the hot recycle gas, it is not good design practice to simply modulate the quench valve with a slow-acting temperature controller. For example, if the surge control system step-opens the anti-surge valve, providing a sudden increase of hot refrigerant recycle gas, the much slower quench controller would allow the recycle gas to be delivered too hot to the LP suction drum for an unacceptable amount of time.

Dynamic decoupling between the anti-surge controller and the quench controller is required so that when the anti-surge valve opens by a significant amount, the quench controller would also immediately open the quench valve by a suitable and proportionate amount and then allow the slower temperature control response to determine the required final opening of the quench valve based on the set-point.

The quench controller’s control action needs to be further coordinated with the anti-surge control system so that when the anti-surge valve is closed (or open up to a configurable minimum opening) the liquid quench valve is kept closed regardless of its recycle gas temperature measurement. Also, when the compressor is stopped, the liquid quench valve needs to be forced closed.

Traditional quench controllers operated with a fixed downstream recycle gas temperature set-point. However, the actual saturation temperature of the vapour in the LP suction drum mentioned above is a function of its operating pressure. Ideally, the temperature set-point of the quench controller must be adjusted as the operating pressure in the LP suction drum changes. Not only that, it should typically be calculated slightly higher (but close to) the current saturation temperature in the LP suction drum.

The latest state-of-the-art quench controllers actually calculate the temperature set-point as being offset (or biased) slightly from the saturation temperature as expressed by the pressure-enthalpy saturation curve characteristics for that particular refrigerant.
There are, however, considerable limitations on the use of quenching that must be considered.

If the compressor is operated ‘blocked-in’, that is isolated from the process chillers for any length of time, the addition of liquid quench into the necessary recycle gas will simply cause the total refrigerant inventory to accumulate and build up in what is in practice a closed circuit. Hence the shaft load of the compressor will rise and possibly exceed the capacity of the driver, causing a compressor trip.

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