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Reciprocating Compressor Control Methods

By Joe Honeywell

 

Introduction

There are many types of gas compressors found in industry. All of them have specific characteristics that make them will suited to a particular application. A centrifugal compressor may be well suited for a process that operates at very high flow rates and moderate pressures. Applications that operate at low flow rates and low pressures may find that a screw compressor is the best selection. Systems that operate at low flow rates and relatively high pressures may use a reciprocating type compressor. While there are many other variables to consider besides flow rate and pressure in selecting a compressor the user has many compressor options to choose from in determining the type of machine best suited to an application. One of the most important considerations, however, is how the machine will be controlled. While there are many other considerations, for some applications it can be the most important.

Every compressor must regulate the amount of energy supplied to the process. Without some type of control the machine would add energy regardless of the process requirements. If the process operating conditions should change then the added energy would be either too much or not enough to meet the process requirements. Keep in mind that the energy supplied by a reciprocating compressor must match the energy required by the process, according to the First Law of Thermodynamics. That is why some type of process control device is needed that can regulate energy supplied to the process.

Another purpose of control devices is to protect the machine from harming itself. One example is a centrifugal compressor which must have surge controls to help prevent the machine from going into an unstable low-flow condition. Reciprocating compressors are no different. They should have control devices that not only regulate the amount of energy they produce but also prevent the machine from harming itself.

This article will discuss the various types of control methods used by reciprocating compressors. It will begin by discussing the theory of the compression cycle and how changes in the cycle influence the machine’s performance. Three types of performance diagrams are presented that illustrate the compression cycle and how they are affected by changing control parameters. This topic is followed by the listing of useful equations that define a compressor’s performance and potential limitations.

The next section presents the six methods used to control a reciprocating compressor. Examples of each method are given along with their advantages, disadvantages and how each method affects the compression cycle. Many illustrations are shown of the various control devices and how each device affects the compressor’s performance and power curves. This section also demonstrates how a combination of control methods is used to better serve an application.

Control devices and methods are also utilized in the startup and shutdown of reciprocating compressors. The various events, their purpose and sequence of activities are described. A list of control devices commonly utilized during startup and shutdown is also discussed.

An important requirement of any control method is to prevent the possible damage to a compressor. To emphasize this theme two example problems are given that demonstrates the application of control devices to real-world applications. This is followed with other examples and illustrations of how a reciprocating compressor can be protected by the various control devices. Finally, this article gives a summary of the topics discussed.

Before beginning the discussion of reciprocating compressor control methods it is important to note certain limitations of this article. Readers should have a basic understanding of compression thermodynamics and gas physical properties. These topics are important to the understanding of control methods. The reference section includes articles that readers may find helpful in refreshing their memory of these topics. Another prerequisite is that readers should also have a basic understanding of the various types of instruments that are commonly found with reciprocating machines. While this article will focus on the control methods and the use of related devices, instruments such as temperature and pressure switches used to monitor the machine are not included. Only the instrumentation that is necessary for the control device to function properly is shown.

 

 

 

 

 

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