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Operation of a compressor in a system
Operation of a compressor in a system Introduction The definition of a process system System resistance curves The operating point A positive displacement compressor in the process system A dynamic compressor in the process system
Introduction In this chapter we will define a system, examine system resistance curves (head required by the process) and define the operating point of a compressor. Again, the principles discussed can be applied to a pump operating in a system. A system is a set of connected things or parts that work together. In a typical process a set of components (vessels, exchangers, fijrnaces, control valves, etc. and piping) work together to produce a resistance to flow at the turbo-compressor flanges. Examining a simple system one can see that any process system relative to the compressor is comprised of two parts, the suction system and the discharge system. The objective of any compressor in a system is to remove the flow from the suction vessel at the same rate that the flow enters that vessel. In doing so, the pressure is reduced from the suction vessel to the compressor flanges. In the discharge system the objective is to push the required amount of flow through the system resistance to achieve the final discharge system terminal pressure. This pressure may be in a vessel, may be in a ship or could be in a pipe line. The discharge pressure at the compressor flange
43
Compressors WMBMMBWMMW^
then is the accumulation of the system resistance (pipe, valve, exchanger, etc.) from the terminal point (delivery point) back to the compressor discharge flange. An important point to remember is that the compressor differential head or energy required is the net effect of the discharge system resistance and suction system resistance. Increasing discharge system resistance with a constant suction resistance will result in higher compressor energy required. Also increasing suction system resistance with a constant discharge system resistance will result in increasing compressor energy required. Many times the concept of increasing suction system resistance is misunderstood (suction throttling). If we review the previous chapter and examine the operation of both positive displacement and dynamic compressors in a simple system, two important facts concerning the operation of these machines in a system become evident. A positive displacement compressor, since it increases the energy of the gas by operating on the gas in a confined space, will always increase the energy, provided sufficient power is available and the machine is designed to meet this objective. Therefore a positive displacement compressor will be relatively insensitive to gas composition changes. In addition, since the positive displacement compressor as previously defined has variable head capability, it will be relatively insensitive to system changes. As a result, a positive displacement compressor will operate at approximately the same delivered volume flow regardless of system resistance or gas composition. On the other hand, since the characteristics of a dynamic compressor are to increase gas energy (a function of mass and velocity) by working on the gas with blades. Velocities and gas mass (density) play an important part in the make up of this compressor's characteristics. Anything that will result in a velocity and/or density change at the tip of the dynamic compressor blade will result in a different produced differential pressure and a corresponding flow change. Therefore, the dynamic compressor will be extremely sensitive to gas composition changes since these changes will produce mass and velocity changes within the compressor blades or impellers. In addition, this compressor type will also be sensitive to system resistance changes since an increased system resistance requirement will force the compressor to operate at a lower volume throughput. This is because the only way this compressor can produce higher delivered energy is at a lower velocity throughput. Therefore one can readily see that the operating point of a dynamic compressor will be much more sensitive to both changes in system resistance and the velocities inside the impeller. Both system resistance and the dynamic compressor curve can change when plotted on flow vs
Operation of a Compressor in a System
pressure coordinates. As will be explained later flow vs head coordinates reflect a dynamic compressor which changes very litde provided that the gas composition changes within the limits of approximately 20%. The compressor operating point can be defined as the equilibrium between the required net process system energy and the compressor produced energy. Remember, that the positive displacement compressor characteristic results in relatively constant flow regardless of system required energy, whereas the dynamic compressor characteristics results in significantly large flow changes for changes in system resistance. One additional point to understand is that the process system characteristic curve can also change. The system resistance curve (head required by the process) is a result of the terminal pressure, that is the discharge pressure at zero flow and the system resistance (square function) as the flow changes. A recycle loop for instance is comprised of friction loss only and will have a relatively steep system resistance curve, whereas an instrument air compressor pumping into a receiver vessel with a relatively small system resistance will have a system resistance curve that will approach a horizontal line. One can see that the combination of a flat dynamic compressor curve and a flat system resistance system curve can lead to a very unstable situation. Remember that the operating point is determined by the intersection of a compressor and system resistance curves. For turbocompressors both of these curves can change and certainly do.
The Definition of a Process System The definition of a generic and process system are presented in Figure 4.1. The process system characteristics and the gas composition determine the head required by the process system. System - Definition 'A system is a set of connected things or parts that work together' In our case, a set of components (vessels), exchangers, furnaces, control valves, etc. and piping) work together to produce a resistance to flow at the turbocompressor flanges.
Figure 4.1 System - definition
Compressors
TO SYSTEM
i
SUCT. VESSEL
DISCH. VESSEL K^
fej
^
FLOW-ACFM
FLOW-ACFM
Figure 4.2 A simple system
A simple process system schematic is presented in Figure 4.2. Based on the characteristics of positive displacement and dynamic compressors presented in the previous chapter, draw both the pressure ratio vs flow and brake horsepower vs flow on the appropriate graphs.
System resistance curves Figure 4.3 shows a system where the system resistance curve (head required by the process) intersects the compressor curve (head produced) at the design point (100% flow). In Figure 4.4, two different system resistance curves representing two different process systems are shown. The curve starting at zero differential pressure represents a process system where only system resistance is present. When the compressor is shut down, the pressure in the system is zero. This would be the system resistance curve that would describe a recycle process. The curve that starts at approximately 50 PSI differential pressure shows that a pressure differential of approximately 50 PSI exists when the compressor is shutdown in this system. This process system would be typical of a system in which both vessel pressure and system resistance are present. One can see from this figure that the higher the percent of vessel pressure in the process system, the flatter the system resistance curve is.
46
Operation of a Compressor in a System 140
120 tu 3
100
ILJS a. a.
80
iij
O
oc
<
o X (/) o S5
60
40
20
20
40
60
80
100
120
140
% FLOW - ACFM Figure 4.3 % Discharge pressure vs. % flow TERMINAL PRESSURE « 0
Uf (A OC M
COMBINATION OF TERMINAL PRESSURE AND RESISTANCE
NOTES THE PRESSURE AT 0% FLOW DEFINES THE TERMINAL PRESSURE (END PRESSURE) IN THE SYSTEM
ui O OC 3 3 <0
THE PRESSURE AT ANY OTHER FLOW IS EQUAL TO THE TERMINAL PRESSURE PLUS THE SYSTEM RESISTANCE TO THAT FLOW
S8 5" 40
60
80
% FLOW • ACFM
Figure 4.4 The system resistance curve for the discharge system
System resistance characteristics for the three basic types of process systems are shown in Figure 4.5. Note that the steepest curve, the recycle loop, will result in the most stable operation regardless of the compressor curve shape. The instrument air compressor process system curve on the other hand tends to be very flat due to high terminal pressure in the discharge (receiver) vessel. This system will experience instability if the compressor characteristic curve is flat.
47
Compressors
I Friction Only
il High Terminal Pressure
ill Intermediate
v^^ ^ Recycle Loop
Instrument Air Compressor
Off Gas Compressor
k U:UJ
100 %ACFM
Figure 4.5 System resistance characteristics
Many times, a compressor has not been considered technically acceptable for purchase because of a relatively flat head curve (less than 5% head rise to surge). If the process system that the compressor will operate in is either Type I or Type III in Figure 4.5, this compressor should be considered acceptable based on the fact that the process system steadily rising head characteristic curve will provide a stable
THE DESIGN POINT 1 IS BASED ON THE CUENTS DATA
THE TURBO-COMPRESSOR CURVE IS BASED ON COMPRESSOR DESIGN, GAS CHARACTERISTICS AND VELOCITY
FLOW RATE - ACFIVI Figure 4.6 Turbo-compressor characteristics for a constant speed - the design point
48
Operation of a Compressor in a System operating point. Figure 4.6 shows that the compressor curve selected is based only on a single operating point specified by the client. Industry specifications only require that one operating point be guaranteed. Therefore the compressor curve shape is not guaranteed.
The operating point The operating point is defined as the equilibrium condition that exists between the head produced by the equipment and the head required by the process system. (See Figure 4.7). The operating point Is determined by the intersection of the turbo-compressor and system resistance curves - both can change and do! Figure 4.7 The operating point
If the process conditions that actually exist are properly specified and if the equipment is properly selected the operating point will occur at the best efficiency point. Figure 4.8 shows a case where the head required by the process is greater than the head produced by a centrifugal compressor at the rated point. z Ul
o
THE OPERATING POINT DEPENDS ON THE SYSTEM RESISTANCE, GAS CHARACTERISTICS AND COMPRESSOR CONDITION
FLOW RATE - ACFM Figure 4.8 The operating point
49
Compressors iiiiiiM^^^^
The result is that the equilibrium point falls to the left of the best efficiency point resulting in a lower efficiency, lower flow rate and lower horsepower.
A positive displacement compressor in the process system Referring back to Figure 4.2 of this chapter, draw any of the three typical system resistance curves on the pressure ratio vs flow graph. Notice that regardless of the change of resistance, the flow rate of a positive displacement compressor does not change. In addition, the flow rate will remain constant regardless of the density of the gas (molecular weight, inlet temperature or inlet pressure). Refer to Figure 4.9 which presents the characteristics of a positive displacement compressor. Compressor characteristics Positive displacement •
Constant volume delivery
•
Variable head capacity
•
Not self limiting
•
Not flow sensitive to system resistance
•
Not flow sensitive to gas density
Figure 4.9 Compressor characteristics
Based on the results drawn in Figure 4.2, a positive displacement compressor is not flow sensitive to either system resistance or gas density.
A dynamic compressor in the process system Again, refer to Figure 4.2 and observe how the system resistance curves previously drawn affect a dynamic compressor's operating point. A dynamic compressor's flow rate is affected by system resistance change. In addition, a change in the gas density will also influence the system resistance (head required curve) and therefore change the flow rate.
Operation of a Compressor in a System
Refer to Figure 4.10 which presents the characteristics of a dynamic compressor. Compressor characteristics Dynamic
• • • • •
Variable volume delivery Fixed head capacity (for a certain flow) Self limiting Is flow sensitive to system resistance Is flow sensitive to gas density
Figure 4.10 Compressor characteristics
Based on the results of Figure 4.2, a dynamic compressor is flow sensitive to both system resistance and gas density.
51
Introduction In this chapter we will define a system, examine system resistance curves (head required by the process) and define the operating point of a compressor. Again, the principles discussed can be applied to a pump operating in a system. A system is a set of connected things or parts that work together. In a typical process a set of components (vessels, exchangers, fijrnaces, control valves, etc. and piping) work together to produce a resistance to flow at the turbo-compressor flanges. Examining a simple system one can see that any process system relative to the compressor is comprised of two parts, the suction system and the discharge system. The objective of any compressor in a system is to remove the flow from the suction vessel at the same rate that the flow enters that vessel. In doing so, the pressure is reduced from the suction vessel to the compressor flanges. In the discharge system the objective is to push the required amount of flow through the system resistance to achieve the final discharge system terminal pressure. This pressure may be in a vessel, may be in a ship or could be in a pipe line. The discharge pressure at the compressor flange
43
Compressors WMBMMBWMMW^
then is the accumulation of the system resistance (pipe, valve, exchanger, etc.) from the terminal point (delivery point) back to the compressor discharge flange. An important point to remember is that the compressor differential head or energy required is the net effect of the discharge system resistance and suction system resistance. Increasing discharge system resistance with a constant suction resistance will result in higher compressor energy required. Also increasing suction system resistance with a constant discharge system resistance will result in increasing compressor energy required. Many times the concept of increasing suction system resistance is misunderstood (suction throttling). If we review the previous chapter and examine the operation of both positive displacement and dynamic compressors in a simple system, two important facts concerning the operation of these machines in a system become evident. A positive displacement compressor, since it increases the energy of the gas by operating on the gas in a confined space, will always increase the energy, provided sufficient power is available and the machine is designed to meet this objective. Therefore a positive displacement compressor will be relatively insensitive to gas composition changes. In addition, since the positive displacement compressor as previously defined has variable head capability, it will be relatively insensitive to system changes. As a result, a positive displacement compressor will operate at approximately the same delivered volume flow regardless of system resistance or gas composition. On the other hand, since the characteristics of a dynamic compressor are to increase gas energy (a function of mass and velocity) by working on the gas with blades. Velocities and gas mass (density) play an important part in the make up of this compressor's characteristics. Anything that will result in a velocity and/or density change at the tip of the dynamic compressor blade will result in a different produced differential pressure and a corresponding flow change. Therefore, the dynamic compressor will be extremely sensitive to gas composition changes since these changes will produce mass and velocity changes within the compressor blades or impellers. In addition, this compressor type will also be sensitive to system resistance changes since an increased system resistance requirement will force the compressor to operate at a lower volume throughput. This is because the only way this compressor can produce higher delivered energy is at a lower velocity throughput. Therefore one can readily see that the operating point of a dynamic compressor will be much more sensitive to both changes in system resistance and the velocities inside the impeller. Both system resistance and the dynamic compressor curve can change when plotted on flow vs
Operation of a Compressor in a System
pressure coordinates. As will be explained later flow vs head coordinates reflect a dynamic compressor which changes very litde provided that the gas composition changes within the limits of approximately 20%. The compressor operating point can be defined as the equilibrium between the required net process system energy and the compressor produced energy. Remember, that the positive displacement compressor characteristic results in relatively constant flow regardless of system required energy, whereas the dynamic compressor characteristics results in significantly large flow changes for changes in system resistance. One additional point to understand is that the process system characteristic curve can also change. The system resistance curve (head required by the process) is a result of the terminal pressure, that is the discharge pressure at zero flow and the system resistance (square function) as the flow changes. A recycle loop for instance is comprised of friction loss only and will have a relatively steep system resistance curve, whereas an instrument air compressor pumping into a receiver vessel with a relatively small system resistance will have a system resistance curve that will approach a horizontal line. One can see that the combination of a flat dynamic compressor curve and a flat system resistance system curve can lead to a very unstable situation. Remember that the operating point is determined by the intersection of a compressor and system resistance curves. For turbocompressors both of these curves can change and certainly do.
The Definition of a Process System The definition of a generic and process system are presented in Figure 4.1. The process system characteristics and the gas composition determine the head required by the process system. System - Definition 'A system is a set of connected things or parts that work together' In our case, a set of components (vessels), exchangers, furnaces, control valves, etc. and piping) work together to produce a resistance to flow at the turbocompressor flanges.
Figure 4.1 System - definition
Compressors
TO SYSTEM
i
SUCT. VESSEL
DISCH. VESSEL K^
fej
^
FLOW-ACFM
FLOW-ACFM
Figure 4.2 A simple system
A simple process system schematic is presented in Figure 4.2. Based on the characteristics of positive displacement and dynamic compressors presented in the previous chapter, draw both the pressure ratio vs flow and brake horsepower vs flow on the appropriate graphs.
System resistance curves Figure 4.3 shows a system where the system resistance curve (head required by the process) intersects the compressor curve (head produced) at the design point (100% flow). In Figure 4.4, two different system resistance curves representing two different process systems are shown. The curve starting at zero differential pressure represents a process system where only system resistance is present. When the compressor is shut down, the pressure in the system is zero. This would be the system resistance curve that would describe a recycle process. The curve that starts at approximately 50 PSI differential pressure shows that a pressure differential of approximately 50 PSI exists when the compressor is shutdown in this system. This process system would be typical of a system in which both vessel pressure and system resistance are present. One can see from this figure that the higher the percent of vessel pressure in the process system, the flatter the system resistance curve is.
46
Operation of a Compressor in a System 140
120 tu 3
100
ILJS a. a.
80
iij
O
oc
<
o X (/) o S5
60
40
20
20
40
60
80
100
120
140
% FLOW - ACFM Figure 4.3 % Discharge pressure vs. % flow TERMINAL PRESSURE « 0
Uf (A OC M
COMBINATION OF TERMINAL PRESSURE AND RESISTANCE
NOTES THE PRESSURE AT 0% FLOW DEFINES THE TERMINAL PRESSURE (END PRESSURE) IN THE SYSTEM
ui O OC 3 3 <0
THE PRESSURE AT ANY OTHER FLOW IS EQUAL TO THE TERMINAL PRESSURE PLUS THE SYSTEM RESISTANCE TO THAT FLOW
S8 5" 40
60
80
% FLOW • ACFM
Figure 4.4 The system resistance curve for the discharge system
System resistance characteristics for the three basic types of process systems are shown in Figure 4.5. Note that the steepest curve, the recycle loop, will result in the most stable operation regardless of the compressor curve shape. The instrument air compressor process system curve on the other hand tends to be very flat due to high terminal pressure in the discharge (receiver) vessel. This system will experience instability if the compressor characteristic curve is flat.
47
Compressors
I Friction Only
il High Terminal Pressure
ill Intermediate
v^^ ^ Recycle Loop
Instrument Air Compressor
Off Gas Compressor
k U:UJ
100 %ACFM
Figure 4.5 System resistance characteristics
Many times, a compressor has not been considered technically acceptable for purchase because of a relatively flat head curve (less than 5% head rise to surge). If the process system that the compressor will operate in is either Type I or Type III in Figure 4.5, this compressor should be considered acceptable based on the fact that the process system steadily rising head characteristic curve will provide a stable
THE DESIGN POINT 1 IS BASED ON THE CUENTS DATA
THE TURBO-COMPRESSOR CURVE IS BASED ON COMPRESSOR DESIGN, GAS CHARACTERISTICS AND VELOCITY
FLOW RATE - ACFIVI Figure 4.6 Turbo-compressor characteristics for a constant speed - the design point
48
Operation of a Compressor in a System operating point. Figure 4.6 shows that the compressor curve selected is based only on a single operating point specified by the client. Industry specifications only require that one operating point be guaranteed. Therefore the compressor curve shape is not guaranteed.
The operating point The operating point is defined as the equilibrium condition that exists between the head produced by the equipment and the head required by the process system. (See Figure 4.7). The operating point Is determined by the intersection of the turbo-compressor and system resistance curves - both can change and do! Figure 4.7 The operating point
If the process conditions that actually exist are properly specified and if the equipment is properly selected the operating point will occur at the best efficiency point. Figure 4.8 shows a case where the head required by the process is greater than the head produced by a centrifugal compressor at the rated point. z Ul
o
THE OPERATING POINT DEPENDS ON THE SYSTEM RESISTANCE, GAS CHARACTERISTICS AND COMPRESSOR CONDITION
FLOW RATE - ACFM Figure 4.8 The operating point
49
Compressors iiiiiiM^^^^
The result is that the equilibrium point falls to the left of the best efficiency point resulting in a lower efficiency, lower flow rate and lower horsepower.
A positive displacement compressor in the process system Referring back to Figure 4.2 of this chapter, draw any of the three typical system resistance curves on the pressure ratio vs flow graph. Notice that regardless of the change of resistance, the flow rate of a positive displacement compressor does not change. In addition, the flow rate will remain constant regardless of the density of the gas (molecular weight, inlet temperature or inlet pressure). Refer to Figure 4.9 which presents the characteristics of a positive displacement compressor. Compressor characteristics Positive displacement •
Constant volume delivery
•
Variable head capacity
•
Not self limiting
•
Not flow sensitive to system resistance
•
Not flow sensitive to gas density
Figure 4.9 Compressor characteristics
Based on the results drawn in Figure 4.2, a positive displacement compressor is not flow sensitive to either system resistance or gas density.
A dynamic compressor in the process system Again, refer to Figure 4.2 and observe how the system resistance curves previously drawn affect a dynamic compressor's operating point. A dynamic compressor's flow rate is affected by system resistance change. In addition, a change in the gas density will also influence the system resistance (head required curve) and therefore change the flow rate.
Operation of a Compressor in a System
Refer to Figure 4.10 which presents the characteristics of a dynamic compressor. Compressor characteristics Dynamic
• • • • •
Variable volume delivery Fixed head capacity (for a certain flow) Self limiting Is flow sensitive to system resistance Is flow sensitive to gas density
Figure 4.10 Compressor characteristics
Based on the results of Figure 4.2, a dynamic compressor is flow sensitive to both system resistance and gas density.
51