You met your match, you met your match
You met your match, you met your match
You met your match, baby

("You Met Your Match," by Stevie Wonder, L. Hardaway, and D. Hunter, 1968)

 

MATCHING COMPESSOR TYPE TO COMPRESSOR JOB 

PTOA Readers and Students recently learned in PTOA Segment #216 that the two main classifications of Compressors are Dynamic Compressors and Positive-Displacement Compressors.

PTOA Readers and Students recently learned in PTOA Segment #217 that there are a wide variety of jobs Compressors and compressed gases perform in a processing facility.

The industrial jobs that Compressors and compressed gases perform include:

• Boosting gas PV Pressure so that it can flow into a pipe or vessel that has somewhat lower PV Pressure. Sometimes boosting is necessary because the discharged gas has a long way to flow to be used in a different part of an integrated plant.
• Moving a compressed gas from Point A to supply Point B via gas headers.
• Recycling Compressed gases improves process economy by reusing a process gas which would otherwise be wasted.
• Purging air from process lines and vessels prior to Start-Up.
• Pressuring Up process lines and vessels prior to Start-Up.
• Maintaining a PV Pressure which will enhance chemical reactions.

 

Brilliant PTOA Readers and Students …

meaning those who read the PTOA Segments in the intended, sequential order …

can extrapolate what they have learned about Dynamic Centrifugal Pumps and Reciprocating Positive Displacement Pumps to predict which process services Dynamic Centrifugal Compressors and Reciprocating Positive-Displacement Compressors are best suited for.

 

 

 

DYNAMIC CENTRIFUGAL COMPRESSORS COMPARED TO DYNAMIC CENTRIFUGAL PUMPS

As PTOA Readers and Students will soon learn, Dynamic Centrifugal Compressors operate pretty much exactly the way Dynamic Centrifugal Pumps work with the noteworthy exception that the gases that flow through Compressors are "compressible" whereas the liquids that Pumps process are not.

The nearby schematic depicts a single-stage Centrifugal Pump. A single-stage Centrifugal Pump has one Impeller. Centrifugal Pumps were featured in PTOA Segments #163 through #176.

Compare the nearby Centrifugal Pump schematic to the nearby graphic which depicts a cutaway photo and the gas flow pathway through a Centrifugal Compressor.

Not much difference in form, eh?

Guess what?

Not much difference in function, either.

The desirable characteristic of both the Centrifugal Pump and the Centrifugal Compressor is the smooth, continuous fluid flow discharged from the equipment with a higher PV Pressure.

In both the Pump and the Compressor:

• The liquid continuously flows into the Pump or the gas continuously flows into the Compressor through the Suction Line.
• The fluid is directed to the eye of a rotating Impeller and thence flung outward to the edge of the Impeller, all the while swapping Fluid Velocity for gains in PV Pressure (PTOA Segment #159 featured the PV Pressure↔Fluid Velocity Swap).
• Upon entering the Volute, the fluid's velocity is further diminished and the PV Pressure increases. Centrifugal Compressors are designed so that half of the increase in PV Pressure takes place in the Volute.
• The liquid exits the Pump and gas exits the Compressor through the Discharge Line at a significantly higher PV Pressure compared to the PV Suction Pressure.

 

PTOA Readers and Students will recall the nearby Performance graph from PTOA Segment #205 . The X axis of the graphic shows that a Centrifugal Pump has a range of flow Capacity (aka Flowrate) whereas the Positive-Displacement Pump has essentially a single Capacity. The same relationship exists for Centrifugal Compressors versus Reciprocating PD Compressors.

In other words, Centrifugal Compressors are best suited for process applications that require the gas flowrate to be able to vary because of unexpected-things-that-happen-upstream or unexpected-things-that-happen-downstream from the Compressor.

The point is that Dynamic Pumps and Dynamic Compressors can provide a wide range of fluid Capacity/Flowrate and smoothly deliver the gas into the Discharge Line at an elevated fluid PV Pressure.

Therefore ..

Dynamic Compressors  … both Centrifugal and Axial models … are used when the process requires smoothly-delivered gas at a varying Capacity/Flowrate.

The Compressor Services that need a wide Capacity range of smoothly-delivered gas are:

• Boosting gas PV Pressure so that it can flow into a pipe or vessel that has somewhat lower PV Pressure. Sometimes boosting is necessary because the discharged gas has a long way to flow to be used in a different part of an integrated plant.

• Smoothly Moving a compressed gas from Point A to supply Point B via gas headers. Note: a Rotary-Motion PD Compressor and gas cylinders can also smoothly deliver a small Capacity of compressed gas.

• Recycling Compressed gases improves process economy by reusing a process gas which would otherwise be wasted. Recycle Compressors are often Centrifugal Compressors.

• Purging air from process lines and vessels prior to Start Up.
• Pressuring up process lines and vessels prior to Start Up.

• Maintaining a PV Pressure which will enhance chemical reactions.

 

Dynamic Compressors are used for smoothly moving a variable Capacity of gas from point A to Point B, recycling gases, and maintaining desired process unit PV Pressure.

 

RECIPROCATING PD COMPRESSORS COMPARED TO
RECIPROCATING PD PUMPS

Who would be surprised to learn that the form and function of a Reciprocating Positive Displacement Pump mirrors the form and function of the Reciprocating Positive-Displacement Compressor?

Same noteworthy caveat … gases are compressible and liquids are not.

The nearby gif shows how liquid is pumped through a Reciprocating PD Pump. PTOA Readers and Students learned about Reciprocating PD Pumps in PTOA Segment #209 and #210.

The nearby gif of a Reciprocating PD Compressor is extremely similar to the PD Pump gif. So much so that the labelled hardware shown in the Reciprocating PD Pump gif likewise applies to the unlabeled hardware components shown in the Reciprocating PD Compressor gif.

Common features of both PD Pumps and PD Compressor include:

• The Suction and Discharge Valves are synchronized to coordinate the entry and exit of a CONFINED VOLUME of liquid and gas for the Reciprocating PD Pump and Reciprocating PD Compressor, respectively. The single Capacity (aka Flowrate) through a Reciprocating PD Pump and Reciprocating PD Compressor is determined by (and limited by) the size of the Cylinder, the length of each stroke of the Piston and the speed of the Driver.
• A Piston and Crosshead are connected to a Connecting Rod which converts the rotary motion of the Driver into the back and forth motion of the Piston.
• When extended, the Piston squishes the liquid and gas into a much smaller volume thus significantly increasing both the density of the gas and its PV Pressure. The Discharge Valve will open at a set Discharge PV Pressure.
• When contracted, the Piston pulls the Suction Valve open to allow more gas to enter.

 

As stated above, the nearby graphic compares the Performance of a  Centrifugal Pump and a Reciprocating PD Pump. The information on the graph can be extrapolated to compare Centrifugal Compressors with Reciprocating PD Compressors.

The Discharge Pressure of each type of Compressor is shown as feet of Hydraulic Head on the Y axis.

The graph clearly shows that

The Reciprocating PD Compressor is limited to a single, lower range Capacity (determined by the Cylinder volume).

However the Discharge PV Pressure of a Reciprocating PD Compressor is much greater than the Discharge PV Pressure that a Centrifugal Compressor can achieve at any Capacity.

Therefore,

Reciprocating Positive-Displacement Compressors are used in process industry when a relatively small Capacity (aka Flowrate) of high PV Pressure gas is needed.

The Compressor Jobs best suited for a Reciprocating PD Compressor are:

• Boosting gas PV Pressure so that it can flow into a pipe or vessel that has somewhat lower PV Pressure. Sometimes boosting is necessary because the discharged gas has a long way to flow to be used in a different part of an integrated plant.
• Moving a compressed gas from Point A to supply Point B via gas headers.
• Recycling Compressed gases improves process economy by reusing a process gas which would otherwise be wasted.
• Purging air from process lines and vessels prior to Start Up.
• Pressuring Up process lines and vessels prior to Start Up.
• Maintaining a PV Pressure which will enhance chemical reactions.

 

Reciprocating Positive-Displacement Compressors will be used for boosting the PV Pressure of a gas so that the gas can flow a long way away or flow into a somewhat lower PV Pressure area,

Reciprocating Positive-Displacement Compressors are also used for Purging and Pressuring-Up a process prior to Start-Up

 

THE BIG DIFFERENCE BETWEEN COMPRESSORS AND PUMPS

The big difference between Compressors and Pumps is the compressibility of gases.

When the gas is compressed to a higher PV Pressure, the PV Temperature will increase. PTOA Readers and Students learned all about the applicable common sense "Gas Laws" in PTOA Segment #152.

In a rigid container …

exactly like the Cylinder of a Reciprocating PD Compressor  …

an increase in PV Pressure will also increase the PV Temperature. The density of the gas also increases as the gas gets squashed into a smaller volume by the piston.

The PV Temperature increase is significant; so much so that it limits the size of the Cylinder and Piston.

Centrifugal Compressors also have a rigid container … the Compressor Housing.

The PV Temperature increases while the gas that flows through the  Centrifugal Compressor is whirled around and gaining PV Pressure.

Sometimes both the smooth, variable Capacity/Flowrate of a gas compressed by a Centrifugal Compressor AND a high Discharge PV Pressure achievable with a Reciprocating PD Compressor are needed.

In that situation Centrifugal Compressors will be situated "in series" so that the gas discharged from the Stage 1 Compressor flows into the suction of the Stage 2 Compressor. .

Because each Centrifugal Compressor will increase the PV Temperature of the gas, intermittent Fin Fan Heat Exchangers are needed to cool the gas down prior to entering the next Compressor.

The nearby chart shows how three  Centrifugal Compressors "in series" can  increase an initial Stage 1 Suction PV Pressure of 793.1 kPa/115 psia to a final Stage 3 Discharge PV Pressure of 19724.1 kPA/2860 psia.

Note that the intermittent Fin Fan Heat Exchangers maintain the Suction PV Temperature at each stage at 40.6 °C/105 °F.

 

TAKE HOME MESSAGES: Centrifugal Compressors are used when there is a need for a varied Capacity/Flowrate of gas delivered smoothly into the Discharge Line. Centrifugal Compressor jobs include:

• Delivering gas from Point A to supply Point B, often via headers.
• Recycling gas via Centrifugal Recycle Compressors.
• Maintaining the Process Unit target PV Pressure.

 

Reciprocating Positive Displacement Compressors are limited to one Capacity/Flowrate but their Discharge PV Pressure is much higher than the Centrifugal Compressor can attain at any Capacity/Flowrate.

Compressor Jobs suited to Reciprocating PD Compressors include:

• Boosting the PV Pressure of a gas so that it can flow into an area of somewhat lower PV Pressure and/or flow a great distance from the Booster Compressor's Discharge Valve.
• Purge a process unit prior to Start-Up. (Compressed gas in cylinders might also be used for purging).
• Pressure-Up a process unit prior to Start-Up. (Compressed gas in cylinders might also be used for Pressure-Up).

 

Gases are compressible; an increase in PV Pressure results in a significant increase in PV Temperature. For this reason, Reciprocating PD Compressors and Centrifugal Compressors are size limited.

When a varied Capacity/Flowrate of gas is needed with a final, high PV Pressure, Centrifugal Compressors "in series" will rely on intermittent Fin Fan Heat Exchangers to cool the gas down before the gas discharge from one Centrifugal Compressor flows to the suction of the next Centrifugal Compressor. .   

©2021 PTOA Segment 0218

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT COMPRESSORS

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