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I SPY A RECIPROCATING-ACTION POSITIVE-DISPLACEMENT COMPRESSOR

The Process Technology and Operator Academy Posted on by

But you're the one who did it, who did it, who did it!
As sturdy as Gibraltar, not a second did you falter.
There's no doubt about it … Yoooooou … did it!

("You Did It," by Frederick Loewe, 1964)

 

THE PTOA ROTATING-EQUIPMENT FOCUS STUDY FINISH LINE IS HERE!

Hallelujah!

This PTOA Segment concludes the PTOA Rotating Equipment Focus Study!

Completing the PTOA Rotating Equipment Focus Study is A BIG DEAL!

Process Industry employers want to employ prospective Process Operators who understand the fundamentals of Rotating Equipment.

Why? Because understanding the fundamentals of operating and maintaining Rotating Equipment  is a prerequisite to keeping this expensive equipment on line,  continuously creating PV Pressure energy and infusing that energy into liquids and gases until the next scheduled maintenance interval.

Unscheduled shutdowns due to catastrophic equipment failure are very costly because no money can be made while the Rotating Equipment is being repaired.

Let's admit it!

Learning about how Rotating Equipment and Tribology, Bearings and Seals, and Prime Movers/Drivers all play a role in creating energy in the form of PV Pressure …  either by static or dynamic means … has also made the brain work overtime! Well done!

The last featured type of Rotating Equipment is the Reciprocating-Action Positive-Displacement Compressor.

Since the PTOA content associated with Rotating Equipment comprises the vast majority of the PTOA PV Pressure Study Area, PTOA Readers and Students are also almost finished learning about how the PV Pressure is created and used in process industries.

 

COMPRESSOR FAMILY TREE REVIEW

The Compressor Family Tree was first featured in PTOA Segment #216.

Brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order … already know that there the two subsets of Positive Displacement Compressors:

  • Reciprocating-Action Positive Displacement Compressors
  • Rotary-Motion Positive Displacement Compressors

The two lineages of Positive-Displacement Compressors are shown in YELLOW BOXES.

Rotary-Motion Pumps operate as Rotary-Motion Compressors do. Remember that when gases are compressed their PV Temperature increases.

Guess what?

PTOA Readers and Students already possess sufficient fundamental knowledge regarding the form and function of Rotary-Motion Positive Displacement Compressors.

Rotary-Motion Positive-Displacement Compressors do not operate much differently than the Rotary-Motion Positive-Displacement Pumps that were featured in

.

Except for one big caveat that Fred remembers, right Fred?

Here is that caveat:

The PV Temperature of a gas rises as the gas is compressed whereas the PV Temperature of a liquid only increases slightly while the liquid is being pumped.

 

FOCUS ON RECIPROCATING-ACTION PD COMPRESSORS

ISA symbols for Positive-Displacement Compressors. PD Compressors are either Reciprocating-Action or Rotary-Motion Compressors.

Those same brilliant PTOA Readers and Students are already quite knowledgeable about the operation and industrial uses of Reciprocating-Action PD Compressors.

The ISA symbols representing Positive Displacement Compressors was previously introduced in PTOA Segment #216.

The operating differences between Reciprocating-Action PD Compressors and Dynamic Compressors was likewise featured in PTOA Segment #216.

Brilliant PTOA Readers and Students learned that the operating theory of Reciprocating-Action PD Compressors is simply smashing gas particles into a smaller volume which thus increasies the PV Pressure (and subsequently the PV Temperature) of the gas.

A more theoretical explanation of the operating theory of a Reciprocating-Action PD Compressor can be explained by the Ideal Gas Law which was featured in PTOA Segment #154.

The various industrial services for Compressors was featured in PTOA Segment #217 and optimally matching the Compressor type to the industrial compressor service was featured in PTOA Segment #218.

Positive-Displacement Compressors generate high PV Discharge Pressures but can only handle lower range Capacities with success.

PTOA Readers and Students learned that Reciprocating-Action PD Compressors are used when a high PV Discharge Pressure and relatively low Capacity is needed for the industrial service

The typical Reciprocating-Action PD Compressor is used to generate PV Discharge Pressures up to 12,000 psi (828 bar).

Specialty Reciprocating-Action PD Compressors (for example the compressors used to compress low-density polyethylene) can achieve PV Discharge Pressures of 50,000 psi (3500 bar)

But guess what, Fred?

Some Reciprocating-Action PD Compressors can have nearly atmospheric Suction PV Pressures, even a vacuum Suction PV Pressure!

It goes without saying that a Reciprocating-Action PD Compressor with a vacuum Suction PV Pressure must have fail-safe leak protection. Exactly zero air must be able to leak into the Cylinder via slipping through the packing on the "Power Side" of the Compressor. A Real-World experience of sealing failure is recounted  below.

Critical Reciprocating-Action PD Compressor Hardware

The hardware that the compressed gas comes in contact with is the Suction Check Valve, Suction Valve, Cylinder Head Volume which is expanded and contracted by the Piston, Discharge Valve and Discharge Check Valve.

The crucial hardware associated with Reciprocating-Action PD Compressors was featured in PTOA Segment #218; PTOA Readers and Students learned that the critical hardware associated with a Reciprocating-Action PD Compressor has a lot in common with the critical hardware associated with the "Single-Acting" Reciprocating-Action  PD Pumps featured in PTOA Segment #209.

As the gas that is to be compressed flows into the Reciprocating-Action PD Compressor, the critical hardware the gas contacts within the "Pressure-Generating Side" of the Compressor includes:

Single-Acting PD Compressor on the Left. Double-Acting Single-Ended PD Compressor on the right. The Suction and Discharge Valves look like springs.

  • Inlet Snubber aka Pulsation Dampener.
  • Suction Valve and Inlet Check Valve.
  • Cylinder.
  • Outlet Discharge Valve and Discharge Check Valve.
  • Discharge-Side Snubber aka Pulsation Dampener.

 

Pulsation Dampeners were first featured in PTOA Segment #204.

The white, long cylindrically shaped container shown on the top of the three Reciprocating-Action PD Compressors is a "Snubber" which provides a constant volume of gas flow into the Suction Valves of the Compressors..

Unlike the delivery of gas into a Dynamic Compressor's discharge line, the PV Pressure of the gas discharged from a Reciprocating-Action PD Compressor pulses.

"Snubbers" (aka Pulsation Dampeners) are situated on both the Suction Side and Discharge Side of the Reciprocating-Action PD Compressor.

The Suction Side Snubber ensures that a constant volume of gas is ever-ready to be sucked into the Compressor and thence compressed. The Discharge Side Snubber ensures that a constant volume of pressurized gas is available to flow into the Gas Discharge Header.

Crucial hardware located on the "Power Side" of the Reciprocating-Action PD Compressor includes:

  • A Crankshaft that converts the Prime Mover/Driver power into rotary motion.
  • A Connecting Rod which converts the rotary motion of the Crankshaft into back and forth "reciprocating" action.
  • A Piston Rod (not labelled in the graphic) which is connected to the Connecting Rod and the Piston.
  • A Piston which is repeatedly extended into and contracted from the Cylinder.
  • A Crosshead which guides the Connecting Rod and allows this hardware to freely move outside of the Cylinder.

 

Who amongst the brilliant PTOA Readers and Students needs to be reminded that such a strong back-and-forth, reciprocating action would tear up any machinery that didn't have a sturdy foundation and Axial Thrust Bearings?

Stabilizing Bearings and Seals/Packing perform the crucial function of preventing leaks from developing within the "Power Side" of the Reciprocating-Action PD Compressor.  The heat generation that accompanies any gas compression plus the oxygen in air from a leak can combine with any source of fuel (e.g,. lube oil) to create an explosive environment.

Thanks to "PetroWiki" for allowing use of the below drawing which clearly labels the critical hardware associated with the "Power-Side" and "Pressure-Generating Side" of a Double-Acting Reciprocating-Action PD Compressor.

POWER SIDE OF COMPRESSOR: Crankshaft, Connecting Rod, and Crosshead on the left. Piston Rod in the Middle .PRESSURE GENERATING SIDE OF CMMPRESSOR: Piston and Cylinder on the Right Note the Packing used to prevent leakage into the housing as well as separating the process gas from the Power Side of the Compressor.

 

All Reciprocating-Action PD Compressors Look Alike.

This bank of Baker-Hughes Reciprocating-Action PD Compressors looks a lot like the nearby photo of the GE version that are not yet completely installed.

Unlike the Centrifugal Compressor, Reciprocating-Action PD Compressors do not have the option of Horizontally-Split and Radially-Split Casings.

The outcome of requiring a Piston-Cylinder architecture is that all Reciprocating-Action PD Compressors look the same.

 

 

 

 

These two GE Reciprocating-Action PD Compressors are not yet installed but still look a lot like the Baker Hughes Compressors nearby.

Many industrial Reciprocating-Action PD Compressors will be "double-acting" which was described in PTOA Segment #209.

A "Double-Acting" or "balanced" Reciprocating-Action PD Compressor has two Suction Valves and Discharge Valves.

The gas is continuously compressed and discharged with each expansion and contraction action of the Piston. Ergo, the "Double-Acting" Reciprocating-Action PD Compressor is more efficient because the cost of utilities used to move the Piston back-and-forth pressurizes gas in both directions.

Besides doubling the Capacity of the Compressor, the 'Double-Acting" architecture balances the Axial Thrust created by the moving Piston which results in less vibration and Bearing wear and tear.

In a "Double-Acting" Reciprocating PD Compressor, when gas is sucked into the Compressor on one side it is discharged from the compressor on the other side. "Double-Acting" architecture has more Capacity, is more efficient, and causes less wear and tear on the Compressor.

 

Reciprocating-Action PD Compressors are Less Expensive to Buy
But Are Less Reliable and Less Efficient to Operate 

The simple geometry of the Reciprocating-Action PD Compressor compared to the precision of manufacture needed to fabricate a Centrifugal Dynamic Compressor makes the capital investment of a Reciprocating-Action PD Compressor cheaper than a Centrifugal counterpart.

However, Centrifugal Compressors are more efficient to operate and  more reliably stay on line compared to Reciprocating-Action PD Compressors.

 

RECIPROCATING-ACTION PD COMPRESSORS ARE NOT CHOOSY
THEY WILL COMPRESS ANY KIND OF GAS

Recall that Dynamic Centrifugal Compressors are not suited to pressurize all types of gases. The Pressure Head of a Centrifugal Compressor is produced by increasing the gas's velocity which is subsequently converted into the PV Pressure. The Molecular Weight of the gas directly impacts the gas's velocity; naturally a heavy gas cannot move as fast as a light one.

Reciprocating-Action PD Compressors are not picky about how light or heavy the gas they are squishing is.

Hydrogen is in the upper right of this chart. Hydrogen has a Molecular Weight = 2 and is smaller than the other gases and alcohols shown..

Otherwise stated, they don't care what the Molecular Weight of the gas being processed, any gas is game for compressing!

Light gases … like hydrogen, the lightest of all gases … must be absolutely sealed off from entering the "Power Side" of the Compressor.

Heavy gases … gases with a high molecular weight … will impact the operating efficiency of the Reciprocating-Action PD Compressor.

 

CAPACITY CONSTRAINTS OF RECIPROCATING-ACTION PD COMPRESSORS

Brilliant PTOA Readers and Students will remember the comparison of Pressure-Head and Capacity for Centrifugal and  Positive-Displacement Rotating Equipment which was first featured in PTOA Segment #205.

The comparison of Pressure-Head vs Capacity clearly indicates that Reciprocating-Action Positive-Displacement Compressors generate much greater Pressure Head (aka PV Discharge Pressures) but are limited to a single Capacity.

The Capacity of a Reciprocating-Action PD Compressor is limited by:

  • The Cylinder Size
  • The Stroke Length. The Stroke Length is determined by the number of "throws" available, A "throw" is a location on the Crankcase where the Compressor's Cylinder can be attached.
  • The available Driver speed.

 

Hey, Fred! Doesn't the Capacity limitation of a Reciprocating-Action PD Compressor just make sense!

The amount of pressurized gas discharged from the Reciprocating-Action PD Compressor is determined by how many times per minutes the Driver can generate the rotational movement that the Crankshaft converts into the "reciprocating" back-and-forth action needed to repeatedly extend the Piston into the Cylinder Head thus displacing the gas and sending the pressurized gas into the Gas Discharge header.

 

Maximum Reciprocating-Action PD Compressor Capacity

So why not just build a Reciprocating-Action PD Compressor with an extremely large diameter Cylinder and Piston to attain the equivalent Capacity of a Dynamic Compressor?

The many equations with Latin letters used to explain the thermodynamic size limitations of Reciprocating-Action PD Compressors are boing, not enlightening.

Nor shall PTOA devote verbiage explaining why the Compression Ratio of a Reciprocating-Action PD Compressor is limited by the PV Discharge Temperature of the gas and the load on the Piston that the Compression Ratio creates. The bottom line is that the maximum designed Compression Ratio for a Reciprocating-Action PD Compressor is 4.0.

Instead, here is a Real World experience which illustrates why there are common sense size limitations for  Reciprocating-Action PD Compressors:

Once Upon a Time in a foreign land,  a new petrochemical process and complex were being started up.

A large bore diameter Reciprocating-Action PD Compressor was placed in service to boost the PV Pressure of hydrogen gas which contained five volume percent light hydrocarbons. The Compression Ratio of this Compressor was over 6.

The constant heaving of the Piston within the Cylinder stressed the Compressor's Bearings and weakened the Seals.

As any PTOA Reader or Student can recite in their sleep by now … the act of gas compression can easily result in attaining the PV Temperatures that permit the Combustion Reaction to occur.

Just a few months after being placed in service, the hydrogen-hydrocarbon gas leaked into the "Power-Side" of the large diameter Reciprocating-Action PD Compressor. .

The percussion wave of the explosion blew out some of the windows in the Compressor House. The windows in the Control Room 40 yards away rattled.

This H2 Booster Compressor blew up because of excessive vibration causing seal failure. The Piston and Cylinder Head were damaged and spares are not stored in the Warehouse. Several industrial processes were shutdown for weeks until the Compressor was repaired.

The Piston and Rotor had been blasted into the Cylinder Head and were bent beyond repair.

All downstream users that depended upon the hydrogen supplied by the blown up Booster Compressor went into Emergency Shutdown mode.

Depending upon a large bore Reciprocating-Action PD Compressor to circulate a large Capacity of hydrocarbon-contaminated hydrogen was a poor design decision that resulted in a catastrophic shutdown that lasted for weeks.

Fortunately the outcome of this catastrophic failure was just lost time and money. No Outside Process Operators, Mechanics, or Instrument Techs were in the building at the exact time of the explosion.

A train of multi-stage Centrifugal Compressor with intercooling can compress a large quantity of gas from 115 psia (799 kPa) to 2995 psia (19724 kPa). The capital investment in a Compressor train and a larger Compressor House would be greater but that is preferable to blowing up a PD Compressor.

A train of Centrifugal Compressors in series with intercooling could have safely provided the amount of high PV Pressure hydrogen needed for the process.

 

 

 

HALLELUJAH!  

PTOA Readers and Students  … including Fred … have crossed the PTOA Rotating Equipment Focus Study Finish Line!

Next Goal?

Finishing the PTOA PV Pressure Study Area with a focus on the detection and measurement of PV Pressure.

And after the PTOA PV Pressure Study Area ends, the PTOA PV Flowrate Study Area begins!

No rest for the weary! ONWARD!

 

TAKE HOME MESSAGES: The two types of Positive-Displacement Compressors are Reciprocating-Action and Rotary-Motion.  PTOA Readers and Students already possess sufficient knowledge regarding the form and function of Rotary-Motion PD Compressors from previous PTOA Segments, thus the content will not be repeated.

The final content in the PTOA Rotating Equipment Focus Study Area is the Reciprocating-Action PD Compressors. PTOA Readers and Students were already aware that: Reciprocating-Action PD Compressors:

  • Cause the PV Temperature of the gas to increase upon being compressed. .
  • Are best used when a single, low Capacity of High PV Pressure gas is needed for the Process, for example boosting the PV Pressure of a gas so that it can be distributed to other users..

 

PTOA Readers and Students learned the following about Reciprocating-Action PD Compressors:

Critical "Pressure-Generating Side" Hardware includes:

  • Suction Side Snubber
  • Inlet Check Valve and Suction Valve
  • Cylinder (Head Volume) and Piston
  • Discharge Valve and Check Valve
  • Discharge Snubber

 

Critical "Power-Side" Hardware includes:

  • Crankshaft
  • Connecting Rod
  • Piston Rod
  • Piston
  • Crosshead
  • Seals and Bearings

 

Capacity is limited by:

  • Cylinder Size
  • Stroke Length
  • Driver Speed

 

Capacity is also limited by:

  • Discharge Temperature
  • Max Compression Ratio of 4

 

Reciprocating-Action PD Compressors can process light and heavy gases and are cheaper to purchase but less efficient and reliable to operate over their lifetime compared to Centrifugal Compressors.

©2021 PTOA Segment 0225

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

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