Reciprocating Compressors

Reciprocating compressors are commonly known as recips or piston compressors. These are positive displacement compressors with a long history of dependable service. And, their ability to operate under partial load makes them very energy efficient.

The 2 stage recip is the most common design. This style of compressor has been made in a variety of shapes and sizes, by several manufacturers for decades. The shapes may be different, but the fundamental operations are the same.

Trouble Shooting

Compressor trouble shooting is the act of identifying the cause of a failure or the source of a problem. The accuracy and speed of the diagnosis will depend on the understanding of the various interdependent systems on a compressor, and knowing how to use the available signals.

Trouble shooting skills help you start at the right place, rather than changing parts to solve the problem through a process of elimination. And, a good trouble shooter will solve many problems before they morph into catastrophic failures

Our Trouble Shooting Guide was developed to improve and speed up the trouble shooting process. It provides information on the primary systems, explanations for key components and tips for trouble shooting the most common problems.

You can use this as a foundation for improving the reliability of the compressors under your watch.

To order the Trouble Shooting Guide for just $59 <click here> and fill out a short order form. In just a few minutes, you will be given a confirmation that your order has been entered and will be shipped within 3 days. The 31 page guide is delivered via email, as a 1.7 MB pdf file.

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The following tips and suggestions have helped Engineers and Maintenance Personnel extend the service life and lower the maintenance expense of reciprocating compressors larger than 100 horsepower.

Packing Rings

Packing rings provide an important, and often overlooked, function in a reciprocating compressor. The rings seal the pressure in the cylinder during compression.

These sealing rings have evolved over the years, from simple tar rope to sophisticated, mechanical devices. And, new materials have opened the door for innovation and problem solving in the most demanding applications.

Packing rings that are worn will waste energy by leaking. And, this can quickly lead to expensive damage to your piston rods and packing cases. 

Our research continues to find packing rings installed wrong. The only conclusion is that repair technicians are no longer being trained on the procedures for installing and maintaining packing rings. 

We have organized some of the forgotten procedures into training tips that will improve the performance and service life of your packing rings. Send an email to rings@compressorwise.com for a copy of our report on pressure packing rings.

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Cylinder Repair

Years of service or harsh operating conditions can wear the cylinder bore to an oversized or out-of-round condition.  A compressor in this condition will not produce as designed, and will require special attention to deal with the added clearance.

There are two common methods of restoring the cylinder bore to its original diameter – metal spray and relining. 

The metal spray approach usually has a lower initial price.  However, the relining option has proven to cost less, because it lasts longer.  

Our observations, and the feedback from compressor operators with first hand experience, indicates that the metal sprayed on the bore will peel and flake off long before you will have problems with a relined cylinder.  This is especially true when a compressor uses a multi step regulator to operate under various load conditions.

The relining process is a mechanical repair.  The cylinder bore is machined to an oversized diameter to accept a liner.  Then, a custom made liner is fitted into the cylinder to return the bore to original size. 

The liner material depends on the application.  However, a majority of cylinders use a centrifugally cast liner because of its ability to provide the correct heat transfer away from the piston.  It also adds strength to the mechanical repair for a long lasting solution.

The best installation process will use liquid nitrogen to shrink the liner enough to slide into place.  The alternative is to heat the cylinder, but this creates a  potential for material distortion in the cylinder casting.

The cleaning process is also important.  You will get the best results by using a chemical treatment because many cylinders have internal passages that will trap solid particles.

The chemical cleaning process will take a little longer than the sand blasting approach that is used by many vendors.  However, it eliminates the extra time spent trying to clean out the blasting agent and the solid debris.  And, it is always best to avoid the potential of solid contaminants contacting to the wear parts of a cylinder.

Send an email to cylinder@compressorwise.com if you would like information on a cylinder repair vendor in your area.

Would you like more information on how to keep your reciprocating compressor cylinders in top shape?  “Cylinder Repair Tips” is our special report on this subject and is exclusively available to subscribers of our newsletter. 

You can scroll down this page or use the Free Compressor Newsletter button to enter your subscription.  Then, after you receive your Introductory Issue, simply request Issue 2 from Year 2003.

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Compressor Valves

The valves in a reciprocating compressor serve as check valves for the inlet and discharge passages of the cylinder, and will open and close once for every revolution of the crankshaft.  If the compressor is running at 500 rpm for 24 hours a day, the valves will open and close 720,000 times every day!

Valves have to perform with minimum resistance or power will be wasted, and components will wear at accelerated rates.  The time and effort you spend on improving valve life will reduce the costs associated with loss production, unscheduled repair work and other downtime expenses.

Here are some ideas for extending the service life of your valves:

Internal Specifications

The internal dimensions regulate the lift and flow area of the valve.  These specifications, when maintained, make it possible for the compressor to perform according to design standards.

You should have a specification sheet for each valve.  If you do not, use your influence with the manufacturer and persuade them to furnish this information. Remember, tolerances are as important as the dimensions which means copying the dimensions off an existing valve will not tell the whole story.

It is possible to run reciprocating compressors with valves that are out of tolerance, and it happens more than you think.  It is just not the best plan for getting the most from your compressor.

Synthetic Lubricant

A lubricated compressor will have oil pumped into the cylinders to handle the friction between the metal moving parts.  The traditional choice of lubrication is an inexpensive, petroleum based oil. 

An upgrade to a synthetic lubricant will extend valve service life.  Synthetic lubricants cost more than petroleum oils, but you get a lot more for your money.

A synthetic lubricant eliminates the carbon and varnish build up on the wear parts in the cylinder, especially the valves.  And, it will also provide greater safety from the hazards of flash fires in the discharge piping.

There are obvious advantages to making the switch, but be aware that the solvent additives in synthetics are cleaning agents. The oil carryover can begin to clean the years of petroleum oil deposits in your compressed air system piping.

It is possible that a dark, sticky residue will form in the piping and find its way into filters, regulators and other equipment located downstream from the compressor.  This is a short term maintenance concern that should be explained to everyone involved with the air system.

There are several types of synthetic compressor oil on the market.  Consult with someone you trust for help in choosing the right lubricant for your application.

Another idea is to install individual filters on each line carrying lubrication to the cylinders.  This keeps any solid contamination that may enter the lubrication box from entering the cylinders.  A sintered bronze filter is available for less than $100 US for this application. 

Send an email to lube@compressorwise.com if you want our help finding a vendor to trust for synthetic oil or lube line filters.

Monitor Inlet Air

Inlet filter maintenance is a low priority in today’s world of smaller maintenance departments, especially if the filter housings are not easily assessable (i.e. on the roof).  However, dirt and other airborne particles cause damage and reduce valve life.

The new compressors being sold today are typically furnished with a device to monitor filter life.  If your compressor did not come with such a device, you can make a simple modification, and take the guesswork out of filter maintenance.

The best way to determine when to change your inlet filter is to measure the restriction in the piping between the filter and the intake of the compressor.  This restriction can be monitored by a water manometer, an intake filter indicator or a dial gauge that is calibrated in inches of water.   

The idea is to service the filter when the monitor reaches a certain level of restriction.  This specification can vary between 10 and 20 inches of water because of the different designs of filters.  We recommend that you get a guideline from a filter vendor that has experience with your compressor.

Cooling

Overcooling a water cooled cylinder can cause condensation to form in the compression chamber.  This liquid will damage valve parts.  The problem is worse for lubricated cylinders, because the condensation can wash away the oil.

Check your owner's manual for a recommended water outlet temperature.  The temperature setting may need to be adjusted for the seasons or to compensate for deposits formed inside the cylinder water jackets

Installation

Valves must be correctly installed and seated in the valve pockets.  And, the proper torque should be applied to secure the valve.  Check your owner’s manual for a recommended torque value.

The valves with straight ports (strip, feather and channel) must be installed with the ports of the valve parallel to the piston rod.  This allows the valve to open evenly, and avoids the erratic wear caused by the rolling action of the internal sealing part.

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Contact Us

Send an email to recips@compressorwise.com if you have questions about any aspect of a reciprocating compressor or if you would like a recommendation for a vendor in your area.

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