Improving Reciprocating Compressor Valve Life

Author : Dan Wise
Compressorwise

INTRODUCTION

The purpose of the valves in a reciprocating compressor is to let air in and out of the cylinder during the compression process. These valves are considered automatic because they are operated by the difference in pressure across the valves. The key to long valve life is to have positive action of the sealing components with minimum resistance.

Compressor valves serves 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. All of this is why valve maintenance should be considered a top priority.

Time and effort spent on improving valve service life will reduce the costs associated with downtime such as loss production and compressor rental. Longer service life means you will not have to buy replacement parts as often. And if done correctly, you can practically eliminate the need to replace the expensive valve bodies.

The following are some tips and suggestions for improving valve service life. These ideas are from mechanics with experience dealing with compressor maintenance.

Internal Valve Specifications

The starting point of valve maintenance is to make sure that all existing valves are operating within original specifications. The internal valve dimensions and tolerances establish the lift and flow area of the valve. These, when maintained, make it possible for the compressor to perform according to design standards.

The people responsible for your valve maintenance, either in-house or at an outside vendor, should have a specification sheet for each valve. If you do not have internal dimensions and tolerances for your valves, 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.

The nature of a reciprocating compressor can lull you into a false sense of security by functioning with valves out of tolerance. The reality is that the components are slowly building to failure and downtime with an unknown degree of severity.

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

Making the switch to a synthetic lubricant.

If you operate a lubricated style of compressor, you should consider upgrading the cylinder lubricant from petroleum oil to a diester based synthetic. This move has proven very effective at reducing valve maintenance.

The diester lubricant eliminates the carbon and varnish build up on the wear parts, especially the valves. In addition, this type of oil is specially blended to provide stabile viscosity, resist oxidation and reduce friction.

The combination of these factors make it possible to increase valve service life. The diester lubricant will also provide greater safety from the hazards of flash fires in the discharge piping.

There are many advantages but you should be aware that a change to synthetic might increase your maintenance work in the short term. This is because the solvent additives in synthetics are cleaning agents. The oil carryover will begin to clean the years of petroleum oil deposits in your compressed air system piping.

Some customers have reported that a dark, sticky residue formulated in the piping and found its way into every filter, regulator, etc. downstream from the compressor. This is a short term maintenance concern that should be explained to everyone involved with the air system.

Synthetic lubricants cost more than petroleum oils but you get a lot more for your money.

Cleaning up the oil going to your cylinders.

If you operate a lubricated style of compressor, you will want to avoid supplying dirty oil to the cylinders. The lubricator boxes used on most industrial compressors will hold between ½ gallon and 2 gallons. This small reservoir must be checked and filled often.

The process of refilling the box often leaves the opportunity for particles to find their way into the lubrication. It is common, for instance, to find the fill cover was left off after the last refilling or has been lost altogether. Also, the fill process is usually to pump oil from a 55 gallon barrel into a small container which accumulates particles of dust between uses.

The end result is an accumulation of sludge at the bottom of the lubricator box. This is agitated by the lube box cam shaft and leads to dirty oil being injected into the cylinders.

There are a few things you can do to improve this situation. The obvious strategy is to clean up the process of filling the lubricator and to make sure all lube boxes are cleaned out on a regular basis.

However, it might be easier to eliminate some of the maintenance rather than add more to your crowded work schedule. You can accomplish this by installing an auxiliary lube reservoir.

These extra reservoirs are available in 5, 15, 30 and 55 gallon sizes and connect directly to your existing box. The are easy to install and reasonably priced from about $350.00 US for the 5 gallon unit to just under $500.00 US for the 55 gallon unit.

This type of auxiliary lube supply system is furnished with a stand, gauge, float valve, hose kit and a tank with a sight to monitor the oil supply. The float valve mounts to an open port on the lubricator. It automates the gravity flow of oil from the tank.

Another idea is to install individual filters on each line carrying lubrication to the cylinders. There is a sintered bronze filter that sells for about $60.00 US that has been successfully used in this application for many years.

The lube line filters and the above mentioned auxiliary lube reservoir systems are available from Direct Supply. You can reach them at lube@compressorwise.com if you have any questions or would like a quotation.

An easy way to monitor your inlet air filter.

Dirt and other airborne particles will cause damage and reduce valve life. The solution is to furnish the compressor with clean inlet air and to stay on top of filter maintenance. This is "easier said than done", especially for the older stationary compressors in today's world of smaller maintenance departments.

The new compressors being sold today are supplied with a variety of devices 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.

Compressor operators have many options available to monitor inlet filter efficiency. There are 2 very simple types of indicators being used by industrial customers. They are both easily installed into the inlet piping, about 24" about the connection to the compressor inlet flange.

The first type is called a manometer which monitors differential pressure across the filter. A manometer should show between 0 and 1 psi with a new filter. A rule of thumb is to replace the filter when it reads 10 psi.

You can buy manometers, for under $60.00 US, from any industrial supply company, such as Grainger. If you call them at 1-800-487-3279 or at 1-847-535-1000, they can put you in touch with their nearest office. They can also help you pick the right product for your application.

Another product that can be used to monitor inlet filters is called a restriction indicator. This measures the restriction on the filter and provides a signal to indicate the filter condition. The indicator is green when the filter is new or still has service life. However, if the indicator turns red, the filter is contaminated and needs to be replaced.

Restriction indicators are available for under $40.00 US from several sources. One manufacturer is Donaldson Company in Minneapolis, MN. You can reach them at 1-800-374-1374 or 1-612-887-3131 and they will put you in touch with the distributor in your area. They can also help you size an indicator for your specific application.

Other tips

Monitor the temperature of the water cooling the cylinders to avoid condensation forming in the cylinders. A visual inspection rarely detects this unless there has been significant liquids and a chance for rust to form.

The condensate will damage the valve parts. The problem is worse in lubricated cylinders because the condensate will wash away the oil.

The temperature setting may need to be adjusted for the seasons or to compensate for deposits formed inside the cylinder water jackets. Check your owner's manual for their recommended water outlet temperature. A rule of thumb is to keep this temperature between 100 degrees F and 130 degrees F.

Do you operate valves with straight ports, including strip, feather and channel valves? If so, they need to be installed with the ports parallel to the piston rod. This lets the valve open evenly and avoids the erratic wear caused by the rolling action of the internal sealing parts.

The final point is to make sure you can trust your supplier of valve parts. The best choice is any company, not necessarily the OEM, that has control of the manufacturing and quality control processes.

Ask for a tour of their manufacturing plant or to see their quality control manual. Spending the time to find a company committed to providing high quality valve parts can have a dramatic impact on your maintenance program.