QUESTIONS AND ANSWERS ABOUT COMPRESSOR VALVES

Q: What is a compressor valve?
A: A compressor valve is a device that controls the inward flow of lower pressure and the outward flow of higher pressure gas from a reciprocating compressor cylinder. Normally these valves open and close automatically, solely governed by the pressure differential in the cylinder compared to the intake or exhaust line pressure.

Q: What are the basic components of a compressor valve?
A: Most valves have five basic components:

1. Seat
2. Guard (guard, stop plate, buffer, plate, etc.)
3. Sealing element (valve plate or valve ring, channel, poppet, feather strip, ball, etc.)
4. Damping element (coil springs, cushion plates, spring plates, damping plates, etc.)
5. Assembly element (bolts, nuts, retainer ring, etc.)

Q: Are there different kinds of compressor valves?
A: There are several different kinds of compressor valves: plate valves, ring valves, channel valves, feather valves, poppet valves, ball valves, reed and concentric valves, to name just a few. Each design has specific criteria with regard to the sealing element and all the other components are designed accordingly.

Q: Is there any such thing as an "all purpose valve"?
A: Not really. Each valve is designed for a certain application and is best used in the range of operating conditions for which it was designed.

Q: What does a compressor valve do?
A: A compressor valve regulates the flow of air or gas in a compression cylinder There is at least one suction valve and one discharge valve for every compression chamber. Each valve opens and closes with every cycle of the piston. To demonstrate this further, a valve used in a compressor operating at 1200 rpm for 12 hours a day and 280 days a year, opens and closes 72,000 times per hour or 864,000 times per 12 hours in a day or 241,920,000 times per year.

Q: What must a compressor valve do?
A: There are essentially two musts, (a) the valve must be efficient, and (b) the valve must be durable and quiet in service. One can of course refine the above criteria and include both the aerodynamic flow efficiency and the volumetric efficiency. Under durability, one can include maintenance free operation for over several thousand hours plus relative ease in servicing and repair

Q: How can I compare two valves of equal size, but different design?
A: By going back to the basic requirements a valve should fill, and comparing the efficiency and durability of both valves. Price should not always be of highest priority. Remember, what may look like a bargain may cost much more to operate.

Q: Can such a comparison be made without running an actual test and total instrumentation of the compression cylinder?
A: There are two phases in analyzing valves:

1. Comparing design criteria and data without actual tests. This can go as far as computer simulated performance diagrams, based on experimental data gathered in actual tests.
2. Installation of valves in the cylinder and complete analysis of the results combined with a subsequent life test

Q: I am interested in pre-analysis How do I go about it?
A: There are two objectives efficiency and durability.
First let's talk about efficiency. The aerodynamic flow efficiency depends greatly on the restrictions in the valve. The gas has to pass through the seat (seat area) then pass around the fully opened valve plate (free lift area) and finally escape through the guard. From two competitive valve designs each area must be analyzed and compared. On most valves the minimum flow area is the free lift area and this is the area most frequently compared. It takes into account that the valve is fully open
The general rules are the higher the area the more efficient the valve. If a more meaningful comparison is to be made, one must compare the free lift area of two valves at equal lift. A higher valve lift may mean higher area, but also means less durability.
The flow resistance is further influenced by other factors such as surface friction, directional changes in the gas flow, turbulence within the valve, springload, etc. Seats with cast slots are not as good as milled slots tapered and properly deburred; slotted seats in general are better than seats with drilled holes, particularly if the seats are made with a few drilled holes of large diameter. Hanging or open guards are often better than safety guards with respect to aerodynamic flow. Most valve manufacturers will supply design information such as valve lift, free lift area, and seat and guard area for comparison. In some instances, theoretically calculated valve losses can be quoted. This will come pretty close to the actual readings from a test.

Q: What is the equivalent area of a valve?
A: Another parameter for valve performance is equivalent area; each valve of a defined geometry has a respective equivalent area. The equivalent area is the orifice area in square inches of a valve which also has a given pressure loss. By comparing the stated equivalent area of two valves one can establish the potential performance of each valve; the higher the equivalent area, the better the flow efficiency.

Q: Are there other criteria to efficiency?
A: Yes. The volumetric efficiency of a compression cylinder may be influenced by a valve conversion. It is affected by changes in the clearance volume, which is that volume in one cylinder end which is not swept by the movement of the piston. Depending on the size of the cylinder it can range from 2% to as high as 30% of displacement, and sometimes more. It includes spaces between piston and head at the end of the stroke, space under the valves, and of course clearance within the piston side of the valve itself. The rule is that the higher the clearance the poorer the volumetric efficiency of the cylinder.

Q: What do you mean by valve dynamics?
A: Every compressor valve has to open and close with every compression cycle. The timing and pattern of the opening and closing events are referred to as valve dynamics. It is important that the valve opens and closes at the right time and doesn't flutter. Compressor valve dynamics are important since they influence valve life and compression efficiency. The valve dynamics can be influenced through proper springing and/or the mass of the moving components. For proper performance valves must be designed for the specific operating window.

Q: Aren't there some cylinders with built-in extra clearance?
A: Yes. There are cases where the output is controlled by purposely adding clearance to a cylinder. In low compression ratios the volumetric efficiency is of minor influence

Q: You mention compression ratio. What is it?
A: Compression ratio is the ratio of absolute discharge pressure to absolute inlet pressure.

Q: So there are ways to pre-analyze the valve efficiency?
A: Yes. By going about it through engineering analysis, much information can be predetermined or at least anticipated. A person who does not exercise pre-analysis and buys on price or availability only, is buying a pig in a poke, and cannot claim to have done a complete valve analysis of two seemingly equal products.

Q: We talked about efficiency. How about durability?
A: Although it is difficult (and in some cases downright impossible) to predict how long a valve will work, there are certain criteria that will increase or decrease the probability of lasting valve Performance. Here are some:

• The lower the rpm. the longer the life.
• The lower the valve lift the longer the life. Simple sealing elements (plates, rings) tend to have a higher fatigue resistance than parts of relatively intricate shape. Good mechanical cushioning and damping elements tend to minimize impact forces and definitely increase life. Air cushioned valves are not a mechanically controlled damping system. They depend solely on maintaining correct production tolerances, both in manufacture and after repair. Today many valves utilize plastic sealing elements. Glass-fiber filled composite materials, such as nylon and PEEK, are more frequently used than steel plates or rings. These new materials have proved very effective in valve applications. Composites can work better in difficult services enduring contaminants and liquids in the gas stream. Non-metallic sealing elements tend to leak more than steel plates when new. This condition typically corrects itself once the compressor is running for several hours and the valve has "worn in". Remember the number of cycles a valve lives through; each cycle means one opening and one closing impact. The sealing element will eventually beat itself to pieces.

Q: It seems that the most critical part is the sealing element, the part that moves constantly and has to survive through all this abuse?
A: Yes, it is certainly the vital part. An experienced engineer-by analyzing the part with regard to design, material and production process-can often separate the good sealing elements from the poor.

Q: If a valve breaks prematurely, is it always due to improper design or inadequate production?
A: No, a compressor valve is an integral part of a larger piece of equipment. Often problems show up in the valve that are caused by outside factors. Carryover of foreign material can cause valve failure. Pulsations in the system can sometimes cause valve flutter. Changes in operational use from original design conditions are often the cause of frequent breakage. Improper valve repair and maintenance also cause short life. The list of possibilities is long.

Q: Does an operator have to live with such "iffy" conditions?
A: No, one should never become resigned to poor performance. Try to analyze your problem. Ask industry specialists for advice and consultation. Remember: for every problem there is normally a solution

Compressor valves in reciprocating compressors significantly affect the operating performance, efficiency and life of the machine. Installed in the direct gas or air stream, and subjected to much abuse in doing their everyday job, it's remarkable - not that we have valve troubles, but that we have as few as we do.

A valve opens and closes with every stroke of the piston. In some cases this means they hang in there in excess of 500 million cycles a year. The valve is frequently subjected to entrained liquids, foreign particles, corrosive gases or solids to destructive forces such as tension, compression, impact, twisting, bending, abrasion, of cold. When erosion to extremes of heat or any these cause the valve to malfunction, the compressor cannot do its job and must be brought down. So whatever improves or prolongs the operation of the valve is meaningful to machine operation and profitability

Valve Failure Analysis

When we undertake valve failure analysis, we investigate effects to determine causes, Effects are usually excessive wear, fatigue, fracture, or a combination of these. They fall into two groups-environmental, and due to abnormal mechanical action.

Environmental Effects
Under this term we combine influences on valve life and performance derived from the gas itself.

These are:

• corrosive elements
• foreign particles
• liquid entrapments and carryover
• improper (excessive or inadequate) lubrication
• formation of carbon or other deposits

Abnormal Mechanical Action
This term covers effects that substantially alter the normal opening and closing motion of the valve. They include:

• valve flutter
• slamming from delayed closings or other pulsations
• multiple impacting from excess pulsations

Needed for proper diagnosis of problems is solid knowledge of how a valve works, which components move or compress in the opening/closing cycles of a given design, where it has to seal when closed to prevent gas leakage, and which surfaces are subject to friction or impact. Examination of a worn or broken valve will often disclose the reasons for premature breakage.

Keep Records
It helps if you keep records on the frequency and nature of failures you experience with compressor valves. Tag the valve, with the following info on the tag:

• Date of failure
• Unit number
• Cylinder
• Location & valve type
• What failed
• Suspected reason
• Estimated hours of operation

Often, across a number of failures, a pattern emerges which will suggest proper corrective action.

Inspection

Inspection should be done in a well-lighted work area, where assembly vises can be used to carefully dismantle the valve. Take care to keep the parts of each valve together and don't wipe or otherwise clean the component parts, or you may "erase" valuable clues.

Try to relate wear of the valves to its hours of operation, for proper perspective. Compressor valves are expected to operate from one turn around to the next which may mean as long as a year. But in highly contaminated environments even 2,000 hours of uninterrupted performance may be acceptable.

Diagnostic Analysis

Once you've determined that performance is inadequate, wear excessive, or downtime pre- mature, the analytical search from effect back to cause can be undertaken. This is essentially a process of elimination. Look first for obvious reasons for failure:

Improper Assembly
Was the valve assembled right? Did the components meet manufacturer specs, assuring that the valve could function properly at an acceptable life and efficiency level?

Repair Frequency or Error
Is the valve performing below par due to frequent repair? Improper remachining? Wrong assembly? Reassembly with poor quality parts?

To repair a valve to rebuild specifications, it pays to consult with the manufacturer or an authorized repair facility. Such things as improper remachining of a seat face (failure to provide proper spacing of recesses between seat lands) failure to remove burrs after such rework installation of the wrong coil or leaf spring elements incorrect depth of pockets in the guards, etc. can affect valve life adversely.

Coil springs are an important component of a valve. In addition to providing cushioning on the opening impact, they control valve timing. Springs should not be tampered with, or replaced with inferior quality products.

Environmental Effects

Corrosive Elements
It will probably show up on the valve if the gas has a substantial amount of corrosive contaminants. Even small amounts, though they will not rust away the valve, can cause stress corrosion and lead to such damage as breakage of the sealing element (valve ring and plate). Certain compounds become corrosive only if moisture is present in the system, or develops after shutdown. This moisture, in combination with the contaminates in the gas, can corrode valves. A gas sample should reveal whether any of these factors are at work.

Coping with corrosiveness may require upgrading of materials in valve plate or ring, or in severe cases the material of all valve components. Since this involves a major expenditure, be sure it's what you need. Metallurgical labs today can analyze broken components as long as the zone around the breakage is not too heavily scored,

Although hydrogen is not corrosive by itself, it can cause embrittlement due to molecular penetration of the metal - at least the top layer The ensuing cracks lead to deterioration and subsequent fracture. Hydrogen embrittlement can be prevented by changing the material. Consult NACE specifications as a guide to material selection.

Foreign Material and Impurities in Gas
Despite proper scrubbing, "aliens" can get wedged in the valve and prevent its proper operation, causing damage. Examine the seat lands and impact surfaces of the valve plate for traces of them. Minor indentations and imprints of the particles may show up between the valve plate and seat, too. Make certain separators, knockout pots and drains are working right and are sized to handle any impurities from upstream.

Liquid Carryover
Liquid slugs can have a devastating effect on valves. The plate will be subjected to extremely destructive forces and will crack. Slugs occur when entrapments get carried through. They're formed when saturated gas contacts the cylinder wall.

Prevention involves raising the cooling water temperature 10 or 15 degrees above the incoming gas temperature, as the water enters the cylinder jackets.

If liquids are coming from the upstream feeding separators can be checked and much of those liquids eliminated.

Improper Lubrication
Valve life is shortened by excessive lubrication particularly of suction valves. It can cause sticking of the valve plate, which delays reseating of the plate. A delayed closing normally results in excessive slamming forces.

Excess lubrication acts like a liquid carryover, and can cause the same slugs as water contamination.

Too much lubrication of the discharge valve, especially if it is exposed to higher temperatures, often causes coking. A carbon buildup will form on the valve surfaces and cause potential failure of the valve.

Mineral oils (especially with high ash content) will coke up more readily than synthetic oils. Synthetics have a higher resistance and greater lubricating effect, and should be used in smaller amounts than mineral oils, When switching to synthetics, strip down the compressor and clean all surfaces exposed to gas. Otherwise the lubricant will free all buildup of coke or contamination and convey it down the system, with resultant valve failure and possible piston ring and cylinder liner damage.

Abnormal Mechanical Action in Detail

Well-designed valves, with the proper spring load for a given application, will do a good job. Their opening/closing motion will be such that no harmful pulsations will occur However, many valves are standardized for "average conditions of operation:' and when applied outside this range may malfunction

The right springload for a valve depends, among other factors, on its operating pressure, the gas velocity, and the specific gravity of the gas. It substantial changes from original design parameters must be made, contact the manufacturer and have the valve springs re-engineered.

Detecting Abnormal Mechanical Action
It's difficult to detect, but carefully analyze the surfaces where the sealing elements impact against either the seat, upon closing, or the guard or stop plate upon opening. If these surfaces show wear related to impact (a hammered finish), assume that flutter or multiple impact is the problem.

Abnormal mechanical action can also result from pulsations in the gas stream. These may be caused by improper pipe selection or manifold sizing but, with the design technology of major OEMs today, this is rarely the case.

The flow of gas to and from the valve is channeled around cages, through cylinder openings and cavities under and above the valve. The uneven distribution of gas flow can cause the valve to wobble during the opening and closing motion. Uneven flow can also cause multi-ring valves to open unsynchronized - one ring opening first and taking the most severe impact, and the others following with lesser impacts against the stop plate. Such a pattern will cause one ring of the set to fracture more frequently than others.

Computer simulation may be used to analyze the valve opening/closing motion, but provides only a rough indication of damage potential in a given application. Performance analyzers, vibration detectors and ultrasonic leakage detectors may be used for in-the-field diagnosis. These tools are used to detect abnormal mechanical action or leakage through compressor valves or piston rings. When analyzing pressure traces consideration should be given to the possibility of distortion caused by channel resonance.

Corrective Steps
Environmental problems should be minimized by employing proper separators so that the gas is pure, if possible. Corrosive environments, if they cannot be avoided, may be dealt with through proper material selection as discussed above.

Liquid entrapment is in the same category as foreign contaminants. Excess lubrication can be controlled by following manufacturers' recommendations for lubricating each cylinder too little is better than too much.

Help from the Vendor
Dealing with abnormal mechanical action frequently requires assistance from the vendor- using your operating conditions to check for correct springing and to analyze valve motion - can often reveal the need for corrective action. Changing springs may cure problems. Or the mass of the moving component can be changed e.g. by switching from steel to a non- metallic material to change valve pulsation.

A thorough examination may be required, involving full analyzer instrumentation and actual readings of a given cylinder. This type diagnosis is expensive, and should be resorted to only when all else fails.

Efficiency Losses
Efficiency losses are not as frequently considered, as is mechanical failure, but are at least as important. Efficiency of compression is closely related to valve performance. Wear especially when excessive and abnormal mechanical action can greatly affect compression efficiency The slowly increasing loss of horsepower in versus horsepower out can become very costly before it is recognized,

A valve's efficiency must be analyzed over its total life cycle. If by design or through abuse it develops a deteriorating wear pattern, leakage and flow losses will steadily increase. Minor deterioration cannot be detected by measuring the amount of gas pumped, or operating parameters such as temperature, etc.

Valve manufacturers today can provide anticipated pressure drops across their valves, and valve losses, for a specific application. These theoretical figures can then be compared to actual operating conditions to determine whether you are getting optimum performance.

Remember that efficiency and life are trade-off. If valve lift is reduced to rectify a mechanical failure, the free lift area is also reduced, gas velocity through the valve increased, and more power consumed. Lower lift and the resultant decrease in impact forces will extend the life cycle of a valve, but at the price of increased operating losses through lower efficiency.

Ascertain the real reason for mechanical or efficiency failures before trying to cure them.

Preventive Maintenance

Preventive maintenance programs will minimize equipment failures and unscheduled downtime. Equipment failures frequently occur during non- working hours, with resultant overtime expenses, delays, material shortages, patch type repairs, and maintenance being performed only on failed equipment. This, of course, contributes to excessive loss of production time, as well as excessive repair costs.

Key Steps in P.M.

1. Lubrication and simple equipment inspection procedures (performed while equipment is in operation)
2. Lubrication changes, parts changes, and detailed equipment maintenance inspection (performed when unit is shut down for overhaul)
3. Proper scheduling of personnel and material to perform steps 1 and 2.

To bring down a compressor can be expensive not only in downtime, but in loss of gas which
often has to be pumped out. A good preventive maintenance program, based on realistic life cycles, will result in minimum losses in production and maximum maintenance efficiency