|
|
|
|
|
|
|
|
| Home | Exhaust
Valve Exhaustion on Compressors
Most producers use compressors of one kind or another to help get the gas pumped up from the wellhead pressure to the pressure preferred by their friendly local midstream operator or pipeline company. Although there are many different designs of compressor out there, such as the screw, centrifugal vane etc., by far the more common type is the reciprocal unit. Reciprocal units use either a gas or an electric driver to turn a compressor. The compressor frame consists of a number of cylinders. These cylinders use suction valves, which open to let low-pressure gas in, and then discharge valves to let out the high-pressure gas. Some of these compressors can be as small as a motorcycle, or as large as a 2 story house. The smallest part inside these, and yet one of the most crucial to the compressor’s performance is the valve. This valve is usually a simple device that automatically opens and closes based simply on a pressure differential on either side. Reciprocal units consist of cylinders and pistons, much like a car engine. As the cylinder moves back and forth (i.e. reciprocate) they compress gas as the piston moves up in the cylinder. In the process, a lot of gas is forced to occupy a little bit of space and thus compression happens. If you know a little bit about cars, forget everything now. There are no push rods, lifters, valve guides or valves with stems, as you know them from high school shop, or your "Idiot’s Guide to Home Car Molestation". Some great expertise in gas compressor valves comes from the folks at Hoerbiger Corporation of America. These valves are typically a simpler arrangement than those seen in cars, with names such as plate valves, ring valves, channel valves, feather valves, poppet valves, ball valves, reed and concentric valves and more. Each of these valve types is designed with a specific application in mind, and each serves a different purpose. All compressor valves have two common performance criteria: A) they must be efficient, and B) they must be durable and quiet in service. Efficiency of a compressor relates to the volumetric efficiency of the cylinder. This "volumetric efficiency" just relates simply to how much of a volume the piston moves during its cycle versus the amount of volume it actually pushes out the discharge valve. The two volumes can be different because there is a volume in the cylinder that is not swept by the piston. This can range from 3 to 30% of the total displaced volume. As a rule; the higher the unswept volume, the lower the efficiency. How does this relate to valves? Well, every valve has to open and close with every compression cycle. Hoerbiger says that timing of this opening and closing cycle, a.k.a. "valve dynamics" is what influences valve life as well efficiency. Valve dynamics can be altered by playing with the springing as well as the mass of the moving components within the valve. It’s really quite simple. The higher the mass of the moving parts, the more work it will take to get that mass to move, and vice versa. In turn, the stiffer the spring, the more work it will take to get it to move and vice versa. Valves generally do an amazing job within a compressor. It is not uncommon for a valve to go through 500 million cycles a year. Given that in the process, the valve is subjected to elements like solids and liquids as well as corrosive substances in combination with great fluctuations in temperature as well as twisting and abrasion, it is amazing they stand up at all. Malfunction is unavoidable in the end. The ultimate end of any valve, if left to run indefinitely is excessive wear. However, fatigue and breakage are often found in valves that prematurely fail. Hoerbiger finds that failures can be broken down into two major categories. The first is environmental, such as failures caused by corrosion, deposits, liquid entrapment, particulates, or even improper lubrication. The second is abnormal mechanical operation. The environmental failures such as corrosion can range in causes from H2S to corrosion due to liquids to embrittlement due to Hydrogen penetration of the metal. Failures due to impurities that then deposit themselves on moving parts can also be found. Liquids can also cause failures when they "hammer" their way through a compressor valve, rendering an interesting pretzel-like shape if anything can be found at all. As with many things in life, improper lubrication can be a factor also. This occurs often on over-lubricated suction valves according to Hoerbiger, because it can cause the valve to stick, thus delaying closure. This means that when it does close the valve gets slammed hard upon closure with eventual resulting damage. Discharge valves, which often see higher temperatures, can sometimes have excess lubrication turn into a carbon buildup, which can coke up the valve. Lubrication with mineral oils tends to cause more coking than synthetic oils. Synthetics resist coking more and lubricate better. Note: Hoerbiger recommends against switching over to synthetics without prior removal of potential coke deposits inside. Synthetics may tend to loosen up these deposits and carry them further down the system and so facilitating the above-mentioned pretzel effect. The second cause of failure, abnormal mechanical operation, can be caused by fluttering valves for example. This can be detected on the valve’s seating surface, which shows an almost hammered finish. This can be caused by pulsations from elsewhere in the system, causing the valve to experience multiple instead of singular impacts. Another type of mechanical failure can be one caused by uneven flow through the valve. This can be caused by differing internals of the cylinders. These irregularities can cause part of the valve to open prior to another. The resulting uneven distribution of the task causes an uneven wear pattern with premature breakage in the more taxed area. Again; pretzels. So how does one solve these problems, or even identify them? The best method is to remove valves during regular maintenance and to have them analyzed in as close to an "as-found" state as possible. Laboratories can analyze the oil, find possible deposits, and perform microscopic analyses of seating surfaces to determine weak areas in your system. Corrective action in the case of mechanical failure may mean a change of spring, or a change in mass of the moving components. Sometimes switching from steel to non-metallic materials such as synthetic plastics can have dramatic effects on valve pulsation patterns. As a final word, Hoerbiger offers the following key steps in preventive maintenance. (for an in-depth How and Why, visit www.petroassist.com
Application of these 3 simple steps can go a long way towards reducing downtime, production losses, and maintenance costs. This curtails the occurrence of pretzel-like objects to the office trailer where we can all enjoy the effects of unimpeded natural gas heating. |
|
Home |
Product Lines | Tech
Notes | Equipment
Catalog
|