August 13, 2010
This is the second part of a two-part series on making shielding gas systems more efficient.
Editor’s Note: This is the second of a two-part article that takes a look at shielding gas efficiency. The first part appeared in last month’s edition of The FABRICATOR®.
One alternative to a rotor-type device is a fixed-orifice flow device. Small, comparatively inexpensive, and easy to install, these devices can be purchased for almost any flow and pressure specification. However, they do not have visual flow indicators and are nonadjustable. These devices can be configured to install directly into system drops or wire feeders.
While these two types of orifice fittings configurations appear very similar, they are not interchangeable because of incompatible threads. Using them interchangeably will cause a leak.
The same principles and concerns that apply to rotor meter flow devices also apply to the location of these flow devices. When installed correctly and maintained properly, these flow devices offer the lowest and most consistent shielding gas cost. Fixed-orifice flow devices are available with or without strainers. However, because the size of most orifice openings averages 1⁄32 inch, strainers (combined with drip legs) help prevent flow restriction and complete stoppage caused by debris in the system.
Proper system pressure must be maintained consistently across the fixed-orifice’s entire system. Small variations in system pressure greatly affect flow rates. Often, piping must be modified to eliminate a pressure drop across a system before this type of flow device can be used. Even with a typical 1⁄32-inch orifice fitting, a pressure increase as small as 5 pounds per square inch (PSI) will result in a more than 16 percent greater flow rate per occurrence.
System leakage is perhaps the most underestimated source of gas loss in most plants today. Leaks can account for 50 to 75 percent of the total losses in a bulk system or plant. Leaks can take on many different forms and involve more than just piping and hoses. Malfunctioning solenoid valves, blown pressure regulators and diaphragms, etc., also can cause expensive leaks.
Small leaks in compressed-air systems often are tolerated or ignored. In an argon or shielding gas system, there is no such thing as a minor leak, and the cumulative effect of many leaks can be devastating. People who are continuously exposed to workplace sounds may become desensitized to the noise. This may be a reason leaks in the shielding gas system can go unattended for long periods of time even when they are audible.
Most bulk system’s piping and distribution lines are continuously pressurized. This means that leaks do not go home at the end of the work shift; instead, they continue to compound themselves every hour the system is pressurized. Many plants shut off the external shielding gas supply at the end of the day. Even though this practice reduces the cost of leaks, it is treating the symptoms instead of remedying the real problem.
Often, plants that adhere to this policy have prescribed procedures for blowing down the system upon repressurization. This is done primarily to try to remove atmospheric contamination introduced into the system while the pressure was “leaking off” or while the system was depressurized and most susceptible to contamination. Depending on the system size (volume) and the procedure used, blowing down can cost more money in gas consumption alone than the leak that would have occurred had the system remained pressurized, not to mention the potentially costly effects of the resulting contamination.
The amount of gas wasted during the bleed-off process, the amount required to repurge the system, and the amount wasted during blow down should be considered. A 1⁄2-inch ball valve fully opened for one minute at 30 pounds per square inch gauge (PSIG) allows approximately 103.5 cubic feet of gas to escape. This amount of gas may equal the amount required to operate one wire feeder for the entire shift.
Leaks often are tolerated by employees and management simply because they do not fully understand or cannot comprehend their costly cumulative effects. Once they understand the individual flow rate that even one leak can produce, the number of occurrences that can develop in a given system, and attach a dollar value to each of them, even the most skeptical employees will adopt a zero tolerance of leaks.
System leak tests should be conducted on a weekly basis. Plants with a high leak exposure typically have long hose runs, wire feeders that are constantly being moved, old piping systems, piping systems that have been butchered by plant renovations and expansions, and piping systems that are exposed to vibrations and movement. Plants that use maintenance personnel who are not qualified pipefitters and/or allow production personnel to connect, reconnect, and relocate supply lines to wire feeders also may have high leak exposure.
Accomplishing these tasks requires a basic knowledge of identifying fittings and fitting compatibility. Some of the worst leaks can result from not knowing the difference between tapered and straight threads or which one requires thread lubricant.
The pressure decay test is one of the quickest methods to determine the integrity of the system as a whole. When armed with the proper data, employees can make a very accurate assessment of the exact losses in about five minutes. Although this test gives an accurate account of the system as a whole, it will not locate a problem’s source.
Leak detection equipment can help locate sources. Many variations of leak detection equipment are commercially available, with prices ranging from a few hundred to several thousand dollars. Considering the payback potential, this is not a good place to skimp. In addition, how beneficial this equipment is depends on the user’s abilities. Operator proficiency depends on training, frequency of use, and, perhaps most important of all, an absolute knowledge of the system being inspected.
Using this equipment requires a high degree of concentration and can be a confusing and frustrating task. To be effective, the operator must not make assumptions about any segment of the system, leaving no stone unturned. Complacency can occur easily with such a repetitive process. One thing worse than not having a periodic inspection program is having one that is not 100 percent thorough, thus creating a false sense of security. Most electronic leak detection equipment is somewhat fragile and does not fare well with multiple users.
A liquid leak detector works well for detecting very small leaks and also shows the fringes of some large leaks. Several leak configurations exist that liquid leak detectors may or may not reveal, especially involving hoses and tubing. In leaks of this nature, a high-velocity jet of gas is concentrated in a small area. This jet of gas blows too rapidly to allow a bubble to form at the leak source. If the inspector is paying close attention, and if the lighting is just right, he or she may see a quick mist for just a split second. While it is easy to overlook a very large leak or leaks with this method, numerous liquid leak detectors are available that work well with very low flows.
Often, individuals acknowledge the presence of leaks but claim they have bigger fish to fry during the course of a busy day. This attitude indicates employees’ and supervisors’ lack of knowledge. In most cases, once a leak audit has been conducted and repairs implemented in a timely fashion, the resulting savings are usually impressive. In the words of one welding engineer after such an occurrence “short of a complete plant shutdown, we don’t have a bigger fish to fry.” Considering that just one leak with the diameter of a pin needle can cost $9,810 per year, he is probably correct.
The often-overlooked storage tank area and the lines between the storage tank and the mixer are also important areas to consider.
In most applications, the gas supplier’s equipment and piping responsibilities end at the final pressure regulator. For this reason, large portions of piping between the vaporizers and the mixer are a no-man’s land regarding inspections. Quite often, malfunctioning relief mechanisms exist around storage tanks, such as tank fill valves that may not be completely closed off and/or may have packing and bonnet leaks. Even though maintaining these areas is not the customer’s responsibility, the customer will pay for any gas losses occurring there.
Argon suppliers usually develop a good feel for their customers’ consumption rates and develop a trend regarding the amount of liquid argon required to fill a tank to capacity based on the number of loads delivered over time. Any time a customer anticipates a significant drop in consumption, the supplier should be notified so that deliveries can be adjusted accordingly. The supplier also should be notified of any plant shutdowns that will last more that a few days because liquid argon must be stored at supercold temperatures to remain in its liquid state.
If a tank remains at or close to full for a long time, the liquid will absorb some heat, causing it to flash into a vapor. This causes pressure to build in the internal tank, resulting in the release of safety devices such as rupture disks or pressure relief valves. Should this occur, large amounts of gas can be released in a very short period of time. By matching storage tank levels to efficient consumption levels, the chances of pressure-related, inadvertent releases are greatly diminished.
Most suppliers of argon and other gases are concerned about the efficient and safe use of their product. Some provide applications programs, ongoing technical support, and training. While these suppliers perhaps understand the effects of various system inefficiencies more than most, their primary responsibility is to keep the customer supplied with a high-quality product. Because leaks and other system losses are the primary sources for contamination after delivery, suppliers often inform their customers of various types of system deficiencies that may exist, especially if the customer has lodged complaints about quality.
However, it usually is up to the user to correct system inefficiencies. Although some suppliers will provide limited or abbreviated services using their own personnel and resources, for a more thorough and cost-effective analysis, expert, professional, and nonbiased individuals in the field of piping and distribution system analysis should be consulted.
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