A Proper Gas Sampling and Control System is Essential to Safe and Efficient Gas Delivery
A discussion on selecting appropriate pressure and flow control devices would be incomplete without covering the requirements pertaining to a variety of chemical process and quality control applications. These applications generally require delivering the process, calibration, or instrument support gas at specific pressures and flow rates in as unaltered a condition as possible. In order to do this, the diameter of the piping must be sized appropriately. For any piping system under actual flow conditions, there will be a pressure drop from the inlet or starting point of the system to the farthest point in the system. This pressure drop is dependent on the flow rate of the gas and the distance to the farthest point. In general a pressure drop of no more than 10 percent of the inlet pressure is acceptable. Table 1 gives the maximum flow rate of nitrogen possible with a 10 percent or 5 percent pressure drop per 100 feet of the pipe size listed for inlet pressures from 5 to 250 psig.
Maximum flow rate of nitrogen possible with a 10 percent or 5 percent pressure drop per 100 feet of the pipe size listed for inlet pressures from 5 to 250 psig. Using a conversion factor provides data for specific gases.
To convert this table to the specific gas used, you must use a conversion factor that is based on the difference between the specific gravity of nitrogen and the specific gas. A good general factor to use is the ratio of the square roots of their specific gravities:
√nitrogen / √specific gas = factor
Let’s consider what the conversion factor would be for helium. For helium: √0.967 (nitrogen)/ √0.138 (helium) = 0.983/0.371= 2.649 or 2.65; therefore you can flow 2.65 times as much helium under the same circumstances as you can nitrogen. This, however, doesn’t give the entire picture. The source must also be able to flow at this rate based on delivery pressure and inlet pressure. Some gases are delivered in pipelines direct from a process source, while some gases are supplied in high-pressure cylinders or from portable or permanent cryogenic tanks. The source pressure can be reduced to the piping inlet pressure using either a simple single-pressure control device, as pictured in Figure 1, or a computer-controlled system that has a primary and backup supply source may be used, as pictured in Figure 2.
Figure 1 and Figure 2. A simple single-pressure control device or a computer-controlled system with a primary and backup supply source can be used to reduce pressure to the piping inlet.
Whatever this primary pressure control device is, it must be able to accommodate the required flow at the required pressure. This source and the pipeline itself should be sized with an adequate but affordable safety factor to allow for peak uses and future growth. It is not unusual to apply a safety factor of 2 times the current anticipated flow and anywhere between 1.2 and 2 times the required pressures. This ensures that any future additions or changes in process requirements don’t leave the facility having to install a totally new piping system.
When a pipeline has more than one use point, the flow should be calculated based on the total flow of all use points in the system. Using the basis that the total flow would occur at the farthest use point from the inlet, you can ensure that each point won’t be starved for flow. Another way to compensate for differing flow rates at different use points is to run the entire pipeline in a loop, thus ensuring that no one point is any farther from supply than any other point. For large piping systems made with exotic materials this can be cost prohibitive, but for smaller systems or critical-use processes this can be the difference between a system that works and one that doesn’t.
Another point to consider is that there are some safety or supply limitations for specific gases. With acetylene, the pipeline pressure cannot exceed 15 psig, as when acetylene is accidentally released at pressures over 30 psig it can spontaneously ignite in air. Certain gases that come from liquefied or cryogenic sources are limited in their maximum flow capability because the liquefied gas must vaporize in or external to the cylinder. In the cases of carbon dioxide, ammonia, chlorine, or liquid hydrocarbons like propane, the bulk of the product is actually in the source tank or cylinder in a liquid form, and the use valve withdraws gas from the vapor phase. This gas must be replaced by vaporizing liquid, which is a physical state change. When this occurs the liquid actually loses heat, making additional vaporization more difficult.
Make sure that the bulk supply for these types of gases is appropriately sized either by number of cylinders or by the size of both the bulk tank and any external vaporizing device. An example of the limitation of the withdrawal rate for propane based on cylinder temperature and maximum flow from a 100 lb cylinder is listed in Table 2.
Table 2. Withdrawal rate for propane based on cylinder temperature and maximum flow from a 100 lb cylinder.
One gas that typically isn’t recognized as needing special care is oxygen. At concentrations above 23 percent, however, oxygen can be extremely hazardous, and the piping materials must be chosen and installations undertaken with extreme respect, since even a small amount of a contaminant that would not ignite in air will burn or virtually explode when exposed to an oxygen-rich atmosphere. Therefore, all oxygen piping systems and their components must be certified to be cleaned and installed as suitable for “oxygen service.” Many facilities require that piping systems that carry high-pressure oxygen (pressures above 300 psig) must be made of Monel®, which is highly resistant to promoted ignition in pure oxygen. For lower-pressure systems that do not require high-purity components, copper pipe that is silver brazed is typically used. In comparison, copper should never be used for an acetylene piping system because the acetylene will react with the copper and form hazardous, explosive compounds.
In instances where the purpose of the pipeline is to sample a process for composition or purity, the sample required is usually very small. For this type of piping, the pipe should be as small as possible. It is not unusual for it to be 1/8” OD or smaller. It should also be limited in length from the sample valve to the analyzer, with pressure or flow control devices that are also minimal in internal volumes. This ensures that the sample is as consistent with real time as possible.
When it comes to drawing gas samples, the source pressure and required inlet pressure of the analyzer need to be considered along with the required sample size or rate. For analytical processes, the sample needs to be delivered in a gaseous state or at a certain temperature. To do this for samples that are in liquid phase or that have condensable components, a vaporizing pressure regulator similar to those shown in Figure 3 and Figure 4 is normally used.
Figure 3 and Figure 4. Series 452 and 453 vaporizing pressure regulators are used for delivering samples in a gaseous state that are in a liquid phase or have condensable components.
Now that we have looked at most sizing considerations, it is time to consider materials. When it comes to piping materials, today’s facilities designers or managers have more options than ever. New corrosion-resistant alloys like Hastelloy® C-22 have greatly improved the service life of process lines in corrosive service. Improvements in welding processes, joining techniques, and fittings have reduced the labor burden and maintenance on many types of installations while improving leak integrity and purity.
(For information on how to select the appropriate material for your piping system, go to www.labmanager.com/articles.asp?ID=324.)
Certain gases require that special processes be implemented to make the process piping suitable. An example of this is piping intended for fluorine, fluorine mixtures, or certain fluorine compounds such as HF. In addition to selecting high-nickel alloys like Monel®, or for some components pure nickel, it is essential to ensure that all the components and the piping be “passivated” for fluorine service. This process involves slowly increasing the concentration of fluorine in the system until the internal surface reaction sites of the piping and its components have a metal fluoride layer that prevents further reaction with fluorine. Any time a modification or addition is made to these systems, no matter how minor, this process must be repeated.
Another process that can be valuable to the integrity of highly reactive samples, such as multiple-component reduced sulfur samples, is “glass coating,” Silcosteel®, or Siltec®/Sulfinert®, an amorphous silicon deposition process. This process imparts in the inertness and corrosion resistance of glass to the material treated. There are some limitations with regard to the substrate material, but it is suitable for most stainless steels, 316L being the most widely treated material. Other than tubing, this process can be performed on various other components, including valves and pressure control regulators.
The last thing to consider with regard to compatibility of the piping system is pressure. The maximum allowable working pressure of the material selected should be sufficiently greater than the maximum expected operating pressure, with a reasonable safety factor of 20 percent as a minimum and 200 percent as a desired target. With most wall thicknesses of 300 series stainless steels, their working pressure ratings can be as high as 6,000 psig; however, with copper the typical installation is limited to 250 psig.
Now that we have considered the size and materials to be used, the last main issue is installation. When it comes to installation, only qualified, certified professionals should be employed or contracted. The procedures and testing that are required to install and certify a highpurity or hazardous process line are quite specialized. Where practical, compression-type tube fittings make an installation quick and eliminate the need to purge the piping during installation. However, for higher leak integrity, welded connections are sometimes used. Special care and purging of the piping must be done during any welding or brazing operation to prevent oxide and contaminant formation inside the piping. Nitrogen is typically the purge gas when brazing copper lines, while high-purity argon is required not only to shield the weld area but also to continuously purge the interior of the piping until the weld area has cooled. Once installed, the system should be thoroughly leak tested, ideally with a helium leak detector. Failure to install and test the system properly can result in extensive additional costs or damage to valuable instrumentation.
Following these simple guidelines and general considerations should result in a system that is delivered under budget, that is safe, and that provides extended service life, delivering high purity and consistent process results.
Monel® is a registered trademark of Specialty Metals Corporation.
Hastelloy® is a registered trademark of Haynes International, Inc.
Silcosteel® and SiltecR/Sulfinert® are registered trademarks of Restek Corporation.