Sample Cylinder Selection Guide
Sample cylinder selection is a very important step in ensuring that your gas or liquid sample is representative of the source stream. As is the case when performing any task, there are many considerations that should be given when determining the right “tool” for the job. In this article, we will provide some guidance to simplify the selection process and perhaps help you to improve the quality and reliability of your samples along the way.
Purpose and Regulatory
Sample Cylinders are used to collect gas and liquid samples for many reasons, including process condition validation, online analyzer verification checks as well as regulatory compliance to name a few. Before getting into the nuts and bolts of different designs and materials of construction, the very first thing to consider is whether you will be using the sample cylinder to transport a volatile gas or liquid. In the United States, the Department of Transportation Title 49 (CFR-49) is the final authority regarding the handling and transportation of sample cylinders. While greatly respected globally, its jurisdiction and enforcement powers are limited to the United States. European Union member countries fall under the authority of the Transportable Pressure Equipment Directive (TPED) and must comply with directive 99/36/EC (TPED) when transporting gas. Other countries may have their own governing authority. You must make a determination prior to purchasing a sample cylinder as to what the requirements are in your location. Shipping hazardous products in a non-compliant cylinder could result in hefty penalties and fines. We will discuss this topic in more detail in a future Blog.
With the understanding that to collect a gas or liquid sample, the sample cylinder will have to be connected in some way to the source stream. As is always the case, some basic considerations must be given to ensure that the chosen sample cylinder has the proper design to, safely and reliably, collect a sample. Consideration of the maximum allowance working pressure of your sample cylinder to accommodate high pressure as well as material to deal with risk factors is important, as well as the sample being hydraulic or gas in nature.
Sample Cylinder types
There are two primary styles of sample cylinders on the market today, Constant Pressure and Spun End.
If you are dealing with a semi-volatile or volatile liquid and you have concerns about phase changes, you may want to consider using a constant pressure sample cylinder. As the name implies, this type of cylinder maintains a constant pressure on the sample so that it may be stored under pipeline conditions. This is accomplished by incorporating a free-floating piston within the cylinder. The product sample is stored on one side of the piston in the chamber and the other side is pre-charged with an inert gas that maintains the constant pressure. These units are portable and can be used for transporting liquid or gas samples. While more expensive, Constant Pressure Sample Cylinders are, considered to be, the better choice for ensuring a more reliable and representative sample.
NOTE: Constant Pressure Cylinders do not fully follow the design criteria of DOT CFR-49 and require a Special Permit if being used to transport a sample. In this case, the DOT will issue a letter stating the permit status of the style cylinder and specifically notes the areas which they approved that were outside the standard design criteria.
The other type, of course, is the Spun End sample cylinder. Most folks are familiar with this style which is typically constructed from a seamless 316SS (Stainless Steel) chamber with ¼ Inch FNPT Connections (fitting) on either end. They are also offered in other alloy materials such as Hastelloy C or Monel for very aggressive products. They are usually manufactured to DOT or TPED requirements and are hydrostatically tested by the manufacturer prior to shipping. These units can be used for either gas or liquid transportation.
As with constant pressure sample cylinders, should there be concerns regarding phase changes for a liquid sample, a dip tube may be required. Dip tubes sometimes referred to as outage tubes, create a vapor space above a liquified gas within the sample cylinder. This vapor space allows for expansion of the product to prevent over-pressurization of the cylinder. Dip tubes come in varying lengths and will “typically” provide a vapor space equivalent to +/- 20% of the total cylinder volume. A rough calculation is to multiply the cylinder length by .20 to determine the dip tube length. NOTE: It is incumbent on you, the user, to confirm the necessary dip tube length based on cylinder volume and the volatility of the liquified sample.
Application Considerations
Once you have determined which type of sample cylinder is preferred for your product sampling needs, you will need to consider the following:
- Material of Construction: 316SS is the industry standard, however, other alloys are available for more aggressive products. Chemical compatibility is crucial not only to ensure a reliable and representative sample but more importantly to ensure that the sample cylinder’s structural integrity is not compromised. In the case of Constant Pressure Sample Cylinders, elastomers are present in the floating piston and end caps, and their material of construction and chemical compatibility must also be given consideration. This also applies to any valve or other ancillary equipment that will be connected to either type of sample cylinder.
- Pressure:Always confirm that the pressure rating of the sample cylinder meets or exceeds the desired sample pressure. DOT 3E-1800 sample cylinders are rated to 1800 PSIG at 100F and 300CC and 500CC are the most common size and styles in service today. But higher-pressure ratings, as well as larger and smaller sizes, are also available.
- Temperature:In many cases, samples are collected at elevated temperatures, and in some instances, these higher temperatures are maintained by an external heater to ensure better sample integrity. Sample cylinder pressure ratings are typically based on an operational temperature of 100 degrees Fahrenheit. As temperature increases, sample cylinder pressure ratings decrease. You should always refer to the sample cylinder manufacturer’s pressure and temperature rating chart when selecting a sample cylinder for collecting higher temperature samples.
- Special Coatings:The are a few special coatings on the market today. One is a passivation technique that bonds an inert silica layer into the internal surface of cylinders which prevent active compounds in gases from reacting with or adsorbing to the metal. Another is the internal Teflon coating of spun end stainless steel sample cylinders. This option provides a less expensive alternative to sample cylinders constructed of exotic alloys for aggressive liquids.
Ancillary Valves and Components
In this blog our primary objective has been to help guide you in the sample cylinder selection process. However, there is a wide range of ancillary valves and components that we have not discussed that can be used with either of the previously mentioned sample cylinder types. This includes, but is not limited to; isolation valves, rupture disks, dip tubes, pressure gauges, quick disconnects and tube fittings. We will go into more detail about these products in a future blog. Just be aware that consideration must also be given to the chemical compatibility as well as pressure and temperature limitations of any valve or component that you intend to connect to your sample cylinder.
I hope that you find this information helpful when selecting your next sample cylinder. However, if you have a difficult application or questions, please give us a call at 334.309.4042 or send us an email at sales@samplecylinders.com. We welcome the opportunity to be of assistance.