Installing a biogas measurement instrument in an explosive atmosphere

• Know the risks
  
• When dealing with biogas, the high concentration of methane is an obvious concern due to its flammability and possible explosion risk. Despite the fact that methane has a fairly narrow explosive concentration range, with a lower explosive limit (LEL) of 4.4 vol-% and an upper explosive limit (UEL) of 16.5 vol-%, a biogas installation is nonetheless classified as an explosive atmosphere (ATEX, EU) or a hazardous location (HAZLOC, US) and appropriate rules and regulations must be followed.

Other risks worth bearing in mind are the presence of hydrogen sulfide, which is poisonous even at low concentrations, and the absence of oxygen, which may present a suffocation risk in confined spaces. Wearing a personal gas alarm that monitors methane, hydrogen sulfide, and oxygen levels is a must when installing instruments in this type of environment. It is also important to plan ahead and prepare a risk assessment document before commencing work.

• Choosing the right process connection
  
• One of the biggest questions that needs answering early on is what type of process connection will be employed: is there an existing ball valve or threaded port that can be used, or is it necessary to weld a port into the pipeline – or even replace a flanged section of pipe with a new section containing the required sampling ports?

The Vaisala MGP261 biogas measurement instrument is equipped with a 1½” NPT thread with a probe diameter of 40 mm and an insertion depth of 209 mm. It can be inserted through 1½” and 40 mm nominal-size ball valves, provided that the valve is a full-bore design and the joint between the valve and the main pipeline is not narrower than 40 mm. The only way to confirm this is to open up the valve to check the dimensions. Larger valves such as 2” or 50 mm can be used with a thread adapter to 1½” NPT, and they are often a good choice as the probe will fit through more easily.

Process connections are not always clearly marked, and the exact thread type may be hard to identify. In such cases it is best to use calipers to measure the diameter and possible taper of the thread, and a thread pitch gauge to determine the thread type. The MGP261 is shipped with a plastic 1½” NPT plug, which can be used as a thread identification aid.

If a ball valve is not available or wanted, for example because of cost considerations, a simple threaded socket welded to the pipeline is an effective solution. A short 50 mm pipe is used to adjust the position of the biogas probe so that its tip is between one third and two thirds of the pipe diameter. In addition to the main probe sampling port, a pressure transmitter process connection is needed, typically in the ¼ to ½” range depending on the pressure transmitter used.

• A straightforward installation process
  
• Regardless of the process connection chosen, the section of gas pipeline must be purged before installation can begin. This is relatively straightforward if the selected measuring location is next to a device that can be bypassed, like a gas drying system or an active carbon filter; in other cases the installation must take place in conjunction with other major maintenance work when the process is not running. After purging the pipeline with nitrogen or air and using a gas alarm to ensure that the area is safe to work in, installing the probe is straightforward.

• Prepare the probe by wrapping PTFE seal tape on the thread following these simple guidelines:
  
• •    Lay the tape parallel to the thread
  
• •    Do not leave hanging ‘tails’ on the process side of the thread
  
• •    Apply only two or three layers of tape depending on its thickness
  
• •    Once you start tightening the thread do not reverse direction.

In an NPT thread interface some of the male thread can remain visible after the connection is tightened. For further guidance on the correct installation of NPT-threaded equipment, see this blog post by Swagelok.

• Ensuring a safe connection
  
• After installing the probe in the biogas pipeline, the next task is to connect the power supply and signal cabling. The Vaisala MGP261 biogas measurement instrument has three cable metric thread entries: one 16 mm and two 20 mm. The MGP261 can be configured in a variety of ways, with a choice of 4-20 mA or Modbus/RTU for both signal output and pressure signal input; depending on the configuration chosen, one or more of the cable entries may not be needed.

The smaller entry is primarily meant for 4-20 mA input from an Ex i (intrinsically safe) pressure transmitter and it leads to the intrinsically safe side of the connector box, which is isolated with a partition wall. Because the MGP261 contains an Ex i isolation transformer it is possible to directly connect an Ex i device to the probe. The two larger cable entries lead to the other side of the partition wall, where 4-20 mA analog output signals for methane, carbon dioxide, and water vapor are available in Ex e (enhanced safety) terminals together with RS485 (MODBUS/RTU) and 24 VDC power terminals.

Choose cable glands with Ex-approval, including strain relief and impact protection, and the correct cable diameter for the cables you are using. Some Ex-approved cable glands have an X in the approval code, meaning they are limited regarding strain relief and/or impact protection. If you are using this sort of cable gland, the cable must be secured to a cable tray or similar structure on the outside of the cable gland and appropriate impact protection must be built.

Particular attention should be paid to protective earth and signal ground connections, and cable shields should be terminated on potential equalization rails as shown in the MGP261 installation and safety guide. Measure the resistance between the potential equalization rail and the grounding screw on the MGP261 chassis; resistance must be less than 1 Ohm and the test must be documented as part of the installation report. This is particularly important as the MGP261 contains an Ex i barrier for the intrinsically safe pressure gauge as shown by the code [ia] in the Ex marking, and the protection of an intrinsically safe device relies on high-quality potential equalization.

We recommend using an experienced instrument integrator when installing instruments in biogas processes.

Visit the product page to learn more about Vaisala's 3-in-1 Methane, Carbon Dioxide and Humidity Multigas Probe MGP261 for Smart Control of Biogas Quality.