The effects of pulsation on Natural Gas Measurement are a major factor to consider when designing meter installations. An innovative mounting system for measuring instruments can reduce or even eliminate the resulting error.
Pulsations created by compressor, flow control valves, and gas regulators, can produce pulsations in a magnitude that will effect accuracy. Another problem can be created by some piping configurations, producing vortices which create pulsations that also affect accuracy. The resulting error is called Square Root Error (SRE) which is inherent to the piping system and can only be reduced by modifications to the system. A secondary effect of SRE is called Gauge Line Error (GLE), it emerges when the differential pressure at the measuring instrument at the orifice fitting connection is not the same differential pressure at the measurement instrument’s connection.
A study by Pipeline and Compressor Research Council (PCRC) in cooperation with Southwest Research Institute (SWRI) in San Antonio, Texas has shown the gauge line error can be reduced mounting your measurement instrument closer to orifice fitting. The best results is a maximum distance of 45cm combined with measurement sensing lines with a minimum internal diameter of 9.5mm This will allow the pulsations to move in and out of the sensing lines and not build up pressure leading to inaccurate readings. Another factor is keeping both sides of the sensing lines from the orifice fitted to the measuring instrument at the same length. Deviations or unequal lengths of the "Low Side" and the "High Side" sensing lines could result in a Gauge Line Error.
Another consideration is in gas production and gas gathering system designs. Designing an installation to drain condensate and other liquids is important. These liquids will build up in small orifice valves affecting accuracy. In these applications it is important to install the measurement instrument above the orifice fitting connections to allow self-draining of the manifold and sensing lines.
AS-Schneider’s DirectMount Systems is a direct vertical assembly, allowing the measurement instrument to be installed within the required distance and with the minimum diameter orifice of 9.5mm from the orifice to the measurement instruments sensing element. This assembly is a safe and efficient method of close coupling the measurement instrument to the orifice fitting and thus eliminating or reducing the effects of Gauge Line Error from inaccurate measuring systems. The large orifice valves also allow the self-draining of liquids to prevent liquid build which would also affect your measurement accuracy.
Stabilized Connector for more security
At the heart of the 'Schneider DirectMount Systems' is the patent pending, stabilized connector. It is manufactured from carbon steel, stainless austenitic steel or nickel-based alloys depending on requirements. In order to be able to connect the valve manifold to the measuring orifice flange, a special adapter is necessary. This adapter is supported on the body of the measuring orifice flange which provides for more stability, explains Markus Häffner, Head of Design & Development on AS-Schneider: "Normally, the connector is only screwed into the orifice with a conical thread. Yet the entire structure with the 5 valve manifold and measuring instrument has a high net weight. In addition, vibrations from the pipeline can affect this connection so that this thread is often damaged or destroyed
Reliably tight up to 420 bar
On the 5 valve manifold, two shut-off valves ensure a reliable primary shut-off if the measuring instrument has to be dismantled and serviced. The valves are available with different sealing materials such as the plastic PTFE and withstand temperatures from -55°C to 232°C and a pressure up to 420 bar depending on the version. In addition, the 5 valve manifold has a venting valve, over which the static pressure in with closed primary shut-off can escape in a controlled manner as well as two equalizer valves.
The 'Schneider DirectMount Systems' has a continuous inner diameter of 9.5mm as recommended in the PCRC study. In addition, the distance between orifice and measuring instrument is less than 30cm which is also an important recommendation for accurate measurement. The robust design of the stabilized connector ensures high stability and makes the 'Schneider DirectMount Systems' insensitive to vibrations. The user can forego additional lines or valves and thus gets a compact, easy-to-use and robust installation.
More accurate measurement saves costs and avoids lost and unaccounted for gas "With the 'Schneider DirectMount Systems', we can eliminate a significant amount of the pulsations in the differential pressure lines", says Markus Häffner. "Flow measurement is thus much more accurate and less error-prone. Based on the more accurate measurement, you now have fewer errors in the billing. This not only eliminates “lost and unaccounted for gas”, but also saves substantial costs." Due to its robust construction, the 'Schneider DirectMount Systems' is also extremely durable and resistant to malfunctions - even under the often harsh conditions to which it is exposed in the gas pipelines. "This is definitely noticeable with the service and maintenance costs," says Häffner.