May 2015
Gas Transmission
Gas Quality Study for NTS Unconventional Gas Supplies
May 2015
Apr 2016
National Grid Gas Transmission
Alex Ferguson,
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Network Innovation Allowance
The gas quality study will focus on:

  • The impact on the NTS of increasing the oxygen content to 1% for non-conventional gases to bring the transmission system into line with the entry requirements for the distribution networks. The study will use and build on existing knowledge for the oxygen limit exemption for the distribution networks. The proposed gas quality study will also include information from Europe and USA to introduce a best practice and harmonisation approach to the 1% oxygen content study. Corrosion rates can be accelerated if both liquid water and other contaminants are present. The approach will be to derive a probability of corrosion rates to calculate wall losses over the NTS lifetime.
  • Investigating a range of flow ratios to determine the point at which the lack of full mixing becomes an issue for determining the bulk properties of the fluid flow. Use this study to define the characteristics of a well-mixed system.
  • Using Computational Fluid Dynamics (CFD) modelling to determine whether the requirement for a sample point to be 20 pipe diameters downstream from the confluence of two flows is suitable and appropriate (reference ISO 10715). The input for this part of the study requires details from the flow ratio sub-part to select the best test cases to study; it is also likely that the mixing will be dependent on gas velocities so a number of different gas velocities will be explored using the same CFD model. For one proposed design, use the model to highlight the impact of pipeline infrastructure on mixing quality.
  • Using the CFD model to determine whether the requirement for centre one third sampling is realistic (reference ISO 10715). The model will enable the gas quality variation over a cross-section of pipe to be simulated and the system homogeneity to be determined. Again the flow ratio sub-part will be used to select the best test cases to study.
  • Investigating forced mixing systems including the options for bi-directional flow. Forcing the gases to mix using static mixer arrangements will be studied to highlight the potential benefits, and provide an overview of the impact on bi-directional flow

· Conducting a high-level survey of the NTS to look at the prevalence of bi-directional flow both now and in the future. Evaluate the mixing arrangement options to determine if there are alternative approaches to reduce site size.

· Compliance with GS(M)R as well as maintaining the calorific value of the prevailing flow which is a commercial issue. Use the NGGD/NGN Flow Weighted Average Calorific Value NIA project to inform this work.

There is a strong link between this study and the NIA project NIA_NGGT0070 NTS Block Valve Connections. NIA_NGGT0070 focusses on an engineering design solution for block valve connections, and the information and new learning from this work, particularly on sampling and blending will help determine the most efficient design solution.

To provide answers to the following two key challenges to the current process:

1.  Oxygen content: the GS(M)R limit for oxygen is 0.2 mol%. Unconventional gas supplies may have higher oxygen content, and whilst gas distribution networks have an exemption to allow biomethane sites to flow up to 1 mol% oxygen, no such exemption exists for the NTS. Is this appropriate?

2.  Gas Quality Blending: the co-mingled gas quality sample point location is set at 20 pipe diameters from the injection point, in line with current ISO standards. Is this appropriate for smaller unconventional gas connections?

Delivery of a project report detailing the results of the gas quality study and addressing the two key challenges as per the objective.