Qwestrum Engineering360 · Petroleum & Energy · Natural Gas Engineering
Gas Metering and Quality
Gas metering and quality systems ensure accurate custody transfer and fuel interchangeability across networks.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.
Key formulas & points
Skim these first — then read the full notes below.
- Ultrasonic meter no obstruction
- Chromatograph composition for BTU
- Custody transfer accuracy ±0.25%
Topic details
Introduction
Beggs and Ahmed both note that commercial gas disputes usually originate from metering or quality mismatch. In Indian exams, orifice-flow basics and Wobbe index interpretation are repeated themes.
Key relations & formulas
Formulas (Indian textbook notation)
Formulas (Indian textbook notation)
Formulas (Indian textbook notation)
- Wobbe index WI = \frac{HHV}{\sqrt}{SG} interchangeability
Notation and sign conventions
Relation 1 —
Formulas (Indian textbook notation)
Write this relation with symbols exactly as in Ikoku Natural Gas Production — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
Formulas (Indian textbook notation)
Write this relation with symbols exactly as in Ikoku Natural Gas Production — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
Wobbe index WI = \frac{HHV}{\sqrt}{SG} interchangeability
Formulas (Indian textbook notation)
- Wobbe index WI = \frac{HHV}{\sqrt}{SG} interchangeability
Write this relation with symbols exactly as in Ikoku Natural Gas Production — Standard reference before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Concept in depth
Custody transfer combines volumetric flow measurement, pressure-temperature compensation, and gas composition analysis. Orifice meters remain common, while ultrasonic meters reduce pressure loss and maintenance. Wobbe index allows burner compatibility checks for gases of different composition.
Assumptions and validity limits
State assumptions explicitly before using any relation for gas metering and quality — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Natural Gas Engineering viva and GATE descriptive questions, listing valid assumptions often earns separate marks.
Step-by-step problem approach
1. Read the question and list given data with SI units (common in Natural Gas Engineering papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to gas metering and quality.
4. Use equation 1:
5. Use equation 2:
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to gas metering and quality.
4. Use equation 1:
.
5. Use equation 2:
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
Applications & exam relevance
Gas Metering and Quality appears in CGD and LNG projects. In Indian petroleum curricula this topic is tested because it connects theory to processing and transmission of gas.
GATE and semester exams often combine gas metering and quality with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use gas metering and quality?" — answer with a lab, mini-project, or plant visit example if possible.
Common mistakes in exams
Typical mistakes are using uncorrected line conditions for billing, ignoring discharge coefficient, and comparing HHV values without specific gravity effect.
Quick revision checklist
Before attempting gas metering and quality problems, confirm you can:
1. Ultrasonic meter no obstruction
2. Chromatograph composition for BTU
3. Custody transfer accuracy ±0.25%
2. Chromatograph composition for BTU
3. Custody transfer accuracy ±0.25%
Revise the solved examples in Ikoku Natural Gas Production — Standard reference and one previous-year GATE or university paper for this unit.
Worked examples
Try the problem first — open the solution when you are ready to check.
Wobbe Index
Problem
If HHV = 39 MJ/Sm3 and specific gravity SG = 0.64, compute WI = HHV/sqrt(SG).
Solution
WI = 39/sqrt(0.64) = 39/0.8 = 48.75 MJ/Sm3.
Conceptual check — Gas Metering and Quality
Problem
In a Natural Gas Engineering semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of gas metering and quality." What should a complete answer include?
📖 Standard books (India)
Ikoku Natural Gas Production — Standard reference
Read: Syllabus unit
Referenced in Indian B.Tech syllabus
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