Gas Pressure Regulation and Metering Systems

Pressure Regulation and Gas Metering Systems

Chapter Introduction

This chapter covers the fundamental principles of pressure regulation and gas measurement, two essential functions of any gas installation. As a gasfitter Class B, you will be called upon to install, maintain, and troubleshoot regulators and meters. Mastering these systems is not only a requirement of the Red Seal interprovincial exam, but also a public safety obligation. Gas pressure must be controlled with precision to ensure the proper operation of appliances and to prevent the risks of explosion or carbon monoxide poisoning.

Learning Objectives

By the end of this chapter, you will be able to:

1. Fundamental Principles of Gas Pressure

1.1 Essential Definitions

Pressure is the force exerted per unit area. In the gas trade, it is expressed in kilopascals (kPa), inches of water column (in. wc), or pounds per square inch (psi). The following table shows the most common conversions:

Gauge pressure is the pressure measured relative to atmospheric pressure. Absolute pressure is the gauge pressure plus atmospheric pressure (approximately 101.3 kPa at sea level). For gas calculations, gauge pressure is typically used unless otherwise specified.

1.2 Service Pressures for Natural Gas and Propane

Natural gas is distributed at different pressures depending on the application:

Propane (LP gas) is generally stored as a liquid and vaporized before being distributed. The vapor pressure of propane varies with temperature: at 20 °C, it is approximately 830 kPa, but it drops considerably in cold weather.

1.3 Boyle's Law and Charles's Law

Boyle's Law (P₁V₁ = P₂V₂ at constant temperature) explains why the volume of gas decreases as pressure increases. Charles's Law (V₁/T₁ = V₂/T₂ at constant pressure) explains the expansion of gas with temperature. These principles are essential for understanding gas behaviour in piping systems and meters.

2. Pressure Regulation

2.1 Role of the Pressure Regulator

The pressure regulator is a device that reduces gas pressure from a variable supply pressure to a constant outlet pressure. It protects appliances from pressure fluctuations and maintains a stable flow rate.

2.2 Regulator Components

A typical regulator includes:

2.3 Operating Principle

The regulator operates on a balance of forces. The outlet pressure acts on the diaphragm and tends to close the valve. The spring exerts an opposing force that tends to open the valve. When the outlet pressure increases, the diaphragm rises, the valve closes partially, reducing the flow. When the outlet pressure decreases, the diaphragm lowers, and the valve opens further.

2.4 Types of Regulators

2.5 Performance Characteristics

Regulation accuracy is the difference between the set outlet pressure and the actual pressure under varying flow conditions. The capacity of a regulator is the maximum flow it can deliver while maintaining the outlet pressure within acceptable limits.

Lock-up pressure is the maximum pressure reached at the regulator outlet when the flow is zero. This value must be lower than the maximum allowable pressure of the downstream equipment.

2.6 CSA B149.1 Rules Regarding Regulators

According to CSA B149.1, Natural Gas and Propane Installation Code, the following requirements apply:

2.7 Regulator Vents

The vent of a regulator allows gas to be released in the event of diaphragm failure or overpressure. Requirements for vents are critical:

Exam Trap: Never plug a regulator vent. An obstructed vent can cause pressure buildup and diaphragm failure.

3. Gas Metering Systems

3.1 Role of the Meter

The gas meter measures the volume of gas consumed by a customer. There are several types of meters, each suited to specific flow rates and pressures.

3.2 Types of Meters

3.3 The Diaphragm Meter

The diaphragm meter is the most common in residential applications. It operates by the alternating displacement of diaphragms that divide the gas into known volumes. Each displacement cycle corresponds to a precise volume, typically 0.01 m³ or 0.1 ft³.

The meter has an index (dial) that totalizes the volume consumed. The reading is taken in cubic metres (m³) or cubic feet (ft³), depending on the region.

3.4 Meter Installation Requirements (CSA B149.1)

3.5 Meter Connections

The meter is connected to the piping using union fittings or flanges, depending on the type and size. A vent pipe or regulator is often installed upstream of the meter to protect the equipment.

The by-pass is an assembly of valves that allows the meter to be isolated for replacement or maintenance without interrupting the gas supply. The by-pass must be sealed or locked in the closed position when not in use.

3.6 Reading and Billing

Reading the meter involves recording the numbers on the index. Modern digital meters display the volume directly in m³. Mechanical meters have dials or rollers.

Consumption Calculation: Consumption = Current reading − Previous reading

Example: Previous reading = 12,450 m³, Current reading = 12,680 m³

Consumption = 12,680 − 12,450 = 230 m³

3.7 Volume Correction Factor

The volume of gas measured by the meter is a volume under operating conditions. For billing purposes, a correction factor is applied to bring the volume to standard conditions (15 °C and 101.325 kPa).

The correction factor (F) is calculated as follows:

F = (P_abs / P_standard) × (T_standard / T_abs)

Where:

Calculation Example:

Gauge pressure at the meter = 1.75 kPa, temperature = 20 °C

P_abs = 101.325 + 1.75 = 103.075 kPa

T_abs = 20 + 273.15 = 293.15 K

F = (103.075 / 101.325) × (288.15 / 293.15) = 1.0173 × 0.9829 = 0.9998

Corrected volume = Measured volume × F

4. Pressure Drop in Piping Systems

4.1 Principle of Pressure Drop

Pressure drop is the decrease in pressure that occurs as gas flows through a piping system due to friction against the internal walls. It depends on:

4.2 Equivalent Length of Fittings

Each fitting (elbow, tee, valve) adds resistance equivalent to a certain length of straight pipe. The following table gives approximate equivalent lengths for common fittings:

Total equivalent length = Actual length + Sum of equivalent lengths of fittings

4.3 Calculating Allowable Pressure Drop

According to CSA B149.1, the allowable pressure drop between the supply point and the farthest appliance must not exceed:

4.4 Pipe Sizing Methods

Pipe sizing can be done using:

The Spitzglass formula for low pressures (< 7 kPa) is:

Q = 0.00087 × √(ΔP × d⁵ / (L × G))

Where:

Calculation Example:

Inside diameter = 25 mm, ΔP = 0.5 kPa, L = 30 m, G = 0.6

Q = 0.00087 × √(0.5 × 25⁵ / (30 × 0.6))

Q = 0.00087 × √(0.5 × 9,765,625 / 18)

Q = 0.00087 × √(271,267)

Q = 0.00087 × 520.8

Q = 0.453 m³/h

Exam Trap: Always check your units. The Spitzglass formula requires consistent metric units (mm, kPa, m, m³/h).

5. Valves and Safety Devices

5.1 Shut-off Valves

Every installation must have a shut-off valve at the building entrance, immediately upstream of the meter or regulator. This valve allows the installation to be isolated in an emergency or for maintenance.

CSA B149.1 requirements:

5.2 Relief Valve

The relief valve protects the system against overpressure. It opens automatically when the pressure exceeds a predetermined set point and vents the gas to the atmosphere.

CSA B149.1 requirements:

5.3 Excess Flow Valve

The excess flow valve is a device that closes automatically when the flow rate exceeds a predetermined value, typically indicating a line rupture. It is commonly installed on propane tanks and buried piping.

5.4 Integrated Regulator-Meter

Some residential meters incorporate a regulator within the same housing. This configuration reduces space requirements and simplifies installation, but requires periodic inspection of the regulator.

6. Installation and Commissioning Procedures

6.1 Installing a Regulator

6.2 Installing a Meter

6.3 Commissioning and Purging

Purging consists of removing air or an air-gas mixture from the piping before commissioning. This operation is critical to prevent explosion hazards.

Purging procedure:

Exam Trap: Purging must always be done to the outdoors, never inside a building.

7. Troubleshooting Regulation and Metering Systems

7.1 Common Regulation Problems

7.2 Common Metering Problems

7.3 Leak Testing

Leak testing is done with a soap solution (water + detergent) applied to the connections. The presence of bubbles indicates a leak. Never use a flame to detect a gas leak.

8. Regulatory Requirements and Standards

8.1 CSA B149.1 — Natural Gas and Propane Installation Code

Key rules related to regulation and metering:

8.2 Other Relevant Standards

8.3 Pressure Testing

Before commissioning, piping systems must undergo a pressure test:

Exam Trap: The tightness test is done at service pressure, not at maximum pressure. The strength test is done before the installation of appliances.

9. Practical Calculations for the Exam

9.1 Pressure Conversion

Example: Convert 7 in. wc to kPa.

7 in. wc × 0.249 kPa/in. wc = 1.743 kPa

9.2 Calculating Equivalent Length

Example: A 15 m pipe has 4 × 90° elbows and 1 ball valve. Diameter = 25 mm.

Equivalent length of elbows: 4 × 0.6 m = 2.4 m

Equivalent length of valve: 1 × 0.3 m = 0.3 m

Total equivalent length = 15 + 2.4 + 0.3 = 17.7 m

9.3 Calculating Pressure Drop

Example: Flow rate = 5 m³/h, diameter = 25 mm, equivalent length = 20 m, G = 0.6.

Using the Spitzglass formula:

ΔP = (Q² × L × G) / (0.00087² × d⁵)

ΔP = (25 × 20 × 0.6) / (0.000000757 × 9,765,625)

ΔP = 300 / 7,392

ΔP = 0.0406 kPa

This pressure drop (0.04 kPa) is well below the allowable drop of 0.5 kPa.

9.4 Calculating Corrected Volume

Example: Measured volume = 100 m³, gauge pressure = 2 kPa, temperature = 10 °C.

P_abs = 101.325 + 2 = 103.325 kPa

T_abs = 10 + 273.15 = 283.15 K

F = (103.325 / 101.325) × (288.15 / 283.15) = 1.0197 × 1.0177 = 1.0378

Corrected volume = 100 × 1.0378 = 103.78 m³

10. Safety Considerations

10.1 Risks Associated with Regulation and Metering

10.2 Emergency Procedures

In the event of a gas odour:

10.3 Personal Protective Equipment (PPE)

When working on gas systems, wear:

Summary

Traps to Avoid

Review Questions

References

This chapter prepares you for the interprovincial exam questions related to pressure regulation and gas metering. Review the calculations, memorize the key CSA B149.1 rules, and practice with the review questions. Good luck with your preparation!