Pressure and Flow Calculations for Gas Systems

Introduction

This chapter covers the fundamental principles of pressure and flow calculations applicable to gas systems, as required by the Canadian Natural Gas and Propane Installation Code (CSA B149.1). As a Class A gasfitter, you must master these calculations to properly size piping, verify allowable pressure drops, and ensure the safe operation of appliances. The Red Seal exam places strong emphasis on your ability to apply these formulas in practical contexts.

Units of Measurement and Conversions

Pressure Units

Pressure is a force applied over a surface. In the gas trade, several units are used that you must be able to convert quickly:

Unit	Symbol	Equivalence
Pascal	Pa	1 Pa = 1 N/m²
Kilopascal	kPa	1 kPa = 1000 Pa
Megapascal	MPa	1 MPa = 1000 kPa
Bar	bar	1 bar = 100 kPa
Pounds per square inch	psi	1 psi = 6.895 kPa
Inches of water column	in H₂O	1 in H₂O = 0.249 kPa
Millimetres of mercury	mmHg	1 mmHg = 0.133 kPa

Essential conversions to memorize:

1 in H₂O = 0.249 kPa ≈ 0.25 kPa

1 psi = 6.895 kPa ≈ 6.9 kPa

1 kPa = 0.145 psi

1 bar = 14.5 psi

Flow Units

Gas flow is generally expressed in:

Cubic metres per hour (m³/h) — standard SI unit

Cubic feet per hour (ft³/h or CFH) — still used in the industry

Litres per minute (L/min) — for small flow rates

Conversion factor: 1 m³/h = 35.31 ft³/h

Reference Conditions

Gas flow rates are always expressed at reference conditions:

Temperature: 15 °C (60 °F)

Absolute pressure: 101.325 kPa (14.7 psi)

Since gas is compressible, its volume varies with temperature and pressure. It is crucial to always bring measurements back to reference conditions to compare flow rates.

Absolute Pressure and Gauge Pressure

Fundamental Distinction

Gauge pressure (relative): measured relative to atmospheric pressure. This is what an ordinary pressure gauge indicates.

Absolute pressure: measured relative to a perfect vacuum.

Formula: P_absolute = P_gauge + P_atmospheric

Standard atmospheric pressure is 101.325 kPa at sea level. It decreases with altitude (approximately 1 kPa per 100 m of elevation).

Practical Application

In sizing calculations, gauge pressure is generally used for pressure drops, but absolute pressure comes into play in:

The ideal gas law

Flow correction calculations

Determining piping capacity

Ideal Gas Law

Statement

The relationship between pressure, volume, and temperature of a gas is described by:

P × V = n × R × T

Where:

P = absolute pressure (kPa)

V = volume (m³)

n = number of moles

R = universal gas constant (8.314 J/mol·K)

T = absolute temperature (K)

Combined Gas Law

For a fixed quantity of gas, you use:

P₁ × V₁ / T₁ = P₂ × V₂ / T₂

This relationship allows you to correct gas volumes to reference conditions.

Calculation Example

A tank contains 10 m³ of natural gas at 200 kPa (gauge) and 25 °C. What would the volume be at reference conditions (101.325 kPa, 15 °C)?

51.Convert to absolute pressure: P₁ = 200 + 101.325 = 301.325 kPa

52.Convert temperatures to Kelvin: T₁ = 25 + 273.15 = 298.15 K; T₂ = 15 + 273.15 = 288.15 K

53.Apply the law: V₂ = P₁ × V₁ × T₂ / (P₂ × T₁)

54.V₂ = 301.325 × 10 × 288.15 / (101.325 × 298.15) = 28.72 m³

Pressure Drop in Piping

General Principles

Pressure drop is the loss of pressure due to gas friction against the pipe walls. It depends on:

The gas flow rate

The inside diameter of the pipe

The equivalent length (actual length + equivalent length of fittings)

The gas density

The supply pressure

Allowable Pressure Drop

CSA B149.1 requires that the pressure drop between the supply point and the appliance not exceed the following values:

System Type	Maximum Allowable Drop
Low pressure (less than 7 kPa)	0.5 kPa (2 in H₂O)
Medium pressure (7 to 100 kPa)	10% of supply pressure
High pressure (more than 100 kPa)	10% of supply pressure

Important note: For gas appliances, the minimum pressure required at the appliance inlet is generally 1.75 kPa (7 in H₂O) for natural gas and 2.75 kPa (11 in H₂O) for propane, unless otherwise specified by the manufacturer.

Equivalent Length

Equivalent length accounts for the additional losses caused by fittings, elbows, valves, and other accessories. You add the equivalent lengths of each fitting to the actual pipe length.

Typical equivalent length values (in metres of straight pipe):

Fitting	1/2 in	3/4 in	1 in	1 1/4 in	1 1/2 in	2 in
90° elbow	0.3	0.4	0.5	0.7	0.8	1.0
45° elbow	0.2	0.2	0.3	0.3	0.4	0.5
Tee (straight through)	0.2	0.2	0.3	0.3	0.4	0.5
Tee (branch)	0.6	0.8	1.0	1.3	1.5	2.0
Shut-off valve	0.3	0.4	0.5	0.7	0.8	1.0
Plug cock	0.6	0.8	1.0	1.3	1.5	2.0

Pipe Sizing Methods

Pressure Drop Method (CSA B149.1)

The code provides capacity tables for piping based on:

Nominal diameter

Supply pressure

Allowable pressure drop

Total equivalent length

Capacity Table — Natural Gas (supply pressure: 2.1 kPa, drop: 0.5 kPa)

Capacity in m³/h for different lengths:

Diameter	15 m	30 m	45 m	60 m	90 m	120 m
1/2 in	2.8	1.9	1.5	1.3	1.0	0.9
3/4 in	5.9	4.1	3.3	2.8	2.3	2.0
1 in	11.1	7.7	6.2	5.4	4.3	3.7
1 1/4 in	22.7	15.8	12.8	11.0	8.9	7.7
1 1/2 in	34.0	23.7	19.2	16.5	13.4	11.6
2 in	65.2	45.4	36.8	31.7	25.7	22.2

These values are provided for reference only. Always use the current CSA B149.1 tables during the exam.

Formula Calculation Method

For medium and high pressure systems, you can use the simplified formula:

Q = C × √(ΔP × D⁵ / (L × G))

Where:

Q = flow rate (m³/h)

C = constant depending on units

ΔP = pressure drop (kPa)

D = inside diameter (mm)

L = equivalent length (m)

G = relative density of the gas (air = 1)

Sizing Procedure

93.Calculate the total load: add up the flow rates of all appliances supplied by the pipe.

94.Determine the equivalent length: actual length + equivalent lengths of fittings.

95.Choose the allowable pressure drop based on the system type.

96.Consult the CSA B149.1 tables or apply the appropriate formula.

97.Select the diameter that satisfies the required capacity.

98.Verify the residual pressure at the most distant appliance.

Relative Density and Correction

Relative Density of Gases

Relative density (or specific gravity) is the ratio of the gas density to that of air under the same conditions:

Gas	Relative Density
Air (reference)	1.00
Natural gas	0.60 to 0.65
Propane	1.52
Butane	2.00

Effect on Capacity

A gas denser than air (such as propane) requires a larger pipe for the same volumetric flow rate. The capacity of a pipe is inversely proportional to the square root of the relative density:

Q₂ = Q₁ × √(G₁ / G₂)

Example

A pipe carries 10 m³/h of natural gas (G = 0.60). What would its capacity be with propane (G = 1.52)?

Q_propane = 10 × √(0.60 / 1.52) = 10 × 0.628 = 6.28 m³/h

Altitude Correction

Effect of Altitude

At high altitude, atmospheric pressure is lower, which affects:

Gas density

Volumetric heating value

Pipe capacity

Correction Factor

CSA B149.1 requires a correction for installations located above 700 m in altitude. The correction factor is:

F = 1 - (0.0001 × (Altitude - 700))

Application

For an installation at 1500 m altitude:

F = 1 - (0.0001 × 800) = 1 - 0.08 = 0.92

The pipe capacity must be multiplied by this factor, or the load must be divided by this factor.

Heating Value and Equivalent Flow

Heating Value

Heating value is the amount of heat released by the complete combustion of a volume of gas:

Gas	Higher Heating Value
Natural gas	37.3 MJ/m³
Propane	93.2 MJ/m³
Butane	121.8 MJ/m³

Flow Conversion

To compare appliances operating on different gases, you use the equivalent flow rate:

Q_equivalent = Q × (HV_gas / HV_reference)

Example

An appliance consumes 2 m³/h of propane. What natural gas flow rate would be equivalent in terms of power?

Q_NG = 2 × (93.2 / 37.3) = 2 × 2.5 = 5 m³/h

Applicable CSA B149.1 Rules

Rule 6.2 — Pressure Drop

Rule 6.2 of CSA B149.1 states that the pressure drop in piping must not exceed the specified allowable values. The gasfitter must ensure that the pressure at the point of use is sufficient for the operation of the appliances.

Rule 6.3 — Pipe Sizing

This rule requires that piping be sized according to the code tables or by calculation, taking into account:

Supply pressure

Equivalent length

Required flow rate

Gas density

Rule 6.4 — Gas Velocity

The velocity of gas in piping must not exceed 20 m/s to avoid noise and excessive erosion.

Rule 6.5 — Test Pressure

Piping must be subjected to pressure tests before being put into service:

Low pressure: 35 kPa for at least 15 minutes

Medium and high pressure: 1.5 times the service pressure, minimum 350 kPa

Practical Applications

Sizing a Main Supply Pipe

Problem: A residence must supply the following appliances:

Range: 1.5 m³/h

Water heater: 2.0 m³/h

Furnace: 3.5 m³/h

Dryer: 1.0 m³/h

The main pipe is 25 m long with 4 — 90° elbows and 2 tees (branch). Supply pressure: 2.1 kPa.

Solution:

156.Total load = 1.5 + 2.0 + 3.5 + 1.0 = 8.0 m³/h

157.Equivalent length:

Actual length: 25 m

4 — 90° elbows (1 in): 4 × 0.5 = 2.0 m

2 branch tees (1 in): 2 × 1.0 = 2.0 m

Total equivalent length: 25 + 2 + 2 = 29 m

162.Allowable drop: 0.5 kPa

163.From the table (30 m, 0.5 kPa drop):

1 in: 7.7 m³/h — insufficient

1 1/4 in: 15.8 m³/h — sufficient

166.Selection: 1 1/4 in pipe

Verifying Residual Pressure

Problem: The 1 1/4 in pipe from the previous example supplies the most distant appliance. Verify the pressure at this appliance.

Solution:

Supply pressure: 2.1 kPa

Actual drop for 8 m³/h over 29 m: approximately 0.15 kPa (interpolation)

Residual pressure: 2.1 - 0.15 = 1.95 kPa

Minimum required pressure: 1.75 kPa

Conclusion: the pressure is sufficient.

Pitfalls to Avoid

Unit Conversion Errors

Confusing gauge and absolute pressure: always add atmospheric pressure (101.325 kPa) in gas law calculations.

Forgetting to convert temperatures to Kelvin: T(K) = T(°C) + 273.15.

Mixing up in H₂O and kPa: 1 in H₂O = 0.249 kPa, not 1 kPa.

Sizing Errors

Using actual length instead of equivalent length: always include fittings.

Forgetting the total load: add up all appliances, including those that do not operate simultaneously (unless permitted by the code).

Neglecting gas density: propane requires larger pipes than natural gas.

Ignoring altitude correction: above 700 m, capacity decreases.

Code Errors

Excessive pressure drop: never exceed 0.5 kPa in low pressure systems.

Insufficient residual pressure: verify the pressure at the most distant appliance.

Excessive velocity: do not exceed 20 m/s in piping.

Calculation Errors

Rounding too early: keep at least 3 significant figures during calculations.

Reversing the density ratio: Q₂ = Q₁ × √(G₁/G₂), not the reverse.

Forgetting temperature in the gas law: pressure and volume vary with temperature.

Exam Tips

Problem-Solving Strategy

195.Read carefully the problem and identify the given data.

196.Convert all units into a consistent system (SI preferred).

197.Draw a diagram if possible to visualize the system.

198.Identify the appropriate formula or table.

199.Solve step by step showing your calculations.

200.Check the plausibility of the result (a flow rate of 100 m³/h in a 1/2 in pipe is impossible).

Key Points to Memorize

1 in H₂O = 0.249 kPa

1 psi = 6.895 kPa

Maximum low pressure drop: 0.5 kPa

Minimum appliance pressure: 1.75 kPa (NG) and 2.75 kPa (propane)

Relative density: NG = 0.60-0.65, propane = 1.52

Altitude correction above 700 m

Maximum velocity: 20 m/s

Low pressure test pressure: 35 kPa

Time Management

Sizing calculations represent approximately 15 to 20% of the exam questions.

Allocate 2 to 3 minutes per calculation question.

If a question seems too complex, skip it and come back later.

Summary

Pressure and flow calculations are essential for the safe sizing of gas systems. The fundamental points to remember:

216.Units: master conversions between kPa, psi, in H₂O, and bar.

217.Absolute vs. gauge pressure: always use absolute pressure in gas laws.

218.Ideal gas law: P₁V₁/T₁ = P₂V₂/T₂ to correct volumes.

219.Pressure drop: do not exceed 0.5 kPa in low pressure, 10% in medium and high pressure.

220.Equivalent length: include fittings in the total length calculation.

221.Sizing: use the CSA B149.1 tables or the appropriate formulas.

222.Relative density: propane (1.52) requires larger pipes than natural gas (0.60).

223.Altitude: correct capacity above 700 m.

224.Residual pressure: verify that the pressure at the most distant appliance is sufficient.

225.CSA B149.1 Code: rules 6.2 to 6.5 govern sizing and testing.

Regular practice of sizing problems is the best preparation for the exam. Redo the examples in this chapter without looking at the solutions, then check your results. Mastering these calculations will serve you not only for the exam, but also in your daily practice as a Class A gasfitter.

Pressure and Flow Calculations for Gas Systems