SAQCC Gas LPG Commercial/Industrial common mistakes
SAQCC Gas LPG Commercial/Industrial · Understand the traps and how to avoid them.
Calculating LPG vaporisation or cylinder withdrawal rates using the liquid density instead of the vapour properties, or treating propane and butane as interchangeable in a sizing calculation.
Candidates memorise one 'LPG density' figure and apply it to every gas-phase question, forgetting that commercial LPG is a propane/butane mixture whose vapour pressure, relative density and calorific value change with blend ratio and temperature.
What to do instead
Before any sizing or flow calculation, state which phase you are working in and which blend you are assuming; use vapour relative density (~1.5 for propane, ~2.0 for butane, air = 1) for gas-phase work and liquid density (~0.51 propane, ~0.58 butane kg/L) only for liquid volume; show the blend assumption explicitly in the answer.
Sizing LPG relief/vent arrangements or cylinder storage distances from memory of a single rule of thumb rather than from the applicable standard's table, and mixing metric and imperial separation distances.
Many candidates learned on legacy installations or imperial-era codes and never reconciled them with current SANS/installation requirements, so they substitute 'about a metre' for a tabulated value.
What to do instead
Learn the structure of the standard's separation and ventilation tables (what the rows and columns are), then practise looking up values under exam conditions; always quote units and state the standard clause family you are applying rather than a rounded rule of thumb.
Confusing the combustion air requirement with the ventilation requirement for an appliance, and ignoring the dilution of flue products when calculating room ventilation for a natural gas appliance.
Candidates treat 'air for combustion' and 'air for ventilation/dilution' as one number, and forget that natural gas needs roughly 10 volumes of air per volume of gas for stoichiometric combustion plus additional dilution air.
What to do instead
Work the two requirements separately: calculate stoichiometric air from the gas composition (or the standard's stated figure), then add the dilution/ventilation allowance the code requires; state both figures and the total in the answer.
Treating natural gas as lighter-than-air for every purpose, including leak-detection strategy, without distinguishing methane's buoyancy from the behaviour of odorant and of accumulated gas in pits and ducts.
The 'natural gas rises' slogan is over-learned, so candidates answer ventilation and leak-detection questions as if gas can never accumulate at low level, contradicting the code's requirement to consider confined spaces.
What to do instead
Answer with methane's relative density (~0.6, rises and disperses outdoors) but always add the confined-space caveat: ventilate at high level for dispersion yet test at low level and in pits/ducts before entry, and never rely on smell alone.
Selecting refrigerant by 'what the system was designed for' without checking the replacement refrigerant's glide, oil compatibility and pressure-temperature relationship, or mixing refrigerants in a system.
Candidates focus on capacity match and ignore that zeotropic blends have temperature glide, that mineral oil systems cannot simply take an HFC, and that topping up with a different refrigerant creates an uncontrolled mixture.
What to do instead
For any retrofit or top-up question, list four checks before answering: glide and whether the TXV/charging method must change, lubricant and oil-return compatibility, elastomer/component compatibility, and pressure limits; state that the system must be recovered and the refrigerant identity confirmed before charging.
Reading a refrigerant pressure-temperature chart at the wrong pressure (gauge vs absolute) or quoting a saturation temperature without stating whether it is evaporating or condensing, leading to superheat/subcooling answers that are out by a bar or more.
Candidates use gauge readings directly against absolute-pressure tables, or calculate superheat from the suction line temperature without first converting the low-side gauge pressure to the saturation temperature.
What to do instead
Always convert gauge to absolute (add ~1 bar) before entering a PT table, write the saturation temperature down explicitly, then subtract or add line temperature; show the conversion step in your working so a units slip is visible.
Filling a compressed gas cylinder by pressure alone when the target is a specified mass or when the cylinder will be used at a different temperature, and ignoring the settled-pressure rise after filling.
Candidates treat 'fill to X bar' as universal, forgetting that a cylinder filled hot will exceed its developed pressure when it cools or when ambient rises, and that some gases are filled by weight for accuracy and safety.
What to do instead
For each gas state the fill basis (mass for liquefied/liquefiable gases and for accuracy, pressure with temperature correction for permanent gases), apply the fill-ratio or settled-pressure rule, and note that cylinders must be allowed to settle and be checked before release.
Mixing up cylinder colour coding, valve outlet connections and thread handedness between gas services, or assuming a connection that fits is the correct connection.
Candidates rely on visual memory of cylinder colours and 'left-hand thread for fuel gas' shorthand without checking the specific standard for that gas and region, and outlet standards differ between medical, industrial and fuel gases.
What to do instead
Learn the principle rather than isolated colours: each gas family has a designated outlet/connection standard and colour code, fuel gases use left-hand threads at the regulator, and oxidant/fuel connections must be non-interchangeable; when answering, name the standard's connection designation and state that adaptors must never be used to bridge services.
Underestimating oxygen-cleanliness and oil/grease exclusion when answering questions on oxygen or medical gas systems, and treating medical oxygen as 'just compressed air'.
Candidates apply general industrial-gas habits to oxygen service, where any hydrocarbon contamination is a fire/explosion hazard, and to medical gases, where purity, dedicated equipment and traceability are regulated.
What to do instead
For oxygen questions state the rule explicitly: equipment, seals and PPE must be oxygen-clean and hydrocarbon-free, no lubricant on threads or regulators, and only approved materials; for medical gases add dedicated cylinders/regulators, no cross-use, and documented purity and chain of custody.
Answering 'leak test with a flame' or 'soapy water is enough' for LPG/natural gas installations, and omitting the pressure-hold test, purge and test-certificate steps from a commissioning answer.
Candidates compress the commissioning sequence into 'check for leaks', losing the structured strength/tightness test, the purge of air from new pipework, and the documentation that the exam is testing for.
What to do instead
Answer commissioning as an ordered sequence: visual and support check, strength/pressure test at the specified test pressure and duration, tightness test at operating pressure with a recorded drop, purge and prove at the appliance, then issue the test certificate; state that flame testing is prohibited and detection is by approved leak-detection fluid or instrument.