If your business relies on compressed air, you are hosting a highly pressurized thermodynamic storage system on your workshop floor. While we often take air compressors for granted, the South African government does not.
Operating an uncertified or untested air receiver tank is a criminal offense under the Occupational Health and Safety Act (OHSA). If an untested tank fails, the stored energy releases instantaneously, resulting in a devastating explosion.
To prevent these catastrophic accidents, the Department of Employment and Labour enforces the Pressure Equipment Regulations (PER) and the SANS 347 standard.
In this post, we’ll break down what SANS 347 requires, what components must be tested (and what is exempt), the truth about mobile compressors, and legal ways you can optimize your compliance costs.
What is SANS 347?
SANS 347 (Categorization and Conformity Assessment Criteria for Pressure Equipment) is the national standard that determines how a pressure vessel is classified based on its risk level. This risk is plotted using three criteria:
- The type of equipment (vessel, piping, or accessory).
- The physical state of the fluid (gas or liquid).
- The hazard group of the fluid (Group 1 for dangerous, Group 2 for non-dangerous like air and nitrogen).
Because compressed air is a gas that does not pose an immediate chemical hazard, standard air receivers are plotted on SANS 347 Figure 2 (Non-dangerous gas vessels).
The hazard category is determined mathematically by multiplying the vessel’s design pressure ($PS$ in bar) by its total volume ($V$ in Litres). Based on this $PS \cdot V$ threshold, your tank falls into one of five categories:
- Sound Engineering Practice (SEP): Extremely small or low-pressure vessels.
- Category I: Low-risk, requiring manufacturer self-declaration.
- Category II: Requires professional engineer design verification and Approved Inspection Authority (AIA) manufacturing oversight.
- Category III: Medium-to-high risk, requiring full third-party design and fabrication audits by an AIA.
- Category IV: High-risk systems requiring maximum regulatory surveillance.
Why is Pressure Testing Mandatory?
As air is compressed, moisture from the ambient humidity condenses inside the steel receiver tank. Over time, this stagnant water—combined with acidic atmospheric elements and compressor oils—causes localized corrosion and pitting along the bottom of the tank. Because this structural thinning is invisible from the outside, the tank can easily rupture under normal working pressures.
To verify the tank’s integrity, Regulation 11 of the PER requires all operational pressure vessels to undergo a witnessed hydrostatic (water) pressure test to $1.25 \times PS$ at intervals not exceeding 36 months.
A hydrostatic test is used because water is nearly incompressible. If the tank fails during a water test, it drops pressure instantly without the explosive release of energy that would occur if it were tested with compressed air. This test must be conducted and certified by an accredited in-service Approved Inspection Authority (AIA).
Exhaustive Testing Scope: What Needs Testing vs. What is Exempt?
Not everything connected to your compressor requires statutory pressure testing. The law separates high-energy storage vessels from rigid machinery casings that are designed for mechanical strength rather than pressure containment.
Here is a quick reference table of what must be pressure tested and what is exempt under South African law:
| Equipment Type | SANS 347 Status | Testing Requirement | Legal Basis (PER) |
| Main Air Receiver Tank | Pressure Vessel | Hydrotest to 1.25 times design pressure every 36 months | Regulation 11(1)(d) |
| Screw Compressor Separator Tank | Pressure Vessel | Hydrotest to 1.25 times design pressure every 36 months | Regulation 11(1)(d) |
| Compressor Reciprocating Casing | Machinery Casing | Exempt from hydrostatic testing | Regulation 2(3)(d)(i) |
| Pneumatic / Hydraulic Cylinders | Actuator | Exempt from hydrostatic testing | Regulation 2(3) Note (m) |
| Utility Air Piping (at or above 50 kPa) | Piping System | Visual inspection and pressure test post-installation | Regulation 11(1)(b) |
| Water Distribution Piping (below boiling) | Utility Piping | Exempt from pressure equipment regulations | Regulation 2(3)(a) |
Mobile Compressors on Wheels: The Exemption Myth
A common compliance misunderstanding involves diesel-driven, mobile utility compressors on trailers or truck beds. Many fleet owners assume these units are exempt under the “road vehicle functioning” clause of the PER (Regulation 2(3)(c)).
This is a critical error. The vehicle exemption only applies if the compressed air is used exclusively for the functioning of the vehicle itself, such as running pneumatic brakes or suspension.
If the mobile compressor is generating “utility air” to run jackhammers, sandblasting equipment, or drills, it is classified as industrial pressure equipment. Furthermore, the PER explicitly states that mobile or transportable pressure vessels containing air are deemed to be pressure vessels, not transportable gas cylinders.
Therefore, the trailer-mounted air receiver and the compressor’s internal air-oil separator tank must carry valid SANS 347 data plates and undergo witnessed hydrostatic testing every 36 months.
Legal Compliance Workarounds
While the law is strict, there are several legal, safe, and clever engineering strategies to minimize your testing costs, prevent downtime, or bypass 36-month water testing altogether:
1. Design Your System Under the SEP Threshold
The easiest way to avoid AIA inspections, design reviews, and 36-month testing is to ensure your vessel’s pressure-volume product remains within Sound Engineering Practice (SEP) limits. Under SANS 347, a vessel is classified as SEP if its pressure-volume calculation meets this limit :
$$PS \cdot V \le 50\text{ bar}\cdot\text{L}$$
By decentralizing your air storage—using multiple small, localized receivers (e.g., a 4-litre tank running at 10 bar, giving a $PS \cdot V$ of $40\text{ bar}\cdot\text{L}$) instead of one massive central receiver—you fall into the SEP category. SEP equipment is legally exempt from recurrent 36-month hydrostatic testing.
2. Secure a 9-Year Testing Dispensation
If your compressed air network is clean, dry, and operates in a non-corrosive environment, you can apply for an extended testing interval. Under Regulation 11(1)(d), if you can prove through historical ultrasonic thickness testing that the vessel is not subject to active deterioration, you can obtain written approval from an AIA to dispense with the 36-month test for up to nine years.
3. Implement Risk-Based Inspections (RBI)
Large industrial sites can legally bypass fixed 36-month intervals by implementing a registered Risk-Based Inspection (RBI) management system under Regulation 12. This program evaluates real-time metallurgical data and operational risks to set custom testing schedules. To do this legally, your RBI program must be audited and verified by a certification body accredited to ISO 17021.
4. Opt for Pneumatic Testing
For specialized cleanrooms or high-precision instrument air networks, filling an air receiver with water during a hydrotest can cause destructive moisture contamination. Under Regulation 11(2), you can apply to an AIA to execute a pneumatic test using an inert gas or dry air at $1.1 \times PS$ instead of water, keeping your pneumatic lines completely dry.
Compliance Checklist for Business Owners
To make sure your workshop is safe and legally protected, ensure you have the following in place:
- The SANS 347 Compliance File: Keep your Manufacturer’s Data Report, Declaration of Conformity, and pressure test certificates in a single, accessible folder on-site for Department of Labour inspectors.
- Conspicuous Data Plates: Ensure every air receiver and air-oil separator has a permanently fixed metal plate showing the manufacturer, serial number, design pressure (in Pascals), and SANS 347 hazard category.
- Safety Accessories: Every receiver must be fitted with at least one representative pressure gauge and a locked, sealed safety relief valve to prevent unauthorized tampering.
By keeping your pressure equipment documented and regularly tested, you protect your employees from physical harm, safeguard your business against massive legal liabilities, and ensure your insurance coverage remains fully valid.