Flameproof enclosures – pressure testing methods
Overpressure testing is an integral part of the verification of new or overhauled Ex d enclosures. Pneumatic testing presents a potentially effective way to complete overpressure testing and may provide some advantages over hydrostatic alternatives.
This paper looks at the process of pneumatic testing of Ex d enclosures including risks and hazards, as well as the advantages of carrying out such testing.
1 OVERPRESSURE TESTING
1.1 Purpose of Overpressure Testing
Overpressure testing is an important part of verification testing as part of new, overhauled or repaired equipment. Overpressure testing forms part of two forms of enclosure validation:
· Type Testing – One of two principal tests used for validating Ex d enclosure:
o Flame propagation test – used to check efficacy of flamepaths
o Peak pressure test – used to establish peak (reference) pressure resulting from optimised CH4 ignition.
· Routine Testing
o Used to validate the structural integrity of new, overhauled or repaired Ex d enclosures using 150% of the reference pressure.
1.2 Safety Considerations in Overpressure Testing
The risks to personnel, plant and equipment when fluid is compressed and when failure of the pressure vessel (enclosure) or fittings may occur is potentially catastrophic. This is because compressible fluids can store significant amounts of energy.
For example: one cubic metre of water at 1MPa stores approximately 200J, and one cubic metre of air at 1MPa stores approximately 2 000 000J of energy. This energy is in addition to the energy created due to the elasticity of the enclosure itself.
Based on these examples, hydrostatic testing appears ‘safer’ than pneumatic testing, as the potential energy stored is significantly less. Pressure vessels are, however, commonly used for gaseous substances and are generally operated safely by non-experts on a regular basis (gas bottles, SCUBA gear, etc).
Regardless of the substance used, it is critical that demonstrably competent practitioners and sound testing methodology are adopted when performing any overpressure testing to ensure it is carried out safely.
1.2.1 The consequences of a failure of overpressure testing
A few examples of the serious consequences of a failure of enclosures during overpressure testing are described below.
Figure 1 shows the result of a failure due to overpressure during preparation of hydrostatic testing. The tank top blew off due to high water filling rates coupled with a low capacity relief valve.
A KC-135 aircraft was being pressurised at ground level. The outflow valves which are used to regulate the pressure of the aircraft were capped off during a 5 year overhaul and were not reopened. As a result the rear hatch was blown a distance of over 70 yards, behind a blast fence. Figure 2 shows the resulting damage to the aircraft.
The damage sustained to a tank which collapsed during the emptying of water following hydrostatic testing is shown in Figure 3. A plastic sheet blocked the air inlet and created a vacuum.
1.1 Considerations for Overpressure Testing
A number of criteria for overpressure testing should be defined, documented and understood prior to starting tests. This will ensure that the results are accurate and relevant. Some questions that may be considered include:
· What event is being emulated in testing? A gaseous CH4 ignition for example?
· What test method best emulates this event:
o Transient (impulse) testing with gas
o Static testing with gas
o Static testing with liquid?
· What deformation and/or damage is acceptable as a result of overpressuring the Ex d enclosure?
· Is it necessary to identify leaks in the enclosure? Can those leaks be between compartments and/or to outside the enclosure?
· When is the most relevant time to measure (flamepath) deformation - under load or after relaxation?
1.2 Standards Applied to Overpressure Testing
There are many standards which can be applied to overpressure testing depending on the application. In Australian underground coal mining applications there are three principal references:
· AS/NZS60079.1:2007 (IEC60079.1:2007)
· AS/NZS 3800:2012 (IEC60079.19:2010)
· HB239:2011 (IECEx OD 315-5:2013)
NOTE: the term “hydrostatic” only appears in HB239. All other documents use the term “overpressure” when referring to this type of testing.
These standards outline the performance criteria of an enclosure during and following overpressure testing, and the way such testing should be completed. The relevant sections of the above mentioned standards are highlighted below.
AS/NZS 60079.1 – Ex d Equipment
Routine overpressure test – First method (static):
· 1.5 times reference pressure for at least 10 seconds (Clause 18.104.22.168)
· Test criteria:
15.1.1 Type Test
…the enclosure suffers no permanent deformation or damage affecting the type or protection…the joints shall in no place have been permanently enlarged
22.214.171.124 Type Test
15.1.1 and there is no leakage through the walls of the enclosure
16.1 Routine Test
Routine tests intended to ensure that the enclosure withstands the pressure and also that it contains no holes or cracks connecting to the exterior.
16.3 Routine Test
…the enclosure withstands the pressure without suffering permanent deformation of the joints or damage to the enclosure…
The individual parts…can be tested separately…such that stresses are comparable to complete enclosure
AS/NZS 3800 – Repair and Overhaul
An overpressure test shall be conducted where structural repairs of the enclosure have occurred or the integrity of the enclosure is in doubt:
· …1.5 times the reference pressure and held for at least 10 secs
· Pass/fail criteria shall include assessment of structural damage measured at the geographical [sic] centre of an enclosure panel…flamepath joint surfaces shall be measured to verify that there is no permanent deformation.
· Where overpressure tests are carried out on multi-compartment enclosures the tests shall be done on individual compartments separately.
· HB 239 provides practical assistance…for verifying the integrity of explosion protected equipment.
HB239 Guidance on Repair and Overhaul
· Major repairs include…any action that brings the enclosure integrity into doubt…and be verified by pressure test.
· Minor repairs [include] a window lens replacement…
6.8 Hydrostatic Pressure Tests:
· Over pressure testing (static testing) of enclosures is required…where repairs are of a structural nature… [or] where the structural integrity is in doubt.
· It is only necessary to test individual compartments…
· Repairs of a minor nature may not require overpressure testing
Other considerations from Clause 6.8:
· Seal the test piece and fill with the testing fluid… (6.8.3.d)
· Note: Air…is recommended only for pressurised enclosures where relatively low pressures are involved. (6.8.3.d)
· …particular attention to flamepaths…flat sections…may show minor deformation but this may not take away from the structural strength of the enclosure. (6.8.3.j). Cf: AS/NZS 3800 126.96.36.199
· The test should be considered satisfactory if the enclosure has not suffered damage or permanent deformation that may affect its explosion protected properties. (6.8.4)
NOTE: Hydrostatic testing using water is the conventional approach, but it is not the only approach permitted under AS/NZS 3800 and HB 239. An example of overpressure test rigs is shown in Figure 4.
2 PNEUMATIC TESTING
2.1 Why Pneumatic Testing?
There are a number of reasons why pneumatic testing may be considered an effective means of overpressure testing for Ex d enclosures:
· Static forces imposed on an enclosure are independent of fluid used
· Disassembly requirements are significantly reduced
· Errors introduced on re-assembly are minimised
· Turnaround times can be reduced
Pneumatic testing as a method of testing is not new; several testing facilities have the capacity to conduct pneumatic testing. TestSafe established a facility at Londonderry under Jim Munro 30 years ago, and Simtars are believed to have a similar facility.
2.2 Undertaking pneumatic testing
When undertaking pneumatic testing the relevant standards must be considered:
· AS 1210:1989 Unfired Pressure Vessel Code
· AS 4047 Pressure Piping
Furthermore a comprehensive risk assessment should be completed. The results of this assessment will determine additional safety considerations that may be required such as remote monitoring and measurement, and exclusion of personnel from test area.
Minimum test pressures need to be maintained while compensating for leakages through deflected flamepaths.
Pneumatic testing is an effective and compliant means of overpressure testing of new, overhauled or repaired Ex d enclosures. This form of testing simplifies the preparation and reassembly of the enclosure being tested.
Irrespective of substance used, it is critical that safety considerations associated with compressible fluids be addressed, including ensuring the competency of personnel working with pressure vessels.