1. PURPOSE

 

1.1       This global engineering specification defines the minimum Company requirements for pneumatic pressure testing carbon and low alloy steel pressure vessels. Many of these requirements are additional to those required by the vessel design and manufacturing code, but are imposed to ensure safety of inspectors attending the pressure test on Company behalf and to minimise schedule risk in the event of a failure.

 

1.2       Pneumatic testing may be preferred because of difficulties in draining and drying a vessel after hydrotest or issues with cost of the water and/or water treatment (and disposal) needed to inhibit corrosion of vessel internals.

 

 

2.         scope

 

2.1       This specification shall be applied to all new carbon or low alloy steel pressure vessels (including heat exchangers) specified or purchased by Company whose first code pressure test is to be pneumatic.

 

2.2       Exclusions – The following are excluded from the scope of this specification:

 

• Hydrotested vessels that are subsequently pneumatically leak tested at site at a pressure at least 10% below the original hydrotest value (regardless of pressure, volume, or thickness).
• Vessels whose greatest reference thickness does not exceed 10 mm (25/64 in) (regardless of pressure and volume).
• Vessels with a product of pressure and volume not exceeding 3000 bar.litres (1500 psi-ft³) (regardless of thickness).

 

For the purposes of this specification, the reference thickness is:

 

• The actual nominal thickness of butt-welded parts (at the location of the butt-weld).
• For nozzles and compensating plates, the greater of the shell at that location and the reinforcing pad or nozzle wall thickness.
• For flat ends, flanges and tube-plates and other components that do not contain butt-welds; the nominal thickness/4.

 

 

3. related documents

 

3.1       Company Engineering Documents

 

4WPI-EW44001         Pressure Testing of Process Piping and Equipment

 

3.2       American Society of Mechanical Engineers (ASME)

 

BPVC, Section VIII     Rules for Construction of Pressure Vessels, Division 1

 

 

4.         Material Requirements

 

4.1       All pressure boundary material must have demonstrated impact properties of at least 40 Joules (29.5 ft-lbf) average [34J (25.1 ft-lbf) minimum] at minus 20 degrees Centigrade (-4°F) or the value specified in the applicable vessel design code or material standard if this is more onerous.

 

4.2       Pressure boundary material is defined as shell plate, head plate, nozzles, nozzle flanges, manways, manway covers, temporary pressure test blanks, and all items directly welded to any of these. Thus it will include all pads welded to the shell/heads including internals, supports, and the section of skirt directly welded to the head.

 

4.3       Where the head or any other part of the pressure boundary is subject to post-weld heat treatment (PWHT) then the required impact testing must be demonstrated on material subjected to a simulated PWHT (see Section 9).

 

 

5.         Weldng Procedure

 

5.1       All welding procedure qualification tests shall be tested to demonstrate 40 Joules (29.5 ft-lbf) average [34 Joules (25.1 ft-lbf) minimum] at minus 20 degrees Centigrade (-4°F) in the weld and heat-affected zone. Testing must cover all welding processes and consumables.

 

5.2          Any change in trade name of flux coated electrodes, wire and flux combinations, flux-cored wires, or shielding gas used in production or permitted on a WPS, from that used to qualify the procedure qualification test (PQR), shall be considered an essential variable and require requalification.

 

5.3          For flux cored wires (the FCAW process) the following requirements shall be applied:

 

• Dual Shield Process.
• Filler metal used in production shall be from the same lot or heat number used to qualify the WPS, when the base material is carbon or low alloy steel that requires impact testing. (Separate batch testing for impact values is also acceptable.)
• The use of low hydrogen diffusible electrodes (H4 or H8) is mandatory.
• Filler metal wire diameter shall be £1.2 mm (0.045 in).

 

5.4       Where PWHT is required in production, the procedure qualification test plates must have been subjected to a simulated PWHT (see Section 9).

 

 

6.         Non-destructive examination

 

6.1       All longitudinal weld seams, including seams in heads, shall be subjected to 100% volumetric examination.

 

6.2       At least 10% of the circumferential seams shall be subjected to volumetric examination. The areas examined must include all T-joints and other areas equally spaced around the circumference. Each area examined should be a minimum of 150 mm (6 in) and a maximum of 400 mm (16 in) long.

 

6.3       Where weld seams are subject to PWHT the volumetric examination must be performed before and after PWHT.

 

6.4       Manway and nozzle to shell and head welds shall be subjected to the following NDE:

 

	Nozzle or manway dimensions	MT	UT
	≤ 6″ NB and with wall thickness ≤ 12 mm	100%	0%
	>6″ NB   or wall thickness >12 mm	100%	100%

 

To facilitate this examination the welds shall blend smoothly into the components. This may require dressing by grinding at the option of the Company Inspector or NDT Operator.

 

6.5       Where the design code does not require the level of NDE specified in paragraphs 6.1, 6.2, and 6.4:

 

• The above NDE may be performed by ultrasonic examination (UT) at the manufacturer’s option.
• The defect acceptance criteria (RT or UT) for non-planar discontinuities (for example, slag or porosity) may be increased to double that demanded by the design code for a 100% examined seam. Planar defects such as cracks and lack of fusion are not permitted and must be repaired and re-examined. If repairable defects are found, additional areas immediately either side of the defect area shall be examined together with a further 10% of the circumference at equally spaced locations. If any of the further 10% of circumferential areas contain planar or other unacceptable indications, then 100% of the circumferential seam shall be examined and repaired.

 

6.6       There shall be a minimum ‘hold’ period after the completion of welding, before performing non-destructive examination (RT, UT and MT).

 

	Wall Thickness	Minimum Hold Period
	≤ 20 mm	24 hours
	>20 mm	48 hours

 

6.7       Where a percentage examination is required, it shall be performed on all welds, except in the case of nozzles equal to or not exceeding DN150 and small multiple attachments, for example, pipe support lugs. In these cases, similar sized items may be grouped together and 10% of the items shall be subjected to 100% examination.

 

 

7.         weld Production control test plates

 

7.1       Weld production control test plates to verify the impact properties in the weld and HAZ shall be performed for longitudinal seams.

 

7.2       As a minimum, one test plate is required to represent the thinnest shell section, one plate to represent the thickest shell section, and a further one to represent the dished head seams (if any).

 

7.3       If the seams that are represented are subjected to PWHT then the test plates must be tested after PWHT. It is preferred that the test plate is heat treated with one of the components it represents, but a simulated PWHT (see Section 9) may be applied to the test plate with agreement of Company.

 

7.4       The test pieces shall meet an average of 40 Joules (29.5 ft-lbf) [minimum 34 Joules (25.1 ft-lbf)] at minus 20 Degrees Centigrade (-4°F) in both weld and HAZ.

 

7.5       Where circumferential seams are welded to a significantly different welding procedure to that employed on the longitudinal seams, a further test plate to represent the thickest circumferential seam shall be additionally tested. Company shall be the final arbiter on what is significantly different.

 

7.6       For serial built vessels ( that is, vessels built in batches to standard designs), if the longitudinal weld seams are produced by automated processes, the requirement for production weld test plates can be relaxed to 1 plate per every 10 vessels, or 1 plate per batch of welding consumable combination (flux +wire), whichever is the more onerous.

 

 

8.         pneumatic pressure testing

 

8.1       A detailed plan for the pneumatic testing should be submitted to Company for review at least 60 days before the planned pneumatic test. As a minimum, this procedure shall comply with Company specification 4WPI-EW44001.

 

 

8.2       During the test, all personnel should be excluded from the area within the safety distance specified in 4WPI-EW44001, noting that Table 2A applies to outdoor tests and Table 2B applies to indoor tests or tests performed near buildings.

 

8.3       The metal temperature of the vessel at the start of test must be a minimum of 15°C (59°F), or 17°C (30.6°F) above the minimum vessel design temperature (ASME VIII Div. 1 Section UG100) whichever temperature is higher. It is intended that there is a minimum of 35°C  (63°F) between the actual pneumatic test temperature and the temperature at which the plate, forgings, weld procedure, production test plates, and so forth, have demonstrated an average of 40 Joules (29.5 ft-lbf) [34 Joules (25.1 ft-lbf) minimum].

 

8.4       Lowering the material and weld procedure test temperature, together with the minimum vessel design temperature can result in a pneumatic test temperature below 15°C (59°F) but in no case must the pneumatic test ever take place when the vessel metal temperature is less than +5°C (41°F).

 

 

 

8.5       The vessel metal temperature must be measured at the start of the pneumatic pressure test in at least 8 places and the temperature recorded. The measurements must include both heads, the thickest shell section, and the thickest pressure test blank.

 

8.6       It is acceptable to employ supplementary heating to ensure the metal temperature exceeds the minimum value. This must be by the use of, for example, a radiant panel heater or hot air blowers. The use of direct flame impingement or other localised heating is not permitted.

 

 

 

 

9.         simulated PWHT

 

9.1       The simulated PWHT must be within the component heat treatment temperature range and must be equivalent to at least 80% of the actual time at temperature.

 

9.2       To cater for possible repairs after PWHT and subsequent re-PWHT an additional treatment time must be allowed for.

 

9.3       It is permissible to perform the entire simulated PWHT at one time, by using the total possible time at temperature.

 

 

10. VESSEL DESIGN

 

10.1    Vessel longitudinal weld seams shall be located in ‘upper quadrant’ of the vessel (that is, between 315 and 45 degrees). The closer to zero (0) degrees the better although clearly long seam welds should avoid any nozzles or pads situated in the vessel upper quadrant. The reason for this requirement is that if a long seam fails on pressure test, the resulting pressure wave will tend to be directed upwards, pushing the vessel downwards.