Acrylic plastic viewports in diving systems have been in satisfactory use for a number of years. However, the development of testing them by polarised light showed that there are various misconceptions about them.

The International Marine Contractors Association (IMCA) has just released updated guidance on the topic. An initial guidance note which sought to correct these misconceptions was initially prepared as AODC 030 in 1986 following advice from Det Norske Veritas (DNV). Now a revised document has been produced by IMCA through the Safety, Medical, Technical & Training Committee of its Diving Division Management Committee as part of its planned phased review of AODC guidance notes, which are then issued as IMCA guidance.

“Acrylic viewports are used in many pressurised systems under a variety of conditions world-wide and this document has been produced to provide general guidance,” explains IMCA’s Technical Director, Jane Bugler. “It includes additional guidance on deterioration compared to its predecessor, and identifies a reference document which perfectly complements it.

“That document is the ASME ‘Safety Standard for Pressure Vessels for Human Occupancy: In-Service PVHO Acrylic Windows Guidelines, PVHO-2: 2003’ which provides technical criteria and guidelines for the in-service inspection, care, repair or replacement, testing and recertification of pressure vessels for human occupancy (PVHO) acrylic windows.”

The IMCA document comes to five specific conclusions: • Acrylic plastic viewports should be properly certified and marked prior to use. • It is generally agreed that they should be replaced after 10 years of service (PVHO-2: 2003 provides additional guidance when determining the length of service. • Examination of an acrylic plastic viewport by polarised light after the viewport has been in service is of very limited use, as all such viewports will show evidence of inbuilt stress. • The use of polarised light to check that acrylic plastic viewports are correctly mounted and evenly pre-stressed is not a simple matter and should not be used as a principal test • When mounting an acrylic plastic viewport, it is critical, especially when installing new ports in old pressure vessels, to check that the housing is the correct dimension and that the viewport and any sealing gaskets or O-rings are properly seated and of the correct size. A torque wrench should be used to correctly tension fixings diametrically to the manufacturer’s specification when replacing retainers.

Tags: IMCA