Overcoming Integrity Limitations of Conventional On-Stream Line Interventions

Introduction

Line interventions are critical to keeping essential piping and pressure vessels online during repairs, maintenance, and modifications. Traditional line interventions either utilize a single isolation point or are utilized in conjunction with bypass lines so product flow can continue around the isolated section of the one-way feed pipes under repair.

Pressurized piping and pipeline asset operators are increasingly moving away from single isolations to double isolations with a zero-energy space in between to enable safe in-service work on critical systems. The redundancy of these positive isolation methods ensures the system remains effective even if there is equipment failure or operator error.

Conventional DBB Methods

OSHA defines an isolation as “the process by which a permit space is removed from service and completely protected against the release of energy and material into the space by such means as: blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; a double block and bleed system; lockout or tagout of all sources of energy; or blocking or disconnecting all mechanical linkages.”

A double block and bleed (DBB) is a type of double isolation identified as an industry best practice. It replaces the need for using three separate line interventions to perform the same function. Two seals are set in a pipe with a bleed port located between. If pressure leaks past the first seal, it is contained by the second seal and forced to exit the pipe through the bleed port. Utilizing a DBB can save installation time and weight on the piping system and space. A DBB can also considerably reduce the maintenance and installation effort while providing a process safety solution with less potential leak paths for positive isolation on both sides. However, any double isolation method should be fully vetted with a formal risk assessment using established industry risk evaluation methods.

The Future of High-Integrity Isolations

While conventional installations of DBB valve systems have become a regular practice, there are a few extra requirements needed to utilize this method. Conventional DBBs (Figure 1) typically require at least two complete welded fittings and associated equipment to set independent line stop specific fittings. Additionally, a significant amount of pipe must be available to deploy conventional DBBs. This can involve extra time, expense, and/or additional weldments on the pipe being serviced.

New smart DBB technology provides a unique solution to what can be a time-consuming and expensive task of adapting existing pipework to incorporate double isolation. The systems are designed as simplified, smart pipe isolation devices. The technology utilizes a bleed system that can provide a bleed route through a pipe isolation device actuator assembly (e.g., through a control bar forming part of the actuator assembly) to thus avoid the extra items associated with conventional bleed systems. This minimizes associated risks by reducing the need for additional line interventions.