In recent years several industries have recognised the need to isolate or protect the inlet of safety valves with rupture discs, but many have missed the opportunity to fully protect the safety valve from the effect of process conditions entering the outlet side of the valve.
Many years ago, the various industries, in many cases driven by legislators and environmental groups, faced the need to improve the rogue emissions that had for many years been almost ignored as part of the way things were.
Finding Better Safety Valves
The first step was to try and find better safety valves, which for new plants was simple to build into the design. But existing plants were looking at substantial investments to replace older designs with newer ones, not an economical choice in a large majority of cases. Even for new plants there was a significant increase in safety valve costs to try and meet the lower emission levels required for certification and necessary permits to allow start up and be able to continue to run the plants.
While there were increases in the capabilities of safety valves, it was still not ideal or meeting the requirements visualised for future zero targets. The expectation was that safety valves could not meet the requirements and an alternative solution was needed: enter the rupture disc.
The Benefits Of Rupture Discs
Rupture discs have been around for decades and were always seen as the secondary solution for overpressure after safety valves, the second-class citizen of the safety valve industry, a title they do not deserve! The lack of understanding of the rupture disc continues to this day. It is still a mystery to engineers in the mechanical and process disciplines and to the field operatives that install and maintain them, or in a lot of cases ignore them.
Discs are perceived as being a “problem”. They open and let the pressure out, when in fact that is exactly what they are designed to do. It is still unrecognised by many operators that when the disc performs correctly, it is not the problem, but the solution.
How Does The Rupture Disc Help The Safety Valve Perform Better In Use?
Well in the case of isolation it partners the safety valve and brings the superior performance needed to meet zero emissions more than a stand-alone safety valve provides. Yes the safety valve can have good performance levels on its own, but achieving 100% isolation and providing better operational stability requires the use of rupture discs.
For several years now we have regularly seen rupture discs being installed upstream of a safety valve. Operators are now starting to appreciate that a properly engineered rupture disc will help lower their operating costs and increase the up-time for a plant.
The belief that this arrangement adds more cost into a project has been proven to be false: in fact the opposite is the case, costs come down.
Take a typical installation where the safety valve faces a process condition with high concentration of corrosive materials, increased temperatures and an operating pressure close to the safety valve set pressure. This tests the limits of safety valves and we see poor performance below the expected levels needed for operational stability and no leaks. High maintenance costs are needed to keep the valve as close to original specs as possible, increased downtime to the production for routine valve servicing and/or repairs and higher manpower costs to cover the work scopes.
What’s The Answer?
The solution of the safety valve manufacturers is a higher specification valve, more exotic materials with higher CAPEX costs as well as increased cost of spares to maintain the valves.
The use of a rupture disc in front of the safety valve allows for the use of valves in standard materials, isolated from the process by a ‘more exotic’ rupture disc.
Claire Lloyd is with Rembe