An increase in consciousness of environmental issues combined with subsequent legislation means that major oil and gas companies are under pressure to cut their greenhouse gas emissions and several have responded by setting reduction targets over the coming decades.
There are several ways in which operators can work towards emissions reductions. Explosion safety and venting equipment maker Rembe’s focus is on the impact the use of various safety devices can have on this target.
The first point of consideration in this regard should be the safety valves in use. Valves are an obvious place to start as no valve is 100 percent leak-tight, and this decreases every time there is an activation and the valve re-seats. In the building of new plants, it is a fairly simple solution to specify within the design of the plant a valve with a lower leak rate. However, existing plants are looking at substantial investments to replace older designs with newer technologies. Not a viable economical solution in most cases.
While there have been significant increases in the capabilities of safety valves, they are still not the ideal product when considering future net zero targets. No safety valve is 100 percent leak-tight and they struggle to meet the exacting requirements of legislators. An alternative solution is needed.
Secondary relief
Although rupture discs have been around for many decades, they are often considered only as secondary relief, to be used where there is a possibility that the safety valve may fail. There is a lack of understanding among engineers in industry and a number of myths surrounding the use of rupture discs.
A rupture disc is a non-reclosing device and therefore must be completely replaced when there is an activation. Nuisance downtime leads many operators to associate rupture discs as being problematic whereas if a disc is rupturing frequently there is likely a problem with the process. It is still unrecognised by many operators that when the disc performs correctly it is not the problem, but the solution.
How can a rupture disc help to get improved performance from a safety valve? Rupture discs are 100 percent leak-tight. By installing a rupture disc in front of a safety valve you get double protection and a solution which can meet emission requirements. There is no more leakage through the safety valve in normal operation and where there is an over-pressure activation, the valve reseats to seal the process once the pressure is vented.
The belief that this arrangement adds more cost onto a project has been proven to be false, in fact the opposite is the case. A correctly engineered rupture disc will help lower operating costs and increase the up-time for any plant.
In processes where there is a high concentration of corrosive media, increased temperatures and an operating pressure close to the safety valve set pressure, safety valves are pushed to their limits. Poor performance is common-place. High maintenance costs are needed to keep the valve as close to original specifications as possible, increased downtime to the production for routine valve servicing and/or repairs and higher manpower costs to cover the work scopes.
The solution of some 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. But if one considers a typical petrochemical plant with several hundred safety valves the capital expenditure is significant.
Isolating the valve
A rupture disc fitted upstream of the safety valve completely isolates the valve from the process. This protects the safety valve from the process which in turn reduces maintenance requirements. There is also the possibility of reducing costs by sourcing a rupture disc and holder in an exotic material and a standard safety valve. The costs of a disc and holder are usually significantly lower than having to source a high specification safety valve which is compatible with the process media.
The protection of safety valves with rupture discs has become increasingly common in recent years across several industries. However, many operators miss the opportunity to protect the safety valve fully by also isolating the valve from potential corrosion issues on the outlet of the valve.
In many cases, the valve outlet is not a separate discharge line to, but is connected with other parts of the plant via a manifold which allows process gases/vapour to enter the outlet of the valve. If there is a risk that the process media can damage the valve via the inlet, this is also the case downstream.
A rupture disc can also be used to isolate the safety valve outlet and prevent any contact with the process media. The rupture disc will also block any back pressure from entering the safety valve and remove those concerns during valve selection.
With burst sensors installed both upstream and downstream rupture discs can be monitored and connected back to the control room for system reporting across the plant, so operators know instantly which valves and discs are in a green or red state.
Another myth surrounding rupture discs is that they can leak. If the disc is to be installed as the primary safety device, that’s to say, without a safety valve behind it, this can be a concern for operators looking to reduce emissions. The majority of leakages via rupture discs are caused by corrosion or damage during installation by mishandling or incorrect torquing.
Rupture disc technology has improved significantly over the years to ensure that damages caused by corrosion or incorrect handling are all but eliminated. Today’s modern rupture discs no longer use mechanical scoring techniques during manufacturing which can lead to works hardening and corrosion over time. Advanced manufacturing technologies have resulted in robust rupture discs which are no longer sensitive to torque and virtually immune to damage during installation. Most spurious failures from rupture discs can be avoided by working together with the disc manufacturer to select the ideal rupture disc for the process conditions.
Overall, rupture discs can be used as a cost-effective and efficient way to create a leak-tight process and reduce emissions whether on their own or in combination with a safety valve.
