Minimising explosion risks

Adequate safety relief equipment is required to keep the risk of gas engine explosion damage to a minimum, write Klaus Meichle and Roland Bunse

Cogeneration installations fulfill an important role in optimising the use of fuel and in reducing greenhouse gas emissions. Such installations are also well-suited to balancing the volatile output of wind turbines and solar panels.

Due to stringent emissions limits, cogeneration units and on-site power generators are increasingly equipped with catalytic exhaust gas cleaning devices. As a result, the volume and length of the exhaust systems increase, thus increasing the risk of explosion damage in case of engine malfunction. Modern engine management and control systems ensure that such malfunctions occur only sporadically, but any technology can fail. Protective devices must ensure that expensive exhaust pipe components such as the silencer, catalyst and heat exchanger will not be damaged if problems occur. Moreover, a safe environment for the workers in the vicinity of such installations should be guaranteed. Thus, the growing number of on-site power installations requires a proper focus on both safety issues and risk assessment.

Misfires during unsuccessful startup attempts or gas engine malfunctions during operation mean that a potentially explosive mixture of fuel gas and air can enter the exhaust system. This mixture can be ignited by fire from the engine exhaust or by the oxidation catalyst installed in the exhaust system.

An explosion in a system with a length of tens of metres can create a pressure wave – a shock wave with very high pressure. Examples exist where the exhaust silencer and heat recovery boiler were severely damaged, even ripped open, and the catalyst was destroyed.

The Q-Rohr DFE burst disc and flame-catching system
The Q-Rohr DFE burst disc and flame-catching system

Fortunately, no one has yet been injured, but the damage is often substantial for the owner. Both the repairs and the unscheduled downtime can be very costly. Moreover, insurance companies require that effective measures be implemented in order to keep such a calamity from recurring.

Venting the pressure

It is close to impossible to design an exhaust system that will withstand the pressures involved in a shock wave. The design pressure is typically around 1 bar for parts such as compensators and silencers. Therefore, adequate safety relief equipment is required to keep the risk of explosion damage to a minimum. For safe use of gas engines, a fast and highly efficient venting of the explosion pressure during an event must be ensured, along with protection for workers and other engine equipment in the close area around the engine.

REMBE GmbH in Germany has over 40 years’ experience in developing and manufacturing venting devices such as bursting discs to avoid damage from dust or gas explosions and high overpressure. The firm recently designed a combination burst disc and flame catching system (Q-Rohr DFE) for gas engine exhaust pipes. The installation of this system is easy, even in the case of retrofits where space is generally a limitation. REMBE’s solution matches the exhaust pipe flange standards of DIN- 86044.

The Q-Rohr DFE device was tested at a test field belonging to a certified body under a one-to-one scale worst-case scenario. The test demonstrated its proper function as a flameless venting device.

Flameless venting

Figure 1 shows what happens when only a burst disc protects a vessel against damage during an explosion. In this case, the vessel will still experience reaction forces because of the escaping pressure. Such reaction forces can also cause damage to an installation. The same test has been carried out in Figure 2, but this time with a flame-catching housing. This system acts at the same time as a pressure compensator, so no shaking forces are exerted on the exhaust system.

Figure 1. Free venting of a methane gas explosion with DN 800 bursting
Figure 1. Free venting of a methane gas explosion with DN 800 bursting

The result of REMBE’s solution is that vent pipes to the outside of boiler houses and engine rooms are no longer required. The flameless venting system ensures that the overpressure can escape safely within milliseconds to the surroundings, while the flames will be extinguished in the housing.

In addition, the Q-Rohr reduces the sound level during an event to a minimum. It works like a silencer, partly because the outlet is in the boiler house rather than outside the building. Previous solutions where the outlet of a burst disc was directed outside the building caused noise problems during normal operation. The integrated signaling unit atop the bursting disc can be used to shut down the engine, putting it in safety mode. In addition, lights or a horn can be activated in order to further create a safe area and improve worker safety.

Figure 2. Flameless venting of a methane gas explosion with Q-Rohr-3-DFE, DN 800 at 4.5 mà‚³ vessel
Figure 2. Flameless venting of a methane gas explosion with Q-Rohr-3-DFE, DN 800 at 4.5 mà‚³ vessel

The REMBE Q-Rohr provides any cogeneration plant or on-site generation installation with the required safety. Its wide range of system sizes from DN 200 up to DN 1400, and its high venting efficiencies, reduce investment to a minimum. The passive bursting disc technology is nearly maintenance-free and saves service costs. This makes the REMBE Q-Rohr DFE a smart solution for cogeneration’s future.

Klaus Meichle is Sales Engineer, Explosion Protection at REMBE. Roland Bunse is Senior Consultant in Technical Sales Explosion Safety and Product Manager, Venting and Explosion Isolation, also with REMBE

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