Jupiter, FL (PRWEB) April 30, 2007
There has been a trend in recent years for new process systems to be both higher capacity and more compact. This trend can lead to difficulty when explosion protection systems are required. Too often the need for explosion protection is left for consideration after all other elements in the process system have been designed. This approach can lead to serious shortfalls, unnecessary compromise to safety, and needless additional expense.
Bill Stevenson is Vice President of Engineering for CV Technology, an international leader in explosion protection and mitigation. In one recent project for a new sugar milling system the distance from the mill to the receiver was too short to allow the needed time for a fast-acting explosion isolation valve to close in the event of ignition inside the mill. This required an extensive re-design and large expenditure. Stevenson points out, "In general a distance of at least 20 feet (30 would be better) is necessary from the mill to the location for a valve to ensure closure ahead of an accelerating flame/pressure front." He explains that an even better approach would be enough distance between the mill and the receiver to allow a fast acting valve to close from a signal in either direction. In this scheme two possibilities are addressed: Ignited material in the mill would be detected and the fast-acting valve would close to prevent flame and pressure from reaching the receiver; or ignited material in the receiver would be detected to close the valve to prevent flame and pressure from propagating back to the mill. This scheme requires double the distance mentioned above.
With the current trend toward more and more compact design these distances are often not available.
Stevenson claims, "We have seen systems that were so compact that proper protection could not be fitted and in consequence are being operated without protection. We have also seen many examples of systems that are neglected and in worst case situations where safety systems have been disconnected simply because they could not be reached for service."
In another project the fast acting valve was squeezed in just at the end of the pipe and at the inlet to the receiver. This scheme provides protection from the mill to the receiver, but accepts the possibility that a deflagration initiation in the receiver could back-flash to the mill. A second fast-acting valve on the air induction port to the mill was necessary to prevent flame spread directly into the room. This scheme adds considerably to the complexity of the protection system and to the cost for protection.
The problem of flame spread propagation in piping systems handling combustible dusts is made even more difficult to control in cases where the distances are too short for isolation to establish a barrier ahead of the flame/pressure front. In another example, the distance from process machine to aspirating dust collector was less than 12 feet. The insurer wanted isolation and the client wanted to comply, but when confronted with the reality that it would not work reliably in such a short distance an alternative and more risky approach had to be taken. In this case the protection system design allows for the possibility that ignition in the machine could accelerate through the interconnected piping to the dust collector and that the resultant ignition would be very strong indeed. This situation is extremely difficult to address with certainty. There is no test data for initially turbulent conditions or from flame-jet ignition sources. Both are possible in this situation and of course the potential for a much more violent explosion is obvious. The housing for the dust collector needed strengthening and the size of the explosion protection system needed to be considerably larger than would have been necessary if more space had been allowed in the original design. More important than the added complexity and expense of this approach is that it is difficult to be certain that this system is adequately protected for a worst case event.
Another impact factor when process systems are designed to be very compact is finding room to fit explosion protection equipment where needed and allowing reasonable access for service. Says Stevenson, "It is always nice to have room to work, and it is necessary to ensure long term reliability for the sophisticated equipment that is used for industrial explosion protection."
The best first step is to get the protection requirements specified as early in the development phase for a new process system as possible. In that way it is often possible to ensure safe protection that is both more reliable and much more cost effective. Bringing us in early is your best option for a successful outcome.
CV TECHNOLOGY, INC
15852 Mercantile Court
Jupiter, FL 33478
Phone: (561) 694-9588