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Explosion Protection
 
 
A COMMON SENSE APPROACH

Information on Vent and Door Sizing
Information on Retrofitting Explosion Protection

 

Explosion Protection - the Realities

Henderson Industrial Specialties is both a manufacturer and distributor of specialty industrial process equipment including that for explosion protection. At Henderson, we try to instill a recognition that explosion protection is still as much an art as it is a science and that, the scientifically developed technical principles that are available, are slow to find their way into everyday engineering at all but some large Corporate engineering departments.

Pressure-time graph for vented vs unvented explosions

Time versus pressure of typical vented and un-vented explosions.

In far too many cases, a system is designed which is handling something of an explosive nature. Such a system may include storage silos, interconnecting duct work, dust collection systems, pulverizers, mixers or grinders and so on. Typically, the major pieces of equipment are put out for tender with the dust collectors being stipulated as requiring explosion protection. It is totally irrational to isolate a given piece of equipment from the system in which it is going to operate when assessing explosion protection needs.

In many cases, explosion protection requirements are left to the end of a project, long after vessels are built and systems designed. This puts a lot of pressure on those needing to balance the concerns for adequate protection and their budgets. There are times when the requirement for explosion protection gets “written out of the specification” or “withdrawn from the scope”. There are those that would argue that this leaves the company involved open to legal liability and those making such decisions liable personally under the Occupational Health and Safety Act. Putting equipment at potential risk is one thing, endangering personnel is another.

Too often, the devices are not recognized for what they are – on doors, hinges and latches are corroded while vents are impeded from opening by catwalks, ladders etc. Snow loads and icing in northern climates and wind loads everywhere are often an oversight given the presumption that under operating conditions, operating temperatures will melt snow and ice.

Protection technology can range in price dramatically with the least expensive solutions being doors and vents while at the other end of the spectrum one can find explosion suppression and isolation technology. To focus on the most cost effective, doors typically have lower release pressure capabilities and are not susceptible to fatigue failures or subject to changing release pressures with changes in temperature.“They are capable of leak tight service, service temperatures of up to 2,000°F and can be more cost effective in small quantities. Rupture membrane type vents can provide a leak tight seal more readily in most cases, have a relatively broad tolerance on their release pressure and are more readily incorporated into systems with discharge ducts.

There are several fundamental considerations in the review of a system handling potentially explosive dusts, gases or a mixture of the two. Dependent upon the design basis being used, often National Fire Protection Association Guideline 68, the definition of these may vary somewhat. To facilitate providing the reader with an appreciation of the issues rather than a design primer, the following have been limited to the major ones only.

What is adequate protection against explosion?

Limited to personnel safety only? Preventing catastrophic failure or permanent deformation of the equipment, which would put the system out of operation? Environmental concerns given large volumes of gases, dusts etc. that would accompany an event?


Technology for Explosion Protection

Protection technology can range in price dramatically with the least expensive solutions being doors and vents while at the other end of the spectrum one can find explosion suppression and isolation technology. To focus on the most cost effective, doors typically have lower release pressure capabilities and are not susceptible to fatigue failures or subject to changing release pressures with changes in temperature.“They are capable of leak tight service, service temperatures of up to 2,000°F and can be more cost effective in small quantities. Rupture membrane type vents can provide a leak tight seal more readily in most cases, have a relatively broad tolerance on their release pressure and are more readily incorporated into systems with discharge ducts.

Dust Collector with Exploguard explosion vents

Exploguard explosion vent on a dust collector

Exploguard precision laser cut NFPA68 compliant explosion vent in its mounting frame on a dust collector

Exploguard fracture clip release panel

Exploguard Fracture-Clip Release Panel and Assembly

Equipment/Vessel/Duct Ultimate Design Strength

This is often not well defined owing to many being designed to withstand maximum fan design loads only. Sometimes the designer has only considered the maximum vacuum design condition as positive pressure is irrelevant, at least until it comes time for explosion protection.

Explosibility of the Dust or Gas

This may have been established by tests which are designed specifically for the process or previously published. A major factor is the assessment of relief area requirements. Estimating this can lead to a serious lack of protection or, on the other hand, to needless expense. Actual test data from a laboratory however, will not displace the need for risk management decisions, good engineering practice or judgment – labs tests do not necessarily “scale up” to industrial processes nor do they take into account particle size changes due to attrition, dust layering, the presence of other gases or dusts in the system and so on.

System Design

Often the designer has the option of operating a system under vacuum or under pressure. Often overlooked until design is complete is the fact that one is almost always better off in protecting a system that is under vacuum. Discharge Ducts induce significant back pressures so consider oversizing the duct on runs over ten feet and always minimize duct runs – locate equipment that is indoors as close as possible to outside walls. It must be recognized that a system must be reviewed in its entirety – too often one element of a system is isolated for review, such as in the case of dust collectors.

Risk Management

There are always risk management decisions to be made in any system protection. These properly belong to the owner to make.

Material Bridging and Abrasion, Condensation

These are of concern with any relieving technology and also from the point of view of the process itself.

Emissions

Leakage to atmosphere may be of great concern from the environmental point of view, on systems handling carbon monoxide (CO), sulfur dioxide (SO2) or carbon dioxide (CO2) for example.

Release Pressure

The level of protection is always enhanced with the lowest possible release pressure that can be accommodated by the process. This means that it is generally best to design the system to work under vacuum instead of positive pressure, if the option is available. Installation on positive pressure systems can present other issues such as potentially increased fatigue of explosion vents or leakage/emissions of explosion doors.

Pulsation, Vibration and Fatigue

These are three factors that can affect the service life of a rupture type membrane device whereas doors remain relatively unaffected.

Orientation

The orientation of the relieving device can be important so as to ensure personnel safety, to minimize wind and snow loads etc. One must account for the “zone of influence” of the discharge from the venting device, when such is used, as the fireball and products of combustion are an obvious, but sometimes overlooked, hazard.

Retrofitting Explosion Protection

Updates to existing equipment are always a challenge. Even if the original design strength is known years of corrosion, abrasion and fatigue make this a “moving target.” In addition, equipment located inside the plant has often been installed at a  time when the explosive hazards of the products were not known/recognized or before process changes have brought forth new hazards not previously in existence. Thus, the equipment may be located in areas difficult to deal with discharge ducting and preclude the use of explosion vents or doors.

Weather

Factors such as snow loads and icing in Northern climates or wind loads are often overlooked under the presumption that at operating conditions the operating temperatures will melt snow and ice. Part of the Hazardous Operations Review should be the potential of an explosion at start up (often the most likely scenario) – heat tracing or other preventatives may be called for and can pose a hazard themselves.

Insurance Underwriter

The Insurance Underwriter is the plant level’s partner in ensuring plant and personnel safety such that the potential of an underwriting loss is minimized. One is well advised to have the Underwriting Engineer “in the loop” as early in the design process as possible, along with a technically credible equipment supplier, to ensure the most acceptable and cost effective protection solution. There are several agencies that have a say as to whether the end level of protection and the type of device is adequate, no matter what the jurisdiction. In addition to the Insurance Underwriter, the Building Department, the Department of Labor and the Fire Marshall’s office are all involved and may, in fact, have somewhat different views on these matters owing to the legislation that they are charged with enforcing. In addition, the Health and Safety Committee and the Union where applicable, all have a role to play as well.

Maintenance of Existing Protection Devices

Continuous maintenance of existing “protection devices” is critical to plant safety. Too often, the devices are not recognized for what they are – on doors, hinges and latches are corroded while vents are impeded from opening by catwalks, ladders etc. If one considers this issue, it becomes apparent that passive, hingeless “failsafe” devices are best suited to explosion protection. In the field, equipment inside manned buildings has been found to be equipped with explosion doors or vents that discharge into the inside of the building!