Flameless Venting for Indoor Collectors in Food Plants

A dust collector is the vessel in a food plant that most needs explosion protection, and it is almost always indoors. Venting it conventionally would discharge a fireball and burning dust into a production area, which is not an option, so for years plants either accepted an unprotected collector or put it outside at considerable cost.

A flameless device solves the location problem. It has a conventional rupture element and, outside it, a quenching mesh that arrests the flame and cools the gases, so the pressure is relieved without a flame front leaving the device.

The device is not a drop in substitute for a panel, and that is where the design errors happen. Its efficiency is less than one, so the vent area required is larger than for a bare panel on the same vessel. The mesh is a product contact surface in a dusty location and it can blind. And the device has a limited temperature range and a limited number of actuations, which is one.

Isolation is the other half. A protected collector connected by open duct to a mill is not independently protected.

A vent area calculated with the device efficiency included, so the relief capacity is what the vessel actually needs rather than what a bare panel would have given.

A quenching element that is inspected on a defined interval, with a documented criterion for what counts as blinded, since a blocked mesh does not perform as tested.

Isolation in the inlet duct and, where relevant, the outlet, so the collector protection is independent of the equipment it serves.

The same device indoors in a building materials plant is sized against a fuel envelope rather than one product, and a unit sized at commissioning no longer fits once the kiln burns alternative fuels.

The calculation comes first and it uses the measured Kst of the collected dust, the vessel volume, the verified vessel strength and the efficiency of the specific device. Substituting a flameless device for a panel of the same size reduces the relief capacity, which is the opposite of the intention.

Installation then has to respect the device's clearances. A flameless vent discharges hot gas, even if it does not discharge flame, and the manufacturer specifies a volume around it that must stay clear of people, equipment and services. In a crowded food plant that clearance is often the constraint on where the collector can go.

Inspection is designed in. The mesh faces a dusty environment, and a food powder, particularly a fatty or hygroscopic one, will build on it. The inspection interval, the access and the criterion for replacement all belong in the plan, because the performance is the mesh.

Isolation is then provided at the inlet duct. And the device's temperature limits are checked, since the collector may handle warm air from a dryer or a process.

• Vent area calculated with the device efficiency, the measured Kst and the verified vessel strength.
• Manufacturer clearance volume respected for hot gas discharge.
• Quenching element inspected on a defined interval with a replacement criterion.
• Access provided so the inspection is actually possible.
• Isolation in the inlet duct so the protection is independent.
• Device temperature limits checked against the actual collector air temperature.

• Measured Kst of the collected dust, not of the product.
• Vessel volume and verified strength, which set the relief requirement.
• Device efficiency, which makes the required area larger than for a bare panel.
• Clearance available around the device for the hot gas discharge.
• Fouling behaviour of the dust, which sets the mesh inspection interval.
• Air temperature at the collector, against the device limits.