A glass-lined reactor’s agitator is sealed with either a single or a double mechanical seal, and the faces of either run in one of three ways: wet on a barrier liquid, dry and in contact under gas, or lifted apart on a gas film. High-purity seals, built so the seal itself cannot contaminate a pharmaceutical batch, are a further option outside those two choices. On toxic or flammable batches the double seal is the norm.

In every type, the product-side parts carry no exposed metal.

Seal Types by Arrangement

By arrangement, a glass-lined reactor seal is either single or double: one pair of faces, or two pairs with a barrier medium between them.

Some supplier lists show “cartridge” as a third type, but cartridge describes how the seal is built, as a self-contained unit with its own sleeve and hardware, and a single or a double seal can be supplied that way. On a reactor, a cartridge seal may add a rolling-element bearing to steady the faces against shaft motion, and a cooling jacket or spool to keep process heat away from the lower faces.

Model numbers such as 212, 212F, 2009 and 2017B are common on Chinese reactor seals. Ask the supplier which arrangement and which lubrication a given number stands for.

Single Seals

A single seal fits a glass-lined reactor whose batch is neither toxic nor flammable and whose vapor may escape to the plant atmosphere in small amounts.

Makers rate their single reactor seals for very different pressures, and no general ceiling applies. Check the specific seal’s rating against the vessel’s design pressure and vacuum.

Double Seals

A double seal is the usual choice on a glass-lined reactor because its barrier, held above vessel pressure, keeps the batch from reaching the atmosphere.

In the US the arrangement is also a compliance route. Under the EPA’s agitator rule in 40 CFR 63 subpart H, an agitator in organic hazardous air pollutant service must be leak-monitored every month unless it carries a dual seal with a barrier-fluid system. To qualify, that system needs a sensor that detects seal or barrier failure.

A pressurized barrier always leaks a little into the batch, so the barrier must be something the batch can accept. Where it cannot, the barrier is a gas.

Seal Types by Lubrication

By lubrication, a glass-lined reactor seal runs its faces on a barrier liquid (a wet seal), in direct contact under dry gas (a dry-running contact seal), or apart on a thin gas film (a gas-lubricated non-contacting seal).

Wet Seals

A wet seal is a double seal whose faces run on a barrier liquid held above vessel pressure, usually from a pressurized pot or lubricator. It is the type for reactors running at higher pressure or temperature, where the batch can tolerate traces of the barrier liquid.

An agitator turns too slowly for the pumping screw that circulates the barrier on a pump seal, so circulation has to come from a thermosiphon loop or an external circulating system.

Reactor wet seals also carry cooling coils or a jacket in the barrier chamber, or a cooling spool between the seal and the vessel, against the process heat that soaks into the seal from the vessel.

Dry-Running Contact Seals

A dry-running contact seal runs its faces touching with no liquid at all, often as a double seal under nitrogen, so no liquid can reach the batch.

Its limit is the dryness of the gas. Carbon faces need a trace of moisture to run dry, and below a dew point of about −35 °C the gas is too dry for ordinary carbon grades; only molybdenum disulfide grades work there. Check the dew point of the plant’s nitrogen before choosing this type.

Gas-Lubricated Non-Contacting Seals

A gas-lubricated non-contacting seal holds its faces apart on a film of barrier gas, usually nitrogen, so in steady running the faces neither touch nor wear.

Solids in the batch, or crystals that drop out of solution in the seal, migrate into the running gap and erode the faces, so batches that carry solids or crystallize are better served by a contact seal.

High-Purity Seals

A high-purity seal is a glass-lined reactor seal built so that the seal itself cannot contaminate the batch, for pharmaceutical and fine-chemical duties run under GMP. It closes the three routes by which a standard seal can reach the product:

  • Wetted materials: the PTFE, elastomers and ceramics the product touches are grades documented for pharmaceutical contact.
  • Barrier: the faces run dry or on gas, or on a liquid the product can accept.
  • Wear debris: a collector on the product side catches particles worn from the faces before they fall into the batch.

How to Choose a Seal Type for a Glass-Lined Reactor

Choose the arrangement by what may escape from the vessel, and the lubrication by what may enter the batch. A toxic, flammable or emission-regulated batch needs a double seal; the barrier medium then follows from what the batch can tolerate:

  • Benign batch near atmospheric pressure: a single seal, if small vapor losses to the plant are acceptable.
  • Higher pressure or temperature, batch tolerates traces of barrier liquid: a wet double seal with cooling.
  • No liquid allowed in the batch, clean product: a dry-running contact seal, after checking the nitrogen dew point.
  • No liquid allowed, no face wear wanted, no solids or crystals: a gas-lubricated non-contacting seal.
  • Pharmaceutical batch under GMP: a high-purity seal.

Check any type against the agitator’s shaft motion: normal deflection at the seal on a top-entry mixer is 0.03 to 0.150 in TIR (about 0.76 to 3.81 mm). Where the shaft moves more than the seal is rated for, a cartridge with an integral bearing steadies the faces.