Mechanical Seal

What are the types of Mechanical Seals?

• Single Seal => it consists of one primary seal and it seals from process to atmosphere.
  • There is always some leakage either to atmosphere or into process depending on the process pressure.

• Double Seal (Barrier) => consists of two primary seals in various arrangements. There is a barrier fluid between the two primary seals which is at a higher pressure than the process. There is always some leakage of the barrier fluid in process & out to atmosphere. As long as barrier pressure is maintained there is no leakage of process to atmosphere or contamination of process by environment.
• Double Seal (Barrier) – Wet => is a double seal where the barrier is a liquid. It is generally the most robust seal. It has a good pressure, speed and temperature capabilities.
• Double Seal (Barrier) – Dry => is a double seal where the barrier fluid is a gas. There are 2 specific types; Contacting and Non-contacting.
• Gas Contacting Seal – the seal faces are in contact, the soft face wear acts as a lubricant. Low pressure, speed and temperature capabilities and possibilities of wear getting into the process.
• Gas Non Contacting Seal – the seal faces are NOT in contact. They are kept apart by a flow of gas between the faces. Good temperature, speed and pressure capabilities. If operated correctly their is no wear but care must be taken in order to minimise shaft run out.
• Component Seal => is where both the rotating & stationary hardware are connected separately to the pump/mixer.
• Cartridge seal => is where the rotating and the stationary hardware are pre-assembled before mounting onto the pump/mixer. Cartridge seals are much easier to fit & maintain compared to the component seal.

What is the difference between a Shaft Seal and a Lip Seal?

A shaft seal is any type of seal which seals a shaft and stationary part e.g o-ring, lip seal or mechanical seal etc

A lip seal is a specific type of seal, it is a radial seal, where the part that seals against the rotating surface is a narrow cross section, soft material i.e the lip and it is made from an elastomer or non-elastomer material with a backup spring. Most commonly used to seal bearings in Mixers/Agitators. A lip seal rotates too quickly to be used on pumps.

What is a Barrier Fluid?

A barrier fluid is any gas or liquid which is used to pressurise a double mechanical seal. It must be compatible with the process, generally non-toxic and a good lubricant.

Check out our blog post on barrier fluid to find out more.

What is the barrier fluid pressure for water or other?

The barrier pressure should be above the process pressure.

In the case of a pump seal, this is set to a certain value above the stuffing box/seal chamber pressure. For this value, please consult seal IOM.

The seal chamber pressure needs to be calculated as it varies with pump type/impeller type/suction pressure/discharge pressure etc

Please consult the pump manufacturer/distributor for guidance on these calculations for mixer seals, the barrier pressure is set at a certain value above the maximum vessel pressure.

Mechanical Seals vs Packing?

Packing is a material that is stuffed between a rotating shaft and a stationary part gland to maintain pressure. Packing is a low cost alternative to mechanical seals.

However, the advantages of mechanical seals over packing is that they have;

• a lower leakage by a factor of 10³
• there is no shaft wear,
• they are more robust with higher speeds, pressures & temperatures.
• better chemical compatability and are longer lasting.

What is a Mechanical Seal leak test procedure?

There are a number of bench test procedures for pressure testing mechanical seals, the most common one is API 682 which Flowserve use.

– API 682 – Pressurised seal to 25 psig.

– Maximum pressure loss is 2psi/5 mins.