Today we will talk about the problems that can affect the mechanical seals of fluid handling pumps and what to do to minimize their wear over time.

The mechanical seal of a pump is a system that allows to physically isolate the part of the pump in contact with the working fluid (the impeller) from the electric motor. It is a component realized mainly of a stationary fixed part, held in position by an o-ring, and a rotating part, fixed to the shaft with another o-ring. Therefore, the mechanical seal is a component subject to wear, which can be more or less rapid over time depending on the working conditions.

The problems that affect the wear of mechanical seals, in fact, can be many and can be caused by incorrect use of pump itself. Below we will analyse all the problems that may affect mechanical seals.

Dry running wear

When around the mechanical seal there in not presence of fluid, we are on the working condition of dry running. This means that the seal is not correctly lubricated and important frictions work on the seal faces, causing damage. These frictions create temperature increases on the surface between the sealing ring and the o-ring, generally made of elastomeric material. This abnormal temperature increases cause the deterioration of material, burning the elastomeric parts.

Low or no flow wear

When the delivery of the pump is obstructed (ex. The valve on the delivery is closed or the delivery piping is obstructed) there is no passage of fluid between upstream and downstream of the pump. This generate the mechanical frictions on the shaft seal, resulting in a temperature increase on the contact surfaces. This, as previously mentioned, causes the damage of the elastomeric parts of the mechanical seal but, also, increase the risk of dry running.

Wear due to low lubrication

Each mechanical seal is designed and built to work with definite types of fluid and in specific working conditions. Low lubrication of the mechanical seal occurs when:

-          There is presence of air in the hydraulic circuit;

-          The viscosity of the working fluid is too low;

-          The temperature of the fluid is greater than the boiling temperature at the working pressure.

In these cases, frictions are generated locally and they cause local increase of the temperature and, consequently, the deterioration of the seal. The temperature difference between hot zones and cold zones of the sealing ring causes thermic shock and so the rise of slits (cracks).

Wear due to incompatibility of the working fluid

The materials of the mechanical seals of the pumps are selected with attention to avoid problem of different nature. Not all the materials are compatible with the different fluids and for this reason we must pay attention to the correct coupling between seal and working fluid. Because of the incorrect coupling between the materials of the seal and the working fluids, could arise:

-          Seal corrosion;

-          Swelling of elastomers.

Installation and connections

When the mechanical seal is replaced, following deterioration, it is important that it be installed correctly and with the utmost care. An incorrect positioning of the seal could lead to damage or premature breakage of the o-ring or the sealing ring.

It is also important to check that the working pressure of the pump is less than the maximum allowable of mechanical seal. Otherwise, the seal could be damaged.

Finally, it is important to avoid that the seal rotate in the reverse sense of the direction it was designed for. This can happen when:

-          Wrong electrical connections have been made (reversed phases);

-          Reverse return flow with pump off;

-          Pump in parallel without check valve on delivery.

These inattentions can cause the break of the mechanical seal spring.

Irregular wears

As a final section, will be now listed all the other possible causes of wear that can affect the mechanical seal:

-          Excessive wear of bearings;

-          Excessive presence of vibrations;

-          Presence of external bodies in the pumped fluid;

-          Seal faces not perfectly flat;

-          Defective mechanical seal.

So many other causes can generate problems on the mechanical seals, such as an incorrect installation, defects on the components or a wrong materials design.

Over the years, Frigosystem has developed enormous knowledge on the subject. Trust the experience of our technicians by asking for advice here.