Types of Pump Seal Explained

Have you ever wondered why there are so many variations in seal types and what each seal type is used for? 

See below for our quick reference guide as to the purpose of each seal.

Single mechanical seal - this is the most common seal type used which is a seal used to seal the pump head, ensuring the liquid is retained within the pump head and does not leak out via the back of the pump casing where entry is made via the shaft which is driven by a motor. The seal consists of several parts involving a rotating assembly which rotates as the shaft rotates, a stationary part which is fixed and is cooled by the liquid being pumped, and a spring to ensure the seal remains compressed and most importantly airtight.

Seal Quench - A seal quench is where pipework is connected to a mechanical seal to allow cooling when high-temperature liquids are pumped. Seal quenches can use freshwater for cooling, but also barrier fluids for keeping the mechanical seal faces clean when abrasive liquids are pumped. 

Double mechanical seal - This is where two mechanical seals are used within a pump. The first seal retains the fluid within the pump as per the standard mechanical seal above. The second seal contains a barrier fluid to ensure the faces are kept clean and are lubricated preventing failure. Without such liquid, the mechanical seals would run dry and fail. 

Cartridge seal - When a mechanical seal is first inserted within a pump its clearances should be set. By utilising a cartridge clearances do not need to be set as the unit is pre-assembled eliminating the need for time-consuming assembly and spring setting on site.

Packed gland - A packed gland is when a braided material which has an appearance similar to a flat rope is wrapped around a shaft. A packed gland is also known as stuffing as the packed gland is typically wrapped tight around the shaft and inserted within a chamber known as a stuffing box. Packed glands must leak a little to allow cooling from immediate contact with the shaft to prevent burning, and over time do wear shafts. Packed glands were used more historically as they were utilised before mechanical seals were invented. They are still used in some boiler feed pumps where seal failure is common from overheating, cavitation or the presence of high fine solid content is expected as it is a cheap way to seal the pump rather than utilising expensive flushing liquids or double mechanical seals such as in dredging. 

Expeller Seal - An Expeller seal is a shaft sealing arrangement where a rotating part called an expeller is mounted where gland packing or a mechanical seal is normally fitted. Its appearance is similar to that of another impeller, with rigid vanes mounted on the rear of a disc, which as it rotates reduces shaft pressure creating a dry seal around the shaft without a gasket or packing.

However once rotation stops, so does the dynamic sealing meaning a seal must be in place to prevent media leaking. This can be done by either external flush, packing, lip seals or an inflatable bellows.

An expeller seal is useful for slurry pumps used within mines at remote locations where a seal flush is required, which consume vast amounts of water. As water is scarce, and expensive in such areas sizeable savings can be made by switching to such a sealing arrangement.  

Lip seal - A seal which fixes around rotary parts to retain lubricants retain pressure and separate fluid from the atmosphere.

O ring - A circular cross-section seal used on casings, around valves and other small valves which forms a seal around the periphery of the object. 

Flat gasket -  used between adjoining metal parts

Why Use a Hard Faced Mechanical Seal?

Typically Mechanical seals are usually manufactured in Carbon Ceramic, with a Stainless Steel Spring. Carbon is a good general choice for the majority of applications where a fluid is compatible, not contaminated and fairly clean. 

However at times a hard faced mechanical seal may be more appropriate.

Hard faced mechanical seals are typically used when the fluid handled may be harsh, and contain solids. Such seals are usually made in Silicone Carbide or Tungsten Carbide due to its hard wearing construction it is built to withstand unexpected small solids such as Sand, dirt, stone and is a great choice for heavy duty applications.

Do you require replacement mechanical seals and gaskets? Suffering from frequent seal failure or replacement? We can help diagnose the cause and eliminate recurring failures. 

Mechanical Seals Vs Gland Packing