Centrifugal Pumps- Part I- Mechanical seals

In the pumps, shafts rotate. Depending on pump the shaft will cross the casing at one or two points. At these points if there are no provisions foreseen, the liquid will leak out of the casing. These provisions, aka sealings, have different types. The most preliminary type is packings, which will be discussed in other article. In this article we will cover mechanical seals.

1- Necessity of mechanical seals

As mentioned, sealing is required wherever the rotating part, passes through a stationary part. For example as shaft passes through the casing if no remedy foreseen for sealing, liquid will leak through the gap. Of course mechanical seal cannot be in contact with the casing or the friction will destroy the shaft in the matter of minutes.

Mechanical seals, contrary to packings, have a very limited leakage. Thus when the liquid is dangerous, hazardous, expensive or etc, mechanical seals are used to block the leakage to good degree. Mechanical seals are more durable than packings and more reliable than other sealing methods.

2- Mechanical seal main components

Figure 1- Single Mechanical seal cross section

There are five major components for sealing as shown in above picture.

·        Rotator Seal: It is a ring that mounts on shaft. Rotate with shaft at same speed.

·        Stator Seal: This component is a ring that mounts the casing. 

·        Springs: Springs pushes the Rotor ring and Stator ring to each other. Limiting the gap between them as much as possible. However Rotator and stator rings cannot contact each other or the seal will break down.

·        Stationary Orings: As shown in the picture there are other routes for liquid to leak between Stator ring and casing. Since these components have 0 relative velocity an O ring can block this path completely.

·        Rotating Orings: The principal of this O ring is same as stationary O ring. Instead it blocks the path between the rotating ring and the shaft.

3- Flushing plans

As mentioned in previous part, the gap between Rotating seal and Stator seal shall be minimized to reduce the leakage. A liquid is needed to fill this gap. The filler liquid has two major roles: 1- Reduce the friction 2- Cooldown the seals. The filler liquid depends on process. It can be taken either from the pumping liquid or use other liquid. Flush plan duty is to supply the filler liquid. Different flushing plans are identified in API 682. If the process liquid is used as filler liquid, it can be taken either from inlet or discharge of the pump (figure 2)

Figure 2- Sealing liquid intake from inlet (Plan 01) vs. intake from discharge (Plan 11)

If the liquid is hot, then a cooler will be used to cool down the liquid before entering the seal chamber (Figure 3).

Figure 3- Seal plan with cooler

If there are solid particles then a cyclone will be used to separate the solid particles and sent the clean stream back to seal chamber (figure 4).

Figure 4- A seal plan with cyclone

Seal plans can be combined. An example is combined cooler and cyclone as shown in figures 5 and 6.

Figure 5- Combined mechanical seal plan consisting of a cyclone and cooler.

Figure 6- Plan 41 seal plan at field

As mentioned the mechanical seals will not block the leakage completely, thus the pump will always have a leakage. If the pumping liquid is dangerous or hazardous, another liquid or gas will be used called buffer liquid. In these cases buffer liquid will leak to atmosphere shown in figure 7.

Figure 7- A typical seal plan with buffer fluid. This plan is specially used in pumps with variable speed drivers.

4- Seal arrangement

In previous section different seal plans are discussed however the seals themselves can be installed in different ways.

·        Single Seal: One mechanical seal per shaft

·        Tandem: The face of one seal is at the back of other seal. Meaning 2 mechanical seals per one side of shaft

·        Double: To mechanical seals, but the seals are facing each other

Twin seals (tandem or double) are used when the reduction of the leakage is important. Typical tandem and double arrangement showed in figure 8 .

Figure 8- Double mechanical seal (Left) vs. Tandem mechanical seal (Right)

5- Common issues

• Heat cracking: Cracks, usually radial, appear in the seals
• Coking: leaves a black build-up, or abrasive sludge on the atmospheric side of the seal. This build-up causes rapid wear of the seal faces, usually caused by oxidation.
• Corrosion and pitting: Chemical attack is a culprit for pitting and corrosion of mechanical seals. 
• Blistering on seal faces: Blistering is a symptom of thermal attack, and is characterized by small circular sections that appear raised on the carbon seal faces. Blistering is a common cause for seal leakage because the blisters cause separation between the seal faces during operation

Related article:

1- Centrifugal Pumps- Part II- NPSH and Cavitation

References:

1- Exploded seal view is courtesy of Fluid Sealing International.

2- Sealing flush plans are courtesy of John-Crane seals

3- Common issues are reflected from Crane Engineering

#centrifugal #centrifugalpumps #seal #mechanicalseal #johncrane #flushplan #api #sealplan #pumps #mechanical_seals #Seal_Flush_plan