How to (not) kill a mechanical seal part IV

Some of you may have noticed that I changed the title of this article a little bit and for good reason! That “handsome guy” in the photo that is standing in front of a vapor recovery system is trying to prevent seals from premature failing by sharing his knowledge with you. So I did not want to be associated with shortening mechanical seal life. Also because of a copyright objection of a picture I wanted to use, made me rewrite the introduction a bit. I especially like the safety glasses that make me look like I have a mono brow. Maybe I should have chosen another one, I don’t know.. Moving on…

Apart from dropping a seal, shaft deflection, cavitation and dry running there are other ways to end its life span like for example hammering it in place during installation. So a faulty installation may destroy the mechanical seal even before it has a chance to operate. In my experience, most of the maintenance of pumps and seals are sourced out to pump specialists. In some occasions there are still some production- and processing plants with a complete in-house workshop. So please keep in mind that the tips in this article are meant to give you a general guideline! Every type of mechanical seal has its own instruction manual, so read that carefully before instalment. Also bear in mind that if you are really interested in an installation course or training, we can help you out with that. Just send me a message and I will connect you with one of my colleagues such as Chris Dean or our local partners, depending on your needs.

Pre- Installation

Equipment inspection

Before installing a seal, first inspect the condition of the shaft. There should be no pitting, corrosion or fretting as the O-rings will not be able to seal. There should be no sharp edges on the shaft, shoulders, sleeves and keyways. You do not want to cut your O-rings while sliding it over sharp edges on the shaft. Also check the O-ring seal position to shaft and ensure a good lead-in chamfer.

The next steps will be a bit more technical and are prescribed in the API norms 682 or 610 so if you are a casual reader of this article, proceed to Mechanical seal installation techniques.

The shaft or sleeve outside diameter has to be within a tolerance of 0,05 mm according to the API 682 norm, as it requires h6 shaft diameter tolerance.

As for Bearing shaft and sleeve diameters check what the API682 specifies (k6/H7).

The radial run-out & straightness should be TIR< 0,10mm (API 610 < 0,001”).

Bearing clearance where the radial deflection should be within manufacturer’s specification. The shaft Axial movement may have an end play of < 0,005”or 0,13 mm.

Stuffing box perpendicularity to shaft within these specifications:

0,003”/ 0,076 TIR if the shaft is < 4 inch/ 100mm diameter

0,005”or 0,125mm TIR if the shaft is > 4 inch/ 100mm

API 610< 0,0005 (In/In)

Stuffing box concentricity to shaft within 0,004”/ 0,127mm TIR.

Other Dimensional Checks

Check by measuring the following dimensions:

Shaft or sleeve diameter, the stuffing box bore as well as the depth, the Bolt circle (PCD) and the impeller clearance. Measure the first obstruction path, the gland max diameter and the sleeve end position to seal.

Mechanical seal installation techniques

When lubricating the shaft or seal in order to slide in the mechanical seal, never use standard grease or soap, other than the silicon grease as provided with the seal. The reason for this is simple, the standard grease may provoke a chemical attack on your O-rings, causing it to leak.

Once the shaft is lubricated, slide the seal onto the shaft (sleeve) and be careful not to damage the O-rings. Then mount the seal to the stuffing box or seal housing. 

Keep in mind that the equipment must be assembled as if it were ready to run, before you complete your seal installation. Also make sure the clearance of the impeller is set to its specifications to prevent over-compression or under-compression.

Slide the seal up to the stuffing box and install the hardened washers and finger tighten the gland nuts. Firmly tighten the gland nuts in a crisscross pattern and make sure you never overtighten to prevent gland distortion. Also make sure you do not use tubes to extend the length of the wrench. 

Make sure the wrench doesn’t contact the gland as they may cause the gland to move off set to the shaft centerline. 

Do not allow the washers to contact the gland as this may also cause an offset to the shaft centerline. The use of thin washers are not recommended, but if necessary use step washers. Make sure the studs are perpendicular (square) to the stuffing box face and concentric to the shaft centerline.

Make sure the shaft (sleeve) is not coated or hardened. If it is coated or hardened, make sure the correct screws are installed. Tighten the set screws just enough so it touches the shaft, then use an alternating pattern to tighten the screws to the equipment shaft.

Make sure you use the correct torque sequence and settings as the overtightening will distort the seal clamp-ring.

Once tightened, remove and save the setting clips in a safe place. (Do not throw them away). Run one clip around the clip groove to ensure the seal gland is concentric to the shaft. When you notice a tight spot:

1. Replace the clips

2. Slightly loosen the gland nuts

3. Reposition the seal gland

4. Re-tighten the nuts and re-check for concentricity.

Once installed, spin the shaft by hand and to get a feel for whether it runs smoothly or if you feel binding. If the shaft moves freely it means the installation was a success.

If binding occurs, repeat the installation procedure.

Never transport or install the equipment without the seal clips in place! This is why you should save the clips after instalment.

Barrier and buffer fluids for double mechanical seals

So by having read all the articles, there should be a more general awareness by now on how to treat a mechanical seal properly. In some processes all the conditions are met to have a good seal life. The instalment was good, the alignment of the pump is good, the temperature is ok. In other words all the checklist has been checked off and we are ready to go! We still aren’t there yet, as some processes may require buffer or barrier fluids.

So one of the last subjects that I would like to point out are the buffer and pressure fluids in a seal support system. You may even call it “a life support system” for the seal.

So in short, what are the support systems good for? Well a couple of things, like:

1. A positive pressure on both sets of seal faces

2. Keeps the product out of the seal faces

3. Clears heat

4. Prevents product vaporization

5. It allows a lower pressure in the stuffing box

6. Quench for internal faces as a positive pressure is maintained for the external seals

7. Clears debris

8. Applicable for high pressure applications

9. Reduces seal face wearing

What type of buffer fluids are best, depends on the process. We can help you by selecting the correct seal configuration in combination with compatible flushing systems (API plans).

Furthermore I hope these articles were useful to you. If you get stuck or have any questions don’t hesitate to drop me a message. Until next time.