Agitator Double mechanical seal Failure.

[shyamshins]

I am facing problem with Double mechanical seal of agitator.

Problem is as below:-

Process Paramaeters :

Fluid : K2DCSA / slurry
Reactor Press. : 30 Kg/cm2
RPM of reactor : 96
Capacity of reactor : 15000 litres.

When pressure in reactor is 0 kg/cm2 & temp.30deg.celcius 150 drops of barrier fluid comes out from LDP port of reactor as barrier fluid pressure is 40 Kg/cm2.

When reactor pressure is 6 kg/cm2 & syphon pot press. is 40 Kg/cm2 barrier fluid leakage rate through LD port is 38 kg/cm2.

When reactor pressure is 14 kg/cm2 & syphon pot press. is 40kg/cm2, barrier fluid leakage rate through LD port is 24 Kg/cm2.

At 30 Kg/cm2 press. in reactor & syphon pot press. is 40 Kg/cm2 barrier fluid leakage rate observed is 1 drop in 1 min.

Barrier fluid used is Duraclear.

Reactor has vibration & gear box which holds shaft of reactor also has vibration.

What could be reason of failure through LDP port which indicates Outboard seal failure?

No leakage observed from inboard side of seal.

Pls. find attached drwg. copy of seal.

Need your help to resolve this seal failure issue.

 

[bparekh]

Hii...

This is Bhavin Parekh. Am working as an Associate Deputy Manager maintenance in Gulbrandsen Chemicals Pvt. Ltd,

There is always some allowable leak rate per face combination in mechanical seal. It depends upon the seal size, pressure and velocity.
I have few question before answering you question
1) What is seal size? means shaft dia
2) Which face combination are you using?

Note : Also wants to refer you drawing of seal and installion.
Plz share.

Will definitely solve your problem.

Thanks

 

[shyamshins]

Dear Parekh sir,

Forwarding you drawing of Mechanical seal.

Pls. find attached copy.

 

[gyaneshwarp]

From the drawing it is clear that Vessel is top entry .
Therefore only pressure variation is coming on the seal.
Leakage is mainly due to pressure variation.

I will suggest that you go for API Plan 53 C which will maintain constant differential pressure across the inboard seal faces and your problem will get solved.

 

[shyamshins]

Sir with ref to above trailing reply for seal failure leakage is from Outboard faces of seal and not from inboard.

Sir also can you pls. explain me why I should go for API plan 53 C so that I can explain my seniors and convince them to install plan 53 C.

 

[gyaneshwarp]

Please find enclosed comparison of API Plan 53 A, Plan 53 B and Plan 53 C.

In API Plans A and B barrier fluid pressure is maintained at about 2 kg/cm 2 above maximum vessel pressure. Due to this outboard seal is subjected to maximum pressure during stand still as well running condition. Starting torque is more as well seal life less.

In Plan 53 C the barrier fluid pressure automatically gets adjusted to about 2 kg/cm2 above actual vessel pressure. Because of this outboard seal becomes more comfortable.

In vessel normally we find variable pressure and high pressure is for short duration.Therefore API Plan 53 C will be best option.

 

[geesamand]

Agitators can cause special considerations:
- Is there a bearing in the seal?
- How much vertical movement occurs in the agitator shaft? Check unpressurized, pressurized, and operating conditions.
- How much concentricity error is present during startup?
- How much is the shaft deflecting in operation? Check when the unit is at maximum load, or when the liquid level is passing through an impeller. (Avoid keeping liquid level at/near impellers as a general rule for maximum shaft and drive life)
- Is the agitator shaft bent?

 

[georgeverghese]

It looks like this is a plan 53A or 53B seal plan, since seal pot pressure is always maintained at 40barg.

Am not a seal systems expert, but what about seal cooling system - is it working ok ?

Is the reactor temp now the same as what was used when the seal was first purchased ? No details stated on temperature during normal ops. Due the heat radiation from the liquid surface to the vessel top dished head, the seal inboard housing temp would be nearly the same as reactor liquid temp.

If you've got a long agitator shaft, maybe the root issue is the shaft vibration you've mentioned ? Any intermediate agitator shaft supports provided ? if there is, what about the bearings for these ?

 

[shyamshins]

Dear Georgevarghese long agitator shaft has supports near top dish & bottom of vessel, provided with intermediate bearings near gear box.

Support is provided to shaft inside vessel, but when checked reactor shaft in dynamic condition with screw driver a noise is heard from inside of reactor.

Also reactor has vibration which can be seen with eyes. Whole gearbox with motor is found vibrating.

Pls. help me :

Should I go for API Plan 53C ? or

Vibration is root cause for Outboard seal leakage ?

 

[georgeverghese]

An internal inspection is most likely in order - and the gearbox also.

It is better to get an experienced mechanical maintenance engineer to help you with this problem - you may have to shutdown this reactor to investigate.

May be the shaft seal has failed because of the high vibration at the shaft as you say.

 

[gyaneshwarp]

I see that Mechanical Seal is rotating seal (Spring is rotating)

Stationary Seals perform better when there is vibration.

I see there is bearing in the cartridge. Therefore vibration should not matter much as bearing will control run out.

My opinion is

First you change the API Plan to Plan 53 C. Problem should get solved.

Buy if problem remains change the seal to stationary seal with Plan 53 C.

 

[shyamshins]

Dear gyaneshwarp what do you mean by stationary seal?

 

[gyaneshwarp]

In all Mechanical Seals one part will be stationary and other part will be rotating.

Stationary Seals are the Mechanical Seals in which Springs are Stationary.

Rotating Seals are the Mechanical Seals in which Springs are stationary.

I am not able to upload the sketch, therefore request you to search on Google.