Nitrogen Entrainment 1

[sinbj1]

Hello all,

I have an application where I suspect nitrogen entrainment of barrier fluid is an issue but I'm not 100% certain.

For an API Plan 53A what seal pot pressure does N2 entrainment begin to become a concern?

When analyzing failed seal parts what signs of damage may indicate N2 entrainment issues?

Does anyone know of a good web-site that has useful information on N2 entrainment and API Plan 53B and C?

 

[flexibox]

Can you provide more information such as barrier fluid being used, shaft speed, shaft size & N2 Pressures? what is the symptoms you see on the seal faces?

Here is an extract from API682 3rd edition:

On pressurized barrier-fluid systems where the method of pressurization is a gas blanket, special attention
shall be given to the application conditions and barrier-fluid selection. Gas solubility in a barrier fluid increases
with increasing pressure and decreases with increasing barrier-fluid temperature. As pressure is relieved or
temperatures rise, gas is released from solution and can result in foaming and loss of circulation of the barrier
fluid. This problem is normally seen where higher viscosity barrier fluids, such as lubricating oils, are used at
pressures above 1,0 MPa.
The viscosity of the barrier/buffer fluid should be checked over the entire operating-temperature range, with
special attention being given to start-up conditions. The viscosity should be less than 500 mm2/s at the
minimum temperature to which it is exposed.

I always use plan 53B to eliminate this issue for anything above 10bar. If you are using an oil as barrier fluid you may see plucking of the carbon face but this is always difficult to comment on without more information, you may not be using carbon.

Something worth following up on is how do the operators top up the system when required? If they de-pressurize the plan 53A whilst pump on standby or still running, that means that the seals are operating in reverse pressure. Check history as to when the seals fail because this may be a contributing factor!

if you can provide some more data we may be able to help you with more infomation.

 

[sinbj1]

Flexibox - Thanks for the feedback. Some more specific information is below.

The barrier fluid is Royal Purple 22 @ 225psig and 150F. The shaft speed is 3550rpm. The shaft size is 2.875". The inner and outer faces are antimony carbon vs. silicon carbide 1. The seal is a dual pressurized back to back pusher design.

The operators fill the seal pot using a hand pump while the pump is running.

This was a pump that was installed on a new project and has only been in service for about 8 months or so. One seal has been replaced. Unfortunately I did not view the failed seal parts. The sister pump is currently leaking and will be replaced shortly. At that point I can analyze the seal parts.

 

[flexibox]

just some items to look for before you change anything:
1) Is there a high point vent in the seal system. check that the double seal housing is able to vent any entrapped air.
2) check the tubing size to and from seal i would recommend that it be 3/4" with no elbow fittings.
3) How hot is the vessel getting? Do you have cooling coils in vessel?
4) Not convinced it is entrained N2 at this stage. If it is found to be this the you can use API plan 53B.
5) Does seal design include a pumping scroll to circulate the barrier fluid?
6) Not knowing the product are the Orings compatible? check for hang up.
7) RP22 is a good Barrier fluid, When operate tops up whilst pump running is N2 to seal pot turned off or remains on?
8) suggest that you inspect the next seal for some evidence and if you have entrained N2 you may see chips on inner face. check to see if carbon has blistering on face.
9) Speed and shaft dia is high and may be experiencing localized heat at faces. attach a seal cross section for further comments if your problem persists.
trust this hels

 

[sinbj1]

Thanks for the pointers. We plan to pull the leaking sister pump next week. At that point I will have a better idea of what is going on with the seal / pump. Again thanks for the input.