Recommendation required on the Seal Type & Flush Plan for Vaccum Bottom Pump service

[JCBan]

Gents,

Pump Service : Vaccum Bottoms Pump.
Pump Type : Axially split Double Volute Between Bearing pump with Horizontal cooling jacket.
Existing Piping Plan : 32/62 ( 32- with Seal with 220°C, 9 barg & 5-7 Ltrs/ min.)/ 62- with LP Steam Quench )
Seal Type : Stationary Metal bellows, Single arrangement with Steam quench with deflector arrangement.
Seal MOC : Ni- Tungsten Carbide / Sealide C. / Inconel 718/ Carpenter 42 /Grafoil
Speed : 3565 RPM.
Product Temp : 370°C
Viscosity : 1.8cP at operating temperature.
Suction Pressure : -0.04barg
Discharge Pressure : 25 barg
Failure Mode : Fatigue & shear in the bellows & excessive coking in the faces.

We suspect the reason of the failure is bad design of Plan 32 & temperature difference between the external oil (Plan 32) with process fluid is creating vibration & in turn fatigue to the bellows during start up.

Also coking in the faces due to the different temp of both these oils at start- up.

Please suggest us what other information is helpful to determine the ideal seal design & exact reason of failure ( Pump sectional drawing to determine seal chamber pressure, Plan 32 fluid detail, exact mode of bellows failure- fatigue or shear failure etc ).

We are thinking of replacing the single seal to dual seal ( Both stationary Bellows) with plan 53B arrangement.

Please suggest us the suitable barrier fluid & its characteristics if Plan 53B is a right selection.

Please share your thoughts.

Kind Regards,
DPK.

 

[JCBan]

Gents,

Also please share your experience on the best plan combination for this application.
- 32/ 62
- 32/53B
-53B
-53B/62
- Or any other.

Kind Regards,
DPK

 

[JJPellin]

We have three vacuum units. All three sets of bottoms pumps use a double seal arrangement with heavy vacuum gas oil as a barrier fluid. I am not sure of the piping plan number. Filtered and cooled HVGO is introduced at pressure between the two seals. It is controlled for pressure on the inlet. The outlet is controlled for flow and dumps the HVGO back to the vacuum tower. I am surprised you are allowed to use outside flush in this service. The cost of the gas oil lost to asphalt or coker charge probably amounts to several millions of dollars per year. We also use a low pressure steam quench out the outside of the outer seal in our pumps. I believe that all of our seals use rotating metal bellows for both the inner and outer seals. But, I am not certain about that.

The bellows would fail from fatigue if you have excessive seal chamber face run-out (greater than 0.003 inch) or it could be slip-stick at the faces if your flush fluid has poor lubricating properties. HVGO at 250 to 300 °F has acceptable properties and should not result in slip-stick failures.


Johnny Pellin

 

[JCBan]

Dear Sir,

Many Thanks for your suggestion.

I understand from your suggestion that the Filtered and cooled HVGO @ 250 to 300°F is a ideal choice of barrier fluid to be introduced from 53B to cool down the faces @ 371°C

Do you have any suggestion on retaining existing Plan 32 as customer is still insisting us to retain the plan 32 even though it is costlier.

Can we suggest them to change them existing Plan 32 fluid ( Seal oil with 220°C, 9barg & 5-7 Ltrs/ Min ) with same HVGO?. or can we suggest them to plug Plan 32 itself & only use Plan 53B with HVGO barrier fluid or any other heat transfer barrier.

Whats your opinion of using Synthetic Lube oil or any other heat tranfer oils like Dowtherm or Therminol instead of HVGO as these oils have high boiling temp than the process fluid will help cooling the faces & prevent the coking.

I believe, seal chamber face run out is also an issue in the existing pump, which is causing bellows failure. As with customer interaction, it sounds like they are facing problem with one side of the pump.


Please suggest us to make this bad actor seal to better one.

Kind Regards,
JBan

 

[nfinit]

I would be a little concerned with an axially split pump in that temperature. I would suggest a Plan 54 and an upgrade to a radial split pump.