Seal flush plan 2

[zipped]

My query is about relation between seal flush plan and seal arrangement.
To be specific, in case of plan 52,53,54 what could be the seal arrangements in these seal flush plans.
Is a seal flush plan 53 applicable to both back to back & tandem arrangement.

 

[Pumpsonly]

1) Plan 52 is for Unpressurised Dual Seal or used to be referred as tandem arrangement before the introduction of AI 682.

2) Plan 53 is for Pressurised Dual seal a.k.a double seal. The arrangement can be back-to back or face-to-face.

3) Plan 54 is also for pressurised dual seal arrangement as 2) except the flushing fluid is circulated from a remote source.

4) Plan 53 is sometimes used for tandem seal arrangement to reduce the DP across the primary seal faces when the seal design can not withstand the whole DP.

 

[zipped]

Thanx for the reply. Now my next few questions are:-

1) can you pls elaborate point no 4 of your reply.

2) When using double seal with flushing plan 53 or 54, which arrangement is preferred; back to back or tandem. which arrangement is more reliable / efficient.

3) when tandem seal arrangement is used, how much max DP it can withstand. (is there any thumb rule?)

 

[Pumpsonly]

In response to your queries of 12-Jun.

1) If the process side pressure is 60 BarG and the max. seal dp allowable is only 40 BarG, you installed 2 identical design seals in Tandem and pressurized the buffer fluid to 30 BarG.
The primary ( process) seal will see a dp of 30 barG across the faces.

2)Back-Back or Face-To-Face arrangement are used for Plan 53 and Plan 54. Except for the application mentioned in (1) above.

3) It depends on the seal design.

 

[bk19702]

If your process pressure is greater than your barrier pressure, as in your example (60 BarG process, 30 BarG barrier), then this is not a pressurized seal and therefore not a Plan 53. You would be describing a Plan 52 in this instance.

It is best to use the current terminology outlined in API 682 describing dual seals, which is dual pressurized and dual unpressurized. You can have tandem arrangements which are pressurized, in which case the barrier fluid will be at the ID of the inner seal faces with process pressure on the OD. In the back-to-back or face-to-face arrangement, barrier fluid is on the OD of both inner and outer seal faces, with the process fluid located on the ID of the inner seal faces only. In either case, assuming you pressurize your barrier fluid 30 BarG higher than the process, than the inner seal will see a DP of 30 BarG in either configuration.

In my experience, I like the BB or FF version better since you get better cooling of both seals and thermal rotations don't work against you as could be the case with the tandem arrangement. However, what arrangement you use and the max DP you can apply will vary depending on seal size, design, and configuration. Best to get your preferred seal vendor involved in the application if you have specific questions.

 

[Pumpsonly]

The pressurized tandem (face-to-back)seal arrangement with buffer fluid lower than process pressure is not mentioned in the API 682. It is a special arrangement used by some pump/seal vendors to over come the limitation of their seal design.

Bear in mind that in any arrangement, the outer seal will see the full barrier / buffer fluid pressure as the DP across its faces and mech seals have higher pressure rating at the OD side than at the ID side.

 

[zipped]

thanx bk19702 & pumpsonly for your valuable response.

 

[bk19702]

The tandem seal is most certainly mentioned in API 682 - it's either a 2CW-CW (dual contacting wet seal unpressurized) or 3CW-FB (dual contacting wet seal pressurized, fact to back): section 4.3, figures 3 and 5 specifically.

If buffer fluid is lower than process pressure, then the seal is being supported with Plan 52, not 53, as I said originally. It's not some 'special' configuration that is rarely used. All of this is included in API 682 - please review prior to commenting if you are not sure on the terminology with these seals.

Liquid Buffer Fluid - unpressurized (Plan 52), 2CW-CW
Liquid Barrier Fluid - pressurized (Plan 53A/B/C, 54), 3CW-FB, 3CW-FF, 3CW-BB.

 

[Pumpsonly]

TO bk19702,

My post of 15/6

The pressurized tandem (face-to-back)seal arrangement with buffer fluid lower than process pressure is not mentioned in the API 682

.

Your post 15/6

The tandem seal is most certainly mentioned in API 682 - it's either a 2CW-CW (dual contacting wet seal unpressurized) or 3CW-FB (dual contacting wet seal pressurized, fact to back): section 4.3, figures 3 and 5 specifically

.

Figure 3 (a) 2CW-CW ( Face to back ) is a standard Arrangement 2 seal configuration which is unpressurized using Plan 52 system.

Fig. 5 (a) 3CW-FB is a Arrangement 3 seal configuration with pressurized barrier fluid higher than seal chamber pressure using Plan 53’s or 54


Your post of 14/6

If your process pressure is greater than your barrier pressure, as in your example (60 BarG process, 30 BarG barrier), then this is not a pressurized seal and therefore not a Plan 53. You would be describing a Plan 52 in this instance

If 30 Bar is not consider pressurized.What do you call it?

There are obvious conflicts or unclear definition in The API 682
Section 3 Terms and Definition
-3.3
Arrangement 2 seal
Seal configuration having 2 seals per cartridge assembly with a containment seal chamber which is at a lower pressure than the seal chamber pressure.

If the seal chamber pressure is 60Bar, the containment seal chamber can be 30Bar as my above example and still qualified as Arrangement 2 seal as per above definition.

Arrangement 2 seal is also classified as UNPRESSURISED using Plan 52 seal support system if liquid buffer fluid is used.

682 3rd Edition page 126 definition (abstract) for
Pan 52 is used with Arrangement 2 seal 2CW-CW.The buffer fluid is contained in a seal pot which is vented to a vent system, does maintaining the buffer fluid system closed to atmospheric pressure.

The pressure of the buffer fluid in a Plan 52 system depends solely on the back pressure of the vent system and the amount of leakage across the inner seal if the process fluid is vaporizing.

 

[bk19702]

Not sure what point you are trying to make. 30 Bar on the containment seal is not considered pressurized if the process pressure is 60 Bar. Whether the seal is a arrangement 2 or 3 depends on the pressurization between the seals. If your contaiment seal is operating at 90 Bar, it is a pressurized dual seal (90>60); if the containment seal is operating at less than the process pressure, it is a arrangement 2 seal and unpressurized.

You said the pressurized tandem is not mentioned in 682, which it clearly is; everything else you wrote in the above post is irrelevant I never said you couldn't have the containment seal operating at 30 Bar; if the pressure is less than the process pressure, it is unpressurized by API's definition - not sure how many different ways I can say it.

Again, use correct terminology when discussing these configurations and you won't confuse anyone.

Buffer Fluid = Unpressurized (containment seal pressure<process)
Barrier Fluid = Pressurized (pressure between seals > process)

Buffer Fluid = Arrangement 2 seal, 2CW-CW or 2CW-CS
Barrier Fluid = Arrangenent 3 seal, 3CW-FB, 3CW-FF, or 3CW-BB

So, to re-iterate ONE more time - you can have a pressurized tandem configuration, which is listed in API 682 as a 3CW-FB. If the containment seal pressure, whatever it may be, is less than the process pressure, the seal is considered unpressurized.

 

[Pumpsonly]

In your posting 14/6

If your process pressure is greater than your barrier pressure, as in your example (60 BarG process, 30 BarG barrier), then this is not a pressurized seal and therefore not a Plan 53. You would be describing a Plan 52 in this instance

.

You further commented that as long as the process pressure(60Bar) > containment seal pressure (30Bar) it is considered as unpressurised= Arrangement 2.

In API 682 only Plan 52 is recommended for Arrangement 2 seal with liquid buffer.

May be you can enlighten us further on how to use a Plan 52 to maintain a 30bar buffer fluid pressure when the buffer fluid reservoir is connected to the vent system.

 

[zipped]

In this case i agree with pumpsonly.
when a containment seal is un-pressurized (atm pr) it is plan 52.
Whenever it is pressurized, it falls under plan 53. (Irrespective of the process pressure.)

 

[bk19702]

Do you guys even read the posting?

I've said all along, containment seal unpressurized is plan 52; pressurized it is plan 53. You would not maintain a 30 bar buffer pressure if connected to a vent system; I was using the pressures as an example. However, the buffer fluid pressure can be maintained slightly higher than vent system pressure in some instances, which still makes this an unpressurized seal provided the pressure is less than process pressure.

The tandem arrangement can be pressurized, whether you refuse to acknowledge it or not, which was my original comment - you said that it was a unique design not in 682, which it clearly is. Throw the pressures out - as long as process pressure is greater than containment seal pressure, it is considered an unpressurized seal.

Seems like you are arguing my own point back to me, which is ridiculous.

 

[flexibox]

I agree with bk19702. Here is another way to look at it. A pressurized seal will have Barrier fluid as the fluid film between the faces. In the argument Pumpsonly is putting to us the Primary seal will still have Process fluid as the fluid film. nothing really achieved. I have never used or seen an application whereby a Tandem seal is pressurised to overcome the seal limitation!!! This seems to be an out and out special. What you are doing is pressurizing the Tandem seal with a Plan 53A vessel but not operating as a dual pressurized seal. Strange!

 

[zipped]

There is total confusion. Can somebody summarize our discussions vise what are the sealants in inboard and outboard seals for all configurations.

Plan 52-- unpressurized (tandem)
Primary seal OD - ? ID - ?
Secondary Seal - OD - ? ID - ?

Plan 53-- Pressurized (Back to back)
Primary seal - OD - ? ID - ?
Secondary Seal - OD - ? ID - ?

Plan 53-- Pressurized (Tandem)
Primary seal - OD - ? ID - ?
Secondary Seal - OD - ? ID - ?

Plan 53-- Pressurized (Face to face)
Primary seal - OD - ? ID - ?
Secondary Seal - OD - ? ID - ?

 

[Pumpsonly]

Flexibox,
That was the reason I said it is not mentioned in API 682 and it is a special. But I had seen a pump vendor used such arrangement.

 

[flexibox]

Zipped,
I have tried to explain further in here and trust that you are clear now. Perhaps someone will second this to assure you and check my comments.

Plan 52-- unpressurized (tandem)
Primary seal OD - Process Fluid ID - Buffer Fluid
Secondary Seal - OD - Buffer Fluid ID - Atmosphere

Plan 53-- Pressurized (Back to back)
Primary seal - OD - Barrier Fluid ID - Process Fluid
Secondary Seal - OD - Barrier Fluid ID - Atmosphere

Plan 53-- Pressurized (Tandem)
Primary seal - OD - Process Fluid ID - Barrier Fluid
Secondary Seal - OD - Barrier Fluid ID - Atmosphere

Plan 53-- Pressurized (Face to face)
Primary seal - OD - Barrier Fluid ID - Process Fluid
Secondary Seal - OD - Barrier Fluid ID - Atmosphere

 

[bk19702]

I agree with Flexibox's post; the pressures in each area for each configuration are correct.