Bladder accumulators versus regular seal pot

[RJAOB]

Hello everyone,

I have a high pressure, high temp API centrifugal pump application that will run with a plan 53B double mech seal. I am planning to use an accumulator with a bladder as the seal pot to avoid N2 degassing on the seal chamber.

Have you had any experience with this seal support system plan? Any concerns with the use of an acumulator with a blader?

Thanks in advance for your help

 

[bk19702]

Do you have a higher barrier pressure? While API 682 does caution against gas pressurization in direct contact with the barrier fluid when the reservoir pressure is above 150 psig, this value is a little conservative. Provided barrier fluid temperature is less than 250F, direct contact gas pressurization can be used up to 300 psig. These values have been determined from field experience and testing results.

There are disadvantages to the 53B in my opinion. You will need to have a provision in the circulation loop to make-up lost barrier fluid. Since there is a smaller volume of barrier fluid in the closed loop it gets thermally cycled on a more frequent basis which limits the service life of the fluid itself. Additionally, the finite volume of the accummulator requires a designed pressure operating range between refills of the barrier fluid (in excess of those required for Plan 53A) - you need to account for this in the pressure rating of the seals. As you know, there is an external heat exchanger typically required for this system, so you need to account for the additional flow resistance the added piping will create in your system. The Plan 53B can have a much lower circulation rate than an otherwise identical Plan 53A system because of this.

 

[RJAOB]

Thanks for your answer bk19702.

My seal conditions are 554 psig at pump suction and 500 F temperature. Process required positive pressure on the seal side due to safety concerns in case of any leakage. Neither seal nor pump has cooling jackets.
Very helpful the information you provided about the direct gas pressurization limit of 300 psig. I'm trying to convince operations and maintenance that a 53B plan is the best (maybe only) selection, but they are complain that is something new form them...
Do you find this pressure limit in any standard or it is just a best practice?

Thanks again,

RJAOB

 

[bk19702]

The 150 psig number is in API 682. The 300 psig number is based on performance testing my company has done along with field installation experience. You may be cutting it close with the temperature.

Have you considered a Plan 54? You may be looking at more cost but you could custom build the system to provide the cooling flow, barrier pressure, and instrumentation to warn of trouble. I worry with the pressure & temperature you have indicated that the circulation rate generated via a pumping ring may not be enough to remove the seal generated heat. You may want to explore this option - you will still get complaints, but it may be the better suited support system for what you have.

 

[flexibox]

RJAOB
Hello RJ, I have extensive experience in using API plan 53A, B & C. In my opinion if you design Plan 53B correctly you will never look back. In your application you will need a 54ltr accumulator (these are std off the shelf) if you pre-charge N2 to 575PSI and fill the accumulator approx 21/4 gallons (8ltr) then the operating pressure will be 690PSI. You should include a pressure Transmitter as this becomes your level alarm because you cannot have a level gauge in the accumulator. I normally use 1 1/2 gallons (6ltr) for the working volume. This assumes top up frequency once every 30 days. And 1/2 gal (2ltrs) safety volume. Your top up can be automated or manual, this depends on site requirements. I use Haskel or Sprauge air driven pumps. When the pressure drops to 600PSI this is when the low level alarm alerts the operator the system needs top up. At this pressure there is still 1/2 gallon (2ltr) of barrier fluid in accumulator. Ensure you include a PRV for protection. The cooling loop in very important with any dual pressurized seal. Most of the cooling these days is done by natural convection as these do not require any utilities. Make sure that you select the correct barrier fluid as this is very important.
It will be very interesting to know what type of seal you are using, Pusher seal or Metal Bellows Seal? In your seal design you need to include a scroll type pumping ring to circulate the barrier fluid through the cooling loop. Ensure that you include a high point vent. Venting is critical in such a system and try to use 3/4" tubing and minimise number of fittings. Bend the tube instead of using elbows.
Trust this helps

 

[flexibox]

Here is another picture of a plan 53B system for you

 

[RJAOB]

Flexibox,

Thanks a lot for the comments and tips. Seal support manufacturers have reported an increase demand on 53B and your words suggests that too.

RJAOB

 

[RJAOB]

bk19702,

Plan 54 was my fisrt pick, but it was killed by the project budget.

RJAOB

 

[bustox]

RJAOB

Reading the previous comments from BK and Flexibox the conclusion is that it is not easy to decide between API 53B and 54, a normal situation for all of us that have worked m. seals for a long time. Each have pro and cons that need to be considered.
Pls. check this link (ftp://ftp.aesseal.co.uk/downloads/pub/literature/L-UK-COM107.pdf) for a hybrid system that is a combination of both 53A, 53B and 54 and I personally consider it is an excellent option that combine the advantages of both plans.

 

[RJAOB]

Bustox,

Thanks for the picture and comments.
'
RJAOB

 

[RJAOB]

Flexibox,

Answering your question about seal type: I am using a double seal, with a pumping ring. My cooling system has a heat exchanger feed with cooling water. I also have a hand pump for refill operations. I calculated the heat load and we will need close to 1.5 gpm of water to keep the system below 200 F.
The only problem I see now is the accumulator size. I ordered a 5 gal (18.9 l)accumulator.

RJAOB

 

[flexibox]

RJ,
the problem with 5 gal accumulator is achieving a reasonable working volume to have 30 day top up frequency. Also a hand pump is not ideal! Have you ever tried charging a hand pump at those pressures? well let me tell you, it is hard work and you may find that operations will not do and lead to seal failures. invest in an air driven top up unit and increase the accumulator size.

 

[lgoochy]

RJAOB

The compact 107 from AES has the advantage of being able to run many seals from 1 system and does not require a huge accumulator.

I have used these offshore with great success.

 

[longeron]

I assume you're going with a pusher design with pefluoroelastomer o-rings- at least with the outboard seal.

There are some other options available for hot services that require dual seals. Dual gas/steam seals have been put to market recently for high temperature services by at least two of the major seal suppliers. I don't know if anyone of those designs are up to these pressures, or if it is sensible to use #650 steam this way.

Lets talk about start-up for a dual seal under these pressures. If the process side seal is a metal bellows seal it may not like the 550 psig differential pressure it migiht see while the pump is inventoried. What experience does the group have with applications like that?