Mechanical seal selection for low specific gravity pumps 2

[hemvig]

Rated Flow: 38065 kg/hr
Suction pressure: 797 psig
Dishcarge pressure: 1450 psig(low speed) 2513 psig (high speed)
Head:
VFD motor
Liquid: Hydrocarbon
Sp. Gravity: 0.13
Viscosity: 0.016
Temp: 40 deg. C
MDMT: -46 deg. C (Canada)

Please advise mechanical seal selection for low specific gravity Centrifugal Pump.

Thank you

 

[NGLENGR]

What is your vapor pressure? I assume it's probably fairly ridiculous with that kind of suction pressure. Our ethane service has a VP in the 500-800 PSIG range and the gravity can be 0.41-0.36 neighborhood.

Typically in very light hydrocarbon service we use a vaporizing liquid / gas seal such as the John Crane 28VL. They operate on a gas film instead of a liquid film at the seal faces. This requires the product to vaporize at the seal face, so the flush rate is very low (1 gal/min or so) and must be very clean and dry.

The API plan designation would usually be a 12/72/76.

EagleBurgmann also supplies good gas seals and support systems.

 

[flexibox]

Another alternative to NGLENGR comments is a dual pressurized seal with API plan 53B & 62 (N2) to prevent icing on the back of the seal. If you use Dual Pressurized seals then you need to consider contamination of Barrier Fluid into the process. You may want to consider Royal Purple oil as the barrier fluid. You cannot do this with a standard Tandem seal arrangement 28VL or Dual pressurized is the best option for successful operation.

 

[bk19702]

A 53B is ok assuming the original poster has the capability to pressurize the bladder above the seal chamber pressure. If a pressurized seal was desired, a more logical choice in this instance may be a 53C instead. Either pressurized option is assuming that process fluid contamination with the barrier fluid is acceptable. All that aside, the dual pressurized systems will be more complex, more costly, and not necessarily significantly more reliable than the 28VL seal and support system that was originally recommended by NGLENGR. That seal and system would be my choice - it's niche is low SG / high vapor pressure services such as this one.

 

[racookpe1978]

Reference.

 

[NGLENGR]

The biggest issue I've had with 28VLs is water or other non-vaporizing liquids in the fluid stream. These can reduce the lift-off of the seal faces to almost nothing in an instant.

We had a rash of failures on an ethane pipeline this year which was directly attributable to water contamination of only 400 ppm. The seal flush stream was filtered for debris, but there was no coalescing capability so the faces got wet.

We've since installed a revised filtration system with particulate (3 micron) and coalescing as separate stages, with liquid return to suction of the pump. This reduces the need for manual draining on the coalescer. Not the most efficient system, but the flush stream is 1 gpm out of 1200 gpm total flow.

That said, the 28VLs are quite reliable when given the proper working environment. Spend the money up front on a good support system and you won't do much except change filters.

I also have some familiarity with 53B and 53C systems, and they work well too. However, there are some drawbacks, particularly when operating high pressure systems. Barrier consumption is a major concern, so it must be compatible with your process fluid. We also have o-ring extrusion on the barrier/atmosphere side of the seal, which significantly reduces seal life. Maintaining a good pressure margin is critical in these systems, but can be difficult in a 53B if you're swapping products, changing equipment speed, or other "bad practices."

I would use the plan 53 systems when you are swapping products constantly, the product streams are heavily contaminated, or you have a light product that can change in composition quickly (say, Y-grade NGL.)

 

[PumpSmart]

YOU MUST USE AND REVIEW API682 3EDITION TUTORIAL FOR HELP.
IT IS VERY SIMPLE.
AND ALSO YOU WILL TO KNOW:
FLUID VAPOR PRESURE, SEAL CHAMBER PRESSURE, ENVIROMENTAL ISSUES LIKE ICING ON THE BACK OF SEAL BECAUSE OF COLD TEMPERATURE, API PLAN THAT YOU USE, AND IF THE IMPELLER HAS BALANCE HOLES OR NOT, TYPE OF PUMP, RPM, API SEAL CHAMBER SIZE.

THEN DISCUSS YOUR SERVICE WITH SOME GUY FROM JOHN CRANE, FLOWSERVE OR BURGMANN, AND COMPARE.