Back to Back vs Face to Face Seal Arrangement

[h4m]

Could someone please provide any insights on below questions as per his/her experience regarding selection of pump double seal arrangement.

Between Face-to-Face and Back-to-Back arrangement which one is better for handling solid particles of around 2mm size.

Is it necessary that flexible elements be the rotating elements as well in Back-to-Back arrangement.

Are there any published guidelines regarding seal arrangement selection.

If someone could provide some useful references regarding this that would be really appreciated.

 

[SB1976]

Hi,

Regarding your two questions :

1. Solid containing pumped liquid is an abrasive service, for which a shorter length of shaft seal is required in view of reliability considerations. F-B seals requires the longest, followed by F-F seals and then B-B (which requires the least of the shaft seals lengths). But, shorter seal length also implies certain disadvantages : limited axial movement, inside mounted cartridge and potential loss of cooling functionalities by the barrier fluid. All these need to be evaluated before a final selection is made. But the primary choice shall always be B-B type.

2. Whether flexible elements be rotating or stationary, API STD 682 serves as a useful guide for this:
(a) Recommends stationary flexible elements in case seal-face surface speed at mean seal face diameter > 23 m/s.
(b) Design considerations are highlighted (seal balance diameter, misalignment issues, runout issues)

Trust above helps.

 

[Pandeyji]

For liquids with solid, both Face to Face and Back to Back should not be considered.

My recommendation is Face to Back ( Tandem seal) with hard face combination.
There is no published guideline, but individuals take decision as per their experience.

STATIONARY VS ROTATING SEAL
Even though API 682 mentions "Within the scope of this standard, rotating and stationary flexible elements are considered to be technically equivalent"
It also mentions following
Consideration should be given for requiring a stationary flexible element if:
a) balance diameter exceeds 115 mm (4.5 in.) (see 6.1.1.7);
b) pump case or gland plate distortion and misalignment exist due to pipe loads, thermal distortion, pressure distortion, etc.;
c) the perpendicularity of the seal chamber mounting surface to the shaft is a problem, aggravated by high rotational speed; or
d) the seal chamber face runout requirements described in 6.1.2.13 cannot be met (as found in
ASME B73.1 and ASME B73.2 pumps and with some slender-shaft, multistage pump designs).

In view of this Stationary Seal will always perform better than rotating seal.