Page 102 - Improving Machinery Reliability

74 Improving Machinery Reliability

Properly designed stationary seals have given outstanding service in abrasive slur-
ry applications. Figure 2-13 shows one such seal installed in a cone-shaped environ-
ment. This short “tapered stuffing box” promotes the outward flow of abrasive parti-
cles and at all times allows new, fresh fluid to contact the balanced single-acting,
dead-ended seal. It should be noted that long tapers will require vortex-breaker ribs;
these are usually cast or welded into the housing bore.
Generally, stationary-type seals or gas seals should be preferred in applications
encountering face peripheral velocities in excess of 4,500 feet (1,372 meters) per
minute, serious shaft deflection, or coking after the pumpage has crossed the seal faces.
When applying bellows seals in light hydrocarbons, we should look for design
features that prevent torsional windup of the bellows in case the seal faces undergo
slip-stick motion relative to each other. Also, the seal face balance line of bellows
seals may shift when applied without adequate forethought in light hydrocarbon ser-
vices. If the design does not alleviate these concerns, the user may favor spring, and
especially gas-type mechanical seals for light hydrocarbon services.

Bid Comparison

After the bids are received, they must be tabulated and compared. The reviewing
engineer should look for significant differences among the competing bids and
should determine which offer incorporates most of the desirable design features.
Special features beyond those specified by the purchaser may have been proposed by
some bidders and would deserve extra credit for reducing the risk of catastrophic
failure incidents. Each special feature must be given a separate assessment of value.
Alternatively, the purchaser may decide that bidder X’s offer is less expensive than,
but nevertheless technically equal to the offer made by bidder Y. He may now wish
to upgrade his selection by asking X to furnish the seals with optional, although not
previously specified features.
Using optimized seal selection procedures as outlined requires close cooperation
between seal user and seal vendor. This should not be too difficult to achieve and the
benefits to both parties should be quite evident. In this cooperative effort, the user
has the responsibility of disclosing to the vendor fluid properties, application para-
meters, permissible leakage rates, and even maintenance practices. This disclosure
will allow the seal vendor to obtain a better feel for user know-how and sophistica-
tion. The vendor can then effectively plan ways to assist the user. He can remind the

Figure 2-13. Stationary seal with
successful experience record in
.. . abrasive-containinq services.
(Courtesy Burgmar% Seals Amer-
ica, Houston, Texas.)