Subject : A.P.I. (American Petroleum
Institute) and C.P.I. (Chemical Process Industry) merger
Any prediction about the future of the pump and seal business
would have to include the high probability that the CPI will adopt
the API seal standard. The adoption of this standard will be
enthusiastically supported by the CPI insurance companies and will
dramatically increase the price of mechanical seals to the consumer,
as well as bring seals into a commodity status which has been the
goal of some of the largest pump and seal manufacturers all
Recent pump/seal mergers, buy outs, and alliances hint that the
adoption of these new standards will also dramatically increase the
profits of these highly competitive manufacturers.
The API (American Petroleum Institute) standard is the one
universal standard being used by oil refineries throughout the world.
There is on going talk about combining this standard with the
chemical industry ANSI (American National Standards Institute)
standard for a single unified pump standard.
The problem with all standards of this type is that they have
produced a failure rate in mechanical seals that exceeds 85%. The
only part of a mechanical seal that is sacrificial is the carbon face
and in better than 85% of the cases there is plenty of carbon face
left when the seal begins to leak. The A.P.I. specification addresses
just about everything about mechanical seals. The subjects
- Seal design
- Inspection, testing and preparation for shipment.
In this section we'll be looking at just a few of those parts of
the A.P.I. standard 682 that when combined with the C.P.I. standard,
will be affecting your seal purchases in the near future. Most of
this information was taken from A.P.I. Standard 682, First Edition,
dated October 1994. I recommend you get hold of a copy of this and
any future updates to learn the full particulars.
- All standard mechanical seals, regardless of type or
arrangement, shall be of the cartridge design.
- The standard single arrangement pusher seal shall be an
inside-mounted balanced cartridge seal.
- The standard, un-pressurized dual mechanical seal shall be an
inside, balanced, cartridge mounted mechanical seal (with two
rotating flexible elements and two mating rings in series).
- Outer seals shall be designed to the same operating pressure
as the inner seal, but do not have to be balanced.
- Cooling for the inboard seal is achieved by a seal flush.
Cooling for the outside seal is accomplished by a circulating
device moving a buffer fluid through an external seal flush
- The standard pressurized dual mechanical seal shall be an
inside, balanced, cartridge mounted mechanical seal (with two
rotating flexible elements and two mating rings in series). The
inner seal shall have an internal (reverse) balance feature
designed and constructed to withstand reverse pressure
differentials without opening.
- The standard configuration for API single pusher and all dual
mechanical seals is for the flexible elements to rotate. For seals
having a seal face surface speed greater than 25 meters per second
(5000 feet per minute), the standard alternative of stationary
flexible elements shall be provided.
- O-ring grooves shall be sized to accommodate
- For vacuum services, all seal components shall be designed
with a positive means of retaining the sealing components to
prevent them from being dislodged.
- Seal chambers shall conform to the minimum dimensions shown in
Table 1 or Table 2. With these dimensions the minimum radial
clearance between the rotating member of the seal and the
stationary surfaces of the seal chamber and gland shall be 3 mm
- For horizontally split pumps, slotted glands shall be provided
to make disassembly easier.
- Provisions shall be made for centering the seal gland and/or
chamber with either an inside-or outside diameter register fit.
The register fit surface shall be concentric to the shaft and
shall have a total indicated run out of not more than 125
micrometers (0.005 inch). Shaft centering of mechanical seal
components or the use of seal gland bolts is not acceptable.
- Seal chamber pressure for single seals, and for the inner
un-pressurized dual seal, shall be a minimum of 3.5 bar (50 psi.)
or 10 percent above the maximum fluid vapor pressure at seal
chamber fluid temperature. This margin shall be achieved by
raising the seal chamber pressure and/or lowering the seal chamber
temperature. Lowering the temperature is always preferable. Pumps
which develop less than 3.5 bar (50 psi) differential pressure may
not meet this requirement and alternate requirements shall be
agreed upon by the purchaser and the seal manufacturer
- On vertical pumps the seal chamber or gland plates shall have
a port no less than 3 mm, (1/8") above the seal faces to allow the
removal of trapped gas. The port must be orificed and valved.
- For single seals and when specified for dual seals, a
non-sparking, floating-throttle bushing shall be installed in the
seal gland or chamber and positively retained against blowout to
minimize leakage if the seal fails.
- Shaft sleeves shall be supplied by the seal manufacturer.
- Unless otherwise specified a shaft sleeve of wear, corrosion,
and erosion resistant material shall be provided to protect the
shaft. The sleeve shall be sealed at one end. The shaft sleeve
assembly shall extend beyond the outer face of the seal gland
- Shaft sleeves shall have a shoulder or shoulders for
positively locating the rotating element or elements.
- Shaft to sleeve sealing devices shall be elastomeric O-rings
or flexible graphite rings.
- Standard seal sizes shall be in even increments of ten
millimeters. It is preferred that alternate seals be sized in
increments of 0.635 mm (0,25 inches) starting with 38.0 mm (1.5
- Sleeves shall have a minimum radial thickness of 2.5 mm (0.100
- Sleeves shall be relieved along their bore leaving a locating
fit at or near each end.
- Shaft to sleeve diametral clearance shall be 25 micrometers to
75 micrometers (0.001 inch to 0.003 inch
- Drive collar set screws shall be of sufficient hardness to
securely embed in the shaft.
- Shaft to sleeve diametrical clearance shall be 25 micrometers
to 75 micrometers (0.001 inch to 0.003 inches)
- Seal and mating rings shall be of one homogeneous material.
Overlays and coatings shall not be used as the sole source of wear
resistant material. Materials such as silicone or tungsten carbide
may be enhanced by applying additional coating.
- The type A standard pusher seal shall incorporate multiple
springs with O-rings as the secondary sealing elements. When
specified on the date sheet option, a single spring shall be
- One of the seal face rings shall be premium grade, blister
resistant carbon graphite with suitable binders and impregnates to
reduce wear and provide chemical resistance. Several grades are
available; therefore, the manufacturer shall state the type of
carbon offered for each service.
- The mating ring should be reaction bonded silicone carbide
(RBSiC). When specified, self sintered silicone carbide (SSSiC)
shall be furnished.
- Abrasive service may require two hard materials. Unless
otherwise specified for this service, the seal ring shall be
reaction bonded silicone carbide and tungsten carbide (WC) with
- Unless otherwise specified, metal bellows for the type B seal
shall be Hastelloy C. For the type C seal, Inconel 718.
- Unless otherwise specified, gland plate to seal chamber seal
shall be fluoroelastomer O-ring for services below 150°C
(300°F). For temperatures over 150°C (300°F) or
when specified, graphite-filled type 304 stainless steel spiral
wound gaskets shall be used.
- If you are using dual mechanical seals, only mechanically
forced seal flush and barrier/buffer fluid systems shall be
provided. Systems that rely upon a thermo-syphon to maintain
circulation during normal operation are not allowed.
- Seal systems that utilize internal circulating devices, such
as a pumping ring, that rely upon the rotation of the mechanical
seal to maintain circulation shall be designed to thermo-syphon
when the seal is not running.
- If a dual seal buffer/barrier fluid reservoir is specified, a
separate barrier/buffer fluid reservoir shall be furnished for
each mechanical seal
Section 4.4.4 contains numerous references to dual seal system
- The purchaser will specify on the date sheets the
characteristics of the buffer/barrier fluid.
Section 4.6 addresses the circulation of the buffer/barrier
There will be some benefits to the user when the API specification
is adopted in to the CPI industry
- The decision to standardize on balanced seals is a wise one.
It will reduce the seal inventory of most consumers and prevent a
lot of premature seal failures.
- Allowing slotted glands for horizontally split pumps is a good
idea. It should also extend to end suction centrifugal pumps.
- Requiring seal chamber vents on vertical pump installations
- Banning coated or plated seal faces makes sense.
- Requiring the manufacturer to specify the carbon he is
supplying is an excellent idea.
What is the
problem with this API specification as a standard for the Chemical
Process Industry? There are a lot of things I do not like about it in
its present form. If combining with the CPI means a complete
re-writing of the API specification that will be fine depending upon
the final result.
- 2.1.1 Some seal designs do not lend themselves to a cartridge
design. Split seals, as an example. You could mount a split seal
on a split cartridge, but that would be "over kill" in most
- 2.1.2 I do not like the definition of pusher seal in this
standard. The term "pusher seal" is emotionally charged and
misleading. It is used to describe a reliable O-ring seal in the
same category as spring loaded Teflon® wedges, or chevrons,
and non-elastomer "U" cup designs. The implication is that the
"non-pusher" metal bellows seal is a better choice. The fact is
that O-ring seals are usually a better choice because of their
ability to flex and roll and the O-ring provides a built in
vibration damper that eliminates the need for letting a bellows
metal face holder bounce off the shaft or sleeve.
- 2.1.5 The dual seal specification recognizes only tandem or
series mounted rotating seals. It ignores concentric and "face to
face" designs that make sense in some applications where space is
not available for tandem configurations. Over the years the API
has failed to recognize that there are four ways to install dual
seals in a pump. They have played with the terminology over the
years but have never got it simplified. It should be:
- Face to face
- Tandem or series
- Back to back
- Concentric, or one inside of the other.
- 2.1.6 The specification calls for the inner seal of a dual
seal to be either balanced or reverse balanced depending upon
whether high pressure barrier fluid or lower pressure buffer fluid
is circulated between the dual seals. It totally ignores two way
balance of the inner seal that would allow the consumer his choice
between barrier or buffer fluid.
- 2.1.6 The specification call for the dual seals to be mounted
in series (tandem), but almost all gas dual seals supplied to
refineries to date have been supplied in the "back to back"
configuration which is the worst possible installation method for
slurry and abrasive service.
- 2.1.7 The specification approves rotating seals only, and
recommends stationary seals for speeds above 5000 fpm (25 m/sec).
The fact is that stationary seals are almost always a better
choice for leak free and the more severe fugitive emission
- 2.1.7 Stationary seals (the spring or springs do not rotate
with the shaft) can be cartridge mounted if you take precautions
to insure that the rotating face stays square to the shaft when
the cartridge sleeve is set screwed or tightened to the shaft. It
is not an easy problem to solve, but there are several solutions
to the problem. Please see "stationary cartridge seals".
- 2.2.6 The specification calls for O-ring grooves with a larger
groove dimension than normally used to accommodate
- 184.108.40.206 The specification assumes all pump manufacturers have
provided a machined diameter concentric to the pump shaft so that
the seal gland can be machined to register on an inside or outside
diameter. The fact is that most pumps were manufactured for
packing and do not have these concentric machined surfaces
available to the seal manufacturer. In the CPI industry, shaft
centering makes the most sense.
- 2.3.10 Maintaining a seal chamber 50 psi (3.5) bar above vapor
pressure does not make any sense in the majority of balanced seal
- 2.4.1 The specification calls for a shaft sleeve and allows
the manufacturer to reduce the diameter of the solid shaft to
accommodate the sleeve. This increasing of the pump shaft L3/D4
adversely affects the pump and seal performance.
- 2.4.1 The specification calls for sealing the sleeve on one
end, but fails to specify the impeller end except in the case of
O-ring seals. If the seal is on the outboard end, the space
between the sleeve and shaft can fill with solids and hamper the
removal of the sleeve. This can be a major concern in hot oil type
applications where "coking" is always a problem.
- 2.4.9 A shaft to sleeve diametral clearance of 0.001 inch to
0.003 inch is not practical. You will never be able to remove the
sleeve once some solids get between the sleeve and shaft, and they
will get there!
- 2.6.1 The standard seal is equipped with multiple springs, but
the standard does not specify the springs must be located outside
the fluid. If located in the fluid they can easily clog with
- 3.2.3 Reaction bonded silicone carbide is specified as the
standard hard face even though it is sensitive to caustic and
other high pH chemicals frequently used to clean lines and
systems. In most cases alpha sintered would be a much better
- 4.2.1 The term "flush" is misleading. Over the years the API
has failed to recognize the differences in bringing liquid to the
pump stuffing box area and lumped them all under the common term
"Flush". There is better terminology:
- Discharge recirculation connects the discharge of the pump
to the stuffing box to raise stuffing box pressure.
- Suction recirculation connects the bottom of the stuffing
box to the suction side of the pump usually allowing clean
fluid to circulate from behind the impeller into the stuffing
- Barrier fluid describes a higher-pressure fluid that is
circulated between dual seals.
- Buffer fluid describes a low-pressure fluid circulating
between dual seals.
- Quenching fluid is introduced into the seal gland outboard
the seal to wash away leakage and control the environment
outboard the seal.
- Jacketing fluid circulates around the outside the stuffing
box to control stuffing box temperature.
- Flushing fluid is fluid from an outside source introduced
into the stuffing box that dilutes the pumpage. It is seldom
desirable, but sometimes necessary.
- The specification allows spring-loaded elastomers (O-rings)
that do not have the ability to flex and roll.
- The specification allows a single spring seal design even if
it is sensitive to the direction of rotation.
- The specification does not prohibit the use of mechanical
seals that frett shafts and
- The specification should call for the seal's dynamic O-ring to
move towards a clean surface to prevent "hang up".
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