PETROLEUM INSTITUTE) STANDARD 18-4
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 include:
- Seal design
- Inspection, testing and preparation for shipment.
In this section we will 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 in the charts section of this CD. 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 (1/8 inch).
- 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.
- 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 silicon 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 silicon carbide
(RBSiC). When specified, self sintered silicon 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 silicon 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 reservoirs.
- 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 fluid.
There will be some benefits to the user when the API specification
is adopted into 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.
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