API (AMERICAN 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 system.
- 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 perfluoroelastomer O-rings.
- 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 plate.
- 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 inches).
- Sleeves shall have a minimum radial thickness of 2.5 mm (0.100 inches).
- 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 furnished.
- 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 nickel binder
- 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 makes sense.
- 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 cases.
- 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.
On a positive note, I do like their adoption of the terms:
- Barrier fluid, if the fluid between the dual seals is at a pressure higher than stuffing box pressure.
- Buffer fluid, if the fluid between the dual seals is at a pressure lower than the stuffing box pressure.
- 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, because it places the slurry at the inner seal inside diameter.
- 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 sealing.
- 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 perfluoroelastomer O-rings.
- 184.108.40.206 Slotted glands make sense for all pumps not just horizontally split pumps.
- 220.127.116.11 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 applications.
- 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.
- 2.4.3 A shoulder to locate the rotating element will not allow a re-positioning of the seal if an open impeller has to be adjusted. API (American Petroleum Institute) pumps are normally closed impeller designs, but we are talking about the possibility of combining standards with the ANSI design.
- 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!
- 18.104.22.168 The majority of hard set screws are not corrosion resistant. If the set-screws are located out of the stuffing box this could be OK, but many designs have the set-screws positioned in the sealing fluid.
- 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 solids.
- 3.2.3 Reaction bonded silicon 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 choice.
- 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 box.
- 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 (damage) shafts and sleeves.
- The specification should call for the seal’s dynamic O-ring to move towards a clean surface to prevent “hang up”.
- The standard does not recognize seal designs with both faces spring loaded and hydraulically balanced.
- On February 18, 2018