Unusual flush plans for mechanical seals
API-682 edition highlights five plans that can be of benefit.
Mechanical seals are used in millions of process pumps; the many available seal configurations are described in the standards of the American Petroleum Institute (API-682). These standards also describe the many flush plans (piping plans) used by modern industry. Except for automotive, home appliance and similar applications where the pumpage fully envelops the sealing components, a flush liquid stream and associated piping plans are used to remove heat from the seal faces.
There are many manufacturers of mechanical seals and their overall strategies appear similar: each desires to deliver safe products at reasonable cost. However, the business objectives of the very best mechanical seal manufacturers go beyond the obvious. Their objectives are expressed in marketing approaches which consistently represent value.
Superior service and high customer satisfaction are among the discernibly beneficial aspects of good marketing. Additional benefits accrue if the seal’s service and asset provider conveys educational or training updates to the ultimate seal user.
All of the objectives endorsed by client and provider can be summarized in “the three C’s”—Communication, Cooperation, and Consideration. The content of this article is rather typical of the “communications aspect” of a rewarding relationship between the parties. The manufacturer should make it their goal to alert user-clients to new opportunities.
Such opportunities exist based on new flush plans found in the 4th edition of API-682; they are Plans 03, 55, 65A and B, also Plans 66 and 99. Although these five flush plans and their derivatives are little known, they can be of great advantage in certain services.
API Plan 03
The new API Plan 03 (Figure 1) is a great addition; it relates to a taper-bore seal chamber for an API pump. For decades API pumps have been using closed (cylindrical) seal chambers and have relied on piping plans to maintain a chosen seal environment. However, because taper-bore stuffing boxes are now very well proven in American National Standards Institute (ANSI) pumps in contaminated services, we also now can specify tapered bores for API-compliant pumps.
In Plan 03 the flush fluid flows into the pumpage. Circulation between the seal chamber and the pump is facilitated by the tapered geometry. Solids accumulation risk is greatly reduced by the tapering and the former stuffing box is now part of the back pull-out cover of this pump. New pumps can accommodate the tapered design, as will pre-existing pumps through a modification or upgrading process. It should be noted that the taper should be relatively steep; 30 to 45 degree inclination has worked well. Very shallow taper angles should be avoided.
This seal chamber geometry promotes circulation which, in turn, provides cooling for the seal and vents air or vapors from the seal chamber. Flush Plan 03 is most often used in applications where the seal faces generate relatively small amounts of heat. Plan 03 is also used in applications where the old-style cylindrical chamber would have allowed solids to collect. Occasionally, the tapered bore is fitted with anti-swirl vanes (sometimes called “swirl interrupting ribs”) for even greater assurance against solids accumulation.
Note also the floating outboard throttle bushing in Figure 1. This provision allows leakage monitoring, assuming the outlet port is located at the bottom.
Standard Seal Flush Plan 55
In Plan 55 (Figure 2), there is an unpressurized external buffer fluid system supplying clean liquid to the buffer fluid seal chamber. Plan 55 is used with dual (double, tandem) liquid seal arrangements. The buffer liquid is typically maintained at a pressure less than seal chamber pressure and less than 0.28 MPa (2.8 bar or 40 psi).
Plan 55 is similar to Plan 54 except the buffer liquid is unpressurized. The Plan 55 representation in Figure 2 shows an efficient bi-directional tapered pumping ring. This particular ring greatly assists in moving the buffer fluid to and from an external reservoir and/or through an external heat exchanger (cooler). Also, the potential advantages of using a tapered pumping ring can be significant. One such model, as seen in Figure 2, is offered with bi-directional functionality and a wide clearance between its vane tips and the opposing stationary parts. In the event of pump bearing distress, this wide clearance gap protects against scraping and extreme heat generation.
The outboard seal in Figure 2 is a wet containment seal (API calls it configuration 2CW-CW—dual contact wet seal) and is normally used in services where process fluid leakage to atmosphere must be avoided, which is to say minimized and contained. Many users found Plan 55 advantageous in applications where the process was prone to solidify in contact with atmosphere or in applications where additional heat removal from the inner seal was required.
Examining major seal manufacturer Websites allows users to see how Plan 55 differs from Plan 52. In Plan 52 the buffer liquid is not necessarily self-contained; with Plan 52 buffer liquid circulation is created by an external pump or pressure system. If Plan 55 is
specified, carefully consider the reliability of the buffer liquid source and the possible contamination of the buffer flow with process liquid or vapor.
However, suitable supervisory instrumentation may give ample warning of a compromised primary seal.
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