Circulation Systems for Single and Multiple Seal Arrangements (Part One)


Written by:
Gordon Buck and Ralph Gabriel, John Crane Inc.
Advantages
  • No product contamination. The flush source is coming from the pump and going back to the pump.
  • No reprocessing of product. Unlike an external flush, the product does not have to be reprocessed.
  • Simplified piping. There is no external piping on the pump.
  • Useful arrangement on fluids that are highly viscous at normal ambient pumping temperatures so as to minimize the risk of freezing if exposed to low temperatures in external piping plans, such as a Plan 11.
Disadvantages
  • Must be used for clean pumpage only as dirty pumpage could easily clog the passageway and it would require pump disassembly to repair.
  • Flush has to be repumped. The circulation from pump discharge back to pump suction will decrease pump efficiency and increase power required for the application. The volume of the flush is based upon the pump OEM's design and the pressure differential.
  • There is no external way to control flow. Unlike Plan 11, which can have an externally replaceable orifice to control flow, the internal design of a Plan 01 eliminates this possibility.
  • The flush is not usually directed right at the faces, but may come in over the seal head.
Sizing and Controlling

The flow rate is dependent upon the pressure differential in the pump and the design of the line running internal to the pump casing. Changing the impeller design can affect the pressure differential and thus the flush rate. The pump OEM should be contacted to ensure that the flow rate is adequate to maintain a stable condition at the seal faces.

General

This flush system can perform its function well when used properly. Changes in pump impellers, or changing seal designs that can move the seal faces away from the flush hole can cause problems that result in seal failures. This system is not recommended on vertical pumps.

 

Plan 02

Plan 02 is a non-circulating flush plan. In Plan 02 the process is not directed into or out of the seal chamber. Seal generated heat is removed by convection and conduction to the process fluid, pump components, and the surrounding environment. Also, some seal chamber designs promote cooling, by mixing of process fluid between the pump cavity and seal chamber. Often, this plan is used in conjunction with API Plan 62 and/or the optional use of a cooling jacket, which will provide some additional cooling. This plan should only be used for services where adequate vapor suppression can be assured, so that vaporization of the process in the seal chamber or at the seal interface does not occur. Plan 02 is often used with a self venting, open seal chamber, i.e. no throat bushing.

 

  seal flush plan 02.jpg

Seal Flush Plan 02

           

Advantages
  • No external hardware required.
  • Solids are not continually introduced to the seal environment.
  • Pump efficiency is not affected, as there is no recirculation of pumped or externally supplied fluid.
  • Natural venting occurs with an open chamber throat.   
Disadvantages
  • Success of this plan can be difficult to predict, reliance on previous experience with a specific process or pump design is often required.
  • If the fluid in the seal chamber vaporizes, the life of the seal will be drastically reduced.
  • Fouling of the cooling jacket, if so equipped, over time will reduce its effectiveness, resulting in higher seal chamber temperatures.
  • Careful design of impeller/chamber interaction is necessary on low head pumps to prevent air ingression.
General

Low duty, chemical service pumps are often a prime candidate for Plan 02. In these services, it is also advantageous to apply Plan 02 in conjunction with a large (open bore) or taper bore seal chamber. Often, in these services, suspended solids may be included in the process stream. In these cases, devices which encourage seal chamber circulation, while excluding solids from the seal chamber, are available and offered by many OEM and after market suppliers. Applications where these devices have been applied often work well with Plan 02.

Hot, refinery and petrochemical heavy oil services can be successfully sealed with Plan 02. Often, these services congeal or become highly viscous at ambient conditions. This can result in fouling and plugging of the recirculation plans, such as Plan 11, 13, 23 and their derivatives, unless effective temperature control schemes are employed. In these services, Plan 02 offers a relatively simple, cost effective way to obtain reasonable seal life. Only Plans 32 or 54 may be found to provide superior seal life. Use of Plan 02 in hot oil applications normally requires the use of Plan 62, using steam or nitrogen. In most cases, use of a seal chamber cooling jacket is helpful.

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See also:

Upstream Pumping Solutions

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