7/15/2019 API 11 - Recirculation From Pump Discharge

http://slidepdf.com/reader/full/api-11-recirculation-from-pump-discharge 1/2

  API PLAN 11 ANSI PLAN 7311

DESCRIPTION: Recirculation from pump discharge through a

flow control orifice, when necessary, to the seal.

PRIMARY PURPOSE: Dissipate seal generated heat and vents the seal chamber at start-up.

7/15/2019 API 11 - Recirculation From Pump Discharge

http://slidepdf.com/reader/full/api-11-recirculation-from-pump-discharge 2/2

 This flush plan is the most commonly used plan for single seals and is sometimes used for the inboard 

seal in a multiple seal arrangement.

Plan 11 is used when a single seal is operating in a product that is at or near its vapor pressure and coolingis not practical. The objective is to raise the pressure in the stuffing box to maintain the product in its

liquid state. A close-clearance throat bushing helps promote this pressure build-up (approximately .008 to

.012 diametric clearance). Raising box pressure is not as effective as cooling for vapor suppression but it

is usually less expensive.

It can be helpful to take the recirculating flush from a position downstream of the discharge check valve.When this is done, the flush to the seal will be maintained even if the pump momentarily loses suction.

This will make the seal more forgiving to pump upsets especially when using a throat bushing.

POTENTIAL PROBLEMS:

1) The flush line can plug if used in heavy slurries. API Plan 32 is more effective for slurries.

2) Thermosensitive or viscous products can set up in the flush line particularly when the pump

is down. This can be prevented by heat tracing and insulating the piping to keep the liquid at the appropriate temperature or by flushing the lines when the pump is down.

3) Plan 11 can be ineffective when the seal chamber is at or near discharge pressure.  

Because of a lack of differential pressure, flow is greatly reduced or non-existent. Whenthis is the case, API Plan 13 is a better choice.

4) When there is a large pressure differential between the pump discharge and seal chamber,

an orifice or series of orifices must be installed to break down this pressure. In order to

minimize the chance of plugging, the smallest orifice diameter that should be used is

0.125 inches.

5) Another problem with high-pressure differential is erosion of seal parts. This can be

 prevented by using tangential flush ports or by installing the orifice some distance awayfrom the seal chamber to eliminate the sand blasting effect of the flush.