API Plan 14 is a seal flush plan that combines a Plan 11 and a Plan 13: one line takes liquid from the pump discharge through an orifice into the seal chamber, and a second line takes it from the chamber through another orifice back to pump suction. The flow through the chamber cools the seal faces and vents vapor while the pump runs, and the two orifices, with the throat bushing, hold the chamber pressure above the liquid’s vapor pressure.
Plan 14 is used mainly on vertical pumps. It also suits viscous products and any pump whose throat bushing alone cannot give the seal the flow it needs. It needs a clean product that lubricates the faces; solids-laden duties go to Plan 31 or Plan 32 instead.
Plan 13 alone is the usual choice where the seal chamber already sits at discharge pressure, since it vents the chamber and bleeds pressure off to suction. Plan 11 alone flushes the faces and, with a throat bushing, can raise chamber pressure, but it has no outlet from the chamber and so does not vent it.
How Plan 14 Works
Liquid leaves the pump discharge, passes an orifice, enters the seal chamber at the flush-in port, crosses the seal, and leaves at the flush-out port through a second orifice back to pump suction. The chamber sits on a bypass loop between the highest and lowest pressures in the pump, and that pressure difference moves the flow without a pumping ring or any outside supply.
The inlet half works like a Plan 11: the flush-in port aims the incoming liquid at the seal faces, so it carries off the heat the faces generate. With the flush-out port at the top of the gland, the outgoing liquid takes vapor and air with it, so the chamber vents while the pump runs.
The Two Orifices
The inlet orifice limits how much liquid enters the chamber from discharge, the return orifice limits how much leaves for suction, and together they set both the flush flow and the pressure the chamber settles at. Changing one changes the chamber pressure and with it the flow through the other.
Each orifice is sized so that its pressure drop at the wanted flow equals the pressure difference across its own leg: discharge minus chamber pressure for the inlet, chamber minus suction pressure for the return. The flow to size for comes from a flush flow calculation. At shaft speeds up to 3,600 rpm and seal chamber pressures up to 500 psig, a common rule of thumb for Plan 14 flush flow is 1 gpm per inch of seal size.
API 682 sets a minimum bore of 1/8 inch for Plan 14 orifices. On a pump with a high discharge-to-suction differential, a single 1/8-inch orifice passes more flow than the seal needs, so that leg gets several orifices in series, a choke tube or a valve. The two orifices need not be the same size, and either can be larger where the flush might plug a smaller bore.
Seal Chamber Pressure
A Plan 14 holds the seal chamber at a pressure between pump suction and discharge, set by the two orifices and the throat bushing below the chamber. On a pump whose chamber would otherwise sit near suction pressure, that means raising it.
A larger inlet orifice, or a smaller return orifice and a tighter bushing, moves the chamber toward discharge pressure; the reverse moves it toward suction.
The pressure the chamber has to hold is the vapor pressure margin that keeps the liquid from flashing at the seal faces. API 682 Fourth Edition requires a seal chamber pressure at least 3.5 bar (50 psi) above the liquid’s vapor pressure, or a 1:1.3 ratio of vapor pressure to seal chamber pressure where the 3.5 bar margin cannot be met.
Why Vertical Pumps Use Plan 14
Vertical pumps use Plan 14 because vapor and air rise to the top of the seal chamber, around the seal faces, and Plan 14’s return line vents them continuously while its discharge line keeps up the flush flow and the chamber pressure. The vent works only when the gland’s flush-out port sits above the seal faces and as far outboard as possible, with the flush-in port below it.
Plan 14 is the plan for vertical pumps whose seal chamber pressure sits close to suction. In vertical overhung pumps (types OH3, OH4 and OH5), impeller balance holes can bring the chamber close to suction pressure; Plan 14 then raises it where vapor pressure margin is short, and vents at the same time. In vertical turbine and wet-pit pumps the seal chamber already sits at discharge pressure, and Plan 13 alone vents it.