An external seal flush system (API Plan 32) is built from a clean flush source, a strainer, an isolation valve, a check valve, a flow control valve, a flow indicator, a pressure gauge, piping to the seal gland and a throat bushing at the bottom of the seal chamber, with a temperature indicator or a cooler on some lines.

Together these parts feed clean liquid into the seal chamber at a pressure above the process, so the seal faces run in that liquid instead of in a pumped product too dirty, abrasive or hot to cool and lubricate them.

Flush Fluid Source

The flush comes from a clean supply outside the pump. In water-based processes such as paper mills and water treatment, that is plant service water or city water. In other services it is sometimes a nearby process stream with the same base fluid that has already been filtered or cooled. Whatever the source, it has to be compatible with the process.

Strainer

The strainer keeps solids carried in the supply, such as the magnetite found in most plant water systems, out of the seal chamber. API 682’s fourth edition supports strainers in its piping plans but limits them to a “minimum mesh size” of 125 µm.

Valves

A Plan 32 line carries a normally open isolation valve, a check valve, and a flow control valve or fixed orifice that sets the flush rate.

Isolation Valve

The isolation valve lets you shut off the flush to work on the strainer, the flow control valve or the instruments without shutting down the water header that feeds them. It stays open whenever the pump runs.

Check Valve

The check valve stops flow from running backwards up the flush line.

Flow Control Valve

The flow control valve sets the flush rate: enough liquid to cool the seal and keep the chamber clean, and no more, because every gallon of flush ends up in the product.

Three kinds of device do the job:

  • Drilled orifice or choke tube: the simplest and cheapest, but it has to be protected from plugging, especially an orifice smaller than 0.125 in, and it holds the rate only when supply and seal chamber pressures are known and steady.
  • Needle or globe valve with a flow indicator: set by hand against the indicator, which allows accurate tuning but needs regular checking.
  • Automatic flow control valve: holds the rate by itself, at a much higher cost to buy and maintain.

Agree the rate with the seal supplier, starting from the minimum flush flow the seal needs.

Instruments

An external flush line carries a flow indicator and a pressure gauge, and often a temperature indicator. API 682’s fourth edition makes transmitters with local indicators the default for monitoring piping plan parameters, so the same readings can also go to the control room and raise an alarm.

Flow Indicator

The flow indicator shows that flush is actually reaching the seal and at what rate, and it is the reading you set the flow control valve against. A falling reading is also the first sign of a plugging strainer or orifice.

Pressure Gauge

The pressure gauge shows the flush supply pressure, which has to stay above seal chamber pressure for the flush to enter the chamber and keep process fluid away from the faces. Set the flush supply pressure 15 to 25 psi (1 to 1.7 bar) above seal chamber pressure.

Temperature Indicator

The temperature indicator shows how hot the flush is when it reaches the seal. It is optional on most Plan 32 lines and worth fitting when the flush is something other than cool clean water: a process stream, hot water, or liquid coming off a cooler.

Cooler

When a Plan 32 line does carry a cooler, its job is to bring a warm supply, such as a process stream taken from elsewhere in the plant, down to the temperature the seal needs before it enters the gland.

Piping to the Seal Gland

The flush line ends at the flush port in the seal gland, and the liquid enters the seal chamber on the process side of the faces, then leaves through the throat bushing into the pump.

A seal built to API 682’s fourth edition ships with plastic plugs in its gland ports, a bag of metal plugs in the right material and an assembly drawing showing which port takes which piping connection. Connect the flush line to the port the drawing gives it, and replace every remaining plastic plug with a metal one.

Throat Bushing

In a Plan 32 system the throat bushing holds the clean flush in the seal chamber at a pressure above the process, so process fluid cannot come back in. It is a close-clearance ring around the shaft at the bottom of the seal chamber, between the seal and the impeller.

Flush leaving the seal chamber through the throat bushing clearance at about 15 ft/s normally keeps process solids out of the chamber. A restrictor bushing with a clearance typically in the 0.005 to 0.010 in range lets a Plan 32 flush reach a 15 ft/s exit velocity at a low flush flow.