Seal chamber pressure gives early warning by showing that the conditions the seal runs in have changed: the pump’s operating point, the flush circuit, or the clearances behind the impeller. The warning works only when each reading is compared with the same pump’s own baseline at the same operating point.

Chamber pressure does not measure seal wear. The Hydraulic Institute lists what sets it on a Plan 11 seal: the pump’s discharge pressure, the flush orifice, the bushing-to-shaft clearance and the pressure behind the impeller. A worn face shows up in leakage, seal temperature and vibration; chamber pressure tracks the seal’s environment, where many of the causes that make a mechanical seal fail begin.

This article reads the pressure in the seal chamber itself. On dual seals, the barrier pressure of a Plan 53A system and the buffer pressure of a Plan 52 system are read at their own reservoirs, and seal water at its own supply; each of those has its own normal pattern.

Where to Measure It

Measure seal chamber pressure at the chamber, or on the flush line between the orifice and the gland, while the pump runs at its normal operating point. Most pumps are delivered with no gauge in the seal chamber or the flush line, so on most pumps the first step is to fit one, and the next seal replacement is the time to do it.

On a Plan 11 flush, what the gauge reads depends on where it sits. Upstream of the orifice, the line is still close to discharge pressure. Downstream of the orifice, or at a tapping on the process side of the faces (a second flush connection or a tap in the back cover), the gauge reads close to chamber pressure. The quench and drain ports on a single-seal gland are outside the faces and read nothing useful.

Measure it rather than working it out from suction and discharge pressure: the usual estimates are good enough to size a seal but not to set a monitoring baseline.

What to Record With It

Record the pump’s operating point and the pumped liquid’s temperature at the same time as the chamber pressure.

  • Suction and discharge pressure: they place the pump on its curve and show whether a change in chamber pressure came from the operating point moving.
  • Flow and speed: where they are available. A reading is only comparable with the baseline at the same flow and speed.
  • Pumped-liquid temperature: it sets the liquid’s vapor pressure, and with it the floor the chamber pressure must stay above.
  • Operating events: a start-up, a valve change, a product change, a very hot or cold day. A note beside the reading stops the jump it causes being taken for a fault.

Keep the seal data sheet’s conditions beside the log as well: pumping temperature, vapor pressure, specific gravity, suction and discharge pressure.

Setting Alarm Limits

Set two limits on chamber pressure: a band around the pump’s own baseline, which catches a change early, and a fixed floor at the vapor-pressure margin the seal needs. Treat leaving the band as an alert to investigate, and approaching the floor as an alarm to act on.

Before setting either, check the baseline against the chamber pressure the seal was designed for. That value should be on the seal layout drawing. If the drawing does not show it, ask the seal’s manufacturer.

Deviation From Baseline

Take the baseline at a steady, normal operating point once conditions have settled after commissioning or a seal change, and set the alert band for each pump, wider than the reading’s normal scatter at that operating point and on both sides of the baseline. A reading taken at a different flow needs its own baseline.

Margin Above Vapor Pressure

The Hydraulic Institute’s general rule is to keep seal chamber pressure at least 175 kPa (25 psi) above the pumped liquid’s vapor pressure. API 682 requires a vapor-pressure margin of at least 3.5 bar (50 psi) in its 4th edition or, where that cannot be met, a seal chamber pressure at least 1.3 times the vapor pressure.

Use the vapor pressure at the liquid’s temperature in the seal chamber, which runs warmer than the pumping temperature on the data sheet unless a seal cooler holds it below. Compare on one basis too. A process gauge reads pressure above atmospheric while vapor pressure tables give absolute pressure, so convert one of them before subtracting.

How Often to Read It

Read chamber pressure as often as the trouble it guards against develops: weekly or monthly rounds where problems build slowly and the pump is not critical, continuous logging where the pump is critical or problems develop fast.

On top of the routine interval, take a full set of readings, chamber pressure with the operating point and temperature, at these events:

  • At commissioning: the first reference for the pump.
  • Once operating conditions have settled: the reading the baseline is built from.
  • Whenever other monitoring shows a change: a shift in vibration, temperature, seal leakage or noise.
  • Immediately after a seal replacement: a new seal, a reworked flush or a new bushing can shift chamber pressure, so the baseline is taken again.

What a Change in Chamber Pressure Means

A change in chamber pressure points to the pump or its flush, and the first check is the operating point. Read the suction and discharge gauges; if they moved, the chamber followed the pump. If the pump is running steadily in its normal range on the curve, the change most likely comes from a flush-system component, and the flush is checked in order from its source to the throat bushing.

Which pressure the chamber follows depends on how the pump is built. On a single-stage pump with back wear rings and balance holes, the chamber sits at or slightly above suction pressure. On a pump without back rings, discharge pressure fills the casing and the chamber sits slightly below discharge pressure.

Pressure Falling

A falling chamber pressure points to whichever pump pressure the chamber follows falling with it: suction pressure on a pump with back rings and balance holes, discharge pressure on a pump without back rings. Suction pressure drops with a falling tank level, a clogging suction strainer or a partly closed suction valve.

If suction and discharge pressure are steady, look at the flush feed. On a Plan 11 flush, a plugged orifice cuts the feed from discharge.

Pressure Rising

A rising chamber pressure points to rising discharge pressure on a pump whose chamber follows discharge, as when a throttled or blocked discharge pushes the pump back on its curve toward shutoff. On a pump whose chamber follows suction, it points to rising suction pressure. Compare a rise with the design chamber pressure on the seal drawing.

Pressure Swinging

A swinging chamber pressure points first to the process: changes in flow, operation or ambient temperature move the reading, so check whether the swings line up with the operating point. If the pump is steady and the chamber still swings, suspect vapor or gas collecting in a chamber that cannot vent.

A fast swing shows only on a continuously logged reading; a gauge read on rounds catches one random point of it.

Slow Drift Over Months

A slow drift over months points to wear or fouling inside the pump or the flush: the clearing vanes on the back of the impeller, the throat bushing clearance, the flush orifice, and the back wear rings and balance holes.

Worn clearing vanes make chamber pressure creep up. The other parts can move it either way, so their drift is read as a change from the baseline and traced by inspection. On a Plan 11 flush, that means checking the orifice and the throat bushing clearance at the next opportunity.