The Difference Between Flushing And Quenching in Slurry Pump Seals

The terms 'flushing' and 'quench' often seem to be confused or misused when discussing seal support schemes for slurry pumps. As the concepts of a mechanical seal cartridge and a filled seal cartridge are slightly different, I will discuss them separately and in turn.

Mechanical Seals

The basic mechanical seal flushing programme is very simple. It requires that a clear/clean fluid (usually water) is introduced into the space between the actual seal and the impeller side outlet restriction. The flushing fluid is introduced at a pressure higher than the pumping pressure, thus ensuring positive outflow/flushing of the mechanical seal and a clean operating environment.
Flushing is defined as "a fluid which is introduced into the seal cavity on the process fluid side, close to the seal face, and is normally used to cool and lubricate the seal face.

When flushing is required, the slurry pump supplier recommends the API Option 32 sealed pipe arrangement as it is more suitable for services containing solids or contaminants that could damage the seal faces if recirculated in the flushing medium.
A clear clean fluid supplied from an external source is normally used which is delivered to the main sealing surface on the fluid side of the sealing process. By using a closed gap throat bushing, the stuffing box can be back-pressurised to a higher pressure, ensuring that the flushing fluid does not flash across the seal face.

Slurry Pump

A quench plan, as the name indicates, is designed to quench or cool the seal. It is normally used if short periods of dry running are expected. As illustrated bellow, the fluid is introduced into the area between the back of the seal faces and the close fit exit on the drive side of the pump.
It is often used if a short period of dry running is expected. Fluid is introduced into the area between the back of the seal face and the tight-fitting outlet on the drive side of the slurry pump.

Quenching is defined as "the introduction of a neutral fluid (usually water or steam) into the atmospheric side of the seal to prevent the formation of solids that may interfere with movement or be used for other purposes.
Some quench seals are planned to replace the tight fitting outlet restriction with a secondary seal and a top outlet that can be piped to capture the used quench fluid and drain it from the rotating assembly. However the principle is the same, we try to cool the seal rather than flush it in any way.

Caution: A common mistake is to over-pressurise the box and damage the expensive mechanical seal.

Slurry Pump

Packing the box seal

The main objective of all flush water programmes is to prevent contamination of the seals by pumped water.The flush water programme for boxes is therefore very similar to the flush water programme for mechanically sealed boxes. However there are still some obvious mechanical differences.  The most notable difference is the addition of a seal (packing) between the inlet and outlet limits. This minimises the amount of flushing fluid consumed.

From an operational point of view the stuffing box is different as it requires some leakage to ensure lubrication and to prevent heat build up. The mechanical seal box should be leak free.
The rule of thumb for setting the water pressure is the same when using the pack box, whether flushing. Mud pump shut-off pressure plus 10% or plus 20 psi, whichever is greater. However, the flow rate is set differently.

With the standard flush schedule, the flow is usually adjusted by compressing the packings until a few drops of water are observed leaking from the seals on the drive side. In a quench schedule the flow rate is set by adjusting the inlet valve, while a valve is used on the exhaust side to maintain the correct seal tank pressure. If the outlet water from the seal box is too hot, the flow rate is increased until the outlet water cools, while still maintaining the correct seal box pressure.

I hope this short blog has helped to clear up some of the confusion about the seal flush programme. Please always refer to the pump manual for specific details. If there are still questions, welcome to contact us today.