Mechanical Seals 1

[paulbrown]

We have positive displacement pumps which use mechanical seals which are designed for a Max. Inlet pressure of 3.5bar.

It has been noticed that although we can maintain the 3.5bar during normal running conditions, the thermal relief valves on the inlet piping are set at 5.5bar

Does this mean the seals will leak or is the situation different when the seals are not rotating?

Regards
Paul

 

[TZellers]

Sure sounds like it. I'm not familiar with PD seals per se, but alot of mechanical seals are "balanced", which means that the fluid pressure working to open the seal also works to keep it closed. Not a perfect balance and certainly limits apply. An unbalanced seal has to rely on spring or similar supply of force to stay sealed, which become over-whlemed quickly.

 

[dangnm]

i agree with tzellers it sounds like you are going to have a leak but what i think you should do is run a line from your stuffing box to your (suction)inlet line, in this way you will reduce the pressure on your seal faces. by doing this you will try to equalize the pressure and your seal will not see a high pressure. its worth a try, good luck

 

[paulbrown]

Thanks for the advice.
I have noticed that the pump stuffing box is already linked to the pump inlet

Regards
Paul

 

[longeron]

Paul,

Is this a sliding vane or gear pump, or something else?

The tubing you describe not only helps to lower the pressure in the seal chamber but to induce flow around the seal to help remove the heat it generates.

 

[tstead]

Paul,

Are you sure the piping arrangement is not an internal relief line?

The problem with running an unbalanced seal at higher pressures is also that it generates more heat at higher pressures because not only do you have spring force pressing the faces together but you also have suction pressure adding to that which will increase the friction (e.g. heat) at the seal faces. This will lead to shorter seal life.

Tim S.

 

[tonyh]

If I understand aright, the question is whether the seal will leak when shutdown and the system is locked in, leading to a call on the thermal relief valve?
I would ask the seal vendor for his advice, but I would guess that at these pressures there is a fairly good chance that the seal will hold when stationary.