Gear Pump Stuffing Box Pressure 1

[PTT]

Dear all,

We use a gear pump on a polymer transfer line. The suction pressure on this pumps is 8 PSIg and the discharge pressure is 110 PSIg. We use a John Crane double mechanical seal with a barrier fluid. The pressure of the barrier fluid is 80 PSIg. We experienced a failure of this seal and the polymer has leaked into the barrier fluid system. It's obvious that the barrier fluid pressure is not enough to keep the polymer inside the pump in case of seal failure. To calculate the stuffing box pressure on a centrifugal pump we generaly use this equation:

Stuffing Box press.= Suction press. + 10% differential press. (25% for non-API pump)

Is there any equation to calculate or evaluate the stuffing box pressure for a gear pump stuffing box???

Thanks a lot!

PTT

 

[poppeye]

You say you are using a double seal with barrier fluid? Are you also using a barrier fluid tank that is gas charged to maintain this barrier pressure? Calculated stuffing box pressures are just that calculated. Pump wear and increased clearances will determing what the stuffing box pressure is. I would suggest that you tap into the stuffing box at a flush port and accurately measure the pressure during operation. Keep your barrier fluid 15psi above the S/B pressure. Also depending on the type of Polymer you are pumping, some can become quite abrasive, and will set up with temp, flow, pressure causing failure of a mechanical seal. Keeping the polymer fluid and clean around the exposed seal internals is a must or the seal will fail no matter how well you maintain the barrier pressure.

 

[PTT]

Thanks Poppeye for your reply and your solution. We are using on this pump a pressurized tank with nitrogen at 80 psig to maintain the barrier pressure. We are pumping polytrimethylene terephtalate polymer with this pump. It's not very abrasive except for the TiO2(titanium dioxyde) grade that we are pumping maybe 30% on a yearly basis. We are going on shutdown next week for Christmas time. I already planned to check the feasibility to tap a flush port into the stuffing box during this shutdown. That's the real thing to do!

Regards,

PTT