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FREQUENT FAILURE OF MECHANICAL SEAL

FREQUENT FAILURE OF MECHANICAL SEAL

(OP)
Hi Guys,
I head the Mechanical department in my company. We are in the business of storing petroleum products and distributing them into Bulk Receipt Vehicles (tankers). We have two centrifugal pumps which serve the purpose of unloading fuel from the BRV (Bulk Receipt Vehicle) into our storage tank via pipelines. So what we do is couple a flexible hose into the drain port and connect the other end of the hose to the suction of the centrifugal pump. The fuel from the BRV compartment gets into the suction pipeline by gravity. One of the unloading centrifugal pump is dedicated for gasoline transfer and the other for gasoil transfer.
The problem is the gasoline unloading pump mechanical seal keeps failing every now and then. So far this year, we've replaced 4 mechanical seals. This is totally unacceptable. My best bet will be that the gasoline that forms a thin film between the seal faces vaporizes and blows them open. I believe so because for close to a year now, the gasoil unloading pump has been running without any mechanical seal failure. What do you think guys?
Are there any viable solutions to this problem? I was thinking of replacing the centrifugal pumps with positive displacement pumps since they don't really have an issue with low NPSH available ( I believe the reasons why the seals are failing is because of low NPSHa and hence the pumps are cavitating). But my boss says it is too expensive. Are there cheaper options? Any possible modification to the current design?

* We can't increase the suction head because the pumps are already fixed. The head is around 2.5 - 3m.

RE: FREQUENT FAILURE OF MECHANICAL SEAL

If the piping plan to the seal is a plan 13 (line running from seal to pump suction area) then change it to a plan 14 (plan 13 plus a plan 11, line running from seal to pump discharge.) If it is already a plan 14, enlarge the orifice on the 11 (discharge line.) Don't start tinkering without running it by the seal rep first, and you may or may not have an available port to run the line to discharge.

Seal chamber pressure should have a minimum 25 psig margin above the product vapor pressure. While I am sure the seal vendor is happy to have the repairs/sales, they should also be happy to help diagnose your problem and make suggestions to help resolve it.

RE: FREQUENT FAILURE OF MECHANICAL SEAL

(OP)
Thanks 1gibson.
We'll give it a try because I've noticed there are quench ports on our John Crane single cartridge seals. Since the suction pressure is already low given the low NPSHa, we'll try plan 11. Thanks once again.

RE: FREQUENT FAILURE OF MECHANICAL SEAL

Quench port can't be used for plan 11, must be an "F" (flush) or "FI" (flush in) port. Or you can use a port in the seal chamber if available. Again - run it by the vendor before modifying stuff.

Don't just fix this one installation. Determine what the problem was, then incorporate something into your specifications to avoid it in the future.

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