Dynamic and Static Pump Seal Pressure

[Sawsan311]

Dear All,

I need to understand how to estimate the dynamic and static pump seal pressure if I only know my suction system design pressure considering:

- a pump is installed upstream my pump in focus and I know its shut off pressure so I will consider it as my suction system design pressure.
- a vessel is feeding my pump in focus and I know my upstream vessel PSV set point and LAHH liquid level and I am considering no credit of suction friction losses since at blocked outlet my flow would seize.

Appreciate your advice
regards,

 

[TiCl4]

Sawsan311,

It is my understanding that you are looking for the stuffing box pressure for the seal faces, and suction supply pressure is known. When the pump is running, typically stuffing box pressures are something like "Suction Pressure" + 10-25% of pump operating head, depending on pump/impeller design. However, that may change if the pump is deadheaded.

Why not estimate it as the max supply pressure + shutoff head of the pump? It certainly can't be any more than that. If you need a more certain number, I believe a call to the OEM is in order.

 

[Sawsan311]

Thanks TiCl4, I think you are referring to shut off conditions rather than the seal static/dynamic pressure defined by API 610. we usually design the seal system to the maximum suction design pressure not shut off, kindly please correct me

 

[TiCl4]

Sawsan311,

I am working from the definitions below. For maximum static pressure, I agree that it would be the the suction design pressure. However, pursuant of the definition of for dynamic pressure, the maximum dynamic seal pressure is defined as the highest pressure the seal can see during any operating condition.

In the case of starting the pump, there may be an opportunity to have three factors coincident:
[ul]
[li]Maximum suction pressure[/li]
[li]Pump running[/li]
[li]Pump deadheaded[/li]
[/ul]

In this case, the pump would see maximum suction pressure plus any additional pressure generated from the pump running at deadhead conditions. Even during normal running operation (non-deadheaded pump), you would expect the seal to handle maximum suction pressure plus a portion of the the pump head - the 10-25% I mentioned above. At deadhead conditions, I do not know if the stuffing box pressure can increase to full pump deadhead pressure. I don't see why the static sealing pressure should exceed the dynamic sealing pressure.

If you are asking how to estimate the particular rating of the seal, I think you'll have to ask the OEM.

 

[Sawsan311]

Thank you very much TiCl4 for your great support,

I can summarize from your explanation that:
- maximum static seal pressure when pump is in non-flowing condition and after a shutdown and can be attributed tot he maximum suction design pressure.
- dynamic sealing pressure can involve the maximum shut off head delivered by the pump in conjunction with the suction maximum operating pressure. There is a debate on whether to consider the suction side reaching the design pressure along with the pump being in shut off deadhead conditions however, this is mainly attributed to the system configuration.

Thanks

Regards,