Can we use self prime pump to replace vertical line shaft for vacuum a

[Majdie]

Hello:

I have a problem in a small steam power generation unit where I have a vertical shaft pump in a close vessel under vacuum. the pump of course has a mechanical seal. It is pumping condensate water at 80 Deg C. The flow is only 5 m3/hr. It is keep failing in the bushing area and I am thinking to replace it with a horizontal self prime pump, but since it is a vacuum at 0.48 kg abs, I am not sure where it will be a good idea.
We tried many think to resolve this issue but unfortunately not succeeded.
I would appreciate any advice.

Regards

 

[bimr]

A self priming pump will not work in that application because of the low operating pressure.

Many times in the application that you are describing, the tank is elevated above the pump. Is that arrangement possible?

 

[Pumpsonly]

Water at 80 C is very poor lubrication property.
If the failing bushing is at the seal area, you need to have a
Plan 21 or 23 flush plan.
If there are intermediate bushing,you need to look into the material of the bushing.

 

[JJPellin]

A plan 23 would be common for this service. However, a plan 23 does not provide for any flush flow into or out of the seal chamber. As a result, the bushing below the seal may not receive adequate cooling and lubrication. With this type of pump, a plan 11 will not usually work since the seal chamber is under discharge pressure. A plan 13 is a problem since the suction pressure is under vacuum. It is usually not desirable to run the seal faces under vacuum. We have been successful in this service using a plan 14 with discharge flush in and suction flush out with the orifices sized to keep the seal chamber pressure positive and still provide some flow across the bushing.

Johnny Pellin

 

[Majdie]

Thanks for all reply.
In fact we have a place underneath the suction vessel to install a pump. I am thinking to have canned motor pump with external cold flushing so we can fully protect the vacuum system. The service is clean whcih is BFW quality. Any comment on this?

 

[bimr]

Feed pumps for HVAC boilers usually are equipped with mechanical
seals, not packing. The mechanical seal eliminates most of the
leakage and evaporation at the stuffing boxes. The mechanical seals must be designed for the maximum temperature encountered with the hot condensate.

http://www.accessengineeringlibrary.com/mghpdf/0071484264_ar022.pdf