Pumping from a vacuum vessel

[notnats]

For some years we have been manufacturing a system which uses a positive displacement screw pump to remove liquid from a vacuum chamber. To prevent cavitation we keep a metre of liquid above the pump suction port and keep the suction line twice the diameter of the port as close to the pump as possible before reducing to the port size.

I have wondered if a centrifugal pump would have less trouble with cavitation and let us run the vacuum vessel "dry". Or would I still need to keep a head over the pump suction.

Delivery pressure is 2 to 6 bar, vacuum tank pressure above the liquid is 1 to 5 millibar absolute.

Has anybody got experience with this, Don't tell me to ask the pump manufacturer, because we are already successfully using the screw pump outside the manufacturer's specs.

Thanks,

Jeff

 

[poppeye]

Power generation facilities use centrifugal pumps to draw down liquid from their steam condensers throughout the world. What is essential is a vent port to the inlet eye of the impeller so the water or fluid will flow freely into the inlet of the pump. While in the Navy, this was referred to as submergence control. The pump was set up allow for a standard amount of water to flow freely into the condensat pump and maintain a certain level inside the hotwell. On the old ships, ones without pumps fitted without mechanical seals. Leaving the vent valve open on the idle condensate pump would allow for a leak path to occur and loss of vacuum as air entered the shaft stuffing box and enter the condenser through the associated piping. You state that you are using a screw pump to perform this at present time. Is this due to the type of fluid being pumped. Ie. viscosity or just using a pump that will draw an inlet vacuum greater than the vessel to thus draw the fluid into the pump? You will need to look into how free flowing the fluid is to entering the pump. More than one stage may be needed to provide enough discharge head to meet your system needs. Look carefully into NPSHR and NPSHA when choosing to go with a centrifugal pump. Most reputible centrifugal pump factory representatives can look at your system parameters and make a recommendation based on your system and fluid parameters.

 

[Torricelli]

the higher the best, a 10 meter barometric leg would be safe and simple. A side channel pump could be the right choice but a head will always be required (which size of pump are we talking about?).

ET

http://www.vacuum-guide.com

 

[Artisi]

What you are looking for is a "Condensate extration pump" pumps of this nature are designed to operate under the conditions you are describing. I suggest you look round the pump manufacturers and ask a few questions, I'm sure you will come up with the answer.

International College
Naresuan University
Phitsanulok
Thailand

 

[notnats]

Thanks all.

The fluid is transformer oil sg about 0.8, and we are degassing and drying in the vacuum chamber, so we can't let any air in before or after the pump. The equipment is often on portable skids or trailer mounted, so we are limited in height. The reason we are using screw pumps is because the customer we build them for is an agent for these pumps and wants to maximise the amount of their product in the plant, but this might change if it becomes more "cost effective" to use a centrifugal pump. I hadn't thought about a side channel pump.

Thanks

Jeff

 

[Torricelli]

for the recirc. pump between vacuum recipient and transformer you could use a SIHI side channel pump.

talk to the experts for this application listed on

http://www.vacuum-guide.com/firmenlisten/produkte/vacuum_impregnation.htm

ET

 

[25362]

From what I've read in this thread, it appears that the screw pump arrangement works fine, and would be a correct selection for a pumping fixture.