Air Diaphragm Pumps in Series due to Pipe Rack and Filter

[whatittakes]

Hi,

I'm trying to connect two tanks to one air diaphragm pump, which will pump a slurry [1 part sawdust, 5 parts phosphoric acid, 10 parts ethanol, small particle size, 700 cP] up 10-12 feet to the top of the pipe rack, then across the pipe rack for 5-10 feet, then 10-12 feet drop down to the floor, go through another air diaphragm pump closer to the plate & frame filter the company is using to filter the slurry. The plate & frame vendor says there needs to be a pump closer to the inlet of the plate & frame to help build the pressure during the initial filtration. Can I connect two air diaphragm pumps in series, one to pump the slurry out of the tank and another to pump into the plate & frame filter? Should I only have one pump? I can't go over 100 psi in the plate & frame, so the max the air diaphragm pumps can pump is at 100 psi. Thanks.

 

[bimr]

One would think that you are relatively close enough at 30 feet or so that one pump will be adequate. Be sure to confirm that the piping is adequately designed and installed since you are pumping hazardous materials.

This comment:

"The plate & frame vendor says there needs to be a pump closer to the inlet of the plate & frame to help build the pressure during the initial filtration."

does not make much sense. Is your piping undersized? If your piping has adequate capacity, then the piping distance will not matter.

One would expect that you would get higher flow through a clean plate & frame. As the filter run continues, the pressure drop will increase and the flow decreases.

In some processes, the filtrate is recycled back to the feed tank and passes through the plate & frame filter again. Are you doing this?

http://www.youtube.com/watch?v=JWR06prkzIc

 

[micalbrch]

"...there needs to be a pump closer to the inlet of the plate & frame to help build the pressure during the initial filtration." That sounds a bit strange as the pressure during initial filtration is the lowest duríng the whole filtration cycle. But does the plate and frame press stand on the same geodetic level as the pump or is there any height difference? From your discription it looks as it is more or less the same level but that is not really common. Your AOD pump is limited to 100 psi due to the available air pressure. The question is what pressure can be achieved in the press with that pump? The is important for the result of the filtration. bimr wrote the piping distance will not really matter if it is correctly sized and that is true. But if there is some additional height to the press, then it will be different. Ask the press supplier again what their concern is, the initial filling or the final pressure which can be achieved with the pump.

 

[Compositepro]

You are probably misinterprating what the vendor said, or it was poorly stated to you. The vendor's concern is that there needs to be enough flow rate through the filter to create a pressure drop across the filter cake, in order to compact it. Two pumps in series will not increase flow rate. They have to be in parallel. In your case, you should do as bimr suggests and use a second pump to recirculate filtrate though the filter until a filter cake is established. The filter is not as effective until this occurs.

You cannot put AODD pumps in series. They are not designed to work with suction pressures much above ambient. The diaphragms are dished, and positive pressure on the suction will invert the dish on every stroke, causing them to fatigue very rapidly.

 

[ccfowler]

I'm obviously late to this discussion, but I can't help wondering how much of your problem may be due to the inherently substantial flow rate variations (pulsations) that are characteristic of diaphragm pumps.

I recognize that your system is currently "stuck" with this type of pump, and there may well be excellent reasoning behind the choice for this application, but personally, I try to avoid pumps that produce substantial pulsations because of the many potential problems that these introduce for the process and for instrumentation. Additionally, unless there are controlling safety or practicality issues, I try to avoid using compressed air as the energy source for pumping. Compressed air, as a process energy source, is very costly for both initial and continuing needs financially, and it always represents a very substantial, and frequently avoidable, waste of energy resources.

Being limited to the information currently present for this case, I would want to take a careful look at replacing the diaphragm pump with a suitable progressing cavity pump. This type of pump can normally handle slurries very nicely at a small fraction of the total energy cost of operating a complete compressed air system of sufficient capacity to power the diaphragm pump(s). In my experience, compressed air is often presumed to be "free and available" in plants with little regard to the real cost of the compressed air. In addition to the compressed air being a very expensive energy source, the "motor" function of diaphragm pumps uses this relatively precious resource at very poor efficiency. Depending on the details of your specific installation, you may find that thorough consideration of the complete energy costs (both financial and environmental) involved, the use of a different type of pump may prove to be surprisingly attractive.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.