PC Pump System 1

[minnjoe]

Hi

The sewage treatment plant where I work plans to install a 650 gal. heated mixing tank to process grease and scum from the primary settling tanks. Currently, scum from the primary tanks passes through a grinder and is then pumped to a Walker Separator tank (

http://www.walker-process.com/prod_grease_scum.htm);

the separator tank then skims off the grease/scum and deposits it in a dumpster which is hauled off for land application.

The mixing tank will replace the dumpster and a progressive cavity pump will be installed beneath the tank to pump its contents through a 4” glass-lined ductile iron pipe to the point of discharge (approx. 170’). Here are some system details:

High scum level in mix tank: 100.00
Low scum level in mix tank: 94.23
Pump C/L elevation: 91.44
Lowest invert elevation in pipe system: 85.32
4” discharge invert elevation: 89.94

A hot glycol jacket will keep the mixing tank contents at around 120°F. Several days may pass between pump cycles, which means approx. 125 gallons of the grease/scum mixture will remain to cool off in the discharge pipe. Since the entire system is indoors, the lowest the temperature will get is around 70° F.

The sales rep. recommends using a 3 or 5 HP PC grease pump that will deliver 25 gpm at 275 rpm; the data sheet provided assumes a viscous fluid with 3% solids and a particle size range of 0.25” to 0.50”.

Questions:
1.) Where can I find computations (preferably some sample calcs to analyze) to verify/validate the sales rep’s recommendations? I’ve never worked with PC pumps before and my experience with conventional pumps isn’t much broader. So I’m desperate for advice.

2.) Given the above scenario, is there any concern that the velocity in the discharge pipe is so low (≈0.58 fps)? Our Plant Operations folks will not allow anything smaller than a 4” diameter pipe. They’re hoping the glass-lined pipe with jetting nozzles installed will be enough to keep the pipe scoured.

Thanks

 

[bimr]

You may find calculations in Metcalf & Eddy's Wastewater Engineering.

The smallest diameter pipe for pumping sludge/scum is generally either a 4-Inch glass lined or a 6-Inch unlined.

There will be a concern with the low velocity of 0.58 ft/sec. Solids will settle in the piping. A minimum velocity of 2.5 ft/sec will assist in moving the solids that settle out.

You should have flushout connections to allow treated water to be injected for fluahing.

If you expect grease to collect inside the pipe, consider the addition of a steam injection system to flush the piping.

If possible, the length of the scum piping should be minimized.