Sodium Hypochlorite Off Gas Problems 4

[PotableWater]

We have a fairly new GE Osmonics potable water skid. We cannot attain full automatic operation because our sodium hypochlorite pumps become gas bound when they sit idle for more than several hours. We are using Prominent GALa model 1601PPE200UD112100 pumps. With the normal pumps heads we would have to reprime the pumps each time they sat idle to attain chlorine injection. We switched over to auto-degas heads and these did not seem to work either. We raised the chemical tote level up so we have a minumum of 18 inches of head pressure on the suction. We also curled the tubing on the suction line to provide a hight point for the gas to accumulate before being pumped through. No gas is visible in the tubing until the pump starts and it draws gas through an approximate 2 foot PVC header upstream of an isolation valve. The system is tight with no leaks. Once the pump is primed and the gas is pumped through, it functions well. What causes the off-gassing? Should we put a check valve on the suction line at the header to maintain the suction under pressure? Would a vent line on the suction header help? We currently use 1/4" tubing on the suction and discharge of the pump per the vendor manual. We larger tubing help?

 

[bimr]

This is a common problem. The best pump for concentrated (12-15%) hypochlorite solution is probably a peristalic type. Off-gassing occurs with PD pumps. The PD actually will cause offgassing on the suction stroke of the pump.

Marlow & Watson pumps work well.

Offgassing is catalyzed by metallic piping and fittings. Offgassing is also accelerated by exposure to sun light or heat.

http://www.englewoodgov.org/Index.aspx?page=399

 

[PotableWater]

We have eliminated 95% of the metal in the piping. Everything is pvc or tubing and there is a small amount of metal in the tote isolation valve. We are going to try and put a vent line in the pvc header back to the vented tote. It appears that the gas is collecting in this small header. The header is downsloped so we do not understand why the gas would build up in this small section of pvc. Thanks!

 

[bimr]

Have you tried the accu-vent automatic degassing valve?

http://www.primaryfluid.com/accuvent.html

http://www.miox.com/benefits/item/operator_safety/

http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=PeriPumps_NaOCl.htm&ID=537

 

[ch81pc]

I've experienced the same thing with PD pumps pulling off of twin 1" PVC pipework (total pipe run from tank to pump ~ 45'). These pipes had open top vent pipes located close to the pumps. Most of the time the system worked fine, it was only whe we drew the tanks down that we got problems (less static head to drive the NaClO). If there is any way your tanks can be raised or your pumps lowered that would help to maintain positive pressure in your suction side. Also minimising the distance between tank and pump helps. Temperature is also a big one to look at, hotter it is the more gassing you'll get.
Hope this helps.

 

[dfmorvan]

You should set the pumps very close to the hypo tank (less than 2 feet) and use a large diameter for the suction pipe between the tank and the pump (typically 2"). That way the bubbles will get back to the tank and not accumulate in the suction pipe.
Hypo will always degas so this is fairly normal. High temperature will accelerate the degassing.
Also do not forget that for Prominent Gala with NPB (self degassing) head, you shoud not go under 30 % stroke lenght otherwise the volume pumped each time is too small and the degassing device does not work well or at all. Better to use the pumps at 50 % stroke min or at max stroke length if possible.

 

[nackra]

one of my Midwest clients had same problem & tried a series of retrofits with vent hoses routed back to bulk storage tank. Bulk storage tank adjacent to pump had vent, but tranfser pipe to pump was not "sloped" back UP to the tank.

"crystallization" build-up in vent hose restricted performance.

finally switched from WT diaphragm metering to Watson-Marlow "tubing" pump for sodium hypochlorite feed

http://www.watson-marlow.com/watson-marlow/p-rangeus.htm

At second facility we put bulk storage tanks on legs but due to secondary containment requirement we put W-M pumps on pad at elevation above bottom of tank.

Good luck

 

[PotableWater]

Thanks for all of your great advice. We have struggled with this system, but I believe we have a handle on the problem. This is a good example of a system's level technology upgrade without proper training and operating experience.