Top Suction for Sodium Hypochlorite Tank? 5

[drboyd]

I'm getting ready to specifiy a new sodium hypochlorite tank - looks like it will be HDPE. One of the technical folks here at the plant says he's heard that almost all the HDPE tank failures have occured at the pump suction flange, due to the pulsations of the metering pump. He's pushing (nicely) to have the pump suction via a vertical standpipe entering from the top of the tank.

I'm thinking problems with offgassing and losing prime to the pumps. Has anyone tried this? How did it work?

Thanks in advance for the help!

Don

 

[jdarco]

Virtually all the tank manufacturer's recommend a flexible connection at the discharge, yet most people neglect to install them this way. That is typically what causes the failure you describe. You could install a check valve on the drop pipe to maintain prime, but on a large tank this could be a maintenance headache if you lose prime.

John

http://www.crecoinc.com

 

[RGCook]

You say your technical friend "heard" that huh? You need new technical friends. It is true that positive displacement pumps and air diaphragm in particular can tear up piping and/or crack nozzles on tanks (metal tanks too, with weld neck flanges and reinforced...don't ask me how I know...ahem...)--IF they are improperly installed. If you are talking about larger PD pumps, then you should utilize surge supressors and minimize piping distances (pump to suppressor--suction and discharge) Also, properly mounting the pipe to allow the suppressor(s) to take the hammer as opposed to the pipe is just common sense and critical to long term life of the system. Flexible isolation connections are advised.

Most reputable pump manufacturers clearly illustrate proper piping/installation guidelines. Call them and they will give it to you. Do not think their sketches are overkill...they are not. Just do it and your HPDE tank will be fine.

 

[quark]

We have been using top mounted pumps (with a tank height of 1 meter) and don't have any problems. As dosing rates are generally low, and you are using a PD pump there should not be any problem of losing prime. And for air, where do you think it will come from? Still it can be pushed by the pump.

Earlier two posts are good.

 

[bimr]

Don,

The reason for the failures of the HPDE tanks has nothing to do with the pumps. HDPE tanks typically incorporate cheap and poor quality bulkhead fittings. The fittings are installed by cutting a whole in the tank wall and then sandwiching the tank wall between flange rings. The bulkhead fittings fail because there is no reinforcement of the tank wall at the flange rings. Sometimes there are failures of the bolts as well since they penetrate all the way through the tank and contact the stored fluid.

This website illustrates the flange:

http://www.kthsales.com/website/vendors/Assmann/Assmann-Fittings.htm

If you use an HDPE tank, buy one with reinforced nozzles like the one illustrated:

http://www.protectoplas.com/superflanged.htm

The other tank option is to use a gel lined FRP material.

HDPE tank life in hypochlorite service typically will last about 7 years.

If you are planning to install a small tank, you can get by with a drop pipe suction. If you are installing a large bulk tank with truckload capacity, the drop pipe suction is not recommended for the reasons that you stated.

If you are using a pd pump, oversize your pipes by a factor of three and you will have no problems with pulsation.

 

[glennpan]

Dr. Boyd,

There are a couple of factors that will determine the best place to install the pump. What is the size of the tank, and also what the flow rate of your metering pump is.

Metering pumps are usually pretty limited in their suction lift (6 ft max for an S.G. of 1.0), so if you have a large tank, top mounting may not be practical.

The off-gassing problems that you mention are worse with lower flow capacities (by this I mean 4 GPH and under). For those flow rates I would recommend a metering pump with a self-deaerating head.

What size are the fittings that you will be installing? Note that you are not limited to bulkhead fittings, although with Viton gaskets you should not have too many problems. Welded fittings are also an option.

The tank longevity is something that nobody should try to put a number on with tanks for Hypo service. It depends on the quality of the Hypo, and if there are any metal contaminants, if it is an indoor or outdoor application, temperatures, fittings, etc. BE AWARE that many tank manufacturers will not warranty their tank for Hypo service. The tanks we carry have a full 3-year warranty for this service.

For more information on the handling of Hypochlorite, go to

http://www.panner.com

Glenn