I'm looking for a chlorine contact tank that can be used under pressure. I have a project for three small water treatment systems. The sources are groundwater. I want to use the well pump to push the water through the treatment plant and into the distribution system. The sizes I'm looking for are about 3 m3(800 gal.) and 1.5 m3 (400 gal.). Has anyone heard of tanks that can provide contact time under pressure?

Your simplest solution is a pipe. e.g. 20 ft of 30 inch (750mm) pipe holds 730 gallons,  I have found 30 inch DI pipe the be the lowest cost in $ per gallon in longer lengths. Although the end caps may make a smaller diameter pipe lower cost in your situation.  By using pipe you get a 100% usage factor because you can assume plug flow.  If you have the distance to the first customer then you can optimize the size to achieve your CT.
One other advantage is the pipe is buried, and requires no special installation skills.
If you use a tank you may have to perform verification study for your regulators, or be required to install a tank 10 times the size without the study.  Also tanks that size may require a Section 8 ASME stamp.

As Hydrae and Semo pointed out, you can use either pressure tanks or oversized pipe.  But you specifically asked about tanks, so I'll try to answer your specific question.

I am not aware of any company that sells tanks, specifically designed for use as chlorine contact.  But, as Semo pointed out, just about any tank shop can make one, given some basic design input from you.  If you can tell me what area of the country the job site is located, then I can probably give you the names of a few reputable fabricators.

Basically you will want to specify a pressure vessel designed for the maximum working pressure of the system.  If the well pump(s) does not have a pressure relief valve installed on the discharge, then I would design for the maximum shut-off or deadhead pressure of the well pump.

The vessel(s) can be either horizontal or vertical configuration.  The side shell length should be at least 3 - 4 times greater than the diameter.  Most health departments like to see at least 3 - 4 internal baffles or "curtains" designed to cause a serpentine flow pattern and prevent short circuiting.

The vessel is typically carbon steel with an NSF-61 listed epoxy coating on all interior surfaces; however, uncoated steel vessels are not uncommon.  (Note:  uncoated steel vessels will develope a rust coating on the interior walls that, if not scrapped off, will actually reach an equalibrium thickness that protects the metal substrate below it.)

The vessel should be equipped with an automatic air release valve located at the highest point of the tank.  Some manufacturers of air release valves include:  Val-Matic, APCO, Crispin, Golden-Anderson, Hoffman.

The vessel should also be equipped with manways for access to each cell and a drain located at the bottom of each cell.  The manways should be at least 14" X 18".  The drains should be at least 2" diamter with shut-off valves.

The vessel may not require the ASME code label & stamp.  It depends on your preference and/or the particular State's boiler and pressure vessel laws.  The ASME code was amended in 1989 to exclude the requirement of the U-stamp for vessels, in essence, used for "cold" water service and not containing any air.  If you can let me know the State in which the job site is located, I can tell you if the U-stamp is required.

If the vessel is going to be ASME code labeled & stamped, then a pressure releif valve (not to be confused with air release valve) needs to be installed at the high point of the vessel and set to open at a point lower than the maximum allowable working pressure limit of the vessel.  Knunkle is a manufacturer of these valves.  Be sure to specify an ASME rated valve.  It needs to be sized for the maximum flow rate of the system.

I hope this information is useful.  If you need any further assistance, please feel free to contact me.

MORE NEWS