Dip Pipe

By "Dip Pipe" I assume you mean a pipe that is internal to a vessel or tank, originating at an inlet nozzle and extending down into the liquid level so the lower end is always under the liquid.

Is this correct?

If so then.
The min length for Dip Pipe is from the inside of the nozzle to a point half way between the lowest liquid level and the bottom of the vessel or tank.

A dip pipe can be made many ways.
1) It can be attached to a mirror-image nozzle on the inside of the vessel at the point of the liquid inlet. To say it another way would be a second nozzle neck and flange is welded on the inside of the outside nozzle. The "Dip Pipe" is then attached to this inside nozzle. This method does not allow for removal and inspection unless the vessel is totally drained and made safe for entry.

2) The second method would be to to allow removal without draining the vessel. For this I need to make some size assumptions for description purposes. Let say the liquid supply line to this vessel (tank) is a 6" line. So we would make the liquid inlet nozzle a 10". Yes a 10" nozzle. I would place a 10" x 10" x 6" reducing TEE with flanges welded to all openings. Two 10" flanges and one 6" flange. The Flanged TEE would mount on the 10" vessel nozzle. The 10" side that is pointed up would be fitted with a Blind Flange which has a small (1" +/-) vent valve attached. The 6" side would be pointed in the proper direction for connecting to the incoming liquid line. The inlet line would have a 6" valve against the 6" connection to the TEE. The 6" Dip Pipe would be made with a thin plate ring (like a Spacer Blind) attached by welding to the pipe. The annular space (around the O.D. of the 6" pipe and the I.D. of the 10" TEE needs to be blocked. This is done by welding another ring to the inside of the TEE below the 6" entry point. This must be done in such a manner that there is no gap. Both rings would be drilled for bolts. The bolts would be installed and removed from the top. The nuts for the bolts would be "Tack" welded to the underside of the ring inside the TEE. The top of the Dip Pipe would also be fitted with a lifting ring for removal.

This second type of Dip Pipe would be removed by shutting off the 6" gate valve on the liquid inlet line, de-pressure the vessel, open the vent valve for safety, remove the the blind flange, remove the bolts holding the Dip Pipe in place and then with a hoist remove the Dip Pipe.

The space required for remove the Dip Pipe depends on the length of the pipe, the manner of removal and the device used to do the removal.
How it will piped?
With Dip Pipe #1 there is no special piping considerations.
With Dip Pipe #2 you need to have that shut-off valve and the ability to extract the Dip Pipe.

There are no doubt many other ways to construct a Dip Pipe.

By the way:
What is the commodity?
What is the operation Pressure?
What is the Operation Temperature?
How big is the Vessel (or Tank)?
What is the Line Size for the Dip Pipe inlet?
Is there a Platform on top of the vessel (or Tank)?