Can I ground my induction type relay to a metal tank or pipe?

Yes, there are two options for making a grounding circuit.

#1 - GROUNDING TO METAL TANK OR PIPE:

A system ground return circuit is required from the indicated relay terminal to the liquid in order to complete the secondary circuit of relay. Conduit should not be used. Instead, the connection should be made directly to the uninsulated metal tank, or to the metal pipe connected to the tank below normal low liquid level. In wells, connect ground to the pump or metallic water pipe.

#2 - GROUNDING TO NON-METALLIC TANK OR PIPE:

For concrete, wood, or insulated tanks, use an extra (3rd) common electrode extending slightly below the longest operating electrode.

INSTALLATION:

Install electrodes in a tank or well by suspending them vertically. One electrode should be set at the desired start level and one at the desired stop level. For sewage or surface drainage sumps, make sure the electrodes are hung far enough apart so that foreign matter floating on the water cannot foul the electrodes. Size 18 or larger Type TW or THW wire is recommended for connection to the relay.

For electrode installation instructions, follow the link below:

https://support.boshart.com/elb-and-els-electrode-installation-instructions