Low level alarm and high level tripping system

There is more than one way to do this, depending on what sort of liquid level sensing devices you want (or have). If your sensors are small differential, you will need two of them: one near the top and one near the bottom of the service tank, and a relay to control the pump. The lower sensor will energize the relay, which starts the pump, and also has a N.O. latching contact in parallel with the lower sensor. The upper sensor, wired N.C. in series with this combination, interrupts the whole circuit, stopping the pump and resetting the relay for the next cycle. This is just like start-stop circuits.

If the sensor has a large differential that accommodate most of the height of the service tank, you will need only one sensor. Circuit details will depend on the specific design of sensor (such as ultrasonic or radar).

What Tornado has suggested is correct for using the level control sesnosrs, but you need to look at your whole useage set up.

Is the service tank just to feed other day process tanks?.

If so how much and how many times per day is the service tank filled?.

Is it needed?

If it is frequently refilled, you need to reconsider the use of the service tank and look at a Mass flow metering system from your main storage tank....using a VFD controlled pump supplying the oil via the mass flow meter (will need a PLC interface for batch recipe presets with overides if using just one meter when another tank needs filling...called queing). The piping manifold to your process tanks needs to be self draining from the meter and with valve controls with sensors for auto-shut off.

Then you do not need the service tank as flow would be on demand only via a VFD controlled pump.

Using a VFD drive will also save you energy and can be more efficient in the long run....I have always oversized my drives for longevity and never had a failure in 28 years (some have been upgraded with the latest electronics for better feed back and have gone from the PWM to Vector type on a few high heavy duty use ones (got credit on the Old ones as well from the suppliers..used Seimens and AB/Reliance)

Geoff NH

Carl E raises a good point: an additional safety on top of the functional controls (which my basic explanation omitted). For instance, in ammonia refrigeration, we might have an electronic level sensor with up to five control points for various solenoids and pumps, but it would be backed up with a float switch in case the previous system failed.

(I already gave a GA, but I can't do it twice. I hope someone else recognizes the wisdom of this.)

Ultrasonic level sensors are most suitable for oil tank level sensing and these are mounted from top and can sense level on continuous bases. Look at www.turck.com/

and www.banner.com/

We use one version of this: http://www.japsininstrumentation.com/magnetic_level_switches.htm

Install one more high high level controller and give alarm to both (high level and high high level ) in the control room. This is an additional safety and will alert the operator as an emergency.

Every one has covered the required components and suggested some redundancy in sensing devices. I would suggest that when you design the control circuitry you have a look at the criticality of the system in question and design the circuit to be fail safe either by spring loading or power absence.

There are some systems where a pump is so critical it is absent of motor overload protection. This is done so that Murphy's law of failing the transistor to save the fuse will not be effected.

I think you got many answer.Consider of selecting the instrument as per your Hazardous location.

Thanking all for positive response .Basically the proposal seems to install a level transmitter alongwith relay for tripping the motor.The other problem viz. overflow can be tackled.Appreciate if basic specs is furnished to initiate procurement action.