Everything about Ullage-Temperature-Interface detector (a quick guide)

16 December 2021

Cargo onboard tankers is gauged by a device called UTI Tape, which stands for Ullage-Temparature-Detector detector tape.

The tape is a  Gas-tight , Portable equipment that is designed to perform under completely closed

conditions in a single operation.

What is Closed Operation?

As Defined by ISGOTT, 6th Edition International Safety Guide for Oil Tankers and Terminals Hardcover – 22 June 2020.

Closed operations –  Ballasting, loading or discharging operations carried out without recourse to opening ullage and sighting ports. During closed operations, ships will require the means to enable closed monitoring of tank contents, either by a fixed gauging system or by using portable equipment passed through a vapour lock.

So, in this article the requirements,constructionworking of the portable means equipment passing through a vapour lock which includes an interface detector will be discussed. 

Disclaimer – The construction and working of UTI tapes are according to the manufacturer and might differ. This article explains the Honeywell HERMetic UTImeter Gtex.

Why do we measure ullage onboard tankers and not sounding?

When measuring level by ullage method, the sounding tape is inserted only till the bob/ tape touches some part of the fluid top surface.  Instead of measuring the whole depth of the tank, only the free space  (from the top of sounding pipe to liquid surface) is measured . This method is convenient if the fluid in the tank is at a  higher level. In heavy oil tanks and viscous fluids, this method does not spoil the whole sounding tape and avoids a false reading. 

The dipping of sounding tape all the way till the bottom, to know the sounding of the tank will cause all the oil to stick to the sounding tape, which would be required to be cleaned to take the sounding of the next tank, making the procedure unnecessarily hectic. Instead determination of ullage using UTI tape is carried out for cargo calculation.

Why do we need an UTI tape, and what are requirements regarding UTI?

• SOLAS Requirements – 

All tankers fitted with a fixed inert gas system shall be provided with a closed ullage system.  (SOLAS II-2/4.5.5.3.3).  

The SOLAS amendments, that entered into force on 1 January 2016, reduced the threshold

further and require new build tankers 8,000 DWT and over, constructed on or after this date, to be  fitted with an inert gas system when carrying flammable cargoes.Oil tankers above 20,000 dwt were already required to install such systems. 

So Assuming you are sailing on an Ship fitted with a working inert gas system, you’d require a closed ullage system for carrying out cargo operations.

Thus, for measuring Ullage of the tank, temperature of the cargo and interface  ( the boundary between oil and water in tanks ) during cargo calculations a UTI tape is required.

OCIMF VIQ Requirements – 

1)The Fixed gauges are required to be checked regularly against portable tapes, which makes carriage of portable gauging tapes ( tapes) mandatory.

2) Furthermore, in case if a fixed cargo tank gauging system is fitted but is unreliable then  portable tapes are used as main method of ullaging.

3) Portable tapes should be calibrated in accordance with manufacturer’s recommendations and valid certificates of calibration should be provided for each instrument

4)We cannot use the UTI tape if it is not certified to be used in cases of complete gas-tight conditions and volatile and toxic cargo is being carried.

5) There should also be two spare tapes onboard. 

6)  UTI tapes must be bonded before being introduced into tanks.  UTI tapes which have quick couplings to connect the unit to the vapour lock will possibly not require bonding wires.  However, the internal bonding of such units should be checked every six months in accordance with the manufacturer’s requirements.

Construction  

Working

For ullage detection,

The sensing probe ULTRA  consists of a stainless steel tube which ends with a high-tech plastic head which cannot be removed from the tube. The sensing probe includes an ultrasonic liquid level sensor, a temperature sensor and a conductivity electrode.The  sensitivity for ullage and interface measurement is not adjustable. The temperature measurement is calibrated at the factory and does not require subsequent adjustment. 

Ullage detection

The ullage detector consists of two piezoceramic plates and electronic circuits. When the sensor head is immersed in a non-conductive liquid (oil or petroleum), the emitted ultrasonic

signal is detected by the receiver, coded and sent to the instrument unit which activates  the  buzzer with the continuous beep. 

Interface detection 

The principle consists of a conductivity measurement between an active electrode and a grounded electrode. When the liquid is conductive (as water), the ullage sensor detects the presence of the liquid to generate the intermittent beep.

Apart from this for servicing and safe usage, refer manufacturer’s manual.