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Apr-2015

Gas and fire detection in a tankage area

A new detection system in a refinery’s tankage area has transformed the level of safety in the event of gas leakage and fire

NÍYAZÍ BOZKURT
Tüpraş Kirikkale Refinery
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Article Summary
A state of the art gas and fire detection system has been installed in Turkish Petroleum Refineries’ (Tüpraş) Kirikkale Refinery. The purpose of the system is to detect both fire and gas leakages across the entire tankage area. The detection system is based mainly on hydrocarbon gases, so most of the detectors are of the hydrocarbon type and calibrated with reference gases. Furthermore, flame, hydrogen, hydrogen sulphide and linear heat detectors are located wherever appropriate. In total, there are 169 hydrocarbon (HC) detectors, two hydrogen sulphide detectors, 49 flame detectors, one hydrogen detector, and seven linear heat detectors.

In the first step of the installation of the detection system, the tankage area was divided into eight separate segments. The criteria for establishing this separation of the sectors is the area, and the distance between tanks and detectors.

The main objective of the system is to detect both fire and gas leakages in the tankage area. All of the tanks have at least one hydrocarbon detector at their entry or exit points. Besides the tanks, there are four pumping stations to be considered. Flame detectors have been placed at high points on the pumping stations. The most important function of the system is to make operators aware of any liquid leakage around the dyke of the tanks. Hydrocarbon detectors help the system to do this.

Linear heat detection (LHD) systems are installed on the surface of floating roof tanks. Seven linear heat detectors are positioned on the system.

Division of the tankage area into regions
The tankage area has been separated into smaller sections, or regions, for better control of the system. In each of these regions, alerting flash lamps and horns are installed. When the detectors of a region sense a gas escape, the flash lamps and horns are activated. An overview of the tankage area can be seen in Figure 1. As indicated, it is divided into many smaller areas.

The main criterion for determining the area of each of these divisions is the number of detectors required within it. Figure 2 shows a more detailed overview of an individual region.

How to locate detectors
The locations of the detectors are decided according to different criteria for pumping stations and tankage areas, but the main criterion for positioning any detector is for it to be close to a potential explosion point.

Detectors are located at the entry and exit points of tanks since leakages tend to occur where pumps and other equipment that handle materials entering and leaving the tanks are located.

Flame detectors are positioned in the tankage area’s four pumping stations, taking into account items of critical equipment. The flame detectors are placed on high platforms to broaden the area each detector observes. In addition, there are flame detectors in the LPG tanks area.

Hydrogen sulphide detectors are located in the control room buildings, close to the ground. Furthermore, hydrogen detectors are positioned in the higher parts of electrical substations.

To summarise, detectors are located according to their distance from potential explosion points and the molecular weight of the gas each detector is designed to detect. If the molecular weight of the sensing gas is higher than the molecular weight of air, the detector is positioned close to the ground. In other circumstances, the detectors are located in high positions.

Naming detectors
Each detector is named according to the type of gas it senses and the tankage or pumping station number where it is positioned. The name of a detector begins with the name of the tankage or pump station and continues with an abbreviation of the detector type, such as hydrocarbon (HCD), hydrogen sulphide (HSD), flame (FLD), and LHD. Finally, the number of detectors present in a location is added to the name. An example of nomenclature for several detectors is:
4101- HCD- 001: Tank 4101 first hydrocarbon detector
3160- HSD- 001: 3160 Building first hydrogen sulphide detector
3100- FLD- 009: 3100 Pump station ninth flame detector
4218- LHD- 001: Tank 4218 linear heat detector

Carrying signals to the DCS
All detector signalling devices are connected to a control board via gas detection system (GDS) panels. Detector signals are carried to junction boxes via 3X2.5 armored instrument cables. In the junction boxes, these signals are jointed and carried by main cables to the control board. If the distance between a junction box and the control board is not too far, the junction box’s main cables are connected directly to the control board.

Remote input-output (RIO) panels are employed as gas detection system panels in the field. The reason for using RIO is the relatively long distances between the detectors and the control board. Communication between RIO and GDS is achieved by Modbus which is faster and safer at carrying long distance signal applications.

The signals combined in GDS are sent to DCS via instrument cables. A connection diagram of the system is shown in Figure 3.
 
System redundancy
The system is redundant in both the GDS and the DCS panels. A GDS panel sends the duplicated data with separate cards. There is no connection between these two communication cards.

In the DCS, there are two redundant Modbus cards: the master and the slave. Each card receives signals from separate GDS communication cards. The master DCS card scans the field and accepts data from the GDS panel. If there is any fault in the master DCS card receiving data from the field, the slave DCS card begins to accept data from the other GDS communication card.

LHD terminology
LHD becomes compulsory as the risk of fire increases above tanks. LHD terminology is based on resistance change in an electrical cable. As the amount of heat increases, there is an increase in resistance caused by the length of the cable.

The main leakage points for tanks are seals. LHD cable is located close to the seals. At Tüpraş, we have applied LHD systems to floating roof tanks. LHD cable is placed close to the second seal and moves as the roof of the tank moves. There are special mechanisms to protect the cable from mechanical damage during such movement.
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