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Pre-fire plan and emergency response for Diesel Hydrotreater Unit (DHDT) in oil refinery

re-fire plan and emergency response for DHDT in oil refinery

Pre-fire plan and emergency response for Diesel Hydrotreater Unit (DHDT) in oil refinery

The pre-fire plan and emergency response strategy are crucial at oil refineries to mitigate any gas leak, fire and explosion emergency through a proactive approach. The pre-fire plan helps emergency responders to manage any emergency efficiently by identifying the proactive mitigation action line for each credible identified emergency scenario and thus helps to minimize the losses.
The Diesel Hydrotreater Unit (DHDT) is very prone to major accidents, including fires, toxic releases and explosions due to extreme process conditions (high pressure and temperature) and handling highly flammable gas hydrogen. This article provides information on the best mitigation techniques for effective emergency response by preparing a pre-fire plan and the best firefighting strategies for Diesel Hydrotreater Unit emergencies, including fires, explosions and the release of toxic gases.
In this article, we will discuss the different emergencies, credible scenarios in the Diesel Hydrotreater Unit and action plan to mitigate these emergencies by improving the implementation and practice of the pre-fire plan (PFP) or the pre-incident plan (PIP). We start with the general operational process of the Diesel Hydrotreater Unit, what are the products of DHDT, the Hazards of DHDT, the required fire protection system, emergency response planning and firefighting strategies.

Diesel Hydrotreater Unit (DHDT) process in refinery
The prime objective of Diesel hydrotreater unit (DHDT) is to maximize the diesel production by improving the quality of diesel ie. to produce high grade diesel as per present market requirement.
In DHDT unit, feed (viz LGO, HGO, Vacuum diesel, LKO, HKO, heavy naphtha from CVU/VDU, Coker naphtha, LCGO from DCU, LCO from FCCU and steam from LPG treatment unit ) from different units and storage is mixed with recycle hydrogen in present of catalyst in the reactor at temperature about 300-400°C and high pressure of about 35- 125 kg/cm2.
The chemical reactions viz hydrodesulphurization, hydro-de-nitrification and saturation of aromatic and olefin are the main reactions happening in Diesel hydrotreater unit. There is separator column where reactor effluent is separated into gas and liquid. Light Gas and hydrogen sulfide  is removed by amine wash and gas is recycled to reactor along with makeup hydrogen.
The hydrotreatment processes differ depending on the raw materials and the catalysts. Hydrodesulfurization removes sulfur from kerosene, reduces aromatic and gum-forming characteristics and saturates any olefin. Hydroforming is a dehydrogenation process used to recover excess hydrogen and produce gasoline with a high octane number. Hydrotreated products are mixed or used as a raw material for catalytic reform.

Hazards of Diesel Hydrotreater Unit (DHDT)

Below are the major hazards of Diesel Hydrotreater Unit (DHDT)
·        Fire Hazard- Jet Fire/Pool Fire/Flash Fire
·        Toxic Release like H2S
·        Explosion –UVCE/VCE
·        Hydrocarbon Leak & Spill
·        Hazardous Chemical Leak
·        Electrical Fire
·        High pressure and high temperature

Safety consideration in DHDT unit

Inspection and testing of safety relief devices is important due to the high pressures in this process. Adequate process control is necessary to protect against clogging of the reactor beds. Due to operating temperatures and the presence of hydrogen, the hydrogen sulfide content of the raw material should be kept strictly to a minimum to reduce the risk of severe corrosion.
Wet corrosion of carbon dioxide in areas of condensation should also be considered as while processing feed with a high nitrogen content, ammonia, and hydrogen sulfide form ammonium hydrosulfide, which causes severe corrosion at temperatures below the dew point of water.
Since the Diesel hydrotreater unit (DHDT) operates at very high pressures and temperatures, it is important to control oil leaks and hydrogen releases to avoid fires.
As this is a closed process, exposures are minimal under normal operating conditions. There is a possibility of exposure to aliphatic naphtha containing benzene, carcinogenic, emissions of hydrocarbon gases and vapors, hydrogen-rich gases and hydrogen sulfide gases as a result of high-pressure leaks . Large amounts of carbon monoxide can be released during change of catalyst. The removal of catalyst vapor creates waste streams containing acidic water and ammonia.
Safe work practices and appropriate personal protective equipment are required when handling the spent catalyst. In some processes, care must be taken that explosive concentrations of catalytic dust do not form during recharging. The discharge of the catalyst requires special precautions to avoid fires caused by iron sulfide. The coked catalyst should be cooled to below 49 ° C before pouring, or it should be placed in containers with nitrogen until it cools.

Credible Emergency Scenarios in Diesel Hydrotreater Unit

The major fire incident can take place in the process equipment involving the following scenarios.
·        Jet Fire / Flash fire in hydrocarbon
·        Pool fire in flammable liquid
·         Toxic release (H2S)
·        Spillage of Hydrocarbon (No fire)
·        Hydrogen fire and explosion
·        Fire in heater (Furnace)
·        Explosion in heat exchanger
·        Fire in Electrical equipment

Refinery emergency response plan for DHDT

Planning for emergency preparedness and response at a refinery is an important document developed with reference to the quantitative risk assessment (QRA), including modelling of gas dispersion.
Here in this article we discuss the emergency response plan for the Diesel Hydrotreater Unit only. We have already identified the credible scenarios for Diesel Hydrotreater Unit reference to QRA, then need to examine the availability of firefighting facilities and the manpower required to manage the major risks in the DHDT.
The emergency response and the action plan must include the mitigation measures by the operations team, the maintenance team, the site incident controller, the security team, the firefighting crew and medical team applicable to all credible scenarios. Now we discuss here the course of action for all types of emergencies by each key member applicable to the Diesel Hydrotreater Unit.

Emergency response plan - Action by the operational team:
In the event of an emergency, by any person, activate the manual call point (MCP) nearby and simultaneously transmit the message to the operator of the control panel and operational Shift In charge of Diesel Hydrotreater Unit plant via Walkie Talkie.
The control room panel officer must inform the respective plant manager, the chief operating officer and the manager of the refinery shift.
The DHDT panel officer must inform the panels officer of neighbouring units. Then, he must make an announcement via the public address system to stop all work and evacuate the area immediately.
The operating team, including the field officer and the site manager, initiates mitigation measures based on the fire / toxic release / oil leak scenario.
Simultaneously, the operations team must initiate isolation of the equipment / pipeline / section of the locally affected unit / plant DHDT panel in accordance with the requirement to control the situation.
The operation team should assist the firefighting team and the medical team by directing them to the emergency scene. They must inform the scenario, the product involved with the operating parameters. The initial firefighting or any other mitigation measure must be initiated by the operating team, viz. operation of fire extinguishers, water spray system for cooling equipment and structures, operation of fire monitors and fire hydrant, etc.
The operating team must monitor the LEL / toxic concentration during the emergency and after completion.
DHDT incharge must maintain constant communication with the site incident controller on the measures taken and must work under the direction of the site incident controller.

Emergency action by Site Incident Controller
Upon receipt of the emergency call, the site incident controller must reach the incident site and assume overall command in accordance with the refinery's emergency response and disaster management plan.
Assess the situation, establish the command post in a safe place and obtain all the information on the measures taken by the area manager / shift manager, the fire commander, the security officer and the maintenance in-charge.
Depending on the situation, site incident controller will guide the operating team to initiate the necessary isolation or the emergency shutdown of plant, if necessary. site incident controller must ensure that all of the above emergency actions have been taken by the operations team.
The site incident controller will take all relevant information from the fire commander and decide on the strategy to follow in consultation with Fire chief for additional resources, calling for mutual aid and adopting alternative methods to mitigate the emergency.
The site incident controller must ensure that the area is barricaded, that the movement of vehicles is restricted, that evacuation and enumeration at the assembly point are done by security team.
The site incident controller must request the maintenance team to perform a maintenance activity to stop the leak in accordance with their standard operating procedures.
He must ensure that the injured person has been transferred to the health center and also ensure the availability of first aiders and auxiliary firefighters.
The site incident controller must initiate an adequate level of emergency siren after discussion with the chief incident controller (Refinery Head).
He is responsible for declaring "All Clear" after ensuring the safe mitigation of the emergency. Before declaring “All Clear”, he must assess the situation for damage and the environmental / toxic level.

Emergency mitigation action by Maintenance Team
The maintenance lead must ensure that the maintenance team immediately reaches the incident site with the necessary non-sparking tools and resources in accordance with their standard operating procedures and initiates the action line in accordance with safe maintenance practices.
Maintenance in charge must report to the site incident controller and issue directives for the continuation of the maintenance activities to be carried out.

Emergency action by Security Team:
The security officer must report to the incident site immediately after receiving an emergency call and report to the site incident controller.
They must delimit the area and restrict the movement of non-essential people. They must regulate traffic and authorize only emergency vehicles on the emergency site.
Ensure an orderly evacuation to the assembly point. A security officer counts the workforce and rebroadcasts it to the site incident controller.
All emergency team leaders must ensure the safety of their team members and mandatory PPE is used.
So far, we have discussed the general course of action of all key personnel to mitigate any type of emergency in the DHDT unit. Let's start specific emergency mitigation measures for specific types of emergencies, as noted above.

Pre-fire plan

The pre-fire plan or pre-incident plan is prepared to effectively mitigate any emergency to minimize the losses. Generally, the pre-fire plan is developed by emergency responders such as fire and rescue services. NFPA 1620 provides detailed guidelines for developing pre-fire plans to assist emergency responders in effectively mitigating emergencies. When developing the pre-fire plan, it is necessary to ensure the protection of plant peoples, emergency response personnel, property and the environment.

Pre-fire plan for DHDT in oil refinery
Pre-fire plan for DHDT in oil refinery

The pre-fire plan must be prepared for a specific machine / equipment / sphere/ vessel or structure in a specific installation in refinery or oil terminal with reference to the QRA. It must be documented and guarantee its accessibility to all emergency responders. It can be communicated at the facility level for better understanding and to reduce the response time of emergency responders.
Below important points must be considered in the pre fire plan document.
Data collection: QRA report and list of credible scenarios for specific equipment.
Scope: When preparing the pre-fire plan, the physical site condition must be taken into account.
Procedure and Templates: The procedure for developing a pre-fire plan with a standard template must be finalized for standardization.
Human occupation: When preparing the pre-fire plan, the density and type of occupation and the level of exposure to a particular equipment / area must be taken into account.
Fire protection system: During the preparation of the pre-fire plan, collect all data to know the availability of existing water supply and fire protection systems
Hazard Consideration: List all possible hazards and communicate to the responders and mitigation team, including special hazards.
Emergency management action plan and strategy: finalize emergency operations, fire-fighting tactics, dilution and dispersion tactics of toxic gases / flammable gases.
Resources requirement: Estimated resources including manpower, firefighting equipment, firefighting media, etc
Welfare and medical facility: Estimated welfare facility must be considered while preparing pre-fire plan for major emergencies. It must also include ambulance and other emergency transportation facility and first Aid.
Fire drill / Mock drill: Pre-incident plan must be practiced by conducting regular fire drills/ mock drill. It should be dynamic document and should be reviewed and revised with changes in technology and for new hazard identification.

Pre-fire plan for Diesel hydrotreater Unit (DHDT)

Ref. Quantitative Risk Assessment (QRA) of the Diesel hydrotreater Unit, the risk level should be considered while preparing the pre-fire plan and developing an emergency response plan.
The pre-fire plan can be prepared by the fire department in consultation with the operations team.
For preparing pre-fire plan, always ref refinery QRA study and select credible emergency scenarios. Let’s consider that in company QRA, the risk identified as “Jet Fire in hydrocarbon mixed with hydrogen”. Jet fire / flash fire can occur in process equipment (viz. Reactor, columns, vessels, heater charge pumps, feed pumps and other pumps and heat exchangers, etc)   with heat radiation ranging from 1 KW/m2 to 37 KW/m2 as per QRA report. Here we must list out numbers of flash fire / jet fire scenarios as per QRA study.
Pre-fire plan should be prepared for all credible scenarios. Here for understanding, we are considering below major scenarios having maximum impacts.

So, let’s start the preparation of pre-fire plan with an example-1.

Pre-fire plan for jet fire due to leakage of saturated hydrocarbon with hydrogen gas from 16’’ pipeline connecting Separator (No-xvz) to Flash drum (No-abc) through 26mm bore /crack at Diesel hydrotreater Unit (DHDT)


Fire due to leakage of hydrocarbon mixed with H2S and dissolved hydrogen through 26 mm hole/puncture from 16’’ pipeline connecting Separator (No-xvz) to Flash drum (No-abc).  
Process stream is mainly containing hydrocarbon (Naphtha and diesel), dissolved hydrogen, H2S and trace of sour water. In case of leakage, immediately fire will take place due to presence of  hydrogen and lighter hydrocarbon (naphtha) at auto ignition temperature. Quantity of release of material from leak point will be higher due to high system operating pressure. Release of H2S will create toxic and hazardous environment.


For Jet fire scenario, affected range would be 29 meter for
37.5 kw/m2 radiation and 72 meters for 4 KW/m2.
For pool fire scenario, affected range would be 83 meter for max 4 KW/m2. This scenario would result into jet fire, 3D fire with pool of burning liquid at the base and has the probability of BLEVE in Separator or Flash Drum.
If we consider the worst case scenario, jet fire impinging the Separator or Flash drum as it increases the chances of BLEVE and rupture of vessel or the pipeline is quite expected. Very careful strategy to be adopted without time wastage to avoid escalation of emergency.

Incident Location:
Oil leakage at about 6 meters height in Diesel hydrotreater unit (DHDT) near reactor column. Location of incident is north east corner of reactor heater. There are two approach to incident side from north east and north west.

Equipment Description:
Separator (No-xvz) to Flash drum (No-abc) are provided to disengage and individually remove hydrocarbon vapors, liquid and water. Separator splits the Recycle gas stream from both the Hydrocarbon liquid & sour water, which contains Ammonia & Hydrogen Sulphide. Water from Separator is diverted to flash drum water section which is finally routed to SWS unit, Hydrocarbon liquid from Separator is sent to Flash drum under level control & Recycle gas is sent to Recycle Gas Scrubber via Recycle gas cooler to remove H2S. Separator pressure is controlled by PIC. Separator is operated at a temp of 57°c and pressure of 85.02 kg/cm2g.
Liquid from flash drum feeds to Stripper under level control through fractionator bottom-Flash drum liquid exchanger, Effluent-Flash drum liquid cold exchanger and Effluent-Flash drum liquid hot exchanger.
Flash drum pressure is controlled by purging lighters to VGO Union fining unit through. Sour water from Flash drum is diverted to SWS unit. Flash drum operated at a temp of 59°C and pressure of 36 kg/cm2g.

Product Data:
(Note: only relevant product data must be captured in pre-fire plan which would help to emergency responders .i.e. fire fighters)
Name and composition of hazardous material           : HC, H2S, Hydrogen
Important process parameters (operating pressure) : 85 kg/cm2
Important process parameters (operating temperature): 57 Deg. C.
Flammability range of most flammable material involved: 4 – 74% (H2)
TLV for toxic gas                                                          : 10 ppm (H2S)
Flash point                                                                    : >32 deg C.
Autoignition temperature                                          : 256 Deg C.
Vapour density                                                             : 3 to 5 gm/cc
Water solubility                                                            : Insoluble

(Note: When preparing the pre-fire plan, the total number of fire protection systems must be taken into account with their identification / tagging.)
  • Fire Hydrants = ___Nos and location
  • HVLRM (2000 GPM) = ___Nos and location
  • Water cum foam monitors = ___Nos and location
  • Water spray systems (DV) = ___Nos and location
  • Water spray systems (manual MVWS) = ___Nos and location
  • Manual Call points = ___Nos and location
  • Fire & Gas detectors= ____Nos and location and location
  • Fire Extinguishers = ___Nos and location
  • Foam Trolley = ___Nos and location
  • Hose Box = ___Nos and location
  • Booster Pumps for tower monitors: ___ Main pumps & ___ Jockey Pumps.
  • Clean gas system for satellite buildings and control room ___Nos

Fire hydrants, fire water monitors, & MVWS system are
connected with main fire water header having header pressure of 10 kg/cm2, all pumps are in auto mode and pressure shall be maintained by sequential auto mechanism.


Fuel supply reduction method
Basic operational control system helps to shut down of DHDT unit and will be taken by emergency depressurization to reduce system pressure. Isolation of separator and flash drum by closing the isolation valves and other feed.

Safe working area for firefighting
Leakage & fire from the said pipeline will contain mainly HC, Hydrogen & H2S. Gases of combustion would include mainly Oxides of Sulphur - SO2, CO etc apart from high radiation heat. These vapours are hazardous to health.

Possibilities of burns due to high radiation cannot be ruled out. Splashing / 3D rain fire possible. Approach fire from safe distance. All responders to wear three-layer proximity suits along with face shield and SCBA is must. Approach from upwind direction and maintain safe distance. Danger area is considered as 29 meters radius from point source of leakage and fire.


Stop all work and immediate evacuation of all the personnel in downwind or cross wind direction. Isolate the affected equipment immediately and start depressurization.
Immediately start the water spray deluge system by operation peoples to and operate the fire water monitor in spray mode. Water monitor jet-cum-spray application should be angled in such a manner to cut the path of Jet flame if it is impinging on other vessel.
Start the water spray cooling system Deluge valves to protect nearby vessels and Fin fan cooler.
Use available portable firefighting foam trolley to apply foam for fighting ground pool fire.
Water stream from fire hydrants  should be used for cooling the structures and pipelines. Continuous Foam supply shall be ensured by connecting the foam fire engine with Foam Nurser.
In case of suspected BLEVE, entire area should be evacuated including firefighters and other emergency responders. Nearby facility to keep vigilant watch.

After receiving an emergency call, the fire team must present themselves urgently with all appropriate firefighting equipment, personal protective equipment and report to the emergency site.
While taking emergency turnout from the fire station, make sure of the wind direction, be in constant contact with the fire control room. After reaching the emergency site, the fire tender / fire engines should only be parked in the upwind / crosswind direction. The fire tender must be parked in a safe place and directed towards the exit.
Evaluate the scenario, get all the information about the emergency site from the plant shift in-charge or site incident controller and a brief final mitigation action to all firefighters.
firefighting strategy for DHDT in oil refinery

All members of the fire crew must wear self-contained breathing apparatus (SCBA). Two firefighters must ensure the connection of the hydrant to the Fire tender for make-up water and be ready for firefighting.
The rescue operation must be launched at the initial level by two firefighters and the other firefighters must be ready for the dispersion of gas and firefighting.
Since most of the equipment in the Diesel hydrotreater Unit are at a high temperature and pressure, the fire commander should contact the site incident controller to take decision on application of water / foam to the affected processing equipment to avoid thermal stress on process equipment / piping. If necessary, apply foam as well as DCP.
If necessary, call in for additional fire assistance and resources. If possible, cool nearby equipment using the deluge water spray (DV) system.
The fire commander should assess the situation and ask the firefighting team to use a fire suit / heat resistant suit / other specialized equipment.
Operate the groundwater monitor / tower monitor and HVLRM (high-volume long-range monitor) for water / foam application depending on the situation.
Help the operations and maintenance team to mitigate the incident. If there is a demand to close the process isolation valve which is engulf in fire, the responder must wear a fire entry suit with a water spray back-up line. These water spray must continue to provide a cooling effect to the personnel involved in closing the isolation valve.
The fire commander must ensure the continuous application of extinguishing means to control, contain and extinguish the fire. Water curtain must be provided to isolate the radiant heat, if possible. Intensive cooling of the surrounding installations must be ensured to prevent the spread of fire.
The fire chief must ensure the required fire water header pressure of the fire water system in coordination with the operator of the fire water pump house. If situation demand, the fire chief may consult the site incident controller for any further assistance from mutual aid members, the auxiliary team, etc.
All actions to mitigate the incident must be carried out under the instructions of the fire chief. Check the LEL / toxic gas level with the operating team in consultation with the site incident controller.
Ensure adequate drainage of spills and take the necessary measures to eliminate any contaminated surface.
After ALL CLEAR, stop the operation of fire protection system and communicate the fire water pump house about no further demand. Ensure the restoration of all emergency fire equipment / appliances.

 Fire water Requirement
Equipment/ system
Area to be protected
Application rate (lpm)
Water Required
Hand line
Firemen shielding
Portable monitor – 2Nos
Creating water curtain
Fixed monitor -01
Fixed monitor -02
Fixed monitor -03
Finfan, column and vessels
Fire engine
Firefighting stripper and drum

Firefighting foam concentrate requirement
Equipment/ system
Area to be protected
Application rate (lpm)
Foam Required (Ltr)
Monitor – 4 Nos
Spill on floor
Fire engine monitor
Spill on floor
Handline foam branch – FB-10
Left over pockets

Firefighting equipment and appliances requirement
Unit of measurement
Available (Yes/No)
Firefighting suit
Portable gas meter
Foam Engine
Foam nurser
3% foam concentrate
26.5 KL
Fire water
857 KL
500 kg
Foam branch FB 10x
Fire nozzles
Portable ground monitors
Delivery hoses (15 m length)
Dividing breaching
Portable foam trolley
Spare SCBA cylinders

Manpower requirement
Fire fighters : 14 Nos
Fire engine drivers: 4 Nos
Fire officers: 02 Nos
Fire chief: 1 No
Auxiliary fire squad members: 12 Nos
First Aiders: 3 Nos
Addition manpower and reliving manpower: 20 % of total first responders.


Ensure proper draining and flushing of used high quantity water, firefighting foam and spilled oil in WWTP system. Fire standby duty must be continue still plant normalization and maintenance operation.
Total fire water usages is estimated approximately is 3900 KL whereas fire fighting foam concentrate is 27 kl


As the expected duration for mitigation of this emergency is 1 hour, welfare arrangement is required for supporting the emergency handling team. Sufficient drinking water, dehydration material and relaxing facility with manpower replacement facility and transportation need to be ensured.
Ambulance with first Aiders and doctor at Occupation Health center must be ensured till completion of emergency.

Emergency No of fire control room: XXXXXXXX
DHDT panel contact No: XXXXXXXX

******* End******
Above is the one example of pre-fire plan for diesel hydrotreater unit (DHDT) for one emergency scenario. Similarly, pre-fire plan must be prepared separately for each credible scenario with ref to QRA study.

Pre-fire plan template
Here is the standard template for the pre-fire plan developed according to our experience. This template can be revised depending on the site condition. Standard NFPA 1620, latest edition 2020 can be referenced for a pre-incident template, but it is mainly applicable for building and may not be suitable for a refinery.
Click here to download
Standard template for Pre-fire plan for oil Refinery credible scenarios


The Diesel Hydrotreater Unit (DHDT )is extremely prone to major accidents, including fires, toxic releases and explosions due to its severe process conditions. Preparation of Pre fire plan or Pre incident plan for specific equipment in specific operational area is the key for effective mitigation of any emergencies. Quantitative Risk assessment must be referred while preparing the pre fire plan. Effective emergency response can be assured by regular practicing i.e. conducting fire drill based on pre-fire plan. Practical firefighting strategies must be included in pre fire plan for all emergencies, including fires, explosions and the release of toxic gases. Important topics such as diesel hydrotreater unit (DHDT) process description, list of hazards substances with their properties, list of available fire protection system, layout, approach rout marking on layout, line of action and firefighting strategies, etc must be the part of pre-fire plan.
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