Shut Down Unit
Types of shutdown
At the completion of this topic you must be able to:
- determine the type of shutdown required
- shut down individual items of equipment and the entire unit
- shut down in an emergency when required
- shut down to a standby condition if appropriate
- shut down for maintenance when required.
The various types of reactors used in the chemical, hydrocarbons and oil refining industries have differences that affect operation.
The procedures outlined in this section are generic and may not apply to all types of equipment.
Refer to your Standard Operating Procedures for the correct way to operate each type of unit in your workplace
The types of shutdowns used in a plant unit are:
- scheduled shutdown
- maintenance shutdown
- emergency shutdown
- shut down to a standby condition.
The Standard Operating Procedures for each type of equipment must be adhered to when shutting a unit down
A scheduled shutdown is initiated by the operator during normal operation of the unit when:
- maintenance is required
- feed supply is low or exhausted.
The shutdown procedure will depend on the type of equipment and the process chemistry.
Some steps taken in a unit shutdown may include:
- shutting off the feeds to stop processes and heat generation particularly if processes are exothermic (produce heat)
- re-circulating feeds from supply tanks so they do not enter the unit
- shutting off heating or cooling to the unit or feed preheat system
- shutting off mixing and other mechanical operations
- cooling and flushing materials from the unit
When maintenance to the unit equipment is required, the equipment may need to be entered so that work can take place.
The shutdown should be a scheduled or planned shutdown as per Standard Operating Procedures where equipment is:
- isolated (process, mechanical and electrical)
- cooled and depressurised
- purged and gas freed
- gas tested on a continuous basis prior to and during entry.
A planned unit shutdown will prevent:
- plugging of lines or equipment
- possible damage to equipment
- possible injury.
To prepare the unit for shutdown, the unit may need to be:
- thoroughly drained and pumped out to remove chemical liquids
- purged with steam or inert gas to remove vapours
- solvent washed to remove deposits that build up on the equipment's internal surfaces
- flooded with water or a solvent to remove any remaining chemicals
- any chemicals trapped in the unit must be flushed out.
- isolated to prevent the entry of hazardous chemicals
- steam cleaned to remove remaining deposits.
During decontamination, regular sampling of the atmosphere inside a unit vessel is required to ensure toxic or explosive atmospheres do not build up inside the unit that could be a hazard to equipment or personnel.
Gas testing must be carried out before anyone enters the vessel to ensure the atmosphere is not toxic, explosive or oxygen deficient.
An emergency shutdown is initiated in the event of a fire, major spill, instrument failure, power failure, or total loss of control of chemical or physical processes.
Emergency shutdown procedures must be followed during a shutdown sequence.
Shutdown of a unit can be initiated by the automatic shutdown system. The systems may be shut down automatically because of temperatures, fluid levels, pressures or flows that are above or below trip points.
Typical shutdowns initiated by trips may include:
- low liquid level in a vessel
- high liquid level in a storage tank
- high viscosity causing increased load on pumping or mixing equipment
- mixer failure
- pressure to high
- temperature to high
- low feed flows.
Shutting down to a standby condition
When a unit is to be shut down for a short period of time for maintenance on auxiliary equipment, the unit is shut down to a standby condition.
A standby shutdown allows a quick startup of the unit after maintenance is completed in order to minimise lost production time and offspec material.
Standard Operating Procedures must be referred to when shutting down each type of unit to a standby condition.
A typical standby condition may include:
- re-circulating material upstream and downstream
- reduced heating or cooling (sufficient to maintain a safe process condition)
- slow-rolling compressors
- venting process gases to flare
- diverting process streams to temporary storage.
When equipment is prepared for maintenance shutdown it is normal to follow the steps listed below.
Click and drag each step to place them in the order in which they are normally carried out.
Press Check to check your answers and Reset if you want to try again.
You will obtain Feedback in the box below.
PMAOPS300A Operate a Production Unit Start Up Unit Monitor and Control the Unit Change Unit Output Rate, Grade or Specification Maintain Plant Effectiveness Shut Down Unit Types of shutdown Communication for shutdown Equipment changeover Prepare for re-startup Assessment tasks Contribute to Controlling Hazards Resolve Problems