Shutdown Plans / Maintenance Planning and Scheduling

The types of shutdowns used in a plant unit are:

• Scheduled shutdown
• Maintenance shutdown
• Emergency shutdown
• Trips
• Shutdown to a standby condition.

 Scheduled shutdown

A scheduled shutdown is initiated by the operator during normal operation of the unit when:

• Maintenance is required or 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)
• recirculating 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

Maintenance shutdown

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
• cleaned
• gas tested on a continuous basis prior to and during entry.

For example: Purging Air

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
• drained
• 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.

Emergency shutdown 

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.

Trips

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:

• recirculating 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. 

Process Groups and knowledge Areas ( Planning, Executing, monitoring/controlling and closing). Methods Engineering: 5S, Lean, TPM, Reliability, Availability, Maintainability and Safety (RAMS), FMEAC,  KPI'S). Reliability Engineering.

Communication for shutdown

At the completion of this topic you must be able to give advanced warning of shutdown where possible.Where a shutdown will affect upstream or downstream process units, advanced warning must be given to the appropriate personnel to allow them to prepare for, and react to, the changing conditions. Appropriate personnel would generally include suppliers of utilities and feedstocks, and the downstream units that receive the various output streams. Sufficient warning time needs to be given to allow these other areas to be ready for the shutdown, and to have alternative disposal or storage ready when needed.

Advanced warning of shutdowns must be communicated. In the case of emergency shutdowns or a process trip, there is not always time to give advanced warnings, but communication must be made with unit suppliers and customers as soon as possible after the trip or emergency shutdown to minimise the adverse effects of the shutdown.

Equipment changeover

Equipment items such as heat exchangers, pumps and filters are often installed in parallel where one equipment item is on duty while the other is on standby – ready to be quickly brought online when the duty equipment requires servicing.

Filters are changed over when the differential pressure across the filter reaches a certain preset high level.

Heat exchangers can be changed over if one heat exchanger is detected as having:

• leaking tubes causing contamination of a process stream
• fouled tubes.
• Pumps and compressors can be changed over after a preset time period to even out the wear between the two pumps, or if one fails when in service. 

Prepare for re-startup

Resetting trips and alarms 

After a shutdown, all trips and alarms must be reset in preparation for startup.

If the unit is shut down due to a trip, the cause of the trip must be investigated and any fault rectified before the trip can be reset for startup.

Trips that are disabled as part of a normal startup procedure must be reactivated after startup as stated in the Standard Operating Procedures. 

Before alarms or trips are reset check the cause of the alarm or trip

Before startup, checks are required to ensure all equipment, including equipment not worked on during maintenance, is in a safe and operational condition.

Pre-startup checks will ensure that:

• all flanges, valves and instruments are connected
• all blinds and spades are removed
• all lines are connected
• valves are lined up and set to the pre-startup position
• all equipment is in a safe operational condition
•  levels are correct in vessels
• all utilities are available and lined up
• all vents and drains are closed
• all instruments are commissioned. 

The various types of units used in the chemical, hydrocarbons and oil refining industries have differences which will affect operations. For this reason it's very important to know what are the functions, responsibilities inside each department in the plants. Additionally inside  each shutdown type we should apply the best global standards practices as engineering methods ,as well as engineering reliability. Most of all   let's keep in mind first personal security then asset integrity.