To estimate the time and manpower required to clean, inspect and repair as required all heat exchangers in a plant, for example, every heat exchanger must be estimated individually. The reason for this is that they are of different types and sizes, and may require different types of maintenance work.

The work to be performed for every heat exchanger must then be further broken down into every task required to complete the job:

• erecting scaffolds
• installing blinds
• unbolting the channel head cover
• removing the channel head cover
• removing the channel head
• unbolting the bell head
• removing the bell head
• pulling the bundle
• removing the floating head
• transporting the bundle to the cleaning slab
• cleaning (hydroblasting) the bundle
• inspecting the bundle
• [possibly retubing or plugging tubes]
• returning the bundle to the unit
• cleaning heads and shell
• repairing the baffle plate
• replacing anodes
• etc.

Then, every task should be assigned a time duration in hours, and manpower required (specifying the craft or skill involved). Multiplying the manpower by the time duration (in hours) will yield the manhour estimate for every step. Adding the manhour estimates for every step will yield the total manhour estimate for every work order.

Personal, Fatigue and Delay Factors

The time and manpower estimates should include a reasonable PFD (Personal, Fatigue and Delay) factor:

• Personal factors include:
  • breaks to use sanitary facilities
  • smoking breaks
  • visits to the attending nurse for any wounds
• Fatigue factors should be considered for those tasks which require physical strength or are exposed to extreme temperature conditions
• Delay factors include the availability of:
  • tools
  • jigs
  • devices
  • vehicles
  • cranes
  • permits

eTaskMaker® is ideally suited to assisting planners in preparing these customized project plans. After all work orders have been estimated, and the total manhour estimates added up to a grand total for the turnaround, the T/A manager may include an additional amount of manhours as a "cushion" for extra work which has not been defined yet, or for any defects in the work scope definition.

If the work orders have been estimated "tight", with little additional time for PFD factors, then the entire turnaround manhour estimate should be adjusted upward by whatever percentage the T/A manager deems realistic or safe.

Estimating Accuracy

The degree of error inherent to estimating is sharply reduced when work orders are well defined. A well defined work order scope is one in which activities are broken down every time there is a change in crafts or a change in the physical work content. The better defined (detailed) a work order scope is, the smaller the degree of error in estimating; therefore a higher quality estimate. An acceptable range of error for a detailed work order scope is of about ten percent (10%).

On the other hand, when work orders are "guesstimated" without any detail, the degree of error is very high. The error range could span from 25% up to 60% and more.