A. Many pipe specifications have sizing rules to aid in the selection of the optimum pipe diameter. The sizing rules can be based on achieving an optimum average fluid velocity in a pipe run for the design flow rate or a specific pressure drop per unit of pipe length. The pipe run sizing rules are trade-offs by:
- Choosing a small pipe diameter to minimize construction cost
- Choosing a large pipe diameter to minimize pipe run friction loss and pumping cost
The value for the optimum pipe diameter is based on the cost of the piping versus the cost of pumping power. If the pipe diameter is below the optimum diameter, then the cost of the piping is reduced, but the friction loss in the pipe run increases, which results in higher pumping costs. If the pipe diameter is greater than the optimum, then there is less friction loss in the pipe run, resulting in lower pumping costs, but higher construction costs.
Determining the optimum pipe diameter requires a cost analysis for the life cycle of the plant. If the capacity of the existing system will increase in the future, then the additional cost of sizing the pipe run for future loads should be compared to the cost of adding parallel pipe runs to the system at a later date to accommodate increased capacity in the future. A trade-off in pump system design elements affects both initial and life-cycle costs. A number of life-cycle costs depend directly on the diameter and the components in the piping system (such as the initial cost, energy costs and maintenance costs).
Much of the pressure loss in the system is caused by valves, in particular control valves in throttle-regulated installations. In systems with several pumps, the pump workload is divided between the pumps, which, together and in conjunction with the pipe system, deliver the required flow.
The piping diameter is selected based on the following factors:
- Economy of the whole installation (pumps and system)
- Required lowest flow velocity for the application (to avoid sedimentation)
- Required minimum internal diameter for the application (suitable for solids handling)
- Maximum flow velocity to minimize erosion in the piping and fittings
- Plant standard pipe diameters
Decreasing the pipe run diameter has these effects:
- The initial costs of piping and components typically decrease.
- The initial costs of the pump increase because of increased flow losses with a consequent requirement for higher head pumps and larger motors. Costs for electrical supply systems, therefore, will increase. Energy costs will increase because of higher power usage caused by increased friction losses.
Figure 5.1. Key cost components for a pumping installation as related to pump size
Some costs increase with increasing pipe run size while other costs decrease. An optimum pipe run size, therefore, may be found based on minimizing costs throughout the life of the system (see Figure 5.1). For more information about improving pumping systems, see Optimizing Pumping Systems: A Guide for Improved Energy Efficiency, Reliability, & Productivity.