February 1, 2010

The Advanced Digital Fieldbus Option for Nuclear Plants

Jack Y. Zhao, U.S. Nuclear Regulatory Commission

A Digital Control System Trumps a Conventional Control System

The following example describes a typical circulating water system in a nuclear power plant that uses cooling towers in order to demonstrate the installation and hardware cost benefits derived from using digital fieldbus technology. For the purpose of simplicity, this example uses only the FF. However, as described earlier, a mixed system that incorporates both leading digital fieldbus technologies may be a better solution for some applications.

Table 1 lists 166 candidate field devices and 311 related I/O points for this typical circulating water system. The discrete I/O points associated with the pump and fan controls used in the circulating water system are not included because they are more appropriately controlled by using Profibus technology. In addition, this example does not include monitoring of the vibrations caused by the pumps and fans because these devices are usually wired to a packaged vibration-monitoring system. If this example also included discrete control and vibration monitoring using a diverse fieldbus-based system, more benefits could be achieved.

1. Field devices and related I/O points of the circulating water system. Source: Jack Y. Zhao

The following assumptions were made for this comparison:

• Transmitters and valves are connected to I/O cards through junction boxes.

• A maximum of 16 wires, each of which has three terminations (a twisted pair with shielding), can be connected to each junction box.

• A maximum of eight devices can be attached to one H1 segment.

• Each analog input (AI) card can accommodate 10 channels, and each digital input (DI) card can accommodate 24 channels in a CCS. Each field-powered digital output (DO) card selected to provide DO signals can accommodate 12 channels.

• Each FF I/O card can accommodate 64 channels.

• Each double-wide control cabinet, 1,600 by 800 millimeters (mm) in size, can accommodate about 30 conventional I/O cards and the related power supplies, communication devices, and other necessary equipment.

Reduction in Input/Output Terminations

With a CCS, 311 I/O points require three sets of 933 ((311 x 3) x 3 = 2,799) terminations:

• 311 wires from 311 I/O points to 933 terminations into junction boxes

• 933 terminations from junction boxes into I/O cards

In a DCS using FF technology, 166 devices can be divided into at least 21 H1 segments, requiring 540 (498 + 21 x 2 = 540) terminations, as follows:

• 166 wires from 166 candidate devices to 498 terminations (166 x 3 = 498) into junction boxes

• 21 terminations from 21 H1 segments into I/O cards

According to the analysis above, FF technology can lead to an 81% reduction in the number of terminations. This reduction decreases installation costs (assuming $15 per termination installation):

• CCS with 2,799 terminations: $41,985

• FF-based DCS with 540 terminations: $8,100

• Savings with FF: $33,885