A Process Unit with at least 25 WFD provides saturation coverage of the unit, with minimal need for design recommendations. Locate the gateway in the center of the initial network, or in the center of the Process Unit, with its antenna at LOS height (obstruction height + 2 m), and with direct wireless connection to at least 25% of the WFD in the network. To fortify a network, add more WFDs. WFDs can be used as repeaters. That's a quick fix for fortifying the network during design, or after installation.
Your existing network, once formed, is the foundation for scalability, whether you add WFDs to the perimeter of the network, or to the interior of the existing network. Don't forget that this is a mesh network. Devices do not need to be within the range of the gateway, they just need to be close to another wireless instrument.
Specifying WirelessHARTTM instrumentation is easy
WirelessHARTTM devices are all basically the same when it comes to their wireless characteristics. All you have to do is to make sure the device is registered with the HART Communication Foundation. This is the indicator of a good wireless design, which has been tested for interoperability with other vendors' products.
Some devices may offer antenna options that will alter the distance assumptions you may make using a standard WirelessHARTTM antenna. If you are trying to "push" a signal from a very remote WFD at the very edge of the mesh, for example, you might want to consider a higher gain antenna than standard.
The other thing you need to decide is whether you want the devices preconfigured with the network ID and join key entered by your supplier, if it is offered. There are reasons both for and against this. Some end users may wish to enter their own network ID and join key information at commissioning time.
Commissioning Wireless Field Devices
As with wired HART, a HART handheld or PC tool will allow you to join WirelessHARTTM devices to the network. You start with the gateway. Install and apply power to the gateway, and then install the devices one by one, beginning with the device closest to the gateway. To do that, you install or activate the device's power source (whether it is line power or battery) and enter the network ID and the network Join Key. Then set the refresh (update) rate and verify that the device has joined the network either by using a handheld or PC field tool, or at the gateway.
Next, verify the device operation by checking tags, engineering units, PV update rate and if the unit is battery-powered, the battery life. One of the unique features of the WirelessHARTTM specification is that the battery life variable is required to be reported as "net time remaining" rather than a simple voltage or numeric value with no context.
Verify the gateway, checking for a minimum of 5 direct connections, and in large networks, make sure that at least 25% of the devices connect directly to the gateway.
Secure WirelessHARTTM networks
WirelessHARTTM is designed to be highly secure from the beginning, but there are some recommended best practices to help you make the network even more secure. The fundamental security precaution is "key management." Segment the networks using different network IDs for different process units. Provide a global join key for all devices or an individual join key for each device. The join key can be fixed or rotating (controlled by the network manager software), and assignment can be made either at the factory with the order or on the bench in the instrument shop.
The use of "white lists" which only permit approved devices to join the network is another recommended practice for increasing the security of your WirelessHARTTM network.