Coming of age

InTech, Jul 2005 by Lewis, Clifford

The economic case for large-scale use of wireless sensors is overwhelmingly favorable.

Instrumentation engineers have successfully applied wireless communication technology in process plant applications for many years.

SCADA systems routinely send data through cellular telephone links.

Tank farms forward liquid level reports to central control rooms hundreds of yards away.

Offshore oil platforms transmit continuous streams of data at high speeds to land-based stations.

Wireless sensors are now giving plant engineers unprecedented access to plant data previously unavailable or unattainable with wired sensors.

Wireless sensors represent a significant cost savings, particularly for parameter monitoring, and provide a quickly implemented, secure link from the process plant areas to the control room.

Advances in digital technology have made large-scale installations of wireless sensors practical and increasingly common. With no external field-supplied power, wireless sensors can run continuously for over five years on an internal replaceable battery while giving multiple updates per minute for the entire period.

These large-scale wireless sensor installations can network and link to SCADA systems, distributed control systems (DCS) or operate independently.

Wireless sensor economics

The economic case for large-scale use of wireless sensors is overwhelmingly favorable.

In most industrial applications, the purchase price of a sensor and its associated electronics is generally less than 10% of the total installed cost of the device.

Many plants allocate the cost of a wiring pairs at over $8,000 per pair. Other plants estimate the cost of running wires by the foot, which run $40 to $100 per foot, depending on the electrical classification of the area.

Of course, wiring needs to run in cable trays, in conduit or underground and it never goes the shortest route between two points. It is not uncommon to run wires many hundreds of feet just to get a signal across even a single plant roadway.

The wiring costs add up very rapidly, so rapidly in fact, that most plants currently monitor fewer than half the measurement points they should be monitoring. The cost of running the wires is just prohibitive.

Wiring cost is just one dimension to the wireless sensor economics matrix. ease and speed of installation are other important factors. Once a base radio sets up in a plant or on an offshore platform, it covers an area roughly 1,000 feet in radius. The actual signal reception can be easily determined in a quick site survey as noted in the sidebar. Installing a wireless sensor anywhere in the coverage area is as quick and easy as installing a gauge.

Installation time is far less. A typical installation can be up and running in less than fifteen minutes. The installation is so quick that wireless sensors can install in the field on a temporary basis, even by supervisory or engineering personnel. With this rapid installation time, instrumentation becomes flexible. Sensors can move about, test in one location, and then relocate to a more optimal spot if necessary. The low initial cost and speed of installation make it possible to put the first wireless sensor into a plant without the need for capital approval. After an initial trial, further justification is easy.

Tie into existing control

A large-scale installation of wireless sensors in a plant is only good if it can easily tie into an existing control system. Modbus digital output and easy conversion of the base radio digital data stream into 4-2OmA or switch closure signals makes inputting large amounts of sensor data easily achievable. These tie-ins are understandable by plant personnel and reduce implementation time.

Most industrial process plants, offshore oilrigs, or oil producing locations already know numerous places where they would like good, solid data. This may be something simple, like continuous monitoring of relief valves to verify releases to a flare or to the atmosphere. Many tanks or loading facilities need overfill protection devices to prevent unpleasant and often costly situations. Pump efficiencies need periodic monitoring. Employee health and security measures, or safety shower uses need to be improved with alert signals.

All of these applications are applications for use of large-scale installations of wireless sensors. Such installations can typically take place with wireless sensors for less than 20% the cost of a comparable wired solution. With savings like this, coupled with a long battery life and the availability of rugged industrial grade sensors make a very compelling case for wireless sensors.

Large-scale applications

Here is a typical installation in the oil and gas industry. The first photo shows a wireless pressure transmitter mounted on the outlet of a producing oil well.

Small quantities of natural gas often leak from the top of the well, making this an electrical Division I environment. In a typical field, there may be several hundred such wells.

Monitoring of the well casing and well tubing pressures along with a number of other parameters is possible. Running wires to these locations is difficult and maintenance on buried cable wiring is high, making this an ideal application for a sensor requiring no wires.