Level Measurement & Control


Gauge emulation - a cost-efficient solution for tank gauging projects

February 2002 Level Measurement & Control

Most manufacturers of tank gauging equipment have in the past only had one bus alternative for communication between the control room and the gauging equipment on the tank.

This electrical interface and protocol software was more or less specific for one manufacturer and level gauges, temperature devices, etc from another manufacturer could not be connected to this bus. If the user would like to have an extension of the system, partial upgrade or exchange a number of tank gauging units, his only practical alternative was to purchase from the same supplier as before. To install equipment from another supplier required separate cabling for another fieldbus, a second operator display in control room, a second interface to existing DCS system, etc. Most old tank gauging equipment in service today consists of mechanical level gauges as float gauges or servo gauges. It is not uncommon that users accept very high maintenance costs, poor performance and unreliable measurement for these instead of exchanging them to modern radar based level gauges. The reason is often that the cost is too high for exchanging the whole system and a partial exchange creases bus compatibility problems.

To change a complete tank gauging system for all storage tanks on a refinery or a big tank terminal is a major project. The first difficulty is of course to get a budget for a complete exchange, especially if some of the units in the existing system recently have been upgraded.

Another obstacle that could make a user hesitate to completely change the legal gauge system, is that the change of system takes a certain time. During this time period it may be necessary to have a large number of tank gauges out of operation, waiting for the new system bus to be started. This situation requires a special effort from the operation department, often an extensive manual tank gauge action and can of course be a safety issue if not handled with care.

In many tank gauging applications the data from the tank gauging system is also transferred to other high-level computer systems. These systems can be accounting systems, inventory/loss control systems, DCS systems, etc. The exchange to a new tank gauging system may in these cases also mean that the data transmission protocols between the old tank gauging system and the high level computer systems have to be established for the new tank gauging system as well. If the existing high-level computer system is slightly out of date, it may be large costs involved with the connection of the two systems. Often, it proves to be quite expensive to modify protocols for older DCS or administrative computer systems. It can often be more economical to leave the tank gauge master unit as it is, use the already working connection to the high-level computer system, and instead exchange it on the day a major change is made on the hardware and software of the high-level computer. During the purchasing process of the new computer system, it is at that time easy to require the supplier to have a modern interface/protocol to the tank gauging system which will cost a fraction of what a special programming in an old computer system can cost.

The aspects above are just a few that could be a reason why a user still prefers his old level gauge system and thereby accepts having unreasonable high maintenance costs. For some old mechanical systems it may even be extremely difficult to get spare parts and the service offered by the local supplier may be unmotivated and high cost since there is a belief that there is no alternative.

This poor maintenance situation does not have to be accepted, since today there are level gauges available on the market that can easily solve the entire problem described. The solution for an easy and cost efficient upgrade of these systems is tank gauge emulation.

The solution: tank gauge emulation

Emulation means that an existing level gauge installed on a tank can be exchanged by another totally different type of level gauge. After this exchange, the existing tank gauge system will not see any difference between the new emulating level gauge and the old level gauges in the system.

A number of aspects have to be considered in this exchange:

1. The emulating level gauge should be electrically compatible with the existing system bus. Poor compatibility could cause the old system to malfunction, even if the emulating level gauge works perfectly.

2. The emulating level gauge should be software compatible with the existing system. There may, as an example, be software commands sent out from the tank gauge master, which are not relevant for the emulating device. As an example: a radar gauge emulating a servo gauge may receive the command 'raise the displacer to top'. This command is obviously not relevant for a radar gauge which has no displace in the tank, but the gauge must still give a proper response back to the tank gauge master unit, otherwise an alarm message could occur.

3. The software compatibility can often be on different levels for different emulation applications. Most common is that emulation is supported for measurement data only. This implies that all measured data as level, temperature, pressure alarms, etc will be sent from the emulating gauge to the tank gauge system master. Initial configuration of the emulating level gauge cannot be performed from software in the tank gauge system master, but has to be made from some other unit (eg a laptop PC or handheld terminal, etc).

4. All measured data that the tank gauge system master expects to receive from the level gauges must be supported. If, for example, pressure, density, flow rate or some other data is measured by the existing system, then the emulating level gauge must be able to produce the same measurement data.

Which level gauge types can be emulated?

There is still a large number of different old float and servo gauges being used today. Common manufacturer names are Varec, Enraf, L&J, GPE and Whessoe, which were the major mechanical level gauge suppliers in the past. They had all their own solutions for fieldbus communication, which they in many cases use even at this time.

The level gauges from these suppliers are in general particularly easy and suitable to emulate, since they have over a number of years used the same fieldbus hardware and the software protocol has changed very little.

There are not very many suppliers on the market who offer the possibility to emulate other tank gauge types. One example is the Saab TankRadar Rex, a modern radar based level gauge, which has put emulation in focus. The Saab TankRadar Rex level gauge is manufactured by Saab Tank Control and is actually a family of different level gauge types designed for different tank types.

Saab TankRadar Rex has an emulation feature which makes it easy and inexpensive to integrate in any existing system. No need to replace any field cables, network units or software. Just plug-n-gauge
Saab TankRadar Rex has an emulation feature which makes it easy and inexpensive to integrate in any existing system. No need to replace any field cables, network units or software. Just plug-n-gauge

This supplier, like all other level gauge suppliers, has its own type of fieldbus, but in addition the modular design allows exchange of a circuit board for emulation of a large number of other types of level gauges. Especially all major old types of mechanical level gauges can be emulated.

Of particular interest is that the modular design of which type of fieldbus to select is not limited to emulation of level gauge types as above. Instead of selecting an emulation module, the user can select some other type of the common standardised fieldbuses as:

* FieldBus Foundation

* ProfiBus DP

* Tiway

This allows a user to change his selection of fieldbus in future, ie if a policy decision is taken to use FieldBus Foundation in future, the level gauge system can to a low cost be rearranged for this.

Maybe one of the most attractive aspects of a design as above is that the user can anytime switch to the bus he prefers. A real life situation could be: a refinery needs a partial upgrade of a limited number of level gauges. The customer then selects modern radar-based level gauges with the emulation function that corresponds to his existing old tank gauge system.

Over a certain time period he stops spending excessive maintenance on his old level gauges. Instead of spending the maintenance budget on old equipment, he uses it for buying a few new emulating radar gauges. Still he uses the old tank gauge master system.

When budget is available, he decides to switch to a fully radar based system. At this time the customers can also switch to the standard radar level gauge fieldbus, without any additional cost. The explanation for this is that the gauges used for emulation in the old system are always equipped with the standard radar level gauge fieldbus as well. It can be activated anytime with a switch. With the standard bus all facilities for configuration service etc are available for the user.

After some years, a major decision for the refinery is taken to use FieldBus Foundation module instead of exchanging the whole level gauge unit. The refinery has thereby easily and quickly managed to convert the existing tank gauge system to FieldBus Foundation at a very low cost.

Conclusions

The user of tank gauging systems has today the option to mix different types of level gauges from different suppliers without having bus incompatibility problems. There is no reason to accept high maintenance costs and poor performance on old existing mechanical level gauges due to bus incompatibility problems. It could in most cases be far more economical to spend maintenance money on a purchase of an emulating radar level gauge instead of wasting them on worn out mechanical equipment.

Project implementation can be far easier with a minimum of disturbance of production with a successive exchange of an existing level gauge system based on emulation. When full exchange has been achieved, the user has the option to select the type of fieldbus that he prefers with a minimum of costs. The open fieldbus communication architecture is today also available for level gauge systems.

Alpret Control Specialists

(011) 249 6700

alpret.conspec@frco.com




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