Plant manager Willem van der Plas (Noordgas Transport): ‘Our knowledge will
Dr Ir Jaap van Ede , freelance business journalist industrial automation
The on-shore gas treatment plant of Noordgas Transport is located in the northeast of Holland, just besides the seadike and close to a big windmill park and peacefully grazing sheep. The plant is an organized mess of pipes and drums, which gives the impression of a small petrochemical factory.
‘You can see us as a value added logistics provider for natural gas’, plant manager Willem van der Plas explains. ‘Oil companies with production platforms in the North Sea transport natural gas through a network of feeder pipelines to us. In our plant we purify this gas by condensing the liquids, and removing these at low temperature. Next, we channel the gas to the network of the Gasunie. This is the company which distributes the natural gas in the Netherlands. Each day, about thirty million m3 gas passes our treatment plant.’
Simple but complex
The gas treatment process appears to be very simple. So, what is then the purpose of all those pipes and drums? ‘Our gas treatment plant is not more then a big refrigerator. But mind you, it is a very special refrigerator, because it is optimised to regain as much energy as possible.’ The temperature and pressure of the incoming crude gas and the outgoing purified gas are about equal. Thus, theoretically spoken, it should be possible to run the plant without supplying energy. ‘That’s right. We try to come so close to this utopic scheme as possible. This explains the complex structure of our factory, with many heat exchangers, control valves, drums and compressors.’
Van der Plas explains, while he draws figure 1: ‘A typical example is of an old local control circuit is this unit, which keeps the pressure in a conduit-pipe at 8 bar. A pressure sensor measures the pressure here, and sends an analogues signal to a local controller. This controller then checks if the pressure is ok. If it is to high or to low, a pneumatically operated valve is a bit more opened or closed, until the pressure equals the set point.’
‘In this old situation, two extra sensors measure the pneumatic pressure. When this pressure drops below or rises above a critical value, it is assumed that the process is out of control, as the control valve is then completely in an open or closed position. If that happens, the emergency shutdown system is activated and part of the plant is shut down’
‘Of course, such a big disturbance is very rare. However, it is common that a set point has to be adjusted a bit. This will for example be the case if the temperature of the seawater rises, and with that the temperature of the incoming natural gas. Until now, the operator then had to walk to the control circuit, and adjust the set point for the pressure.’
‘We will not need less process operators after the implementation of Experion. Why not, you may ask. Well, our process is running so smoothly, that in many cases you don’t need to change anything all day. The only two external factors which influence the gas treatment process are the amount of the incoming gas and the temperature of it. Therefore, a team of a few operators suffices to keep our plant going. And these persons could still be twiddling their thumbs, if they were not also responsible for coordinating the safety aspects of the maintenance work.’
It is clear that improving the operational efficiency alone, can’t be the reason for installing a new DCS at the gas treatment facility, which was built in 1975. The motive for doing that turns out to be much more simple. Van der Plas: ‘It is becoming harder and harder to get spare parts. Of course, you could exchange the old local control circuits for new ones, but this resembles replacing pen and paper by pocket calculators. We thought it was better to grab this chance, to introduce the most modern technology.’
Van der Plas now draws figure 2. ‘The control circuit for the pressure in the pipe I mentioned earlier will now work as follows. The pressure sensor communicates with the corresponding DCS control station, by sending a digital signal via the Fieldbus-network. Let’s now assume, that the pressure threatens to become low. In that case, the control station gives the valve positioner the instruction, to open the valve a bit more.’
‘Note, that the pneumatically operated valve will be the only old part in the new control circuit. All sensors have to be replaced because the old ones cannot communicate by way of Foundation Fieldbus.’
From the operator room it will become possible to adjust the set points of all the control circuits, which are connected to the fieldbus. And if a certain set point cannot be maintained by the DCS, the operator receives a warning on his computer. He or she can then try to correct this. If this turns out to be impossible, the plant part concerned is brought to a standstill by an autonomous emergency shutdown (ESD) system.
‘Until now, we avoided technical malfunction by doing preventive maintenance. With Experion we hope to change over to condition-based maintenance. Valves and other parts can then be utilized longer than before, and that will bring cost reduction.’
Experion is web-enabled, but for security reasons on the level of visualisation only. Process information can however be made available at different locations. In addition, it is possible to check via the internet if spare parts are available at Honeywell. ‘Another benefit of Experion is, that it can be easily connected to for example a maintenance management system. And that is important to us, considering our wishes in the field of condition-based maintenance.’
Gaz de France, one of the most important shareholders in Noordgastransport, formulated the design specifications of the new DCS. ‘They do all the engineering for us. Gaz de France was already familiar with the use of Foundation Fieldbus at some of their off-shore installations. This explains that they gave preference to this fieldbus type.’
It was however not clear at once, that Honeywell would become the supplier of the DCS. ‘Competitors like Fisher Rosemount, Invensys and Yokogawa proposed similar solutions. However, Honeywell was the only party who also suggested an emergency shutdown system, developed with their own hands. That fact decided us. When buying DCS and ESD from one supplier, the odds are that those systems will connect well. And if this is not the case, we know which party to turn to’.
One by one
‘Now we are transforming the control circuits in the field. This will be done one by one. We hope to have connected about half of them to the DCS this spring. Then we will decide which other control loops we wish to revise and link to Experion.’
The plant will continue to purify natural gas during the implementation of the DCS. This fact is striking. Normally, a factory is shut down when a new process control system is brought into use. However, in this case that is impossible because the supply of crude natural gas from the North Sea can’t be stopped.
Fortunately, Noordgastransport has more reserve capacity then the average factory. The treatment plant is after all a vital link in the gas supply chain. Recently, the reserve capacity was increased further, by replacing the gas/liquid separators by more efficient ones. From that moment on, less chilling of the crude gas was needed to remove the liquid pollutants. This is why there are now five propane compressors, of which there are normally only two in use. ‘Every processing step in our plant is split into several sub steps, which can be executed in parallel. For example, just before the five propane compressors the conduit-pipe of the refrigerant splits up in five pipes. Behind the compressors, these pipes join together again. This makes it possible to freely add more or less compressors, according to the need for cooling capacity.’
process lay-out you see everywhere in our factory. Therefore it is possible to
connect the control circuits to Experion one by one, while the production
continues. Only when we will bring into use the ESD-system, a short shutdown of
the plant is necessary.’
Cooling and heating in balance
Noordgastransport tries to save as much energy as possible. For that purpose, a system with many heat-exchangers was built, to keep cooling and heating in balance.
Willem van der Plas: ‘The refrigerant we use is propane. The way we chill is however not much different from the cooling method in a household refrigerator. Liquid propane is pumped through a vessel, our refrigerator. The crude natural gas streams through this vessel by way of a bundle of pipes. During the passage, the natural gas chills to –25 ºC, so that the heavier hydrocarbons in it become liquid. This condensate is removed later, by gas/liquid separators.’
‘During the chilling process, heat is transferred from the natural gas to the refrigerant, which then partly evaporates. To recycle this gasified refrigerant, it is removed and enters then a separate circuit to make it liquid again.’
The first step in regenerating the refrigerant is pressurizing it, so that the temperature rises to about +70 ºC. The last step is an expansion to chill the refrigerant to –40 ºC and make it liquid again.
Between these two steps, heat should however be withdrawn from the refrigerant. ‘In a household refrigerator this is done by pumping the refrigerant through a radiator, which is located at the back of the machine. Heat is then transferred from the refrigerant to the air. However, in our plant we use this heat to warm up the stream of purified natural gas. So, heat which was first transferred from the crude natural gas to the refrigerant, is later transferred back from the refrigerant to the purified natural gas. You can imagine, that saves us a lot of energy!’
Facts implementation ExperionTM Process Knowledge System
The gas treatment facility of Noordgastransport purifies crude natural gas
coming in via a pipeline from The North Sea.
Many stand-alone (analogues) control loops, no fieldbus present
Distributed control system, Experion Process Knowledge system (PKS) of
Honeywell. This is a fully integrated, digital and redundant process control
solution. Field communication network: Foundation Fieldbus.
Hardware: Honeywell, Amsterdam
Engineering: Gaz de France, Zoetermeer (NL)
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© C.J. van Ede 1997-2014
The Netherlands (Holland)