Water usage optimisation at Hinkley Point B nuclear power station
EDF Energy is committed to continually striving towards being as efficient and environmentally friendly as possible across its fleet of nuclear power stations. One of the key targets at Hinkley Point B (HPB) power station in Somerset is to reduce water consumption. Even though the station has been operating for 40+ years, modern technologies are continually embraced to improve the station's performance.
Author(s): Dr Gabriel Carrillo-Ureta and Callum Grayson
Examples of technologies not commonly associated with older nuclear power stations are ultra-sonic flow measurements, which allow us to monitor our overall water consumption through the main HPB station inlets whilst also having the flexibility to be used in smaller applications, such as monitoring the health/performance of various plant systems and identifying losses or even leaks. Through specific sub systems detailed analysis, the station looks to continually reduce our environmental impact by reducing water consumption across the site. With the use of ultrasonic flow meters in different applications across the station, coupled with the identification of possible areas for improvement and further implementation, it is hoped to be able to significantly reduce losses for specific plant items.
Hinkley Point B (HPB) is the second Nuclear Power station based in Somerset, with Hinkley Point A (HPA) currently being decommissioned and plans for Hinkley Point C (HPC) well under way. Nuclear reactors generate electricity by converting water to steam, which is used to drive a turbine that then generates electricity. This steam is then condensed under vacuum using rankine cycle principles to optimise turbine efficiency.
Due to this large requirement of condenser water, Power Stations are predominantly located on the coast or adjacent to large sources of water; HPB is no different, being based on the Bristol Channel. One of the key goals of EDF Energy Group is to continue to reduce its impact on natural resources, with targets including maintaining sustainable water usage across its fleet. By ensuring that HPB is as efficient and sustainable as possible, it not only limits its effects on the environment but has the additional benefits of reducing resourcing costs across the business.
At Hinkley Point there are three types of water used: sea water, demineralised water and town's main (‘tap’ water). Sea water is required for cooling in the main condensers, converting steam back to water after it has passed through the main turbines. It is also used in coolers and heat exchangers to cool additional auxiliary equipment across the station.
The boiler feed/condensate, which is the water fed to the boilers, converted to steam which then drives the turbine is demineralised water. It is necessary for this feedwater to be of extremely high quality, otherwise impurities imparted to the feedwater system would rapidly concentrate to undesirable levels, forming deposits either in the boilers or turbine cylinders. Either of these would dramatically affect the efficiency of the station if left unchecked. Other uses include being the coolant medium for Reactor Auxiliary and Pressure Vessel cooling systems (RACW and PVCW), which include Gas Circulator and Concrete Pressure Vessel Cooling. At HPB power station, demineralised water is produced on site in the Containerised Water Treatment Plant (CWTP).
The third type of water used across the station is Towns Main. This is the standard water that you receive from the taps at home. It is used for the same purposes, i.e. drinking/sanitation etc., with additional applications including firefighting and in HPB case, to produce demineralised water onsite. Town's main is provided by Wessex Water with two distribution mains leading to Hinkley Point, one to HPB and the other to HPA/HPC.
As a station, the daily/weekly/monthly water usage is monitored so that we can identify areas to reduce consumption and respond accordingly if any abnormal changes are noticed. We can now track a spike or drop in usage and attempt to match work being performed across the station. The station is now over 40 years old, however the technology used to monitor our water usage and assist in reducing consumption is considerably more modern.
Use of technology in water optimisation
Across the station not only is the overall usage recorded and tracked, but we also have the capabilities to measure specific consumption, such as during fire deluge testing or even measuring different fluids such as lub oil usage on the generators. The predominant method of measuring flows across site is through the use of ultrasonic flow meters. Flow meters across site include permanent and other portable meters that can be installed semi-permanently for a variety of different applications. These flow meters are non-intrusive and do not affect the water flow, hence appropriate for a power station.
Ultrasonic flow meters work by attaching sensors to the outside of the pipe that send ultrasound signals through the pipe and liquid. Data can then be collected from the data logger manually, or more recently our permanent solutions can be viewed as a live feed through station computer loggers.
How we are changing in order to lower our water consumption
There is a particular emphasis on reducing the amount of demineralised water and consequently Town's main used on site. Sea water is on an open loop from the Bristol Channel whereas both town's main and demineralised water are on closed loops, with the aim to reuse as much water as possible. Demineralised water costs the most to produce and with a composition of 70–80% Towns Main, any reduction in its use not only decreases production costs in the CWTP but also in town's main overall usage.
To record HPB's total town's main usage, a recent project has been recently completed to install new permanent flow meters that provide a more accurate representation of how much water is used and which lines the water is supplied from, see Fig. 1 . The top graph is the main inlet to site via valve B/TM/5 and the bottom is the cross flow between A and B station through valve B/TM/20. As can be seen, both flow rates cycle between two levels, following each other profile identically but at different volumes. Fig. 1 shows a ‘usual’ water consumption trends from our two main inlets (in cubic metres per hour); any deviations from this trends can then be investigated accordingly.
Fig 1: Live feed of HPB's Towns Main water consumption
Before the new installations, the two main distribution lines to HPB and HPA/HPC had one magnetic flow meter on them each, originally installed by Wessex Water. However, they are not an accurate measure of our consumption as there are cross links between HPA/HPB which these flow meters do not account for; data was only logged every 15 min and there is a day lag before it is able to be collected. With the new ultrasonic flow meters, that have been placed on the main distribution line and cross-linking pipes between the sites, the data is logged every minute and it has a live feed that can be tracked on a station computer logger. This instantaneous read out with intervals between recordings of only 1 min allows for greater accuracy in mapping water usage to work being undertaken on site. An example of this is when town's main water is required to supply heat exchangers; this allows intrusive maintenance to be undertaken on their normal sea water supply pipework or pumps. A general breakdown of key water consumption areas and their dependence can be seen in Fig 2.
Fig 2: Town’s main water applications
After an initial review of all these plant areas from Fig 2, three specific ones were chosen due their known ‘open loop nature’, i.e. water did not return back to the associated tanks or water system, and ‘perceived’ quantities of water been used. Next steps were taken to review actual data and agreed on way forward.
With this, these three areas have been highlighted in red within Fig 2: 400 kV automatic grid wash; start/standby boiler feed pump Gland sealing system (SSBFP Gland sealing) and the 275 kV manual grid wash. These three areas have been targeted as specific areas that water losses can potentially be reduced due to their known high and potentially inefficient usage. In regards to specific water reduction tasks, the portable flow meters play a prominent role in confirming the assumptions made and also help to identify other areas for improvement. It also helps to quantify and review the results when the corrective actions/work has taken place, measuring the amount of water that has actually been saved.
400 kV grid wash
There are two substations at HPB, the 400 and 275 kV. Their purpose is to transform (increase) the voltage in order to reduce losses when transmitting electricity across the National Grid. EDF Energy, as part of an agreement with the National Grid Company, has to provide ‘cleaning’ facilities to both substation's electrical circuit components. The 275 kV substation is manually cleaned with town's main pressure washers directly connected to the HPA town's main system; whereas the 400 kV substation is on an automatic spray system that triggers a grid wash whenever the pollution levels in the substation have reached certain thresholds, this uses demineralised water as it can be seen in Fig. 2 . In order for this automatic wash to take place, an operate technician is requested by the control room to manually start a transfer pump to top up a substation live washing tank that supplies the automatic spraying system.
Initial water surveys carried out on the 400 kV Grid wash quickly identified that a substantial amount of water was being used per wash due to unknown specific duration of the automatic wash and also the total amount of water required, see Figs. 3 and 4 . By performing a tank drop test, the duration and actual volume of water required to complete a 400 kV grid wash were calculated, this is the actual water required for an ‘optimal’ grid wash without any surplus water been added to the tank which would result in an overflow going to waste. A tank drop test is the process of ascertaining the required volume of water or flow rate needed through a system to perform is designed function by means of monitoring the change in fluid level within the tank (or drop) with an isolated fluid supply, i.e. no additional water added in a specific amount of time.
Fig 3: Grid washes at HPB
a 400 kV substation
b 275 kV substation
Fig 4: 400 kV grid wash
a 400 kV grid wash results
b Grid wash permanent flowmeter and sensors
After acquiring this information, an ‘optimal’ grid wash (in terms of duration and volume) was identified. Herewith, results were captured and incorporated into station operating procedures and communicated to all relevant staff in order to improve the efficiency of the system.
Fig 4 shows the installed ultrasonic flow meter and the volume and total duration for an ‘optimal’ grid wash after been implemented, ∼100 m 3 and ∼50 min. The surveys conducted identified that the pumps were sometimes left on for up to 3 h in order to ensure the tank always had water available for the wash, this equates to an average of 175 m 3 being lost per wash. After implementing the identified recommendations, these demineralised water through manual interventions losses were reduced to less than 15 m 3 per wash. An average loss of 15 m 3 with the optimal wash when compared with previous losses of 175 m 3 show an improvement of more than 90% for the process!
In addition to this, the feasibility of an automatic ‘cut off’ system for the top up pumps is also currently under review by the engineering system owner.
275 kV grid wash
As mentioned earlier, within the 275 kV substation live washing takes place manually with pressure washers that are connected via detachable hoses to various standpipes across the substation. This occurs weekly/bi-weekly dependant on the season (washed more frequently throughout the winter months). It was originally thought that due to being a laborious manual process which was never recorded or measured, it could be ineffective and a large amount of water lost per wash. In order to prove this theory, the flow and volume required for the manual live washing in the 275 kV substation required to be accurately calculated.
To achieve this, a portable battery powered ultra-sonic flow meter was used; this instrument was connected to the external pipe which is downstream of the town's main header tanks. Fig. 5 shows half of the total data that is required for a grid wash, from which it has been calculated that approximately 40 m 3 are used for a full grid wash. This was found to be far more efficient than initially thought, especially in comparison with the 400 kV substation, dedicated human intervention in this case helping to optimise the process and reduce water losses!
Fig 5: 275 kV grid wash
a 275 kV grid wash results
b Grid wash portable flowmeter and sensors
The main reason for the large difference in volume used in the 275 and 400 kV substations is that the 275 kV substation has 174 insulators to wash, unlike the 400 kV substation which has approximately 980 insulators. This combined with the insulators being larger on the 400 kV substation means far more water is required. The decision was made that the benefits of pursuing a live washing automated process for the 275 kV were not required given the near optimal water utilisation under the manual grid operator process.
SSBFP Gland sealing system
The third targeted area for water recovery was the SSBFP Gland sealing systems. The electrically driven SSBFP's roles are to provide feed water during startup and shutdown of the reactors when the main steam driven boiler feed pump is not in service. Gland sealing system is to prevent the high temperature feed water from leaking along the bushings and flashing off outside the glands to atmosphere. The excess water then drains off to a collection tank, ready to be sent to the treated water recover tank (TWRT) before being distributed back for other uses (refer to Fig. 2 ). However if the TWRT tank is unable to collect water, then high quality demineralised water then overflows through the discharge pipework, unable to then be recycled.
Due to the flow from the discharge pipe not always being full bore, a different type of flow meter was required. The ‘stingray’ flow meter, unlike the other flow meters, attaches to the inside of the discharge pipe and measures the temperature, flow velocity and height of fluid over the sensor, Fig. 6 . By also measuring the dimensions of the pipe, the volumetric flow rate can then be calculated.
Fig 6: SSBFP Gland sealing tank overflow
a Flowmeter results
b Installed flowmeter (Stingray)
It was found that a substantial amount of high quality demineralised water was being discharged per hour whilst the tanks were overflowing, which has already led to mitigating actions being put it in place. Improvements to the automatic control system are currently being tested that prioritises restocking the service recovery tank such that losses through the drains are dramatically reduced. With the auto control system improvements now in service, the system is being monitored and so far there have been no additional discharges.
Even though the station has been operating for 40+ years, with lots of the equipment on site having been operational for that amount of time, the use of new technology is invaluable at ensuring that we still run as efficiently and safely as possible. We are always looking for new techniques and technologies to continuously improve as an individual station and also as a larger organisation by sharing best practices and operational experience.
One of HPB station's targets is to reduce the amount of Towns Main water usage per year by 10%; with the assistance of technology we are currently delivering on plan. In order to achieve this, several different ultrasonic flow meters have been used to quantify the water use of specific sub-systems and with the further implementation of the recommendations proposed, work completed so far on the SSBFP Gland sealing tank and the 400 KV grid wash have achieved water losses reductions of more than 90% when compared with their specific previous water usage values on these systems. All these means that the station's overall 10% station reduction target will be achieved by the end of the year. Moreover, with the installation of the new flow meters on the main station inlets we now have live data feed (with a higher degree of accuracy) of the stations water consumption to measure our water reductions against.
For the coming years, the next steps in this water usage optimisation will be to concentrate on water leaks around the plant and improvements on material condition of pipes and valves. This is not expected to have a major impact on total volume of water usage but should ensure that the station runs effectively until decommissioning and beyond.