Chris Downing of Siemens Building Technologies explains how it’s possible to design gaseous systems to overcome the most challenging of environments.
Selector valve systems have long been used to offer users substantial savings by allowing more than one area, or compartment, to be protected, so apportioning the cost of cylinders and extinguishant across all of the protected areas. Deciding where the cylinders should be positioned in relation to the protected areas, determining the best routes for the piping and, in some cases, the need to comply with site-specific working conditions can, however, add to the challenges of developing selector valve solutions, as well as single area protection with remote cylinder locations. But with the help of appropriate expertise and equipment, these challenges can be readily addressed.
Following a conventional design philosophy, one of the largest selector valve systems that Siemens Building Technologies has installed in the UK comprised 756 cylinders serving four areas. With all the cylinders sited at one end of the building, this required several large diameter, low pressure feed pipes to be run between the cylinder bank and the protected areas. So a standard selector valve system was used here, albeit a very large one.
For the Dinorwig hydroelectric power station in north Wales, however, six sizeable risk areas had to be protected and, as space for cylinders was limited, again a selector valve system seemed the obvious choice. Here, however, the areas to be protected are arranged in a line inside a mountain, with a total length exceeding 100 metres, which posed a problem for agent flow.
Since the cylinder bank would be 120 metres from the furthest risk area, a single high-pressure welded feed pipe was used to allow operation up to 200 bar. But only by extending the high pressure rating further into the piping network, which significantly reduces friction losses in the pipe, could such a concept be feasible. Selector valves and pressure reduction devices were located only at the point of entry to each protected area, allowing for a shorter run of low pressure pipework (operating at <60 bar).
For this project, the single small-diameter feed offered advantages in terms of price, speed of installation and aesthetics. It also made possible the necessary flow rates, where other solutions might have struggled.
A single low pressure feed
At the other end of the scale, constant discharge technology (CDT), typically used on smaller applications, creates flow through the pipework at a constant rate, avoiding the pressure peaks associated with conventional inert gas solutions.
This is attractive, as it reduces the pressure rating of the selector valves by up to 80%, and reduces the size of pressure relief dampers by approximately 50%, both of which can be costly.
But CDT has limitations and may not be suited to every application. It’s clear that a balance needs to be struck between materials used, space available for cylinders and pipe, time on site, and price – always taking into account the available technology.
High pressure feed
A high pressure feed that does not involve welding the pipework is also possible. This was necessary for an installation where a selector valve system serving a number of areas was required. The many risk areas were spread over an appreciable distance, and were due to be built in phases.
The existing shell of the building could not be modified and, with restricted space for pipework, it made a single small diameter high pressure feed essential. But welding was undesirable in the first phase, and would not be permitted at all in subsequent phases. Furthermore, the future phases were not fully specified and the system had to be flexible – a challenge that can induce nightmares in the calmest of people!
The answer was a standard Siemens 300 bar nitrogen system, which allowed the use of the smallest possible pipes, and required only a single pass through the building. Where the possibility of a future module was envisaged the modular high pressure pipe sections, which are assembled on site without welding, were fitted with flanged tee connectors.
This approach actually allows faster installation than conventional pipe threading, and gave flexibility for the future since, at each section, it is possible to install multiple (selector) valves of varying sizes to cater for risk areas of almost any shape.
With standard modular parts – unlike prefabricated welded and flanged pipe sections where any modification in the high pressure pipe network would require a pipe section to be replaced with one of a different configuration – the new section could just be bolted in place. Prefabricated welded and flanged pipe sections would have required a new section to be specially manufactured, whereas many of the modular high pressure pipe sections are held in stock. So lead times for such on site changes are normally reduced when compared to bespoke prefabrication.
The result is an installation where optimum flow is maintained throughout, the space occupied by pipework is minimised and, for such a complex and bespoke solution, it is cost effective. Time on site is also minimised and the reduced pipe threading carries benefits in terms of cleanliness and waste.
Selector valve systems remain established technology and, while constant discharge technology brings a new dimension, extending the high pressure pipe section further into the system can offer some applications distinct advantages, especially when this can be achieved on site without welding.
On another project, the only cylinder bank location was on the opposite side of the road to the protected areas, which were themselves a substantial distance away and on the first floor. Although not benefiting from a selector valve arrangement, high pressure feeds were prerequisite for the system to perform, and in this instance had to be run under the road.
The fire risks, involving an unusual fuel, were such that the extinguishing concentrations needed were higher than the Lowest Observable Adverse Effect Level (LOAEL) value, so isolation (manual lock-off) valves were necessary. Normal practice would see these isolation valves placed at the cylinder bank, to reduce the amount of high-pressure pipe required, but in this case locating the isolating valves down one floor and across the road was not safe practice. Since it was desirable to locate the pressure reducer adjacent to the protected areas to reduce friction losses, it made sense to run the high pressure feed pipes from the cylinder bank right up to the protected areas.
The isolation valves and pressure reducers were thus located next to the entrance of each area, with suitable controls to ensure the valve was closed and the system locked off – before anyone could enter the protected area.
These examples show the advantages of working with companies which have a full portfolio of products and are able to choose exactly the right solution. Perhaps more importantly, they must also have the necessary knowhow, flexibility in mindset and design capability to utilise them to their full potential.
Chris Downing at Siemens Building Technologies on 07814 461616 or [email protected] or www.siemens.co.uk/buildingtechnologies
