INDUSTRY COMMENT
BSEE
The new BSRIA BG29 2020 PreCommissioning cleaning of pipework systems
Distribuon pipework design is generally taken as a given, aer all it is only the connecon of a Terminal unit with a flow and return pipe and a few valves. What these ‘few valves’ are, has changed over the years
depending on if the system is Constant or Variable volume. The configuraon and types of valves has also changed, as beer understanding of temperature control, flow control and water quality have been achieved, some through trial and error.
flow and return pipe and a few valves. What these ‘few valves’ are, has changed over the years depending on if the system is Constant or Variable volume. The configuration and types of valves has also changed, as better understanding of temperature control, flow control and water quality have been achieved, some through trial and error.
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Therefore, it is surprising that the latest edition of the published BSRIA technical guide contradicts manufacturers’ recommendations. This backward step in an age where following guidance is the safest path, could potentially cause problems during the flushing and long-term maintenance of a system. Of equal surprise is that the guidance seems to only apply to certain types of Terminal Units. Other terminal units such as Heat Interface Units that are subjected to the same water quality standards and are often part of the same system are not covered.
Although the design of distribution pipework is considered straight forward, there is a set program of tasks in order to deliver a working system. Each task depends on the successful completion of the last task before moving on to the next one. The flushing of pipework to remove any dirt or debris is by far the most difficult task to achieve. Even with all the best work practices/programs in place, the challenges faced by any Flushing Company when performing this vital task are often difficult to overcome. Typically hard to access units or limited site/location access due to an ongoing and ever-changing construction environment. Throw in
istribution pipework design is generally taken as a given, after all it is only the connection of a Terminal unit with a
the time pressure of time critical projects using Heat Interface Units, this process can be impeded. Effective flushing is achieved with strong flow velocities, and these are best achieved without restrictions in their path. Remove the restriction and the flow velocity increases and hence removes dirt more efficiently. Flush through a flow orifice or PICCV valve and the velocity is restricted, thus allowing dirt to easily get trapped. So, having no or minimal obstructions in the flow path is desirable.
However, we must also consider the effect of having a restrictive drain as the means for the water to exit the system with an equally torturous path will have on the flushing velocity. Therefore, we can conclude that drains should be of a diameter that allows dirt to pass through easily. Otherwise what is the point?
The new BSRIA guide, led in part by the requirement to reduce the resistance in one leg to allow a good flushing velocity on the flow is recommending opening the return leg to backflushing, which is now part of the BSRIA recommended method of back filling the remaining valve set.
One of the design requirements of a PICCV and PICV valve is the ability of the valve to limit system flow, as limiting/controlling the flow through the system is crucial to the balancing of said system and the systems intended function itself. To achieve this most PICCV/PICV by the nature of their design and manufacture are naturally biased in their direction of operation. Thus, when the PICCV/PICV valve is return mounted, as recommended by BSRIA recently, and the flushing operation is undertaken, water potential is flowing backwards through the PICCV or PICV valve. This is not good as it was not it’s intended direction of flow control and there is no limit to the flow.
The flow will then be determined by the local pressure within that part of the system. However, there is no accurate method of
limiting/controlling this flow apart from the 90-degree opening of the drain valve, which can be considered on/off at best. Therefore, this potentially could bring dirt and forces into a valve that was not designed to ever work with. Because of this the dirt can easily get stuck in the chambers of the valve and adhere itself to the moving parts exposed because of the reverse flow direction. Additionally, the uncontrolled velocity of water moving in a reverse direction can do damage to diaphragms and ‘O’ Rings and clean away essential protective grease and oils that coat the moving parts. The result of this can be clearly seen when hysteresis is affected, and the readings are not repeatable.
If the method promoted by BSRIA of backflushing is accepted by the industry then there is a high likelihood of an increasing number of problem projects using existing technology. If the internals of the valve have been damaged by the action recommended by a BSRIA document that contradicts manufacturers’ instructions (and hence voiding warranties), where does this leave the building owner in the future?
The alternative and perhaps the better solution are to install the PICCV/PICV in the flow along with the flow measuring device and any other component required. The return leg will then be free of components, thus giving the desired maximum flushing velocity through the terminal device and protecting the PICCV/PICV at the same time. Additionally, it can be stated that the PICCV/PICV valve is in its optimum controlling position in this arrangement and therefore operating at peak efficiency.
Wrien by Marn Lowe
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BUILDING SERVICES & ENVIRONMENTAL ENGINEER JANUARY 2021 7
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