Measuring ultra-low flows
Ultra-low flow rates to terminal devices, partly because of larger ∆Ts to increase system efficiency, are difficult, if not impossible, to measure — with the result that design flows cannot be verified. Chris Parsloe has some solutions.
In past days leaky, poorly insulated buildings ensured hefty heating loads with manageably high flow rates. By this I mean that flows would be measureable by standard flow-measurement devices and high enough to avoid air or dirt settlement.
Previous editions of CIBSE Guide B1 for heating systems recommended minimum velocities in pipes of 0.75 m/s for sizes up to 50 mm and 1.25 m/s for larger pipes. These values were then referred to in other CIBSE and BSRIA publications on pipe-system design.
Despite this sound advice, such minimum flow velocities are now routinely ignored by designers, particularly for pipes feeding to terminal units. The reason is that the flows we specify for terminals are much lower than they used to be. As implied above, one reason is that because of changes in the Building Regulations our buildings require less heating input than they once did. Therefore the size of heat emitters and the flow rates they require are smaller.
In addition, there has been an increase in the design temperature differentials specified by designers (i.e. the ∆T). Whereas older systems were designed on a ∆T of 11 K, new systems are typically being designed for 20 or 30 K. This is because many low-carbon heat sources, such as biomass boilers or CHP systems, perform better with return temperatures as low as possible. High ∆T values result in low flow rates.
This is not a problem for main branch pipes which might drop from, say, 50 to 40 mm in size, but which can still be selected to give a achieve a reasonable velocity and a measureable flow rate. However, there is an issue when it comes to pipes feeding terminal units. For a start, the minimum flow that can be measured by orifice-type flow-measurement devices tends to be in the range 0.01 to 0.015 l/s, depending on which device you choose. Lower flow rates will be difficult if not impossible to measure.
Further, connections to terminal units made in 15 mm steel or copper pipe will have velocities well below the old limit of 0.75 m/s (a flow of 0.015 l/s in a 15 mm pipe would have a velocity of only 0.11 m/s). One solution would be to use smaller-bore pipes. Copper is available at 10 mm diameter, and its use for low flows would obviously increase velocity. However, there is reluctance because 10 mm copper is flexible and, if wrongly handled, can easily kink — potentially blocking the pipe. The thought of installing and hiding so much flexible copper pipe in commercial buildings is therefore a concern amongst designers and installers. I would mention here that pipe solutions such as multi-layer pipe are perhaps less ‘kinkable’ at smaller sizes, but all need to be handled with care.
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| One approach to measuring ultra-low flows is provided by this specially designed commissioning module from SAV Systems. The flow to all branches is readily measured. One branch is then isolated and the flow to the others is measured. The difference is the flow to the isolated branch. |
If these design conundrums are not resolved, the problems become apparent during commissioning. The installing contractor and commissioning specialist are faced with a system with flow rates that cannot be measured and with water that appears permanently dirty because of ongoing settlement in terminal branch pipes.
CIBSE Commissioning Code W 2010 and BSRIA Guides BG 2/2010 and BG 29/2011 have gone some way towards recognising and addressing the problems. The commissioning guides now make specific reference to ‘ultra low flow rates’ and define them as less than 0.015 l/s. There are several permissible options for commissioning systems with ultra-low flow rates.
• Regulating ultra-low flows to any type of terminal unit by the temperature balance method (as commonly applied to radiator systems) whereby flows are adjusted until the return temperatures from the terminals are within a ±3 K range.
• Adopting an improvised method of flow measurement such as using pressure tappings across a fixed resistance such as a control valve.
• Temporarily increasing terminal-branch flow rates (by throttling other branches) until measureable flow rates are achieved.
• Adopting the ‘subtraction method’ of flow measurement, whereby the flow through a terminal branch is determined by measuring the reduction in flow at the main branch when that terminal is closed.
With regard to dirt settlement, the BSRIA Pre-commission Cleaning Guide (BG29/2011) now recognises that dirt settlement may be inevitable in terminal branch pipes and therefore differentiates between suspended solids and settled solids when assessing system cleanliness.
These new recommendations should hopefully enable designers and installers to reach agreement on system acceptability at handover without excessive debate. In all the discussions leading to these recommendations, there was agreement that artificially increasing design flow rates is not acceptable. Such an approach would reduce the energy efficiency of low-carbon heat sources and increase pump energy consumption.
Chris Parsloe (Parsloe Consulting Ltd) was author of CIBSE Commissioning Code W and BSRIA BG2/2010 Commissioning Water Systems.
