Dealing with ultra low flow rates

By Chris Parsloe:

In past days leaky, poorly insulated buildings ensured hefty heating loads with manageably high flow rates. By this I mean that flows would be:

 a)      measureable by standard flow measurement devices, and

b)      high enough to avoid air or dirt settlement.

 Previous editions of CIBSE Guide B1 recommended minimum velocities in pipes of 0.75m/s for sizes up to 50mm, and 1.25m/s if larger than 50mm. These values were then referred to in other CIBSE and BSRIA publications on pipe system design.

Despite this sound advice, such minimum flow velocity values 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 for this is that, due to 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 lower.

In addition, there has been an increase in the design temperature differentials specified by designers (i.e. the delta T). Whereas older systems were designed on a delta T of 11°C, new systems are typically being designed for 20 or 30°C. 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 delta T values result in low flow rates.

This is not a problem for main branch pipes which might drop from say 50 to 40mm in size, but 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 litres/sec, depending on which device you choose. Any flow rate that is below this range is going to be difficult if not impossible to measure.

Furthermore, if terminal unit connections are made in 15mm steel or copper pipe, then the old velocity limit of 0.75m/s will be contravened in large order (a flow of 0.015 litres per second in a 15mm pipe would have a velocity of only 0.11m/s). One solution would be to use smaller bore pipes. Copper is available at 10mm diameter, and its use for low flows would obviously increase velocity. However, there is reluctance because 10mm 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.)

As with all such design conundrums, if 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 due to ongoing settlement in terminal branch pipes.

CIBSE Commissioning Code W 2010 and BSRIA Guides BG 2/2010 and BG29/2011 have gone some way towards recognising and addressing the problems. The commissioning guides now make specific reference to “ultra low flow rates” and define these as any flow rate less than 0.015litres/sec. Alternative permissible options for commissioning systems with ultra low flow rates include:

  • 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°C range;
  • adopting an improvised method of flow measurement such as the use of 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 was 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) is the author of CIBSE Commissioning Code W 2010, BSRIA BG2/2010 Commissioning Water Systems, and BSRIA BG 29/2012 Pre-commission Cleaning of Pipework Systems.  

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