Measuring flow through a balancing valve

[TSN79]

My company installed a number of balancing valves (Tour & Andersson, STAD) in a central heating system. These valves are used to measure the flow (in this case water). According to the valve's specs it should be installed with a certain distance of straight pipe both before and after the valve in order for the measurement to be correct. The only problem is that they aren't installed according to this. One valve has a 90° almost immediately on each side. I'm wondering if someone has any idea how much this might affect the measurement of flow?

 

[edgepflow]

I am not familar with the balancing valves you mentioned for measuring flow, but in general most flow meters (except Coriolis style) are sensitive to upstream flow disturbances. The elbow will skew the velocity profile and can result in measurement error. It is difficult to estimate due to many variables, but an elbow may cause between approximately 2% to 8% measurement error. If you can move the elbow at least three, but preferably 10 diameters away from the flow measurement, you will reduce the error significantly.