EPG Companies Inc. - Bulletin 0165
The most important consideration in working with flow meters is the way that the sensor is installed.
The specifications supplied by manufacturers are based on test data taken from textbook installations. The accuracy and repeatability achieved with the sensor is dependent upon its installation.
The rules about placement of the flow sensor should be obeyed implicitly if accurate measurements are required.
Insertion (paddlewheel) flow sensors are velocity devices. For an insertion (paddlewheel) flow sensor to achieve accuracy and repeatability, it requires the pipe it is installed in to be full with a minimum velocity of 2 ft./sec. (check with EPG for lower velocity). These sensors should be installed based on the minimum velocity rather than the discharge or force main pipe size.
UPSTREAM FLOW DISTURBANCE
Although the effect of upstream flow disturbance is more severe on some styles of flow meters than it is on others, the effect of these disturbances can be quite serious with almost any style of meter. There is no reliable way of predicting what effect a given pipe configuration will have upon a particular flow meter since fluid dynamics of each installation is highly complicated and so varied. EPG recommends a minimum of 10 pipe diameters upstream of the flow sensor.
DOWNSTREAM FLOW DISTURBANCES
A common misconception is that once a fluid has gone through a flow meter, the discharge flow path is unimportant. Unfortunately, this is not true. Disturbances caused by a bend, valve, etc. in the discharge piping, can propagate backward against the current for a few diameters affecting a flow meter installed close upstream. Therefore, EPG recommends a minimum of 5 pipe diameters downstream of the flow sensor.
SUGGESTIONS FOR AVOIDING INSTALLATION ERRORS
When installing paddlewheel flow sensors, these steps should be followed to obtain accurate results.
- Remember that swirl is the worst enemy. Above all, try to avoid the classic swirl-generating situations where the flow is forced to make a three-dimensional bend; that is, to turn two successive right angles in two perpendicular planes. The suggested distance from these swirl inducing bends is 100 pipe diameters upstream of the flow sensor.
- If at all possible, install the flow sensor with adequate lengths of straight pipe of the correct diameter upstream and downstream of the flow sensor. The generally approved specification is 10 times the pipe diameter upstream and 5 times the pipe diameter downstream so long as rule #1 is followed. In case where turbulence inducing equipment such as elbows, valves, tees, etc. are present, lengths of 50 diameters upstream and 10 diameters downstream are sufficient.
- If it is impossible to accommodate the desirable lengths of straight pipe, there are two alternatives: either have the flow meter calibrated after it has been installed in its pipe work complete with all bends, valves, etc., or install a flow straightener, which is designed to remove swirl not to correct a distorted velocity profile. Some types of straighteners are designed to improve a badly shaped profile as well as reduce swirl, but other types do not have a very beneficial effect on a bad profile, and often they may actually make a fairly good profile worse.