Air in the piping system comes from three sources. First is a water itself, since the water contains approximately 2% air by volume. Second source of air is a mechanical equipment, and third source is a initial pipe filling when some of the air will be trapped at the high points. The presence of air can impact pipe operation and lead to many problems in a piping system. One of the problems is that air will be accumulated in the high points. By the time air pockets can grow and become so large that will significantly restrict or block the flow. Please see the figure below.
The second and most likely occurrence is that the increased velocity will suddenly dislodge the pocket, pushing it down stream. This sudden and rapid change in fluid velocity when the pockets dislodges and is then stopped by another high point, can, and often will lead to high surge pressure. Please see
the figure below.
You might be wondering what happens with chemical piping, since you will not find any air release valve installed on a process piping. To answer this question, we will take a look on experimental research that address problem of air removal associated with velocity. Although, there a very little study on this topic, some experiments has been made and empirical equation (Mosevoll, 1987) for minimum velocity to remove air pocket is given Vmin = sqrt (0.63(g D)), where g is the gravity in metric and D is the outside pipe diameter in meters. Some other international experiments show that air coefficient of 0.63 can be higher, reaching 0.75 or even 0.93, depending of differential pressure. Recent study (Launchan et al. 2005) compared more or less similar equations found in the
international literature and variations are considerable and no unambiguous conclusion could be made. The figure below shows Vmin calculation for 6″ pipe.
It is evident that Vmin velocity cannot be precisely determined due to limited study, but conclusion can be made that higher velocity will push air thru the piping system, not allowing air pocket formation, which is the case with process piping system where velocity is above 5 ft/sec. However, when we have a small velocity, air accumulation might occur. That is usually the case in a water supply pipeline. An air release valve must be installed on all high points. Please see the figure below.
Pump suction pipe has also small velocity ranging from 0.5 ft/sec to 3.0 ft/sec. To avoid potential air pocket formation, pump suction pipe must be horizontal without any high points. Please see the figure below.
It is wort to mention that air in suction pipe will not produce water hammer, but it might reduce or block a flow. For the same reason; using suction manifold is not good idea and it should be avoided. As rule, the suction manifold is larger in size than the suction pipe and air pocket formation will occur at the top of manifold. Please see the figure below.
Typical configuration for the vertical pump in the pipeline is shown on the figure below. A well service valve is installed to exhaust air on the pump during star-up and admit air during the pump shut down. Second air valve is installed to alleviate water hammer force during the valve closing. In the case of sudden pump stoppage, the control valve will be activated to avoid entering air into the piping system and producing water hammer.
In water supply system, reversible flow might occur and installing air-vacuum valve will remedy this problem. Please see the video below.
Controlling water hammer force due to reversible flow can be achieved by installing an anti surge relief valve or a surge tanks, but that is out of scope in this blog.