This primer presents basic surge control principles and the functions of various valves associated with pumping stations.
Water pipelines and distribution systems are subjected to surges almost daily, which over time can cause damage to equipment and the pipeline itself. Surges are caused by sudden changes in fluid velocity and can be as minor as a few PSI to five times the static pressure. The causes and effects of these surges in pumping systems will be discussed, along with equipment that is designed to prevent and dissipate surges. Reference will be made to typical installations and examples so that an understanding of the applicable constraints can be gained.
Figure 1 illustrates a typical water pumping/distribution system where two parallel pumps draw water from a wet well, then pump the water through check and butterfly valves into a pump header and distribution system. A surge tank and relief valve are shown as possible equipment on the pump header to relieve and prevent surges. Each of these will be discussed in greater detail.
Causes and Effects
Surges are caused by sudden changes in flow velocity that result from common causes such as rapid valve closure, pump starts and stops, and improper filling practices. Pipelines often see their first surge during filling when the air being expelled from a pipeline rapidly escapes through a manual vent or a throttled valve followed by the water.
Being many times denser than air, water follows the air to the outlet at a high velocity, but its velocity is restricted by the outlet, thereby causing a surge. It is imperative that the filling flow rate be carefully controlled and the air vented through properly sized automatic air valves. Similarly line valves must be closed and opened slowly to prevent rapid changes in flow rate.
The operation of pumps and sudden stoppage of pumps due to power failures probably have the most frequent impact on the system and the greatest potential to cause significant surges. If the pumping system is not controlled or protected, contamination and damage to equipment and the pipeline itself can be serious.
The effects of surges can be as minor as loosening of pipe joints to as severe as damage to pumps, valves, and concrete structures. Damaged pipe joints and vacuum conditions can cause contamination to the system from ground water and backflow situations. Uncontrolled surges can be catastrophic as well. Line breaks can cause flooding and line shifting can cause damage to supports and even concrete piers and vaults. Losses can be in the millions of dollars, so it is essential that surges be understood and controlled with the proper equipment.
Some of the basic equations of surge theory will be presented, so an understanding of surge control equipment can be gained. First, the surge pressure (H) resulting from an instantaneous flow stoppage is directly proportional to the change in velocity and can be calculated as follows:
H = surge pressure, ft water column
a = speed of pressure wave, ft/s
v = change in flow velocity, ft/s
g = gravity, 32.2 ft/s2
The speed of the pressure wave (a) varies with the fluid, pipe size, and pipe material. For a medium sized steel line, it has a value of about 3500-ft/s. For PVC pipes, the speed will be far less. For a 12-in steel line with water flowing at 6-ft/s, the magnitude of a surge from an instantaneous flow stoppage is:
H = (3500 ft/s)(6 ft/s) / (32 ft/s2)
H = 656 ft water column
This surge pressure of 656-ft (285-psi) is in addition to the static line pressure; therefore, the resultant pressure will likely exceed the pressure rating of the system. Further, this high pressure will be maintained for several seconds as the wave reflects from one end of the piping system to the other end, causing over pressurization of pipe seals and fittings. Then after a reflection, the pressure wave may cause a negative pressure and vacuum pockets for several seconds, allowing contaminated ground water to be drawn into the system through seals or connections.
Even higher velocities than the pumping velocity are attainable in long piping systems. If the pumps are suddenly stopped due to a power failure, the kinetic energy of the water combined with the low inertia of the pump may cause a separation in the water column at the pump or at a highpoint in the pipeline. When the columns of water return via the static head of the line, the reverse velocity can exceed the normal velocity. The resultant surge pressure can be even higher than the 656-ft calculated above.
Transient analysis computer programs are normally employed to predict column separation and the actual return velocities and surges. transient programs can also model methods employed to control column separation, such as the use of a surge tank, vacuum breaker, or air valve. These solutions will be discussed in greater detail.
Thus far, the changes in velocity have been described as "sudden." How sudden must changes in velocity be to cause surges? If the velocity change is made within the time period, the pressure wave will travel the length of the pipeline and return, the change in velocity can be considered instantaneous, and the equation for surge pressure (S) given earlier applies. This time period, often called the critical period, can be calculated by the equation:
t = 2 L/a
t = critical period, sec
L = length of the pipe, ft