Surge Pressure Computations 1

What is a Surge A pressure surge or a water hammer is a pressure wave caused by a sudden change in water velocity. The word “water hammer” originates from the hammering sound which plumbing makes inside a house when faucets are turned on or off quickly. 2

Why do Surge analysis? Protect the pipeline and its appurtenances Protect human life Reduce pipe costs 3

When do Surge analysis? Consideration should be given to performing a surge analysis when any of the following conditions exist the maximum change of fluid velocity could exceed 4 fps the length of the pipeline exceeds 1000 feet there are fast acting valves in the system there are pumps in the system 4

What are the Maximum Allowable Pressures? The various standards for PVC pipe are not in total agreement regarding the “safety factor “for PVC pipe. The pipeline shall have a pressure class rating greater than the static or working pressure plus surge at any point in the system. For low pressure systems, “Pressure relief valves shall be large enough to pass the full pump discharge with a pipeline pressure no greater than 50 percent above the permissible working head of the pipe.” 5

Allowable Pressures cont.. For high pressure systems, “Pressure relief valves shall be set to open at a pressure no greater than 5 psi above the pressure rating of the pipe or the lowest pressure rated component in the system. The Handbook of PVC Pipe (Uni-bell, 1977) states that “In general, system operating pressure (static pressure plus surge pressure ) should not exceed 150% of the pipe pressure rating or pressure class.” 6

What causes varying velocity? Closing a valve Opening a valve Rhythmic valve operation Starting a pump Stopping a pump Movement of air pockets Sudden release of air Sudden halt in flow when air has been exhausted Recombination after water-column separation 7

Critical time Critical time = 2L/a Valve or pump closure should be greater than critical The last 20% of valve closure most critical. Very dependent on type of valve. 11

What Really Happens? Simple Valve Closure

Wave Diagram t=0 to 2L/a 8

Wave Diagram t= 2L/a to 4L/a

How fast is the pressure wave? Wave speed based on two things Bulk modulus of elasticity of the fluid, usually water. Bulk modulus of elasticity of the pipe walls. 9

How to compute surge pressures 10

Another way is nomographs 12

What happens if there are multiple pipes? The Procedure here is to convert the series pipe to an equivalent uniform pipe 13

Branching Pipelines 14

Effects From Air Movement When an air pocket becomes suddenly dislodged, as by a stream of flowing water, the extent of changes in “local” fluid velocities and the consequent waterhammer pressures are often much in excess of instantaneous closure. 15

Pressure Rating Pressure rating can changed based on hoop stress and number of cycles 16

Pressure Rating Over Time 17

Example Compute the head increase due to instantaneous stoppage of water initially flowing at 5.0 fps in an 10 inch, 80 psi, PIP pieline, a = 860 If the static head at the point of stoppage is 55.0 psi, what is the total pressure acting on the pipe at the moment of stoppage? What is the hoop stress developed at the moment of stoppage? Hs=P*D/2*t If this Hoop stress will occur twice each day, and at each occurrence there are 8 stress cycles of this approximate magnitude, and if the irrigation season is 90 days long, how many years can this pipe be expected to last before failure due to stress cycles?

Summary Actual surge analysis is generally much more complicated than the simple example. The most practical method of solutions is to understand the available protection hardware and to install that equipment at enough locations and with reasonable settings. A fact of life which makes this approach almost necessary is that irrigation systems tend to be added to or changed with time until they may only vaguely resemble the original design. 19