Water Hammer Calculations
Water Hammer Calculations
(OP)
I have looked on other threads. I have looked at technical documents. I have looked at on line calculators.
I am missing something.
There seems to be two different ways to calculate the pressure and/or force exerted when flow is stopped by a quick acting valve, i.e. water hammer.
One way per thread124-18869: SURGE CALCULATION indicates h(wh)=a*DV/gc where a is the velocity of wave propagation, D is the inside pipe diameter, V is the change in velocity of the fluid, and gc is the grav. constant.
Then there is the other approach/method found in several locations including engineering toolbox http://www.engineeringtoolbox.com/water-hammer-d_9... for water hammer which states P=0.07VL/T where V is the velocity, L is the pipe length, t is the valve closing time.
I am assuming h(wh) and 0.07VL/T are analogous in that they represent the surge in pressure on the system caused by the velocity drop, but i do not see how since they use different system variables
Assume
a = 4,000 ft/s
D = 0.5 in
V = 10 ft/s
gc = 32.2
L = 20 ft
T = 0.1 sec
Unless I am missing something, the first equation lacks the density of water (62.4 lb/cf) to make the units work and adding conversions from feet to inches so
lb/in^2 = 10*4000*0.5*62.4 / 32.2*1728
= 22.4
When I use the second equation, which the units do not get to pressure,
lb/in^2 = 0.07*10*20/0.1
= 140
What am i missing?
I am missing something.
There seems to be two different ways to calculate the pressure and/or force exerted when flow is stopped by a quick acting valve, i.e. water hammer.
One way per thread124-18869: SURGE CALCULATION indicates h(wh)=a*DV/gc where a is the velocity of wave propagation, D is the inside pipe diameter, V is the change in velocity of the fluid, and gc is the grav. constant.
Then there is the other approach/method found in several locations including engineering toolbox http://www.engineeringtoolbox.com/water-hammer-d_9... for water hammer which states P=0.07VL/T where V is the velocity, L is the pipe length, t is the valve closing time.
I am assuming h(wh) and 0.07VL/T are analogous in that they represent the surge in pressure on the system caused by the velocity drop, but i do not see how since they use different system variables
Assume
a = 4,000 ft/s
D = 0.5 in
V = 10 ft/s
gc = 32.2
L = 20 ft
T = 0.1 sec
Unless I am missing something, the first equation lacks the density of water (62.4 lb/cf) to make the units work and adding conversions from feet to inches so
lb/in^2 = 10*4000*0.5*62.4 / 32.2*1728
= 22.4
When I use the second equation, which the units do not get to pressure,
lb/in^2 = 0.07*10*20/0.1
= 140
What am i missing?





RE: Water Hammer Calculations
RE: Water Hammer Calculations
The L in the second equation means that the surge pressure increases at the valve when the length increases ( more mass).
Both of these are at best simple worst case scenarios and for short piping lengths probably Ok, but anything more complex or considerably longer, you need to start using a transient analysis program to model pressure wave attenuation due to the friction and the walls expanding.
This also allows the pressure wave to reflect back. It starts to get complex if your lengths and flowrates increase.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Water Hammer Calculations
One of my sources indicated different approaches if the valve closing was less than or greater than the t=2L/a - so I get that.
So, in general, if the valve closes quicker than 2L/a - I should probably use h(wh)=a*DV/gc , but when it is slower, I should probably use 0.07VL/T?
I am looking at trying to wrap my head around the issue of flush valves in a commercial toilet room. Typical flow each is 25 gpm but like to use 50 gpm for two "simultaneous" valves. The length of piping would be around 10-15 feet. We typically put a spring piston hammer arrestor on the piping, but some owners do not like them. We also use a larger manifold pipe to reduce the velocity, reducing the momentum of the water, but this effect is not as straight forward because increasing the pipe size will increase the volume of water which increases the momentum. I am trying to see when I should specify more pipe support to help minimize the pipe movement from the surge
I understand how to approach the problem when the valve is on a straight pipe, but with flush valves you will have a tee off the main to go into the room, then there is the elbow of the flush valve that directs the flow to the actual valve. I don't know if this would constitute use of a transient analysis program or not.
RE: Water Hammer Calculations
My understanding has always been that if the pipe coming to a halt is connected to another pipe which is flowing, the surge wave just vanishes as soon as it hits the tee where flow is occurring. You only really get classic water hammer when you have a length of pipe which is only flowing to a single item. Elbows act to divert the shock wave along the pipe, but get a reaction force on them.
It all depends on what is flowing at the same time
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Water Hammer Calculations
RE: Water Hammer Calculations
1. If the flow is less than 100 gal/min, the small diameter piping has a high pressure rating and can usually withstand the pressure.
2. Pipe network junctions significantly dissipate the pressure waves and do not need to be analyzed.
3. If the velocity is kept below 6 ft/sec, water hammer should not be a concern.
4. Pumping systems with a head greater than 50 ft. if the flow is grater than about 500 gal/min should be analyzed.
For your system, there is little that may be done to minimize water hammer except for limiting the fluid velocity.
Here is another water hammer formula from Garr Jones' Book. This formula is more intuitive.
RE: Water Hammer Calculations
Larger pipe size will also decrease your bulk wave speed (combined wave speed through liquid-filled pipe), which also slightly lowers potential surge pressures.
RE: Water Hammer Calculations
Do you have piping downstream of the valve in question? If so, the t=2L/a does not consider that piping. Then you could include valve flow area vs time and downstream hydraulics.
Sailoday28
RE: Water Hammer Calculations
The gist of what I am reading is that my pipe is not large enough, nor do I really have enough flow for water hammer to be an issue. But pipes sometimes move and they sometimes make noise, which might be called water hammer by the client or contractor, but could just be poorly supported piping in the chase behind the fixtures. Since pipe supports are generally loosely specified and not detailed, what gets installed could run the gambit.
Maybe this is more of a support issue than a flow issue.
RE: Water Hammer Calculations
There are also other factors to consider when determining if the surge pressure will cause damage or not; I recall that the energy of the surge must exceed the energy needed to cause damage to the pipe, so that needs to be checked.
Finally, there are simple measures to reduce the surge pressure or energy that occurs during waterhammer events , including adding a compressible bladder in the pipe system, limiting the speed of closure for the last 5% of valve stroke, and preventing full tight closure. However, programming these tricks in the control system logic is not a failsafe solution; the fatal pipeline failure in Washington state in 1999 was partly caused by a reset of the redundant control systems; during reset it led to a failed -closed fast closure of the stop valves in a 100 mile long gasoline pipe( underground), and the change in momentum of a 100 mile leg of gasoline led to the failure of the pipe at a location was previously damaged by a backhoe.
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