The following Blacoh "water hammer" presentation
is being conducted by Gary Cornell, Chairman/CEO of Blacoh Fluid Control, a manufacturer of
pulsation dampeners, surge suppressors, inlet stabilizers, and other fluid control products,
based in Riverside, California. With a BS degree from California Polytechnic University,
Mr. Cornell has worked in the reciprocating pump industry for more than 35 years, and
is a member of the Hydraulic Institute and the American Society of Mechanical Engineers.
Now, pay attention to this because two things are going on: hydraulic vibration and acoustics.
So what is this phenomenon? It's hydraulic shock. It's a momentary increase in pressure
in a liquid system due to the sudden change in velocity of a fluid. That's the key point.
It's not necessarily stopping the fluid, but a rapid change in velocity. It's called water
hammer because it creates this acoustic sound or pressure wave, or transient, and it sounds
almost like a hammer banging on a piece of pipe.
Now, since it's an acoustic wave -- and we'll talk more about this as we get into it -- since
it's an acoustic wave, it's not just liquid shifting back and forth, but an acoustic wave,
this wave of pressure that's created travels at the speed of sound in water. Now, what's
the speed of sound in air? About 1700 feet per second. In water, it can be as high as
4700 feet per second, this acoustic wave. And that's important for a couple of reasons,
but one of them is that it's very hard for an end-user or customer sometimes to understand
what's going on when a valve closes 500 feet away or 1000 feet away, and at the pump there's
an action almost instantly. Now, we've got our little coil demonstration,
I think most of you've seen, that we built out in the plant, and we can create some of
that time lag in that situation but, people don't equate this acoustic wave with the reaction
that occurs. So they're always synthesizing what happened, where did it happen, why did
it happen, and then it's our job to figure out what to do to fix it. A lot of things
can vary, and we'll get into it a little bit more but, we call it water hammer or hydraulic
shock. There's also a thing called surge, and you
know it's usually called a surge suppressor, but surge is a less intense form of water
hammer and typically happens downstream of any valve closing or anything like that; not
upstream. So, if we have uncontrolled hydraulic shock
or water hammer, what's the potential? You've heard us say before that it can be 4 to 8
times (a momentary increase in pressure), over the normal flowing pressure.
I was in the wrong, it's not 1700; it's about 1125 feet per second versus 4700.
So, with the same intensity, energy pressure in water is 60 times greater than in air and
that's why submarines have torpedoes that explode below the water line. You think about
a tsunami and the energy that's carried for literally thousands of miles without dissipating.
Water or liquid is a very efficient transferor of energy.
A simple formula that's quite accurate in predicting what this pressure rise will be
is using this 60 times the velocity of the liquid in the pipe, in feet per second, times
any specific gravity, divided by time. Typically, in talking about water hammer with a valve
closure which is probably 85 to 90 percent of the applications, we're using one second
as a valve closure time. So, if we use this, and this is just a computer
generated graph of what happens when a valve closes instantly, you get an immediate spike
and then a tapering off of this acoustic wave. It doesn't just stop and reverses from where
it is, when it hits another solid object it reverberates back the other way and keeps
oscillating until something breaks or the energy is dissipated through friction.
[Audience: "What is a second and when they say micro-second, what's the difference? A
micro-second obviously is faster."] Just much faster; much, much faster.
[Audience: "Because I hear that a lot of times too in talking about valves."]
Yeah, a second is pretty fast, but in terms of water hammer it's a long time.
So, we're just going through a little exercise here and we're saying the velocity is 6 feet
per second, specific gravity 1.2, and time is one second. Now, 6 feet per second flowing
in a process system is not all that fast. I mean there are a lot of places out there
that are going 8, 10, 12, even 15 feet per second. Obviously, the faster it flows, the
more velocity you have, the more damage that can be done. Mostly, at about 4 to 5 feet
per second you start being concerned about this phenomenon called water hammer. So, it
equals 432 psi. Now, this is really important because I've
been on the phone, I've heard you guys talking to customers and the customer says, "Well,
why can't I use a plastic dampener? My pressure is only 20 psi." Right? Low pressure system,
so what? Because we don't want to use plastic dampeners for water hammer or as surge devices,
The problem is, this 432 psi is cumulative to the system pressure. So, you may be at
20 psi in the system, but now you're going to add the system pressure to the increase
in pressure and if the system pressure is 100, now we're at 532 psi. Even taking that
away, let's assume we've got plastic pipe, right? Or a plastic valve, some low pressure
gauges, plastic flanges rated to 150 or 200, 250 even; and, we're hitting it with 432 psi
above the system operating pressure. It's a formula for disaster and it happens all
the time. What causes this? We talked about a change
in velocity, rapidly. Right? Valve opening and closing, pump start and stop, pump power
failure -- potentially one of the most dangerous areas there is -- piping profile and direction
change, and column separation. There are other factors that can create or increase the potential
problem here. Entrained air because one of the things we
know about and we'll see later on is that for all intents and purposes, liquid is not
compressible. Right? But if there's a lot of air in it, or if it's hot water there's
a lot more air in it, you get that compressibility factor in there and now we've got a spring
that can actually increase that spike as this water comes crashing to a stop it's still
moving because there's air compressing in it and then all of a sudden you get a secondary
slam. [Audience: "Gary, are engineers not taught
this in college?"] No -- the sad thing is no. We had a young
engineer that had just graduated and worked for a summer in a work program at Blacoh years
ago; Eric. I asked him about that and he said, "You know, in four years of college we touched
on this for about 30 minutes." You can't believe the number -- well, you can -- the number
of systems that are designed without any concern or thought whatsoever for the consequences
of valve closure. I took a course from an instructor several
years ago who was a ASME instructor. We did a bunch of problems and one of them, I'll
always remember it, was a 36 inch diameter pipe, ten miles long and on a slight incline
of 3 or 4 degrees. You calculate -- and you can do this -- you calculate the time to close
the valve which will prevent closing it, or slowing down the velocity, too quickly. It
took 24 minutes to close that valve so you wouldn't have a rise in pressure.
Remember, the whole key is rapid change in velocity. If you can avoid the rapid change
in velocity, you don't have a problem. And of course, that's where the Blacoh dampener
or surge suppressor comes in. Typically closing within 1 ½ seconds but,
depends on fluid velocity, a quick valve closure is similar to a train wreck in a pipeline.
The engine stops, but these cars keep moving. Especially if there's more air in the liquid
or more air between them. That first car stops and the rest of them still move until you
get the big crash. I like to use the example of the crash dummy
test. You've got this block wall. You've got a car travelling at 60 miles an hour -- we'll
get into this a little bit more -- and the car hits the block wall. It stops rapidly.
Velocity changes rapidly. What happens? The energy gets absorbed in the car, right? Into
the test driver and everything. If you put a spring at the wall so that the car came
to hit the spring first, that spring would absorb the energy and slowly decelerate the
car so that by the time the car hits the wall, there's no damage because the velocity is
gone, slowly dissipated. That's exactly what a Blacoh surge suppressor does, and we'll
talk about that more. But, think about it. The valve closes. The liquid is coming full
force into the closed valve. If we have a surge suppressor properly charged and sized
right there upstream of the valve, the liquid is directed up into the surge suppressor,
which is just a big spring. So that liquid has a place to slow down and we change the
velocity speed slowly, and no pressure spike is created. And you can see that in that coil
that we have. Energy concentrates, it reverses direction, the pressure wave moves at the
speed of sound in water, hits the check valve or pump and it reverses direction again back
and forth. Energy is absorbed by friction after several waves.
This is a check valve in a system and we'll do this, I think. Pressure on the downstream
side is usually a drop in pressure depending on what we talked about. The surge which is
on the downstream of the valve. That's the valve closure and hitting a check valve.
Most all pump systems -- see, there's the valve closing -- most pumping systems
will have a check valve at the discharge of the pump to protect the pump when the pump
is turned off. That's the check valve there that's slamming. This is that whole system.
So, to control the valve closure hammer, slow the liquid velocity. Use a slow closing valve,
use stronger pipes and braces; use relief valves, surge tanks or bladder surge suppressors.
You can use any of these. The whole idea is either you're going to have to contain that
energy or prevent it from occurring, or transforming its makeup. I'm not going to go through all
of these carefully. One the things that you can do is control
the valve closure time but, typically they say if I want the flow to stop, I want it
to stop as quickly as I can get it to stop so they use quick closing valves. You can
use a surge tank, which is basically just a stand pipe. The problem there is that you
can't keep the air separated from the liquid so it water logs and loses its effectiveness.
The bladder surge tank is the best permanent, lowest maintenance product you can use to
control this surge or water hammer phenomena because it keeps the gas separate from the
liquid -- we all know that. This is just some examples of what can happen now when you're
running a pump, and typically a centrifugal pump but, it doesn't have to be.
One of the problems that can occur is when you have a pump start and the system line
is full of liquid but stationary. This is the problem you have with big sprinkler systems,
that's why there always has to be some sort of surge suppressor in these systems because
you're starting the flow of liquid against a block wall, and when that flowing liquid
hits the solid liquid that's not compressible and you get a big bang.
The other situation that can cause a lot of problems is when the line is empty, because
now you're pushing liquid rapidly down an empty line that has only air in it. The air
is going to move much more quickly without resistance until you reach some point at the
end which can be a reduction, an elbow, or any other thing that creates, again, a rapid
change in that velocity. Then there are other problems that start making
these things complicated and that's pump profile. You could have liquid here and then a rise.
When you turn the pump off this area can be filled with air and then down below can be
liquid again. You start moving that liquid against air and then it hits a solid, non-moving
piece of liquid again, and then you get all kinds of problems.
Some of these get pretty complicated in the profile. Sometimes we have to get help in
doing these things but, most of the ones we deal with are pretty straight forward. Now,
this is what's called rapid pump shutdown and you're going to see column separation.
This also is a significant problem because, when you have liquid flowing at the discharge
of the pump down the line and you turn the pump off, the flow stops coming out of the
pump but, that fluid will tend to return or reverse and come back because, one of the
main reasons is you've got a section of pipe that now with nothing in it, no air release,
so you lower the atmospheric pressure in this piece of pipe and the liquid gets sucked back
in. It can be actually sub-atmospheric -- you can go below atmospheric pressure that section
of pipe from the pump as the fluid moves away from the pump. So, this is what happens.
Now watch this. See it come crashing back? That's the reciprocating effect. And what
is happened here is -- that's that acoustic vibration reciprocating -- but here we're
flowing along, the flow stops, the pressure reverses and all of a sudden we get a big
gigantic pressure spike as the flow reverses back to the pump. Now we don't know how long
that pipe is but that's happening pretty quickly and that could be a 500 foot long piece of
pipe. This is a failed pipe underground. Cars going
passed, there's water on the ground now. That's significant water hammer. This is big. That's
a truck. There's the manhole cover; it's huge. [Audience: "Where is this?"]
[Audience: "This is a domestic one; this is in the United States."]
Alright, I'm going to go on. So, the ways you control start/stop are to
use air relief valves, vacuum breakers, slowly opening and then slowly closing the valve
at pump discharge, check valve at the pump discharge, surge tanks and bladder suppressors.
Some of the same things you could use before. Again, the goal is to have the velocity of
the fluid change slowly. When you have slow change you don't get a buildup of energy all
at once. The suppressor acts -- we talked a little about the spring and the block wall
-- but, the same thing on stop/start. If you throw the liquid into the line you're going
to have velocity hitting a non-moving column of liquid and a spike. But, putting a dampener
in is like putting in a spring interrupting the system between the pump, which is the
hammer, and the rod which is the liquid stationary in the line. The spring absorbs that rapid
start of energy by allowing the liquid to go up into the suppressor and hold it there
until the speed of the solid or non-moving column of liquid starts moving.
It would be like let's say you're going to push a car that is stalled. You come up to
a too fast and you hit the back to the bumper you get an energy event and you break bumpers.
But, if you put a spring on the bumper of the car you're pushing and then come up on
it and hit that spring first, it will first absorb some energy and then start accelerating
the car in front of you and there's no damage will be done.
Potentially the most dangerous situation of all is power failure in which the pump is
running, producing flow and they lose power which is, unfortunately, more common than
people realize especially in some rural areas and things like that. What happens is the
pump stops flowing liquid. Now, remember our example of the liquid keeps moving and then
reverses direction and it reverses, and I can't give the formula or the exact wording
of it right now but, it reverses at the same velocity that it went out. So now it's reversed
and it's coming back. Just as it gets to the point of the pump, the pump starts again.
It's a momentary power failure. Now, we have a head-on collision. So, we've got the energy
of this liquid coming and the energy of the liquid coming from the pump and they collide
somewhere in this area and it is just catastrophic at that point. And again, putting a surge
suppressor there is going to protect the system because it gives a spring where these two
liquids coming together can go up and decelerate against.
Now, in all of these situations we're talking about, with the exception of downstream valve
surge, this device needs to be placed in the direction the flow is coming. If it's coming
back this way, you would have a check valve here and it would hit the check valve and
go up in in here. If it's coming from the pump, then you want this to flow up into the
pump and the check valve would be downstream. So, the energy side basically is where the
device, or surge suppressor, goes. Pipeline profile, again, can get very complicated
mainly because most customers don't know it -- don't even know their profile. But, as
you drive around the city, did you ever notice on some corners these green little standpipes
with a neck on it and a tank? Did you ever see those? Inside is a float on a hinge they're
designed to let air out of the underground water system so you prevent something like
that. Those are nothing more than air release valves that are connected underground to the
water system. You see them all over the place if you take the time to look.
Water hammer in my review is an acoustic pressure transient or wave. It can occur whenever fluid
velocity changes rapidly and remember, we talked about rapidly can start at 4 feet per
second which is not really high velocity. I remember doing calculations with Wilden
and we'd be up to 8, 9, 10 feet per second and you've really got a potential situation
for disaster when you get to those levels. One of the problems is companies, especially
contractors, like to try and get a low bid -- they'll undersize the pipe. Well, to get
the same flow out of an inch and a half pipe that you get out of a 2 inch pipe, what do
you have to do? You have to increase the velocity, which increases the pressure, which gives
you that base pressure that you're going to raise up or increase when you have this event
occur. So, anyway, I say 5 feet per second there,
start looking at it at 4 feet. This is what we need to know: what can happen, why will
it happen, where will it happen, and then what can we do to prevent it. The toughest
thing, when the customer calls us, we ask "What size pipe is it?" Well, it could be
2, 3 inches. "How long is it?" It's 4, 5, 100 feet long, or 200. You know, we can't
solve the problem unless we get some relatively decent technical information. So, we have
to keep digging and digging and digging. Now, this is a computer generated profile
of a valve closure. What you're going to see is this pump profile and the arrows going
back and forth with the oscillation, they'll change from blue to red and then a graph profile
of this pressure spike. This is the whipping action. See the arrows changing as the oscillation
occurs, and the pressure spikes going all over the place, and in the profile up there
of the pressure spikes dissipating as it oscillates. This is extreme stuff but it happens every
single day. We had one situation with Graham. It was a
diesel filling station for diesel automotives and they had Blackmer sliding vane pump, we
talked about those the other day, that they would use to pump the diesel to fill the locomotives
and these are like four inch lines, done manually. Well, when tank got filled the operator closed
it, sent the shockwave back, blew all the sliding veins out of the pump. So, we ended
up putting, I think in that case it was a 40 gallon unit, out there. But my point is,
this was all designed by an engineering firm -- somebody who should have known the potential;
it wasn't even considered. Ninety percent or more of the applications
we get into involving surge, and we get them almost every day, ninety percent of them are
after the system has been built and operating and then they find they have a problem. But,
after the plant opens and after the first event occurs, then they're scrambling and
sometimes it's really difficult to find a place or a spot to put the units in. And if
you don't put it in the right place, it's not going to work.
Remember we said this is traveling at the speed of sound in liquid, which can be as
high as how much? Forty seven hundred feet per second. So once that acoustic wave starts
moving, you can't capture it. It's going to go right pass the stabilizer inlet. But, if
you put it right upstream of the valve, within 10 pipe diameters, it hits the valve, as its
decelerating it's right there, it's going right up into the surge suppressor which is
the big spring. The spike never happens. When the liquid then stops, the pressure stabilizes.
That liquid that's been accumulated against the gas charge just pushes back into the line.
No harm, no foul, no problem. Another article I wrote was based on a true
story at Behr Paint. I could have told them, just slow the flow. You know, changing from
a ball valve or a butterfly valve to a gate valve - like we've got on our hose pump out
back. It takes about 15 turns to close it but, by that time, you're slowly closing it,
the velocity of the liquid is slowing down -- no pressure increase. But, if you take
a quarter-turn ball valve, you're stopping it quickly.
It was in Ohio in Sherwin Williams' plant. They had a Wilden 2 inch pump bolted to a
concrete pad with welded stainless steel tubing that went up about thirty feet in the air
in the ceiling, across the plant, came down, and a man was filling totes and closed that
valve. It ripped that two-inch pump off the concrete pad, turned it 45 degrees and bent
that 2 inch stainless steel piping. The energy is incredible. Just go stand in the waves
and let a wave hit you with the velocity and the mass of the liquid will knock you over.
