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. We're going to have a little discussion about
acceleration head. Acceleration is the change in velocity and
we can see this in basically a pulsation curve which is a sine curve, a modified sine curve,
depending upon the type of pump used. In piston, in diaphragm, even in a rotating
peristaltic pump, a given amount of liquid is captured and then expelled. What happens in that process is, if you think
about a piston, it pulls back to charge on the inlet stroke and then on the discharge
stroke it starts moving through the chamber and pushing liquid out. In the initial phase of that movement of the
piston there's no flow, but the flow starts and builds or accelerates, if you will, through
the stroke until it reaches midpoint and then the flow starts slowing down until you reach
the point where the piston and the diaphragm pulls back and recharges on the inlet stroke. And the flow, depending on the type of pump,
can actually stop momentarily. Well, this curve is not only a flow curve,
but it's a pressure curve. Because at the start of the stroke there's
no pressure. Pressure builds to the midpoint and then starts
reducing down as the flow decreases through the stroke cycle. Now, pressure also creates a vibration and
other components that occur during the stroke cycle; vibration, all the other elements. For that pulsation, we use a pulsation dampener. But when we start talking about acceleration
head it's the energy, from our point of view, required to accelerate or change the velocity
of the liquid in the system from at rest or zero to some point in the cycle. A race car, for example, starts out at zero,
accelerates to a maximum point through the lights and then stops; decelerates. It would, in effect, be a similar type curve. Relating it to our situation, it takes energy
to move that liquid or accelerate that liquid. On the discharge of a pump, it takes that
energy to accelerate or move the liquid down the pipeline. Think of an analogy of a freeway system and
the freeway going to a large city, the freeway has a bypass around the city. If you take the bypass and hold your speed
at a steady speed of, whatever, sixty miles an hour, you're going to use a certain amount
of energy. Remember Newton's law about a body in motion
tends to stay in motion unless acted upon by an opposite or equal reaction. But, if you get off the freeway and decide
to drive through town and hit every single stoplight, now you're decelerating and then
re-accelerating, then decelerating and then re-accelerating. That takes a lot more energy or more gasoline
than if you stayed at a steady speed driving around the city. Now, the point of this is, and we're going
to talk about the discharge side of the pump but also the inlet side of the pump. But the point of this is -- and as an aside
just remember when we're talking to people that all friction charts that are published,
which is a pressure loss per typically 100 feet in a pipe, are based on steady state
flow; centrifugal flow if you will, like out of a faucet. That is, once you get the liquid moving it
stays constantly moving; takes less energy once it's moving. When you start a centrifugal pump there is
a peak of power used to initially get the liquid moving. Then that peak energy drops down to an average
flow, or average energy use, as the product is flowing. But, with a reciprocating pump that flow is
not constant. So, if you try to use those pressure loss
charts that are published, it would not relate at all to the energy required or the loss
due to friction created by a reciprocating pump because we are starting and stopping
the flow just like going through town. So, if you take a pulsation dampener and put
it at the discharge of the pump (there could be a pump), on the discharge stroke of the
pump a certain amount of liquid goes up and is accumulated into the dampener. Remember, there's a gas charge on the top
side of the dampener, but it's always at a lower pressure than system pressure. So a certain amount of liquid goes up into
the dampener. Now that takes some energy to push up against
this resistance. But when the pump reaches its peak and starts
to stop (or at the end of its stroke) and retracts to recharge, now the pressure in
the dampener is higher than the system pressure so this accumulated or stored liquid is pushed
back into the system by this stored energy that the pump provided, and it evens this
flow so that we don't have a peak or a valley, but we tend to have near steady state flow
like that produced by a centrifugal. Well, when the system is in near steady state
flow, we are not using excessive energy. We're using the pump's energy but we're not
losing it. We're just storing it momentarily and then
re-introducing it back into the system. It's relatively easy on the discharge of the
pump to overcome acceleration head or deal with it because you've got the power of the
pump, whether its compressed air, or a motor or whatever form of energy generation is attached
to the pump, so that even if you don't have a pulsation dampener, you have the power to
do it; takes more energy but, you have the power to do it. Where we can have problems is on the inlet
side of the pump, because now that acceleration head can work against you and you only have
atmospheric pressure as the form of energy or power on the inlet side the pump. Remember, the pump can't do anything until
the liquid gets into it and it applies its energy at that point to discharge it. But up to that point, all we have is atmospheric
pressure -- at sea level 14.72 psi. Now, what's important about that is that if
we end up with a relatively long inlet system before our pump -- we don't have to go back
to that friction loss chart, remember we talked about steady state flow verses reciprocating
or start stop flow. So, the pump is going to create a low pressure
area, assuming that we don't have a lot of inlet pressure. Atmospheric pressure, whether it's a tank
or whatever, is going to push the liquid into the pump but it has to overcome this friction
loss. In other words, every time this pump shifts
-- remember our sine curve -- every time this pump shifts not only does the flow slow down
on the discharge side, but the inlet valves close and the flow slows down and stops -- depending
on the number of pistons in the pump -- stops on the inlet side. Now, you've got to reaccelerate that flow,
right? On the discharge side we're pushing it with
energy, but now we've got to restart that flow. In the loose sense of the word it's coming
in in slugs. Well, if you have a lot of friction loss,
you're going to lose flow because remember we've only got this 14.72 pounds. If we were to install an inlet stabilizer
here, and in some cases depending upon that inlet pressure actually put a vacuum in this,
when the pump's valves close this liquid is starting and stopping but it will accumulate
in the inlet stabilizer, provide a ready source of product or liquid right at the pump's inlet
so that during the shift when this flow slows down we've accumulated liquid in the accumulator
or the inlet stabilizer, so the pump then when it shifts, pulls this liquid out of here,
it's not having to reaccelerate the whole line. Takes less energy, ensures that there's not
cavitation, ensures that we get full fill in each of the chambers, and is particularly
important in multiple piston or multiple diaphragm pumps. And I said, depending upon what the conditions
are, we may actually pull a vacuum in the dampener. The idea is when these valves close on this
pump, this flow instead of stopping altogether cycles up into here, keeps in motion like
the car going around the city on the freeway, so that there is less energy expended when
the pump shifts and the inlet valve opens to the chamber. Now the other scenario that can happen is
that we have too much positive pressure going to the pump. Now, because of this acceleration head and
the pressure involved, when the valve closes we create water hammer and all water hammer
is, is a change in velocity or a change in acceleration, typically rapidly. And that occurs when this valve ball closes
but the product is moving because there's acceleration. We have a force which is the liquid, created
by the mass moving and its speed. That force wants to go somewhere. It can be fairly significant depending again
on the conditions and when the valve ball closes that force is created. It can damage components and pumps. It can damage gauges. It can damage gaskets, flanges, threaded joints. It can cause all kinds of problems because
that pressure spike can be four to as many as eight times greater than the flowing pressure. Well, now we charge this inlet stabilizer
different and it acts like an accumulator and stores energy much like it does on the
discharge side, so that when the valve balls on the inlet of the pump close - instead of
this liquid continuing to move because it's under acceleration, slamming the check valve
and creating a big pressure spike - now the inlet valve balls close, the liquid is cycled
or accumulated up into the dampener, the acceleration is decelerated at a controlled slower speed
so no pressure spike occurs. And we've accumulated liquid again right here
so that when the valve balls opens on the inlet stroke, liquid is immediately available
and this stays in motion. Now, we have effectively changed this acceleration
head, whether its loss or pressure, into steady state flow at a constant pressure and a constant
energy requirement. So, the whole point is to change this reciprocating
flow, and it's either a gain or loss from acceleration head, into a constant steady
state flow so that we eliminate the problems created by this acceleration head. Now there are formulas that allow you to calculate
both pressure and flow loss or energy required to overcome the resistance to flow of the
acceleration head, so that you can properly size the piping and elbows and all the other
components in the system, to maximize efficiency and minimize the use of energy. But, most, many people who do these calculations
to ultimately determine what the total dynamic head of the system is -- total dynamic head
basically is all those components, the friction loss, pipe size, elbows, all the other things
-- to determine what size pump is needed to produce the flow at the discharge pressure
required, if you don't use inlet stabilizers and pulsation dampeners, you can't use a key
component that formula which is the friction loss created by the steady state flow in the
pipe. So you can't use those charts. You can use acceleration head components and
bring that into the equation but, with the installation inlet stabilizers and pulsation
dampeners, you can just basically go to the standard friction loss charts and use those
and they will be accurate. In addition obviously, to minimizing the energy
use, we're going to protect system components, gauges, valves, flanges, any other components,
diaphragms, and the components in the pump. The inlet valve balls, diaphragms; all the
various other components that would receive shock or in the case of cavitation, the damage
caused by the collapse of high-pressure energy when it hits the piston face or the diaphragms
in the pump. It can be rather complicated but it can be
reduced down to a relatively simple proposition and calculations if we just have everybody
understand what this acceleration head or force is and how to minimize it, maintain
it or control it. In order for the inlet stabilizer, or the
pulsation dampener on the discharge, to control or minimize acceleration head, it must be
properly located. For example, the inlet stabilizer is going
to handle this acceleration head up to the point where the dampener is installed in the
system. Not between the pump and the dampener, but
from the beginning source of the liquid to the dampener. So, if we move the dampener here, for example,
everything from here on is going to have this acceleration problem because the valve is
opening and closing and affecting this length of pipe flow. So the stabilizer or the dampener needs to
be installed as close as possible to what is causing this acceleration head loss or
positive pressure. Now, we recommend within 10 pipe diameters
of the pump's inlet or 10 pipe diameters of the pump's discharge. But it can't be effective if it isn't acting
on the full flow volume of the liquid in the system. Size, location and charge is exactly what's
required. Not only to minimize pulsation, but to control
and reduce acceleration head. In effect, with the inlet stabilizer or the
pulsation dampener located within these ten pipe diameters, we have eliminated acceleration
head. You won't totally eliminate all of it, but
we have eliminated the majority of it to the point where it no longer has to be a consideration
on the inlet side and everything past the dampener on the discharge side. We in effect have turned this, into this. You'll see it if you look at a pressure gauge
at the discharge of a reciprocating pump, because at this point of peak acceleration,
the gauge may be at a hundred. At the valley, depending on the type of pump,
the number of cylinders or pistons, the gauge may be reading 20. But when you've minimized or eliminated acceleration
head, and we have this flow, you'll reach a steady state, again depending on the type
of pump, the number of cylinders, somewhere in the neighborhood of 40 psi. That's the true number that you would get
if you size the system and figured the total dynamic head using the friction loss charts
on a centrifugal pump.
