LDi Catalog Printable PDF P4, P5, P6, P7, P8, P9
The danger is that the pipe system will generate a frequency that it amplifies.
1. Find the effective compressibility for the system.
2. Determine the speed of pressure transfer for "softness".
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Feet per second or |
Meters per second = Frequency/Hz. |
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feet pipe run |
meters pipe run |
4. Compare System Hz. with pump Hz. and Pipe XY&Z natural shaking Hz.
5. Where there is coincidence, or at a power of, any of them, at any temperature, or for any one nodal length, your pipes are a problem
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The speed of the fastest projectile (2800 feet per second) is half the speed of pressure transfer (5600 feet per second) in a cold liquid de-gassed in a hard pipe.
Rate of pressure transfer a.k.a. "acoustic velocity". |
Easy to prove - Change the pressure, measure it a mile away a second later.
But your flow rate is between 3 ft / sec and 25 ft / sec so flow fluctuation could be addressed with a bottle on a "T".
Pressure is 250+ times faster than flow, so:
Pressure pulsation can not be addressed without interception.
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It doesn't matter how much you have to begin with, what counts is how much the system turns it into.
Large pipes - with no delta "P" do not scrub out pressure waves - are potential amplifiers.
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LDi Page 5
Pulse interception plus flow fluctuation reduction
Or just use one of our connections, if all you want is volume accumulation.
Then save a "T", and use the second connection for flush, drain, relief valve, or the PI tap.
It's too simple, trying to pass through a sudden increase to a larger diameter dampens high frequency pressure pulsation (large relative to system pipe diameter).
Pressure waves go 500 mph in hot gas, but travel 3500 mph in cold liquid.
A "T" piece could not intercept them.
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LDi Page 6
Truth proof about discharge pulsation
Pipe systems cause pulsation. Hard systems destroy pumps.
Same pump, no pulsation
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Same pump, lots of pressure pulsation
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Pumps make flow fluctuations, Systems convert them to pressure pulsation.
As truth is defined by "what if-what if not" you proved to yourself that pipe systems make pressure pulsation.
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When we were seventeen and could run a 1/4 mile in 60 seconds, our pumps could work because our veins and arteries were soft and "dilatable".
When we die in bed with out boots on, and the quack say "of natural causes", his pronouncement is simply that our "pump" was tired because our "pipes" became hard.
Restrictions make systems "hard". Hard systems kill pumps.
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LDi Page 7
Pulse damping - infested with "witchcraft"?
How much pulsation was there before the "pulsation dampener" was fitted?
And how much after fitting the single connection "pulsation dampener".
It can be impossible to know, without data capture at kHz and very fast response characteristic transducers.
Can a human eye see above 30 Hz? Or a 20 ms response gauge measure a 1450 meters / sec pressure pulse go by?
Spring weight
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How fast they shake, how far they swing depends on the weight and the spring - not on what excites them. |
Eight gauges connected to the same point at the same time, which to believe?
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With glycerin and choke |
With glycerin, no choke |
No glycerin but with choke |
No glycerin, no choke |
Gauges connected through a capillary |
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Gauges connected through a 1/2" pipe |
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LDi Page 8
Suction side pulsation
The problem - absorbed air or gas
"Started" for Reciprocating pumps means each suction stroke.
When a pump is started, there is an instant pressure fall at the pump end of the system.
Unless intercepted, a negative pressure wave travels back up stream to the point of supply.
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The sudden pressure reduction causes gasses to come out of solution. They form bubbles, which join then act as springs between slugs of liquid.
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The slugs are excited into oscillation by the rebounding negative wave. The slugs then alternately slam then starve the pump suction. The higher the pressure the harder the slam.
Air / gas out of solution
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Excited mass oscillation between two springs |
If the pump has suction check valves, they are knocked open when they should be closing. (see "VE" below)
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The force from the mass velocity of the slugs is more than check + spring |
Volume is good, force is bad |
Do not increase force / pressure |
When you are thirsty, you lift and pour the liquid in, you don't force a pressurized hose down your throat!
The biggest enemy of good volumetric efficiency for a reciprocating pump, is too much pressure on the suction side.
"VE" - Volumetric efficiency falls, and pulsation shakes the pipes, which causes even more pressure pulsation.
See Page 8 item A
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LDi Page 9
Suction side pulsation
The answer - 2 do's and 2 don'ts
"Started" for reciprocating pumps means each suction stroke.
Local suction volume elasticity is an essential.
2. Do not insert orifices or restrictions they will cause wave rebounds at a higher frequency, and cause frothing.
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